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Fourier-transform infrared spectroscopy

Fourier-transform infrared (FTIR) spectroscopy is a technique that measures the absorption or emission of infrared radiation by a sample to produce a spectrum representing molecular vibrations, providing a unique "fingerprint" for identifying chemical composition and structure in solids, liquids, or gases.^[1] In this method, broadband infrared light from a source passes through an interferometer, where it is split into two beams by a beamsplitter and recombined after reflection from fixed and moving mirrors, generating an interferogram that encodes all wavelengths simultaneously.^[2] A computer then applies a mathematical Fourier transform to convert the interferogram into a conventional spectrum plotted as intensity versus wavenumber (typically in cm⁻¹), focusing primarily on the mid-infrared region (4000–400 cm⁻¹) where fundamental vibrational modes occur.^[1] The core component of an FTIR spectrometer is the Michelson interferometer, invented by Albert A. Michelson in 1881 for precision measurements of light wavelengths, which laid the groundwork for encoding spectral data efficiently.^[3]^[4] The first practical Fourier transform infrared spectrum was recorded in 1949 by Peter Fellgett, marking the beginning of FTIR as a viable spectroscopic tool, though commercial instruments did not emerge until the 1960s and 1970s with advancements in computing power for fast Fourier transforms.^[3] Early FTIR systems overcame limitations of dispersive infrared spectrometers, which scanned wavelengths sequentially using prisms or gratings, by enabling multiplex detection (Felgett's advantage) for faster acquisition—often in under one second—and higher throughput (Jacquinot's advantage) for improved sensitivity.^[1] Additional benefits include mechanical simplicity with only one moving part (the mirror), internal wavelength calibration via a helium-neon laser, and reduced noise through signal averaging, making FTIR non-destructive and suitable for trace analysis without external standards.^[2] FTIR spectroscopy excels in qualitative identification of organic and some inorganic compounds by matching absorption bands to specific functional groups, such as C-H stretches around 2900 cm⁻¹ or carbonyls near 1700 cm⁻¹, and supports quantitative analysis through Beer's law for concentration determination.^[5] In biological and medical applications, it enables label-free, non-perturbative imaging of tissues and cells, distinguishing healthy from diseased states (e.g., benign versus malignant tumors in breast or prostate samples) via biochemical fingerprints in regions like amide I/II bands (1500–1700 cm⁻¹).^[6] Broader uses span materials science for polymer characterization, environmental monitoring of pollutants, pharmaceutical quality control, and forensics for substance identification, with modern extensions including microspectroscopy for spatially resolved mapping down to micrometer scales.^[5]

Introduction

Conceptual overview

Fourier-transform infrared (FTIR) spectroscopy is an absorption spectroscopy method that utilizes a broadband infrared light source, an interferometer, and a detector to generate infrared absorption or transmission spectra via mathematical Fourier transformation of the acquired interferogram. This technique enables the identification and quantification of molecular species by measuring how infrared radiation is absorbed by a sample.^[7]^[8] The fundamental principle of FTIR relies on the interaction of infrared light with matter, where photons are absorbed at specific wavelengths corresponding to the excitation of molecular vibrations, rotations, and phonons in solids. These absorptions occur when the energy matches the frequency of bond stretching, bending, or other modes, producing distinct bands characteristic of particular chemical bonds or functional groups.^[8]^[7] The output of an FTIR measurement is a spectrum plotted as absorbance (or transmittance) against wavenumber, typically in the mid-infrared region spanning 4000 to 400 cm⁻¹, where most organic functional groups exhibit strong absorptions. A notable advantage of the FTIR approach is the multiplex (Fellgett's) advantage, which arises from simultaneously detecting all wavelengths across the spectrum, thereby enhancing the signal-to-noise ratio compared to sequential scanning methods.^[9]^[7] For instance, the C=O stretching vibration of carbonyl groups, such as in ketones, produces a prominent absorption peak near 1700 cm⁻¹, aiding in the structural elucidation of organic compounds.^[10]

Historical development

The discovery of infrared radiation in 1800 by Sir William Herschel marked the inception of infrared spectroscopy, as he measured elevated temperatures beyond the red end of the visible spectrum using a prism and thermometers, revealing the existence of invisible heat rays.^[11] Practical infrared spectroscopy emerged in the mid-20th century with the development of dispersive instruments in the 1940s and 1950s, which employed prisms or diffraction gratings to isolate wavelengths sequentially for analysis of molecular vibrations in organic compounds. However, these systems were hampered by slow scanning times, often requiring minutes per spectrum, low signal-to-noise ratios due to narrow slits limiting light throughput, and poor sensitivity for weak absorbers, necessitating more efficient alternatives like interferometry.^[12] The foundational technology for Fourier-transform infrared (FTIR) spectroscopy stemmed from the Michelson interferometer, invented by Albert A. Michelson in 1881 to measure the speed of light with unprecedented precision through interference patterns.^[13] In 1951, Peter Fellgett theorized the multiplex advantage of interferometric techniques, wherein simultaneous measurement of all wavelengths enhances signal-to-noise performance compared to dispersive methods, a principle that would prove pivotal for FTIR's sensitivity gains; Fellgett had earlier recorded the first practical Fourier transform infrared spectrum in 1949 during his doctoral work.^[14]^[15] This concept gained traction in the 1960s when Larry Mertz and Henry A. J. Macleod adapted the Michelson design for infrared applications at Block Engineering, demonstrating the first viable FTIR system by converting interference patterns, or interferograms, into spectra via mathematical transformation.^[16] Concurrently, Pierre Connes and collaborators in France pioneered high-resolution FTIR using advanced step-scan interferometers and precise wavelength calibration for planetary and atmospheric studies.^[17] The commercialization of FTIR accelerated in 1969 with Digilab's introduction of the FTS-14, the first fully automated commercial instrument, which integrated a dedicated minicomputer for data processing and marked a shift from laboratory prototypes to accessible tools.^[18] The 1970s computing revolution, driven by affordable microprocessors and fast Fourier transform algorithms, revolutionized FTIR by enabling rapid interferogram processing—reducing computation times from hours to seconds—and broadening adoption across chemistry and materials science.^[19] Advancements in the 2000s and 2010s transformed FTIR into portable, battery-operated systems for on-site analysis in environmental monitoring and forensics, alongside hyphenated setups like gas chromatography-FTIR for enhanced compound identification.^[20] Synchrotron-based FTIR emerged as a high-impact variant, leveraging brilliant infrared beamlines at facilities like Elettra for microspectroscopy with spatial resolutions below 10 μm and flux increases up to 1000 times over conventional sources, facilitating studies of biological tissues and nanomaterials.^[21]

Principles of Operation

Interferometry basics

Interferometry is a technique that involves the superposition of light waves to produce interference patterns, where the resulting intensity depends on the path length differences between the waves. This principle allows for the precise measurement of wavelength-dependent phase shifts in optical systems. In the context of spectroscopy, interferometry encodes spectral information into a time- or space-domain signal rather than dispersing light by wavelength.^[22] In Fourier-transform infrared (FTIR) spectroscopy, interferometry plays a central role by replacing traditional grating-based dispersion with a time-domain modulation approach, enabling the simultaneous encoding of all infrared wavelengths into a single signal. This multiplex method, first conceptualized by Peter Fellgett in 1949, provides a significant throughput advantage over dispersive instruments by collecting data across the entire spectrum at once, reducing measurement time and improving signal-to-noise ratio for broadband sources. The technique modulates the infrared light through interference, capturing the full spectral content in a raw form that can later be decoded.^[23]^[24] The optical path difference (OPD) is the key parameter in interferometry, defined as the extra distance one light beam travels relative to another, which varies systematically from zero to a maximum value to generate the complete interference signal. At zero OPD, the beams are in phase, producing maximum constructive interference, while increasing OPD introduces phase shifts that depend on the wavelengths present. This variation in OPD effectively samples the interference across the spectrum.^[1] For a broadband infrared source, akin to white light in the visible range, the interference pattern exhibits a sharp central burst of intensity at zero OPD, where all wavelengths constructively interfere, followed by a rapid decay in amplitude as the OPD increases due to the dispersion of phases across different wavelengths. This characteristic envelope arises because shorter wavelengths lose coherence faster than longer ones, limiting the visibility of fringes to small path differences. The resulting pattern encodes the superposition of contributions from each wavelength component in the source.^[22]^[23] The interferogram is the raw output of the interferometric process, plotted as the intensity of the recombined light versus the OPD, representing a time-domain signal that contains the encoded spectral information from the infrared source and sample. This plot typically shows a high central peak tapering off symmetrically, with the entire curve serving as the input for subsequent Fourier transformation to recover the frequency-domain spectrum.^[25]^[1]

Michelson interferometer design

The Michelson interferometer serves as the core optical component in Fourier-transform infrared (FTIR) spectroscopy, enabling the generation of an interferogram by modulating the optical path difference (OPD) of infrared light. It consists of a beam splitter, a fixed mirror, and a moving mirror arranged in a configuration where the infrared beam from the source strikes the beam splitter at approximately 45 degrees. The beam splitter divides the incident radiation into two equal parts: one portion transmits to the fixed mirror, while the other reflects to the moving mirror, both positioned perpendicular to their respective paths. Upon reflection, the beams return to the beam splitter, where they partially transmit and reflect to recombine and exit toward the detector, producing interference patterns as a function of the OPD created by the moving mirror's position.^[26]^[27] The beam splitter is critical for achieving balanced division and recombination of the infrared beam, typically designed as a thin film-coated substrate that provides a 50/50 split across the relevant spectral range. Common materials include potassium bromide (KBr) for mid-infrared applications (covering approximately 400–4000 cm⁻¹), calcium fluoride (CaF₂) for broader mid-infrared coverage, and other substrates like zinc selenide (ZnSe) or barium fluoride (BaF₂) depending on the wavelength range, with dielectric or metallic coatings such as germanium or gold to ensure efficient splitting and minimal absorption losses. These materials are selected for their transparency in the infrared region and ability to maintain a consistent splitting ratio over the operational spectrum.^[28]^[29]^[27] The moving mirror is driven along a linear path to vary the OPD, typically using a voice coil actuator or linear motor for smooth, precise motion at constant velocity, ranging from 0.1 cm/s for high-resolution scans to several cm/s for faster acquisitions. This mechanism ensures uniform scanning, often over distances of a few centimeters, to produce the required interferogram. Stepper motors may be employed in some designs for discrete positioning in step-scan modes, but continuous scanning with voice coil drives is preferred for rapid, vibration-free operation in standard FTIR systems.^[30]^[31] Precise alignment and stability are essential to minimize tilt errors and maintain optical path integrity during mirror motion. A helium-neon (HeNe) laser reference beam, operating at 632.8 nm, is commonly integrated into the interferometer to track the moving mirror's position by monitoring interference fringes, with sampling triggered at zero-crossings for sub-wavelength accuracy (on the order of 0.3 μm). This laser provides a stable reference for velocity control and OPD calibration, compensating for mechanical vibrations and ensuring long-term stability in commercial instruments.^[32]^[33] Variations in the standard Michelson design enhance performance for specific applications. Double-sided interferometers, where the moving mirror scans past the zero-OPD point to record symmetric interferograms on both sides, improve signal-to-noise ratio and efficiency by allowing background subtraction and extended path differences without doubling scan time. Corner-cube (retroreflector) mirrors replace plane mirrors in some configurations to reduce sensitivity to misalignment and tilt, as the retroreflectors automatically return the beam parallel to the incident direction regardless of small angular deviations, enabling robust operation in portable or field-deployable FTIR systems.^[34]^[35]^[36]

