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Absorption spectroscopy

Absorption spectroscopy is a spectroscopic technique that measures the absorption of electromagnetic radiation by atoms, molecules, or other materials as a function of wavelength or frequency, producing an absorption spectrum that reveals characteristic patterns akin to fingerprints for identifying and quantifying substances in a sample.^[1]^[2] The fundamental principle underlying absorption spectroscopy is the interaction of photons with matter, where radiation is absorbed when its energy matches the difference between quantum mechanical energy levels in the sample, leading to transitions such as electronic, vibrational, or rotational excitations.^[3]^[2] This process is quantitatively described by Beer's Law, which states that the absorbance (A) of light is directly proportional to the concentration (c) of the absorbing species, the path length (d) through the sample, and the molar absorptivity (ε), expressed as A = εcd.^[3] Historically, the technique traces its roots to the early 19th century with Joseph von Fraunhofer's observations of dark lines in the solar spectrum, laying the groundwork for modern applications.^[4] Absorption spectroscopy encompasses various types, including ultraviolet-visible (UV-Vis) for electronic transitions, infrared (IR) for vibrational modes, and X-ray for core electron excitations, each tailored to specific wavelength ranges and sample properties.^[2] Its applications span analytical chemistry for qualitative identification and quantitative analysis of compounds in solutions, environmental monitoring for detecting pollutants, astronomical studies to analyze stellar compositions, and biomedical research for probing molecular structures.^[1]^[2] Widely used due to its sensitivity and versatility, the method is essential in fields like pharmaceuticals, materials science, and remote sensing, enabling non-destructive characterization of samples at trace levels.^[5]^[2]

Principles

Basic Mechanism of Absorption

Absorption spectroscopy is based on the process by which atoms or molecules absorb photons of electromagnetic radiation, resulting in the excitation of electrons, vibrations, or rotations from lower to higher energy states.^[6] This phenomenon was first systematically observed in 1814 by Joseph von Fraunhofer, who identified hundreds of dark absorption lines in the solar spectrum using a prism spectrometer, laying the groundwork for understanding spectral absorption in stellar atmospheres.^[7] At the quantum mechanical level, absorption occurs through discrete transitions between quantized energy levels, where the photon's energy must match the difference between initial and final states, given by E = h\nu, with h as Planck's constant and \nu as frequency.^[8] These transitions are governed by selection rules derived from quantum electrodynamics, particularly for electric dipole interactions, which require a change in orbital angular momentum quantum number of \Delta l = \pm 1, such as s to p transitions in atoms.^[9] The probability of absorption at a given wavelength depends on the energy gap between levels, with shorter wavelengths corresponding to larger energy differences typically involving electronic excitations.^[6] Molar absorptivity, denoted \epsilon, quantifies the intrinsic probability of these transitions for a specific molecule or atom at a given wavelength, reflecting how strongly the species interacts with the radiation.^[8] Common examples include electronic absorption in the ultraviolet-visible (UV-Vis) region, where valence electrons are promoted (e.g., \pi \to \pi^* in organic molecules), and vibrational absorption in the infrared (IR) region, involving changes in molecular bond stretching or bending modes.^[6] The Beer-Lambert law provides a quantitative framework for relating this absorption intensity to concentration, but the underlying mechanism remains rooted in these quantum transitions.^[8]

Beer-Lambert Law

The Beer-Lambert law, a foundational principle in absorption spectroscopy, combines the contributions of Johann Heinrich Lambert and August Beer in quantifying light absorption by matter. In 1760, Lambert demonstrated that the intensity of light diminishes exponentially as it passes through a homogeneous medium, with the decrease proportional to the distance traveled.^[10] August Beer extended this in 1852 by applying it to colored solutions, showing that absorption also depends linearly on the concentration of the absorbing solute, thus enabling quantitative analysis in solution-based measurements.^[10] The law states that absorbance A is given by

A = \epsilon c l,

where \epsilon is the molar absorptivity (specific to the substance and wavelength), c is the molar concentration of the absorber, and l is the optical path length.^[6] This form arises from the differential equation describing infinitesimal intensity loss, dI = -\alpha I \, dl, where \alpha = \epsilon c is the linear absorption coefficient; integrating from incident intensity I_0 to transmitted