Interferogram acquisition and processing

In Fourier-transform infrared (FTIR) spectroscopy, interferogram acquisition begins with the detector capturing the intensity of the recombined infrared beam as the movable mirror in the Michelson interferometer scans linearly, generating a time-domain signal known as the interferogram that encodes the sample's absorption information across all wavelengths simultaneously.^[7] This raw signal is sampled at the Nyquist rate—typically twice the maximum wavenumber of interest (e.g., around 4000 cm⁻¹ for mid-infrared spectra)—to avoid aliasing and ensure faithful representation of high-frequency components, with sampling often triggered by zero-crossings of a reference helium-neon laser interferogram for precise optical path difference control.^[7]^[37] The analog interferogram undergoes digitization via an analog-to-digital converter (ADC), commonly with 16–24 bit resolution to provide sufficient dynamic range for weak signals amid strong background intensities, enabling accurate quantification of subtle spectral features.^[7] The sampling interval \Delta x directly influences spectral resolution, defined as \Delta \nu = \frac{1}{2L}, where L is the maximum optical path difference achieved during the scan (e.g., L = 0.5 cm yields \Delta \nu = 2 cm⁻¹).^[27]^[7] Pre-processing of the digitized interferogram prepares it for Fourier transformation by addressing distortions and improving spectral quality. Zero-filling appends zeros to the interferogram array (e.g., doubling or quadrupling its length) to enhance interpolation and smoothness in the resulting spectrum without altering true resolution.^[7] Apodization applies a mathematical window function, such as the Happ-Genzel function—which balances sidelobe suppression with minimal resolution loss by tapering the interferogram edges—to mitigate ringing artifacts from finite scan lengths.^[38]^[7] Phase correction compensates for mirror misalignment or sampling offsets by adjusting the complex phase spectrum (e.g., via the Mertz method), ensuring the output is purely real and absorbance-positive.^[7]^[39] Background subtraction removes instrumental and environmental contributions, such as solvent or atmospheric absorptions, by ratioing the sample's single-beam interferogram (processed to a spectrum) against a reference background single-beam spectrum acquired under identical conditions without the sample.^[40]^[41] This single-beam approach, inherent to FTIR, simulates double-beam operation through software, enhancing signal stability and reproducibility compared to traditional dispersive methods.^[42]/4:_Infrared_Spectroscopy/4.3:Fourier-Transform_Infrared_Spectroscopy(FT-IR)) Common artifacts, including cosmic ray spikes from high-energy particles hitting the detector and non-linearities from detector saturation, are handled via software filtering techniques such as median replacement or polynomial interpolation to identify and correct aberrant data points without manual intervention.^[43]^[44] These corrections preserve the integrity of the interferogram for subsequent Fourier transformation into a usable spectrum.^[43]

Fourier transform computation

The mathematical foundation of Fourier-transform infrared (FTIR) spectroscopy relies on the relationship between the interferogram in the path difference domain and the spectral intensity in the wavenumber domain. The interferogram I(\delta) is expressed as

I(\delta) = \int_0^\infty B(\nu) \cos(2\pi \nu \delta) \, d\nu,

where B(\nu) represents the spectral intensity at wavenumber \nu (in cm⁻¹), and \delta is the optical path difference (OPD). This equation arises from the interference of infrared radiation across all wavelengths simultaneously in the Michelson interferometer.^[45] To recover the spectrum from the measured interferogram, the inverse Fourier transform is applied. For real-valued, even functions typical in FTIR, a one-sided cosine transform is used:

S(\nu) = \frac{2}{T} \int_0^T I(\delta) \cos(2\pi \nu \delta) \, d\delta,

where S(\nu) is the recovered spectral intensity, and T is the maximum OPD. This transform decodes the multiplexed information, yielding the frequency-domain infrared spectrum.^[46] In practice, interferograms are digitized into discrete samples, necessitating the discrete Fourier transform (DFT). The fast Fourier transform (FFT) algorithm, particularly the Cooley-Tukey implementation, enables efficient computation with a time complexity of O(n \log n) for n data points, compared to O(n^2) for direct DFT evaluation. This efficiency, developed in 1965, made real-time spectral processing feasible in FTIR instruments.^[27] The spectral resolution \Delta \nu in FTIR is fundamentally limited by the maximum OPD, given by \Delta \nu = 1 / (2 L_{\max}), where L_{\max} is the maximum mirror displacement (with OPD = 2 L_{\max}). To mitigate artifacts like sidelobes from finite interferogram truncation, apodization functions are applied prior to transformation. The boxcar function preserves maximum resolution but introduces ringing artifacts, while the triangular function reduces sidelobes at the cost of slight peak broadening, offering a common trade-off for improved signal fidelity.^[27]^[38] Modern FTIR software implements these computations in real time, often using optimized FFT libraries for rapid interferogram-to-spectrum conversion during data acquisition. Aliasing, which can fold high-frequency components into lower frequencies violating the Nyquist criterion, is prevented through anti-aliasing filters—both optical (via beam divergence control) and electronic (low-pass filtering before digitization)—ensuring accurate spectral reconstruction.^[47]^[40]

Instrumentation

Infrared light sources

Infrared light sources for Fourier-transform infrared (FTIR) spectroscopy must provide broadband emission across the mid-infrared region to enable the capture of molecular vibrational spectra. The most common sources approximate blackbody radiators, operating at elevated temperatures to emit continuous radiation suitable for interferometric detection. Selection criteria emphasize high emissivity, thermal stability, and compatibility with the desired spectral range, typically prioritizing sources that deliver sufficient intensity without introducing noise that could degrade spectral resolution.^[48] A widely used source is the Globar, consisting of a silicon carbide (SiC) rod, approximately 5 mm in diameter and 50 mm long, electrically heated to 1000–1650 K. This source emits broadband radiation from about 4000 to 400 cm⁻¹, with high emissivity in the mid-infrared, making it ideal for routine FTIR applications. Its output is stable over extended periods, though water cooling is often required for the electrical contacts to prevent degradation.^[48]^[49] The Nernst glower, made from rare-earth oxides such as zirconia and yttria, operates at higher temperatures up to 2000 K and provides greater intensity than the Globar, particularly in the near- and mid-infrared. However, it requires preheating and has a shorter lifespan due to mechanical fragility, limiting its use in high-throughput settings.^[48]^[50] For near-infrared applications, incandescent wire sources, such as tightly wound coils of nichrome or Kanthal wire heated to around 1100 K, offer a simpler alternative with lower intensity but extended operational lifetime compared to Globars or Nernst glowers. These are particularly suited for systems requiring visible-to-near-IR overlap. Tungsten-halogen lamps provide another option for near-IR with stable output up to 4000 cm⁻¹.^[48]^[51] These thermal sources are modeled as graybody approximations to ideal blackbodies, where the spectral radiance follows Planck's law:

B(\nu, T) = \frac{2h\nu^3}{c^2} \frac{1}{e^{h\nu / kT} - 1}

Here, h is Planck's constant, \nu is the frequency, c is the speed of light, k is Boltzmann's constant, and T is the temperature; this equation predicts the emission intensity as a function of wavelength and temperature, guiding source optimization for FTIR.^[52]^[48] While continuous-wave sources dominate mid-IR FTIR, pulsed options like high-pressure mercury arc lamps are employed for far-infrared extensions, offering higher effective temperatures around 5000 K but with variable output requiring stabilization. Since their development in the late 1990s, with broader adoption in the 2010s and 2020s, quantum cascade lasers (QCLs) provide tunable narrowband emission for targeted applications, enabling higher brilliance than thermal sources at specific wavelengths.^[53]^[54] Typical power outputs from these sources range from 1 to 10 mW in the mid-infrared beam after optical coupling, with stability better than 0.1% essential for reproducible interferograms and quantitative analysis. Recent advances since 2015 include LED-based infrared sources, which offer compact, low-power operation for portable FTIR systems, achieving broadband emission through array designs while reducing size and energy demands.^[55]

Beam splitters and optics

In Fourier-transform infrared (FTIR) spectrometers, the beam splitter serves as a critical optical component within the Michelson interferometer, functioning as a semi-transparent plate or thin pellicle that divides the incoming infrared beam into two equal parts with approximately 50% transmission and 50% reflection to enable interferometric modulation.^[27] This division is essential for creating the path length difference that generates the interferogram. Common materials for beam splitters are selected based on the spectral region: potassium bromide (KBr) for the mid-infrared range (typically 4000–400 cm⁻¹), cesium iodide (CsI) for the far-infrared (below 400 cm⁻¹), and calcium fluoride (CaF₂) or fused silica for the near-infrared (above 4000 cm⁻¹). Germanium (Ge)-coated substrates extend coverage into the mid- to near-infrared.^[56]^[57]^[58] Beam splitters are typically coated with multilayer dielectric films to achieve broadband efficiency across the desired wavelength range while maintaining the 50/50 split ratio. These coatings, often consisting of alternating high- and low-refractive-index layers such as Ge and ZnSe on a KBr substrate, have thicknesses on the order of a few micrometers to minimize chromatic dispersion and ensure uniform performance.^[59]^[60] Additional optics in the FTIR beam path include gold-coated collimating mirrors, which provide high reflectivity exceeding 95% across the infrared spectrum (0.7–20 μm) to direct and parallelize the beam with minimal loss. Parabolic mirrors are commonly used for focusing the beam onto the sample or detector, optimizing energy throughput in the interferometer assembly. To mitigate atmospheric interference, purge gas systems deliver dry, CO₂- and H₂O-free air through the optical path, reducing absorption bands from water vapor (around 3700–3500 cm⁻¹ and 1600 cm⁻¹) and carbon dioxide (around 2350 cm⁻¹ and 670 cm⁻¹) that could distort spectral data.^[27]^[61] Standard FTIR beam splitters exhibit minimal polarization effects due to their isotropic dielectric coatings, preserving the unpolarized nature of typical broadband infrared sources. However, for specialized polarimetry applications, wire-grid polarizers can be integrated as beam splitters, using metallic nanowires on a substrate to separate orthogonally polarized components with high extinction ratios in the infrared.^[62] Maintenance of beam splitters is crucial, particularly for hygroscopic materials like KBr, which can absorb moisture above 40% relative humidity, leading to fogging, deliquescence, and degraded performance; regular desiccant replacement and instrument purging are recommended to extend lifespan. Non-hygroscopic alternatives, such as zinc selenide (ZnSe), offer robust options for humid environments, transmitting effectively from 0.6–20 μm without moisture sensitivity.^[63]^[64]

Detectors and signal handling

In Fourier-transform infrared (FTIR) spectroscopy, detectors convert the modulated infrared interferogram into an electrical signal for subsequent processing. Thermal detectors, which respond to incident radiation through heating effects, are widely used due to their operation at ambient temperatures. The deuterated triglycine sulfate (DTGS) detector, a pyroelectric type, generates a voltage proportional to the rate of temperature change induced by absorbed IR photons, offering a spectral range of approximately 4000 to 400 cm⁻¹ in the mid-infrared region with a response time on the order of milliseconds.^[65]^[66] This makes DTGS suitable for routine mid-IR measurements where high speed is not critical, as it provides good linearity and sensitivity without cooling requirements.^[67] For far-infrared applications, bolometers serve as thermal detectors, where IR absorption raises the temperature of a resistive element, altering its electrical resistance; these devices, often microfabricated, achieve response times around milliseconds and extend coverage to wavelengths beyond 400 cm⁻¹.^[65]^[66] Quantum detectors, relying on photon-induced electronic transitions, offer superior performance for demanding applications. Mercury cadmium telluride (MCT or HgCdTe) detectors operate photoconductively or photovoltaically, requiring cooling to 77 K with liquid nitrogen or thermoelectrically to suppress thermal noise, and exhibit high specific detectivity (D*) values around 10¹⁰ cm Hz¹/²/W with response times under microseconds.^[68]^[65] This enables MCT to capture fast interferogram modulations, supporting high-resolution spectra (e.g., down to 0.5 cm⁻¹) and low-concentration analyte detection, such as 5 ppm CO, though their spectral cutoff is typically around 1000 cm⁻¹.^[67]^[69] The raw detector output, a time-varying voltage representing the interferogram, undergoes signal handling to preserve fidelity. Low-noise preamplifiers boost the weak signal immediately after detection to minimize added noise, followed by lock-in amplification techniques synchronized to the interferometer mirror velocity for enhanced rejection of environmental interference.^[70] Analog-to-digital (A/D) conversion then samples the amplified signal at rates like 20 kHz, guided by a helium-neon laser reference to ensure uniform interferogram sampling across scans.^[70] As referenced in interferogram acquisition, this digitization captures the full optical path difference for Fourier transformation.^[71] Noise in FTIR signals arises primarily from Johnson (thermal) noise in resistive components and 1/f (flicker) noise at low frequencies, degrading the signal-to-noise ratio (SNR) and limiting sensitivity.^[65] SNR improves through signal averaging over multiple scans, where random noise reduces by the square root of the number of accumulations (√n), enabling detection limits as low as parts-per-million for routine analyses.^[72]^[67] Since the 2010s, modern FTIR systems have incorporated uncooled microbolometer arrays for hyperspectral imaging, leveraging arrays of tiny thermal sensors (e.g., vanadium oxide elements) to simultaneously detect spatial and spectral information without cryogenic cooling, achieving noise-equivalent temperature differences below 50 mK for mid- to far-IR mapping. As of 2025, microelectromechanical systems (MEMS)-based detectors continue to advance compact, portable FTIR instrumentation.^[73]^[74]^[75]

Sampling methods

In Fourier-transform infrared (FTIR) spectroscopy, sampling methods are critical for positioning diverse sample types—solids, liquids, gases, and thin films—within the infrared beam to measure absorption spectra effectively. These techniques minimize preparation while optimizing signal-to-noise ratios and spectral fidelity, with selection guided by sample properties and analytical goals. Common approaches include transmission, reflection, and specialized accessories, each exploiting different interactions between the IR radiation and the sample.^[76] Transmission mode involves directing the IR beam through the sample, providing bulk compositional information. For solids, the potassium bromide (KBr) pellet technique grinds the sample to a fine powder (typically <2 μm particles), mixes it with anhydrous KBr (ratio 1:100 to 1:300), and compresses the mixture at 10–15 tons into a 1–2 mm thick, 13 mm diameter pellet transparent to mid-IR radiation. This method yields high-quality spectra but requires dry conditions to avoid moisture interference from KBr's hygroscopic nature.^[77] For liquids, demountable or sealed cells with windows of sodium chloride (NaCl) or other IR-transparent salts contain the sample in path lengths of 0.01–1 mm, adjustable via spacers to match absorbance levels and prevent saturation. These cells accommodate volatile or corrosive liquids while ensuring beam alignment in the spectrometer's sample compartment.^[78] Attenuated total reflectance (ATR) enables surface analysis without sample dilution or thin-sectioning, ideal for solids, liquids, and pastes. The sample contacts a high-refractive-index crystal (e.g., diamond with n=2.4 or ZnSe with n=2.4), where the IR beam reflects internally at angles >45° (typically 45° incidence), generating an evanescent field penetrating 1–5 μm into the sample depending on wavelength and refractive index contrast. Multiple reflections (often 3–10) amplify signal; diamond crystals withstand abrasion and chemicals, while ZnSe suits softer samples but is more fragile. Post-measurement, a background spectrum of the clean crystal corrects for its absorption. Reflection techniques capture spectra from surfaces or scattered light, suiting opaque or powdered materials. Diffuse reflectance (DRIFTS) scatters the IR beam onto powders or rough solids in a sample cup, collecting hemispherical reflected light with mirrors; it excels for quantitative analysis of layered or mixed powders, though particle size (<10 μm) and packing density affect depth profiling up to ~1 mm. Specular reflection measures coherent reflection from smooth films or coatings on metal substrates at near-normal incidence, providing thickness and uniformity data for layers >1 μm. Grazing-angle reflection directs the beam at 80–85° to the surface on reflective substrates, enhancing electric field at interfaces for monolayer sensitivity (~0.1–1 μm effective depth). Gas sampling employs cells to extend interaction paths for dilute analytes. Long-path cells use mirrored chambers with optical paths up to 20 m via White or Herriott configurations, folding the beam for ppb-level detection in flowing or static gases without condensation issues. Photoacoustic methods detect modulated IR absorption as pressure waves in a sealed cell, using a microphone; this non-contact approach suits corrosive or high-temperature gases, with sensitivity scaling by cell volume and modulation frequency.^[79] Microsampling accessories handle minute quantities or extreme conditions. Diamond anvil cells compress 1–10 μg samples between opposing Type IIa diamond tips (0.3–1 mm culet) at pressures to 50 GPa, transmitting IR through the diamonds (cutoff ~2000 cm⁻¹) for in situ high-pressure studies of phase transitions. Fiber-optic probes, using chalcogenide or silver halide fibers (transmission 7000–900 cm⁻¹), couple remote samples to the spectrometer via 1–10 m cables, enabling non-invasive analysis in reactors or field environments.^[80]

Advantages and Limitations

Performance benefits over dispersive methods

Fourier-transform infrared (FTIR) spectroscopy offers several key performance advantages over traditional dispersive infrared methods, which rely on grating or prism monochromators to sequentially scan wavelengths. These benefits stem from the interferometric design, enabling simultaneous detection across the spectrum and efficient light utilization, leading to superior signal-to-noise ratio (SNR), faster acquisition times, and higher precision.^[2] A primary advantage is the multiplex or Fellgett's advantage, named after R. J. Fellgett's 1951 PhD thesis, where all wavelengths are detected simultaneously during a single scan of the interferometer. In dispersive systems, wavelengths are measured sequentially, limiting the time available for signal averaging per channel and resulting in poorer SNR for weak signals. In contrast, FTIR distributes the total measurement time across all spectral channels, yielding an SNR improvement of approximately √N, where N is the number of resolution elements or channels—often hundreds to thousands in typical IR spectra. This gain is particularly beneficial for low-light or noisy samples, allowing higher-quality spectra with less averaging time.^[24]^[2] Complementing this is the throughput or Jacquinot's advantage, identified by P. Jacquinot in his 1954 publication on spectrometer luminosity. Dispersive instruments require narrow entrance slits to achieve resolution, which restrict light throughput and reduce sensitivity, especially with extended sources like globar lamps. FTIR interferometers lack slits, permitting a larger étendue (the product of area and solid angle) and thus higher energy collection—up to 100 times greater in some configurations—without compromising resolution. This enables the use of brighter, more efficient sources and enhances overall sensitivity, particularly in mid-infrared applications.^[24] FTIR also excels in acquisition speed, capturing a complete spectrum in seconds (typically 1–10 s per scan) compared to minutes required by dispersive methods for equivalent coverage. This rapid throughput facilitates time-resolved studies, such as kinetic reactions or in situ monitoring, where dispersive scanning would be too slow to track dynamic processes effectively.^[2]^[81] Wavelength accuracy benefits from the Connes' advantage, developed by J. Connes in her 1961 thesis, through an internal helium-neon laser reference that precisely tracks mirror position during scans. This self-calibration achieves accuracies better than ±0.01 cm⁻¹, far surpassing dispersive systems where grating alignment drifts over time, leading to calibration errors of several wavenumbers without frequent manual adjustments.^[24]/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/How_an_FTIR_Spectrometer_Operates) Additionally, FTIR supports internal comparison via real-time ratioing of sample spectra against a background scan, acquired under identical conditions shortly before or after. This approach minimizes artifacts from atmospheric absorption (e.g., CO₂ and H₂O bands) and instrumental variations more effectively than the sequential beam splitting in dispersive double-beam setups, yielding cleaner, more reproducible absorbance spectra.^[2]

Resolution and sensitivity factors

The spectral resolution in Fourier-transform infrared (FTIR) spectroscopy is fundamentally determined by the maximum optical path difference (OPD), denoted as L, in the Michelson interferometer. The resolution \Delta \nu, expressed in wavenumbers (cm⁻¹), is given by the relation \Delta \nu = \frac{1}{2L}, where the factor of 2 accounts for the double-sided interferogram traversal.^[82] For instance, achieving a resolution of 0.5 cm⁻¹ requires an OPD of 1 cm, enabling the distinction of closely spaced absorption bands in complex spectra.^[82] Apodization functions are applied during interferogram processing to mitigate Gibbs ringing artifacts from finite OPD truncation, but they broaden the instrumental lineshape and thus affect the effective resolution. The unapodized (boxcar) function yields the nominal resolution with a sinc-shaped lineshape, but it produces significant sidelobes leading to spectral ringing. In contrast, the Blackman-Harris apodization, a multi-term cosine series (e.g., 3-term with coefficients 0.42323, 0.49755, 0.07922), suppresses sidelobes to below -92 dB while broadening the full width at half maximum (FWHM) of the main lobe by approximately 50%, resulting in an effective linewidth of about 0.9/L compared to 0.61/L for the unapodized case.^[7] This trade-off enhances spectral fidelity for quantitative analysis but reduces the ability to resolve fine structure.^[7] Sensitivity in FTIR systems, quantified by the minimum detectable absorbance, typically reaches 10⁻³ to 10⁻⁴ absorbance units (AU) under optimal conditions, allowing detection of weak molecular vibrations. This limit arises primarily from the infrared source power, which provides the incident flux, and the detector's noise equivalent power (NEP), the minimum incident power yielding a signal-to-noise ratio (SNR) of 1 in a 1 Hz bandwidth; for common mercury cadmium telluride (MCT) detectors, NEP values are on the order of 10⁻¹⁰ to 10⁻¹¹ W/√Hz.^[83] Additional factors influencing sensitivity include beam convergence, which introduces convolution broadening of spectral features due to angular divergence (half-angle \alpha \approx \sqrt{\Delta \nu / \nu_{\max}}, where \nu_{\max} is the maximum wavenumber), smearing the interferogram and degrading resolution beyond the nominal OPD limit.^[84] Similarly, non-uniform sample thickness in transmission measurements causes path length variations across the beam, leading to dephasing and peak broadening that reduces effective sensitivity for thin or heterogeneous samples.^[85] High-resolution variants, such as step-scan FTIR, extend capabilities for time-resolved studies by incrementing the mirror position in discrete steps to achieve longer effective OPDs without continuous motion artifacts, enabling resolutions as fine as 0.001 cm⁻¹.^[86] In this mode, the interferogram is sampled at fixed OPD positions while modulating external perturbations, preserving high spectral detail for dynamic processes like photochemical reactions.^[86]

Practical constraints and mitigations

One significant practical constraint in Fourier-transform infrared (FTIR) spectroscopy arises from atmospheric interferences, particularly absorption by water vapor and carbon dioxide, which exhibit strong bands in the mid-infrared region and can obscure sample signals. These gases are ubiquitous and fluctuate with environmental conditions, leading to variable background absorption that reduces spectral accuracy and complicates quantitative analysis.^[87] To mitigate this, instruments are often housed in purged enclosures where dry nitrogen or air is continuously flowed at rates such as 30 standard cubic feet per hour, effectively removing most water vapor and CO₂ within 2-4 minutes and stabilizing spectra for low-concentration samples.^[87]^[88] Additionally, software-based subtraction algorithms apply reference spectra of water vapor or CO₂ to correct residual interference post-acquisition, enhancing data quality in non-purged setups.^[88] Sample preparation poses another challenge, especially for strong absorbers like water in aqueous or biological matrices, where intense O-H stretching bands around 3300 cm⁻¹ overlap with analyte features and saturate the detector. This necessitates dilution of the sample in non-absorbing media or alternative sampling techniques to prevent total absorbance and maintain signal integrity. Attenuated total reflectance (ATR) modes are particularly effective here, as the evanescent wave limits penetration to a few micrometers, allowing direct analysis of hydrated samples without dilution while minimizing water's impact.^[89] FTIR systems involve high initial costs and operational complexity, with benchtop models typically ranging from $20,000 to $100,000 depending on resolution and features, which can limit accessibility for routine laboratory use. Advancements in miniaturization and electronics since the early 2000s have introduced more compact benchtop designs that reduce size, power needs, and overall expense while preserving performance, making them suitable for diverse environments.^[90]^[91] Scanning speed is constrained by the moving mirror's velocity in the interferometer, where higher velocities enable rapid data collection but trade off with the maximum optical path difference, potentially limiting resolution for time-sensitive experiments. Modern fast-scanning configurations achieve rates exceeding 100 spectra per second at moderate resolutions (e.g., 16 cm⁻¹), supporting real-time monitoring of dynamic processes like chemical reactions.^[92]^[93]^[94] Maintenance requirements further complicate operation, including periodic realignment of interferometer mirrors to counteract drift from mechanical vibrations or thermal expansion, which can degrade resolution if unaddressed. Detector cooling for sensitive mercury cadmium telluride (MCT) types traditionally relies on liquid nitrogen dewars, necessitating frequent refills and handling precautions, but Stirling cryocoolers offer a maintenance-free alternative by providing reliable cryogenic operation without consumables.^[95]^[96]^[97]

Spectral Regions

Near-infrared region

The near-infrared (NIR) region in Fourier-transform infrared (FTIR) spectroscopy spans approximately 12,500 to 4,000 cm⁻¹ (0.8 to 2.5 μm), where absorption arises primarily from overtone and combination bands of fundamental molecular vibrations observed in the mid-infrared range.^[98] These overtones represent higher-order transitions, such as the second harmonic of C-H stretching around 6,000 cm⁻¹, while combination bands involve simultaneous excitations of multiple vibrational modes, like C-H and C-O stretches.^[99] This spectral region enables the probing of molecular structures through weaker, higher-energy transitions that are harmonics or sums of the stronger fundamental vibrations.^[100] A key advantage of NIR spectroscopy is its deeper penetration into samples, often several millimeters, due to the lower molar absorptivities of overtone bands, making it suitable for non-destructive analysis of heterogeneous or turbid materials like foods or tissues.^[101] Additionally, the compatibility with quartz optics and silica-based fiber optics allows for remote sensing and flexible sampling configurations, such as in-line process monitoring.^[102] Water interference is minimized compared to mid-IR, as NIR features overtones of O-H stretching with relatively weak absorption, facilitating measurements in aqueous environments.^[103] Instrumentation for NIR-FTIR typically employs tungsten-halogen lamps as sources, which provide continuous output from visible to NIR wavelengths.^[104] Detectors such as indium gallium arsenide (InGaAs) are used for their sensitivity in the 0.9–1.7 μm range, offering fast response times and low noise.^[105] Quartz optics are preferred for beam splitters and windows, as they transmit effectively up to about 3.5 μm without significant absorption.^[106] In applications, NIR-FTIR excels in quantitative analysis of organic compounds, such as determining moisture, protein, and lipid content in agricultural products through characteristic overtone bands.^[107] For proteins, amide I and II overtone and combination bands in the 4,000–5,000 cm⁻¹ region allow estimation of secondary structures like α-helices and β-sheets in aqueous solutions, supporting non-invasive studies in biochemistry.^[108] Despite these strengths, NIR bands are inherently weaker, with absorbances typically 10–100 times lower than mid-IR fundamentals, necessitating longer path lengths or concentrated samples to achieve detectable signals and limiting qualitative identification of complex mixtures.^[100]

Mid-infrared region

The mid-infrared (MIR) region in Fourier-transform infrared (FTIR) spectroscopy spans approximately 4,000 to 400 cm⁻¹ (corresponding to wavelengths of 2.5 to 25 μm), encompassing the fundamental vibrational transitions of molecular bonds.^[109] This range is the primary focus of FTIR due to its sensitivity to the stretching and bending modes of polar covalent bonds, such as C-H, O-H, N-H, and C=O, which absorb energy and produce characteristic spectral bands.^[110] For instance, the O-H stretching vibration in alcohols and carboxylic acids appears as a broad band between 3,700 and 3,100 cm⁻¹, while the C-O stretching mode in ethers and esters occurs from 1,300 to 1,000 cm⁻¹.^[110] A key feature of the MIR region is the fingerprint region, spanning 1,500 to 400 cm⁻¹, where complex patterns of bending, rocking, and skeletal vibrations create a unique spectral signature for each molecule, enabling definitive identification by direct comparison with reference spectra.^[110] The high specificity of these fundamental bands allows for reliable detection of functional groups, supporting both qualitative identification of molecular structures and quantitative analysis through peak intensity measurements, often with detection limits in the parts-per-million range for purified samples. Challenges in MIR-FTIR arise from strong atmospheric absorptions, particularly from water vapor (broad bands around 3,700–3,000 cm⁻¹ and 1,600 cm⁻¹) and carbon dioxide (sharp peaks at approximately 2,350 cm⁻¹ and 670 cm⁻¹), which can obscure sample signals and necessitate purging the instrument enclosure with dry nitrogen or inert gas to minimize interference.^[111] Common detector choices for this region include deuterated triglycine sulfate (DTGS) pyroelectric detectors for routine room-temperature operation and mercury cadmium telluride (MCT) semiconductor detectors for higher sensitivity and faster response times, typically cooled with liquid nitrogen. Optical windows, such as potassium bromide (KBr), are frequently used due to their broad transmission in the MIR but require careful handling as they are hygroscopic and can absorb moisture, leading to spectral artifacts. Recent extensions in MIR-FTIR have incorporated step-index optical fibers made from chalcogenide glasses (e.g., based on sulfur, selenium, or tellurium), enabling remote sensing applications by transmitting MIR light over distances up to several meters with low attenuation in the 3–12 μm window, as demonstrated in developments since the early 2010s.^[112]

Far-infrared region

The far-infrared (FIR) region in Fourier-transform infrared (FTIR) spectroscopy spans approximately 400–10 cm⁻¹ (corresponding to wavelengths of 25–1000 μm), focusing on low-energy molecular transitions that require specialized instrumentation due to the region's unique challenges and applications.^[113] This range probes subtle vibrational and rotational dynamics not accessible in higher-frequency infrared domains, enabling analysis of complex material properties.^[114] Spectral features in the FIR region primarily include rotational transitions in gases, lattice vibrations in solids (phonons), and intermolecular modes. Rotational transitions are prominent in gases with permanent dipole moments, such as water vapor or hydrogen chloride, appearing as discrete absorption lines in the 200–10 cm⁻¹ range due to low-energy rotational energy levels.^[113] Lattice vibrations, or phonons, manifest in solids as collective oscillations of atomic lattices, often observed below 200 cm⁻¹; for instance, in crystalline materials like bismuth selenide, these modes reveal phonon densities and carrier interactions through broad absorption bands.^[115] Intermolecular modes, involving weak interactions like hydrogen bonding or van der Waals forces, produce low-frequency absorptions (typically 100–10 cm⁻¹) in liquids and molecular crystals, such as the cage rattling in tetraethylammonium iodide at ~70 cm⁻¹.^[116]^[117] Instrumentation for FIR-FTIR differs from mid-infrared setups to accommodate lower photon energies and transmission requirements. Beam splitters are typically constructed from high-density polyethylene (HDPE) films, which offer high transparency and low absorption in the 400–10 cm⁻¹ range, enabling efficient interferometry.^[118] Detectors commonly include deuterated triglycine sulfate (DTGS) pyroelectric sensors for room-temperature operation across the FIR, or liquid-helium-cooled silicon bolometers for enhanced sensitivity at longer wavelengths, achieving noise-equivalent powers as low as 10⁻¹⁵ W/√Hz.^[118]^[119] Light sources are selected for their output in the low-energy regime: silicon carbide globars provide stable emission above 100 cm⁻¹, while high-pressure mercury arc lamps extend coverage to sub-100 cm⁻¹ with higher radiance than thermal sources.^[120] Key challenges in FIR-FTIR arise from the inherent limitations of the spectral regime, including low source intensity, which reduces signal-to-noise ratios compared to mid-infrared measurements, often necessitating longer integration times or brighter synchrotron sources for practical spectra.^[118] High atmospheric attenuation, particularly from water vapor absorptions in the 400–200 cm⁻¹ window, severely distorts spectra, requiring operation under vacuum purging or dry nitrogen flushing to minimize interference from rotational-vibrational bands of H₂O.^[87]^[121] Applications of FIR-FTIR are specialized, leveraging its sensitivity to low-frequency modes in targeted analyses. In organometallic compounds, it elucidates metal-ligand bending and stretching vibrations; for example, vapor-phase spectra of pentacarbonyl manganese derivatives (e.g., CH₃Mn(CO)₅) reveal terminal M–C–O deformations around 100–50 cm⁻¹, aiding structural elucidation.^[122] For explosives detection, FIR spectra provide characteristic phonon and intermolecular signatures; measurements of compounds like RDX and PETN show distinct absorptions at 80–50 cm⁻¹, enabling non-contact identification even in trace amounts.^[123] Advances since the 2000s have integrated FIR-FTIR with terahertz (THz) extensions, bridging 10–100 cm⁻¹ for non-ionizing imaging in security applications. These hybrid systems, often using photoconductive antennas or quantum cascade lasers, enable real-time standoff detection of concealed threats through dielectric contrasts in explosives and contraband, with resolutions improved to sub-millimeter scales via computational reconstruction.^[124]^[125] Such developments have enhanced throughput, reducing acquisition times from minutes to seconds while maintaining high specificity.^[126]

Applications

Material science and polymer analysis

Fourier-transform infrared (FTIR) spectroscopy plays a pivotal role in material science by providing detailed insights into the molecular structure and composition of polymers, composites, and nanomaterials, enabling the identification of functional groups and phase behaviors critical for engineering applications. In polymer analysis, FTIR excels at detecting vibrational modes associated with backbone chains and side groups, facilitating non-destructive characterization of synthetic materials. This technique is particularly valuable for assessing material quality control, processing effects, and performance optimization in industries such as packaging, automotive, and aerospace. For polymer identification, FTIR identifies chain conformations through characteristic C-H bending vibrations, such as the out-of-plane rocking modes of methylene groups in polyethylene, which appear as distinct bands in the mid-infrared region. Crystallinity assessment relies on the splitting and intensity ratios of these modes; for instance, in polyethylene, the appearance of separate peaks at 720 cm⁻¹ and 730 cm⁻¹ indicates orthorhombic crystalline phases, with the relative intensity of the 730 cm⁻¹ band increasing with higher crystallinity levels.^[127]^[128] These spectral features allow differentiation between amorphous and crystalline domains, aiding in the evaluation of thermal history and mechanical properties without sample preparation beyond thin films or pellets. In polymer composites, FTIR probes filler-matrix interactions by monitoring shifts or broadening in vibrational bands due to chemical bonding or hydrogen interactions at the interface. For example, in polypropylene-silica composites, two-dimensional correlation analysis of FTIR spectra reveals sequential changes in polymer chain vibrations near silica particles, confirming enhanced interfacial adhesion through silanol group interactions.^[129] Quantitative determination of water content in plastics uses the broad O-H stretching band around 3400 cm⁻¹, applying the Beer-Lambert law to correlate absorbance intensity with concentration, as the integrated area of this band directly reflects absorbed moisture levels affecting material stability.^[130] For nanomaterials, FTIR confirms surface functionalization, such as silane coupling agents on SiO₂ nanoparticles, through the emergence of Si-O-Si and C-H stretching bands at approximately 1100 cm⁻¹ and 2900 cm⁻¹, respectively, indicating successful grafting that improves dispersion in polymer matrices. In the far-infrared region, size effects manifest as shifts in phonon modes; smaller nanoparticle dimensions lead to broader and lower-frequency lattice vibrations due to confinement, observable in spectra of nanocrystalline materials like WO₃, where peak positions vary inversely with particle size below 50 nm.^[131]^[132]^[133] Specialized FTIR techniques enhance analysis for diverse sample forms: attenuated total reflectance (ATR) mode is ideal for surface characterization of solid polymers, probing the top 1-5 μm without solvent interference, while diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suits powdered or rough composites by collecting scattered radiation from irregular surfaces.^[134]^[135] FTIR mapping, or hyperspectral imaging, assesses spatial homogeneity in composites by generating distribution maps of functional group intensities, revealing filler dispersion variations across micrometer scales in blend gradients.^[136] A representative case is the study of oxidative degradation in polymer composites, where FTIR detects emerging carbonyl (C=O) bands at 1710-1740 cm⁻¹ and hydroperoxide (O-O-H) features around 3400 cm⁻¹ as oxidation products form during thermal or UV exposure. In high-density polyethylene-wood flour composites, these bands quantify degradation progression, correlating spectral changes with reduced mechanical integrity due to chain scission and cross-linking.^[137]^[138]

Biological and pharmaceutical studies

Fourier-transform infrared (FTIR) spectroscopy plays a pivotal role in biological studies by enabling the analysis of protein secondary structures through characteristic amide I and amide II bands. The amide I band, primarily arising from C=O stretching vibrations around 1600–1700 cm⁻¹, provides insights into hydrogen bonding patterns, with α-helices typically absorbing at approximately 1650–1658 cm⁻¹ and β-sheets at 1620–1640 cm⁻¹ or 1680–1690 cm⁻¹.^[139] The amide II band, involving N–H bending and C–N stretching near 1550 cm⁻¹, complements this by highlighting conformational changes, particularly when partial deuteration shifts overlapping signals to improve differentiation between ordered structures like α-helices and disordered regions.^[139] This approach has been widely applied to peptides and proteins, such as in evaluating folding stability and environmental interactions without requiring extensive sample preparation beyond standard transmission or attenuated total reflectance (ATR) modes. In lipid membrane research, FTIR spectroscopy assesses bilayer organization and dynamics by monitoring C–H stretching vibrations in acyl chains, which indicate chain order and fluidity. Symmetric and asymmetric CH₂ stretches around 2850 cm⁻¹ and 2920 cm⁻¹, respectively, shift to higher wavenumbers upon increasing disorder, reflecting gauche conformer formation in fluid phases versus all-trans configurations in gel phases.^[140] For phospholipid headgroups, bands in the 1000–1300 cm⁻¹ region, such as PO₂⁻ asymmetric stretches near 1220 cm⁻¹, reveal hydration states and electrostatic interactions, aiding studies of membrane-protein associations or phase transitions in model systems like phosphatidylcholine bilayers.^[140] These spectral signatures facilitate non-invasive probing of lipid packing and headgroup orientation, essential for understanding cellular membrane functions. In pharmaceutical applications, FTIR excels at identifying polymorphs of active ingredients, where distinct crystal forms exhibit unique vibrational fingerprints due to differences in intermolecular hydrogen bonding. For aspirin (acetylsalicylic acid), form I and form II polymorphs are distinguished by variations in carbonyl and OH stretching bands around 1750 cm⁻¹ and 3000–3500 cm⁻¹, respectively, allowing quantitative analysis via peak intensity ratios in solid-state spectra.^[141] This technique ensures product consistency, as polymorphic transitions can alter solubility and bioavailability. Additionally, FTIR evaluates excipient compatibility by detecting shifts or broadening in drug-specific bands upon mixing, such as disappearance of characteristic peaks indicating hydrogen bonding or ionic interactions; for instance, studies have confirmed compatibility of various APIs with fillers like lactose or magnesium stearate through unchanged spectral profiles after stress testing.^[142] FTIR-based spectral histopathology has advanced cancer diagnostics by mapping biochemical alterations in tissue sections, providing label-free molecular contrast for tumor identification. In colorectal and other cancers, elevated glycogen content in tumor cells manifests as intensified peaks around 1020–1150 cm⁻¹, correlating with metabolic reprogramming and enabling differentiation from healthy stroma with sensitivities exceeding 90% when coupled with multivariate analysis.^[143] This approach supports rapid, non-destructive imaging of tissue heterogeneity, aiding in grading and margin assessment during surgery. In nano-bio interfaces, FTIR characterizes protein coronas on nanoparticles by tracking amide band shifts that signify conformational changes upon adsorption. For silver or magnetite nanoparticles exposed to serum proteins like bovine serum albumin, the amide I peak broadens or shifts from 1650 cm⁻¹, indicating partial unfolding and surface-induced β-sheet formation, which influences cellular uptake and toxicity.^[144] Surface functional groups, such as carboxyl versus amine, modulate corona thickness and stability, as evidenced by altered C=O and N–H intensities. FTIR also aids in virus characterization by capturing envelope and capsid vibrational modes for identification and infectivity assessment. In poliovirus studies, spectral changes in the 1500–1700 cm⁻¹ region during infection reflect viral protein accumulation in host cells, enabling quantification of viral loads with detection limits below 10⁴ PFU/mL through principal component analysis of amide and phosphate bands.^[145] This method supports rapid, reagent-free profiling of viral strains in biological matrices.

Environmental and process monitoring

Fourier-transform infrared (FTIR) spectroscopy plays a crucial role in environmental and process monitoring by enabling real-time, non-destructive analysis of gases, liquids, and dynamic reactions in industrial and natural settings. Its ability to detect multiple species simultaneously through characteristic absorption bands makes it ideal for compliance with emission standards and optimizing operational efficiency. In environmental applications, FTIR facilitates the quantification of pollutants at trace levels, supporting regulatory frameworks such as those outlined by the U.S. Environmental Protection Agency.^[146] In gas analysis, FTIR is widely employed for stack monitoring in industrial facilities, where it detects sulfur dioxide (SO₂) emissions via its strong absorption band at approximately 1360 cm⁻¹. Portable automated measuring systems based on FTIR have demonstrated high accuracy in quantifying SO₂ concentrations in stack gases, achieving detection limits suitable for continuous emission monitoring under European standards. For broader plume assessments, open-path FTIR systems extend measurements over path lengths of up to 1 km, allowing the simultaneous identification and quantification of multiple contaminants such as volatile organic compounds (VOCs) and inorganic gases directly in ambient air without sample extraction. These systems have been validated for real-time monitoring of emissions from sources like industrial vents and volcanic activity.^[147]^[148]^[146]^[149] FTIR also supports water quality assessment by characterizing dissolved organic matter in wastewater and detecting nitrates in aqueous samples. Attenuated total reflectance (ATR)-FTIR spectroscopy has been used in situ to quantify nitrate concentrations in industrial effluents, with curve-fitting techniques enabling precise determination down to environmentally relevant levels. For dissolved organics, FTIR spectra reveal functional groups associated with humic substances and other contaminants, aiding in the evaluation of treatment efficacy in municipal and industrial wastewater streams.^[150]^[151]^[152] In process control within chemical plants, FTIR enables in-line monitoring of reaction kinetics, such as in esterification processes where the formation of ester carbonyl (C=O) groups around 1740 cm⁻¹ is tracked to assess conversion rates. This real-time capability has been applied to biodiesel production, where FTIR quantifies product yields during transesterification without interrupting the flow. Such monitoring optimizes catalyst use and reaction conditions, reducing waste in large-scale operations.^[153]^[154] Remote sensing applications leverage aircraft-mounted FTIR instruments for atmospheric profiling, particularly of ozone layers. Airborne solar occultation FTIR systems measure vertical profiles of ozone and other trace gases by analyzing solar absorption spectra during flight, providing data on stratospheric distributions with high spatial resolution. These deployments have contributed to global ozone assessments by integrating with networks like NOAA's monitoring efforts.^[155] Advancements in portability have introduced handheld FTIR units for field-based VOC detection, particularly in post-2010 models equipped with compact interferometers. These devices allow on-site identification of VOCs in soil, air, and water matrices at industrial sites, with sensitivities reaching parts-per-billion levels for rapid environmental compliance checks. Mobile FTIR systems have been demonstrated in field trials for detecting fugitive emissions, enhancing response times in hazardous waste investigations.^[156]^[157]

Hyphenated techniques and imaging

Hyphenated techniques integrate Fourier-transform infrared (FTIR) spectroscopy with separation or thermal analysis methods to enable multidimensional characterization of complex mixtures. Gas chromatography coupled with FTIR (GC-FTIR) separates volatile compounds via chromatography and identifies them through infrared spectra of functional groups, providing retention times and structural information for applications such as detecting environmental pollutants like pesticides and polycyclic aromatic hydrocarbons.^[158]^[159] This approach excels in analyzing unknowns by correlating chromatographic peaks with mid-infrared absorption bands, offering complementary data to mass spectrometry without fragmentation.^[160] Thermogravimetry coupled with FTIR (TG-FTIR) monitors evolved gases during material decomposition under controlled heating, revealing pyrolysis products and reaction mechanisms in polymers. For instance, it identifies volatile species like water, carbon dioxide, and hydrocarbons released from coal or plastic samples, aiding in the study of thermal stability and degradation pathways.^[161]^[162] The technique uses a heated transfer line to direct gases into the FTIR gas cell, enabling real-time spectral analysis synchronized with mass loss curves.^[163] High-performance liquid chromatography hyphenated with FTIR (HPLC-FTIR) extends these capabilities to non-volatile or liquid-phase analytes, separating compounds online and detecting them via flow-cell infrared spectroscopy. It is particularly useful for characterizing polar molecules in mixtures, such as in pharmaceutical impurity profiling, where solvent elimination interfaces minimize spectral interference from mobile phases.^[164] This method provides molecular fingerprints for species not amenable to gas-phase analysis, though sensitivity is often limited by aqueous eluents.^[165] FTIR microscopy enhances spatial resolution by mapping infrared spectra across samples, often employing focal plane array (FPA) detectors for rapid 2D or 3D imaging. FPA-based systems collect hyperspectral data from thousands of pixels simultaneously, achieving lateral resolutions of approximately 5–10 μm in the mid-infrared region, limited by diffraction but sufficient for heterogeneous materials like tissues or composites.^[166]^[167] These arrays, typically using mercury cadmium telluride (MCT) sensors, enable chemical imaging of functional group distributions, such as protein aggregates in cells.^[168] For nanoscale imaging beyond the diffraction limit, synchrotron-based FTIR and scattering-type scanning near-field optical microscopy (s-SNOM) provide sub-micron resolution. Synchrotron sources deliver bright, broadband infrared radiation to nano-FTIR setups, achieving spatial resolutions down to ~20 nm by coupling with atomic force microscopy tips that scatter near-field signals.^[169] s-SNOM, often integrated with broadband laser or synchrotron illumination, probes local dielectric properties and vibrational modes at scales approaching λ/20, where λ is the infrared wavelength, enabling analysis of nanomaterials like protein nanostructures or mineral phases.^[170] Correlative Raman-FTIR spectroscopy combines the complementary strengths of both modalities—Raman's sensitivity to symmetric vibrations and FTIR's to asymmetric ones—for comprehensive molecular profiling. This approach, often implemented on shared microscope platforms, overlays vibrational maps to resolve overlapping bands, as demonstrated in metabolic fingerprinting of biological samples where it distinguishes subtle structural variations in lipids and proteins.^[171]^[172] Such integration enhances identification accuracy in complex systems without requiring separate sample preparations.^[173]

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Data acquisition and interferogram data treatment in FT-IR ...
Aug 7, 2025 · Traditional FT-IR spectrometers use the zero crossings of an interferogram from a control HeNe laser as a trigger signal for digitizing the ...
[36]
[PDF] Fourier Transform and Apodization --- 2 ABC of the FTIR ... - Shimadzu
Dec 1, 2010 · The Happ-Genzel function is often used to achieve a good balance between ripple size and resolution. Applying it to the Fourier transform image ...
[37]
Research on Fourier Transform Spectral Phase Correction Algorithm ...
Linear phase errors primarily arise from the offset between the actual sampling positions and the ideal sampling positions. This offset causes a misalignment ...
[38]
FT-IR Spectroscopy Definitions of Characteristics - Newport
Background Spectrum: A single beam spectrum acquired without a sample in the infrared beam. The purpose of a background spectrum is to measure the signal ...
[39]
The FFT in FTIR - Essential FTIR
Although zero-filling increases the number of data points in the spectrum, it cannot increase the optical resolution of the spectrum. Prepare Phase-Correction ...Missing: pre- | Show results with:pre-
[40]
FTIR instrumentation and theory - Oregon State University
Dec 28, 2020 · In double beam operation, a beam splitter separates the incident beam in two; half goes to the sample, and half to a reference. The sample ...
[41]
ARCHER. A new algorithm for automatic removal of cosmic spikes ...
These techniques enable the identification of wavenumbers that are supposed to contain cosmic ray artifacts and their automatic correction without supervision.
[42]
An Algorithm for the Removal of Cosmic Ray Artifacts in Spectral ...
Apr 22, 2019 · In this paper, a cosmic ray artefact removal algorithm is proposed that works in a similar way to the double acquisition method but requires ...
[43]
[PDF] LA-UR-16-25235 - OSTI
Jul 19, 2016 · I(δ) = Z +∞. −∞. B(ν)cos(2πνδ − θ(ν))dν. (12). The addition of the phase error for equation 12 introduces a sine component to the transform.Missing: ∫ | Show results with:∫
[44]
[PDF] First Trace Gas Measurements Using Fourier Transform Infrared ...
B(ν). 2 cos 2πνδ dν. (2.17). Equation 2.17 is one half of a cosine Fourier transform pair. The inverse transform is written as. B(ν)=2 Z. +∞. −∞. I(δ) cos 2πνδ ...Missing: ∫ | Show results with:∫
[45]
None
### Extracted Mathematical Equations and Explanations
[46]
FTIR: Hardware - Chemistry LibreTexts
Aug 29, 2023 · The beginnings of the first commercial FTIR spectrometer have been attributed to the work of M.J. Block and his research team in the small ...
[47]
[PDF] Introduction to FT-IR Spectroscopy
The Globar source is a silicon carbide rod (5mm diameter, 50mm long) which is electrically heated to about 1500 K. Water cooling of the electrical contacts ...
[48]
6.11.2: Infrared Sources and Transducers - Chemistry LibreTexts
Jun 27, 2025 · Common examples of IR sources include the Nernst glower (a ceramic ... incandescent wire (a nichrome wire heated to approximately 1100 K).
[49]
Infra red Absorption Spectroscopy - Instrumentation
The incandescent wire source is a tightly wound coil of nichrome wire, electrically heated to 1100 K.
[50]
The Emissivity of Globar for Use in Temperature Measurements with ...
Globar has often been used as a radiation standard substitute for the blackbody in the infrared. In this capacity the globar output is predicted by Planck's law ...
[51]
Infrared output from a compact high pressure arc source
In the far-infrared the arc temperature is similar to that of the standard mercury lamp with an effective temperature approaching 5000 K. Previous article ...
[52]
https://opg.optica.org/ao/abstract.cfm?uri=ao-8-8-1737_1
[53]
Advanced Mid-Infrared Sensors for Molecular Analysis
Mar 28, 2025 · These advancements suggest the scalability of the design, rendering LED-based light sources well-suited for optical communications and gas ...
[54]
[PDF] Methods for Determining the Infrared Complex Refractive Indices N ...
Sep 6, 2022 · a KBr window for the mid-IR, a glow bar source and a series of beam splitters (Ge/KBr, Ge/CsI, 12 μm Mylar, and 23 μm Mylar) were employed ...
[55]
[PDF] Infrared spectroscopy of conjugated organic molecules under high ...
Some materials that are used as beamsplitters include polyethylene terephthalate (Mylar), germanium, CsI, CsBr, and KBr. The two most commonly used materials ...
[56]
IR Plate Beamsplitters - Newport
Coated with a multi-layer dielectric beamsplitter coating, the optics are optimized for IR use with a 50% +/- 10% splitting ratio. 50/50 beamsplitting for 2 to ...
[57]
[PDF] Lehigh Preserve Institutional Repository
Two types of beam splitters were available to cover different frequency ranges; a KBr beam splitter coated with Ge/ZnSe for the 450 cm-1 -. 5000 cm-1 range ...
[58]
[PDF] Effect of Purging a Sealed and Desiccated FTIR Spectrometer ...
The purpose of providing a dry, CO2-free purge to an FTIRspectrometer is twofold. The purge prevents deterioration of the beamsplitter by moisture and also ...Missing: H2O | Show results with:H2O
[59]
Wire Grid Polarizers for FTIR Spectroscopy - Optometrics
Wire grid polarizers create polarized light and transform it from one state to another. ZnSe, CaF2, BaF2, KRS-5, Ge from 2.5μ-30 microns.
[60]
Repair of Beamsplitters - FTIR.com
The KBr does not fog from moisture at a relative humidity below 40%. It is best to keep your FTIR powered on over long weekends or when you are away on vacation ...
[61]
ZnSe configuration (optional) - Thermo Fisher Scientific
Aug 12, 2025 · The non-hygroscopic ZnSe configuration for your instrument replaces the KBr sample compartment windows, beamsplitter, and detector window with ...Missing: alternative | Show results with:alternative
[62]
[PDF] Infrared Detectors - Agenda INFN
The detector is suspended on legs which are connected to the heat sink. The signal does not depend upon the photonic nature of the incident radiation. Thus, ...Missing: transform | Show results with:transform
[63]
[PDF] Characteristics and use of infrared detectors
This manual describes major characteristics and applications of Hamamatsu infrared detectors, including InGaAs PIN photodiodes, PbS and PbSe photoconductive ...
[64]
[PDF] Thermoelectrically cooled MCT detectors enable high speed and ...
FTIR spectrometers are often configured with two main types of detectors: DTGS (deuterated L-alanine doped triglycine sulphate) and MCT (Mercury Cadmium ...Missing: bolometers | Show results with:bolometers
[65]
[PDF] Detectors for Fourier Transform Spectroscopy - Nicolet CZ
From the applications perspective, this means that in high-throughput applications, we must limit the beam energy reaching the MCT detector to prevent.Missing: bolometers | Show results with:bolometers
[66]
FT-IR Spectrometer Detectors - Newport
Mercury Cadmium Telluride (MCT) detectors exhibit fast response times, a wide dynamic range, and broad spectral response. A two-stage, low noise amplifier is ...
[67]
The effects of two noises from passive FTIR (Fourier transform ...
May 26, 2011 · ... signal processing part. Lastly, the signal processing part performs ... Noise Ratio(SNR) such as detector noise, jitter of moving ...
[68]
[PDF] Signal and Noise Analysis in an Undergraduate Instrumental ...
Jun 30, 2025 · FTIR. NMR standards are ... Signal-to-Noise Ratio, Signal Processing, · and Spectral Information in the Instrumental Analysis Laboratory.
[69]
Signal-to-Noise Ratio, Signal Processing, and Spectral Information ...
Using an FTIR spectrometer, the signal-to-noise ratio is obtained as a function of spectral resolution and frequency range. The S/N increases with decreasing ...
[70]
Uncooled infrared focal plane arrays - Wiley Online Library
Sep 21, 2017 · This paper reviews the basics of and advances in uncooled IRFPAs, including IRASs. Some emerging trends in business and applications are also discussed.2 Basics · 3 Development Trends · 4 Business Trends And...
[71]
(PDF) Uncooled infrared detectors and focal plane arrays
Aug 9, 2025 · Over the past several years, uncooled IR detectors and focal plane arrays have been rapidly developed. Impressive progress has been made in ...
[72]
Fourier Transform Infrared Spectrometry | Wiley Online Books
Jun 30, 2006 · ... Fourier Transform Infrared Spectrometry brings this core reference up to date on the uses of FT-IR spectrometers today. The book starts with ...
[73]
Should I be using KBr pellets in FTIR Spectroscopy - Specac Ltd
KBr pellets remain a popular choice with several advantages over ATR. This application note outlines these advantages and the basics behind the technique.Missing: mode DRIFTS gas microsampling review
[74]
[PDF] Introduction to FT-IR Sample Handling - Thermo Fisher Scientific
How does it work? The transmission technique does not require a separate accessory. The user simply places a sample directly into the infrared (IR) beam.Missing: review | Show results with:review
[75]
Gemini FTIR Gas Analysis Cells | International Crystal Laboratories
Long path cells address the most demanding gas sampling requirements of industrial and field deployed FTIR and IR instrumentation. For applications requiring ...Missing: photoacoustic | Show results with:photoacoustic
[76]
Miniature Diamond-Anvil Cells for FTIR-microspectroscopy of Small ...
Jul 23, 2018 · Here we propose a novel method for the analysis of small quantities of biosamples by FTIR-microscopy of dry films using a diamond-anvil cell ( ...Missing: fiber optic probes
[77]
How an FTIR Spectrometer Works and FTIR Analysis
Apr 12, 2024 · An FTIR instrument uses an interferometer (Figure 4), which consists of a source, beam splitter, two mirrors, a laser and a detector. The energy ...
[78]
[PDF] Introduction to Fourier transform spectroscopy - GovInfo
... resolution; (2) that the limit of resolution decreases with increasing i nterferogram bandwidth, or in other words the maximum optical path difference.
[79]
A New, World-Class Facility for Infrared Calibrations | NIST
Oct 21, 2015 · The figure of merit for sensitivity in such detectors is called noise equivalent power (NEP, expressed in nW/Hz1/2), and lower values represent ...
[80]
[PDF] FT-IR Spectroscopy - Newport
1. The interferometer consists of a beam splitter, a fixed mirror, and a mirror that translates back and forth, very precisely.
[81]
FTIR-ATR spectroscopy in thin film studies - ResearchGate
Aug 6, 2025 · This result can be explained by the fact that when the film thickness is lower than the penetration depth of the IR light, reflexion on the ...
[82]
[PDF] a step-scan Fourier-transform interferometer - University of Toronto
Step-scan FT spectrometers obtain high temporal resolution in a different manner. With these in- struments, the moving mirror is translated (stepped) to a fixed ...
[83]
[PDF] Impact of active gas purge on FTIR measurements
A gas purge helps to minimize or remove atmospheric contributions in FTIR spectra and improves the ensuing library search.Missing: H2O | Show results with:H2O
[84]
A New Criterion to Evaluate Water Vapor Interference in Protein ...
These methods include purging with dry air or nitrogen during spectral acquisition, subtraction of reference water vapor absorption spectrum from protein ...Missing: CO2 enclosures<|separator|>
[85]
Advances in ATR-FTIR Spectroscopic Imaging for the Analysis ... - NIH
Jun 12, 2023 · In this review, the power of ATR-FTIR spectroscopic imaging is demonstrated by presenting a number of successful applications of this chemical imaging ...
[86]
Buy Infrared and FTIR Spectrometers For Sale, New & Used Prices
Free delivery over $1M 30-day returnsNew IR and FT-IR systems typically range from $15,000 to $150,000, depending on the system's resolution, sensitivity, and additional features such ...
[87]
Portable FTIR spectrometers get moving - American Chemical Society
To make a traditional benchtop FTIR spectrometer portable, the interferometer and electronic circuits are miniaturized. Many developments in portable FTIR have ...
[88]
None
### Summary of Mirror Velocity and Scanning Speeds in FTIR
[89]
Step Scan / Rapid Scan Interleaved TRS - Bruker
The Step Scan technique allows the monitoring of the temporal progress of very fast reproducible events (transients).Missing: OPD | Show results with:OPD
[90]
Using FTIR to Observe Chemical Reactions in Real Time
Jan 4, 2021 · Traditional FTIR units typically can collect 80-150 scans per second, at low resolution.Missing: rate | Show results with:rate
[91]
Common Problems and Precautions in the Operation of FTIR ...
Improper FTIR instrument operation may cause baseline drift, signal-to-noise ratio degradation, and even damage to optical components. Mastering standard ...Missing: alignment | Show results with:alignment
[92]
What's the Appropriate Infrared Detector for FTIR Analysis and How ...
Dec 17, 2024 · Traditional MCT Detectors: When high sensitivity and fast response times are required, traditional MCT detectors are preferred. They are ...
[93]
Shepherd FTIR - Cerex Monitoring Solutions
Mar 12, 2025 · IR source life time is approximately 2 years; Stirling cooler (detector cryocooler) guaranteed for 2 years before refurbishment is required.
[94]
Fourier Transform Near-Infrared Spectroscopy and Chemometrics ...
Aug 1, 2018 · Near-infrared spectroscopy (NIRS) uses optical absorbance from 780 to 2,500 nm based on molecular overtone and combination vibrations of ...
[95]
Original spectra of (a) FT-NIR and (b) FT-IR - ResearchGate
The typical absorptions in the NIR region ( Fig. 1(a)) are dominated by CH second overtone bands (7692-8695 cm -1 ), NH and OH first overtone bands (5882-7142 ...
[96]
Combination Bands, Overtones and Fermi Resonances
Jan 29, 2023 · Combination bands and overtones generally have lower intensities than the fundamentals, and Fermi resonance causes a spilt and shift in ...
[97]
Why FT-NIR spectroscopy? - Bruker
The NIR light not only analyzes the surface, but penetrates deeper into the material. This makes it ideal for measuring heterogeneous samples.
[98]
Analytical Vibrational Spectroscopy - NIR, IR, and Raman
Oct 1, 2011 · For instance, the NIR region is still compatible with glass optics, which means that fiber optics can be used. However, the detectors used in ...Missing: deeper | Show results with:deeper
[99]
[PDF] Near Infrared perturbation spectroscopy: Water in action seen as a ...
This spectral range has the advantage that water absorption is relatively weak and overtones of various X-H bonds, with X typically being N, O or C within the ...
[100]
[PDF] Thermo Scientific Nicolet iS50 FTIR Spectrometer Specifications
Spectrometer. Polaris High Stability, Long Lifetime Mid-IR Source. Standard. Tungsten-Halogen Near-IR/Visible Source. Option. Four Position Source Mirror.
[101]
Principles and Applications of Miniaturized Near‐Infrared (NIR ... - NIH
Oct 29, 2020 · The NIR‐S‐G1 spectrometer has extremely compact dimensions (82×63×43 mm3; <145 g weight) and uses a tungsten halogen source and InGaAs detector.Missing: FTIR | Show results with:FTIR
[102]
IR Fused Quartz - Optical Material
IR Fused Quartz is an optical material that exhibits excellent transmission properties across a wide range of infrared (IR) wavelengths, from 260nm to 3500nm.
[103]
Near-infrared spectroscopy and imaging in protein research
This review provides some explanation of how one can correlate the combination modes with protein secondary structure using several examples. One of the most ...
[104]
Near-infrared analysis of protein secondary structure in ... - PubMed
The normalized second-derivative near-infrared spectra of proteins in aqueous solutions suggested that some bands indicated alpha-helix (4090, 4365-4370, 4615, ...
[105]
Experiment 7 - Columbia University
The Fourier transform of this interferogram gives the conventional spectrum. The relationship between the interferogram and the spectrum is given by equation (5) ...
[106]
[PDF] INFRARED SPECTROSCOPY (IR)
Carbonyl compounds are those that contain the C=O functional group. In ... band in the middle of the spectrum around 1710 cm-1 corresponding to the C=O.
[107]
A Process for Successful Infrared Spectral Interpretation
Jan 1, 2016 · Methods for trying to minimize atmospheric peaks include purging the instrument with dry nitrogen, sealing and desiccating the instrument, and ...
[108]
Advances in Mid-Infrared Spectroscopy for Chemical Analysis
Jun 12, 2016 · Mid-infrared supercontinuum generation to 12.5μm in large NA chalcogenide step-index fibres pumped at 4.5μm. Opt. Express 22:1619169–82 ...
[109]
http://www.columbia.edu/cu/chemistry/ugrad/hssp/EXP_7.html
[110]
Far-Infrared Spectrum - an overview | ScienceDirect Topics
Far-infrared spectra refer to the range of wavenumbers from 200 to 10 cm⁻¹, where large-amplitude anharmonic vibrations occur.
[111]
Far-Infrared and Raman Spectroscopy Investigation of Phonon ...
Jun 24, 2016 · A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered ...
[112]
Far‐Infrared Spectroscopy - Griffiths - 2001 - Wiley Online Library
Aug 15, 2006 · Intermolecular vibrations of small molecules that absorb in the far infrared include hydrogen bonds and liquid lattice bands. The phonon bands ...
[113]
https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/Infrared_Spectroscopy
[114]
[PDF] Instrumentation for Far-infrared Spectroscopy
Synchrotron sources are used for far-infrared, with Golay detectors and mercury lamps as detectors. Sources should be hot with high emissivity.
[115]
[PDF] Bolometer - IRLabs
Bolometers are detectors used to measure incident. Infrared radiation. They are very sensitive to thermal radiation and are predominantly used in the ...
[116]
Instrumentation for Far‐Infrared Spectroscopy - Wiley Online Library
Aug 15, 2006 · Poly(ethylene terephthalate), also known as Mylar or Melinex, is the most common material used to fabricate far-infrared beamsplitters but other ...
[117]
Vacuum benefits for high-end research using FT-IR techniques
Atmospheric disturbance is a major problem in Fourier-transform infrared (FT-IR) spectroscopy. The different vibrational modes of water vapour and carbon ...<|separator|>
[118]
Far-infrared and NMR spectroscopic studies of organometallic ...
Nov 19, 2015 · The vibrational spectra of XMn(CO)5 compounds (where X= CH3, H, CF3, Br, Cl) in the vapour phase have been studied in the far-infrared region.Missing: FTIR | Show results with:FTIR
[119]
Spectroscopic characterization of explosives in the far-infrared region
Far infrared spectra of 14 commonly used explosive samples have been measured by using Fourier Transform Infrared Spectroscopy (FTIR) and THz Time-Domain ...Missing: detection | Show results with:detection
[120]
High-throughput terahertz imaging: progress and challenges - Nature
Sep 15, 2023 · However, recent advancements in terahertz imaging systems have greatly increased the imaging throughput and brought the promising potential of ...
[121]
Terahertz Spectroscopy and Imaging for Defense and Security ...
Aug 5, 2025 · In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz ...
[122]
Toward real-time terahertz imaging - Optica Publishing Group
The objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution ...
[123]
Crystallization in the Thin and Ultrathin Films of Poly(ethylene−vinyl ...
However, in crystal state, the band splits into two narrow bands located at 730 and 720 cm-1. Figure 9 shows the FTIR spectra of LLDPE crystallized ...
[124]
Complementarity of Raman and Infrared Spectroscopy for Structural ...
Feb 19, 2019 · Two IR bands at 720 and 730 cm–1 can be assigned to the in-plane rocking mode of CH2. This pair of peaks indicates crystallinity in the sample: ...
[125]
Study of matrix-filler interaction of polypropylene/silica composite by ...
We demonstrate a Fourier transform infrared (FTIR) spectroscopic imaging combined with two-dimensional (2D) correlation analysis to elucidate matrix-filler ...
[126]
Vibrational Spectroscopic Analysis of Water Absorption in ...
Oct 1, 2025 · Quantitative Analysis of the ν(OH) Band. Figure 10 shows the variations in the integrated area of each spectral component in the ν(OH) band ...
[127]
On the Silica Surface Modification and Its Effect on Charge Trapping ...
Jul 22, 2020 · It was found that the incorporation of the silane onto the surface of silica nanoparticles not only improved the dispersion of the nanoparticles ...Introduction · Experimental Section · Results and Discussion · Conclusions
[128]
Surface Modification of Silica Particles with Adhesive Functional ...
Jan 17, 2023 · The prepared and commercial silica particles were analyzed using FTIR to confirm successful surface functionalization with silane compounds and ...Missing: SiO2 | Show results with:SiO2
[129]
Size effect on structure and infrared behavior in nanocrystalline ...
From the results of Fourier Transform Infrared (FTIR) spectra and X-ray Diffraction (XRD), we found that there exists size effect on the structure and ...
[130]
Depth-Resolved FTIR-ATR Imaging Studies of Coating Degradation ...
Jun 30, 2022 · This study provides a method for detailed assessment of coating degradation, giving a good estimation of its durability.
[131]
Diffuse Reflectance Infrared Fourier Transform Spectroscopy
Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) allows collecting spectra from powdered samples with a minimum of sample preparation ...
[132]
[PDF] Characterization of Combinatorial Polymer Blend Composition ...
Fourier transform infrared microspectroscopy mapping technique was used to determine the composition of polymer blend gradi- ents. Composition gradients were ...
[133]
Surface chemistry changes of weathered HDPE/wood-flour ...
Fourier transform infrared (FTIR) spectroscopy was used to monitor the development of degradation products, such as carbonyl groups and vinyl groups, and to ...
[134]
Real-time analysis of the thermal oxidation of polyolefins by FT-IR ...
Secondary oxidation products, such as γ-lactones in the case of polypropylene, were observed from the onset of the degradation, which is consistent with a model ...
[135]
Evaluation of protein secondary structure from FTIR spectra ... - NIH
Feb 3, 2021 · It is well known that amide I (about 80% C = O stretching) and amide II (mainly δ(N–H and ν(C-N)) bands are the most useful for secondary ...
[136]
Membrane lipid phase transitions and phase organization studied ...
In this review, we discuss how FTIR spectroscopy can be utilized to study lipid phase structure and organization, and lipid phase transitions, in lipid model ...
[137]
Polymorphs of Aspirin – Solid-state IR-LD spectroscopic and ...
Polymorphism caused of an existence of more of one crystalline form of given compound has been intensively studied due to it remaining poorly understood and ...Missing: identification | Show results with:identification
[138]
A Review: Drug Excipient Iincompatiblity by Ftir Spectroscopy
Apr 17, 2023 · This review emphasizes on various examples of drug interaction with a number of excipients on the basis of Fourier Transform infrared spectroscopy data
[139]
Digital Histopathology by Infrared Spectroscopic Imaging - PMC
This review summarizes IR imaging instrumentation especially suited to histopathology, analyses of its performance, and major trends.
[140]
Effects of surface functional groups on the formation of nanoparticle ...
Here, we combined the SPR and FTIR spectroscopy techniques to study the effect of surface functional groups on the formation of protein corona. We investigated ...
[141]
Detection and quantification of poliovirus infection using FTIR ...
Dec 5, 2011 · In this work poliovirus (PV1) was used to evaluate the utility of FTIR spectroscopy with cell culture for rapid detection of infective virus particles.Missing: characterization | Show results with:characterization
[142]
[PDF] Method 320 - EPA
Aug 2, 2017 · Barrow, chapters 12, 13, and 14, McGraw Hill, Inc., 1979. 5. “Fourier Transform Infrared Spectrometry,” Peter R. ... required detection limit.
[143]
Laboratory Infrared Spectra (2.3-23 μm) of SO2 Phases
As observed in spectrum (a) of Fig. 11, a very intense band appears during the adsorption with a wave number of 1360 cm−1, that can be assigned to SO2 in the ...
[144]
Full article: SO2 emission measurement with the European standard ...
This study presents the results obtained when three portable automated measuring systems (P-AMS), based on Fourier-transform infrared (FTIR) spectroscopy, non- ...
[145]
Measuring chemical emissions using open-path Fourier transform ...
OP-FTIR spectrometers pass infrared light along an open beam path (up to ∼1 km) to simultaneously measure and identify a wide range of contaminants directly in ...
[146]
In Situ Determination of Nitrate in Water Using Fourier Transform ...
Dec 10, 2020 · FTIR-ATR spectroscopy combined with deconvolution curve-fitting can be conducted to determine the nitrate content in aqueous solutions.Missing: dissolved wastewater
[147]
In-situ monitoring of nitrate in industrial wastewater using Fourier ...
Quantitative prediction of nitrate contents in different industrial wastewater was carried out using Fourier transform infrared attenuated total reflectance ...Missing: dissolved | Show results with:dissolved
[148]
Comprehensive characterization of industrial wastewaters using ...
Jan 20, 2022 · EEM fluorescence, FT-IR and 1 H NMR are useful techniques for wastewater analysis. Organic matter from industrial effluents was characterized by all three ...
[149]
Fourier Transform Infrared Spectroscopy (FTIR) Method To Monitor ...
An analytical method has been developed using Fourier transform infrared spectroscopy (FTIR) to determine biodiesel content in the reaction mixture to ...
[150]
Online ATR-FT-IR for Real-Time Monitoring of the Aspirin ...
The ATR-FT-IR spectroscopy technique can monitor and analyze the changes of raw materials and products with time, which is more reliable to determine ...Missing: plants | Show results with:plants
[151]
[PDF] The CU Airborne Solar Occultation Flux Instrument
Jan 21, 2022 · The SOF technique uses a solar tracker coupled to a Fourier transform infrared spectrometer (FTS) and is capable of measuring many relevant ...<|separator|>
[152]
Field Demonstration for Mobile FT-IR for Detection of Volatile ...
A mobile system has been developed at Kansas State University (KSU) to measure volatile organic compounds (VOCs) in the atmosphere using a Fourier transform ...Missing: 2010 | Show results with:2010
[153]
Portable Fourier Transform Infrared Gas Analyzer
MODEL 3080FT portable Fourier transform infrared analyzer, using Fourier transform infrared spectroscopy technology (FTIR), does not require sample separation.
[154]
Trends Applications of gas chromatography-Fourier transform IR ...
In addition, GC-FTIR techniques have been applied to the analyses of such environmental contaminants as the priority pollutants, pesticides, polycyclic aromatic ...
[155]
Gas Chromatography—Fourier Transform Infrared Spectroscopy for ...
Jul 24, 2020 · GC-FTIR allows to quick identifying functional groups in unknown substances, based on the retention behavior of the analytes and the IR ...
[156]
FT-IR detection in gas chromatography - ScienceDirect.com
MS is a favoured technique to be hyphenated with FTIR, providing complementary information of both molecular/fragment m/z ratios and possible compound ...
[157]
Application of Thermogravimetric Fourier Transform Infrared ...
The objective of the present work was to use TG−FTIR to examine the pyrolysis profiles of oxygen-containing gases H2O, CO2, and CO of the Argonne premium coals ...
[158]
Evolved Gas Analysis of Polymers by Thermogravimetry Fourier ...
A hyphenated TG-FTIR technique has been developed with a unique interface system. The IR beam of a FTIR spectrometer is led directly into the TG equipment, the ...
[159]
Thermal Characterization of Materials Using Evolved Gas Analysis
Thermal analysis combined with evolved gas analysis has been used for some time. Thermogravimetry (TG) coupled with Fourier transform infrared (FTIR) spect.
[160]
Hyphenation of Ion Exchange High-Performance Liquid ...
Oct 15, 1997 · Fourier transform infrared spectroscopy (FT-IR) is presented here as a molecular-specific detection system for high-performance liquid ...
[161]
R. Andrew Shalliker (Ed): Hyphenated and Alternative Methods of ...
In practice, hyphenated methods usually refer to spectroscopic methods linked to GC and HPLC.
[162]
A Focal Plane Array (FPA) FTIR and multivariate analysis - Nature
Nov 21, 2019 · FPA µ-FTIR 2D Imaging is an advantageous and feasible approach when investigating samples that have heterogeneous structure and composition at ...
[163]
Spatial resolution in infrared microspectroscopic imaging of tissues
The spatial resolution in a given IR imaging setup strongly affects the character of the infrared spectral patterns obtained from the biomedical samples.
[164]
Fourier Transform Spectroscopic Imaging Using an Infrared Focal ...
A wide-field imaging approach for simultaneous super-resolution surface-enhanced Raman scattering bioimaging and spectroscopy.
[165]
Synchrotron Near-Field Infrared Nanospectroscopy and ...
May 24, 2024 · While s-SNOM refers to imaging at a single wavelength, nano-FTIR involves illuminating the tip/sample interface with a broadband IR source and ...
[166]
Rapid Infrared Spectroscopic Nanoimaging with nano-FTIR ...
Sep 21, 2020 · Here we combine s-SNOM, nano-FTIR, and synthetic optical holography (SOH) to achieve infrared spectroscopic nanoimaging at unprecedented speed.
[167]
Combining Raman and FT-IR Spectroscopy with Quantitative ...
Apr 1, 2015 · In this study, for the first time we have combined Raman spectroscopy and Fourier transform infrared (FT-IR) spectroscopy, as metabolic fingerprinting ...
[168]
In Situ Raman and Fourier Transform Infrared Spectroscopy Studies ...
Jun 9, 2025 · Combining FTIR and Raman spectroscopy enables a comprehensive understanding of the reaction mechanisms and interfacial processes occurring at ...
[169]
Optical photothermal infrared spectroscopy with simultaneously ...
Nov 5, 2022 · In this paper, we demonstrate a systematic comparison of IR (FTIR and O-PTIR) and Raman spectra (collected via simultaneous O-PTIR + Raman and ...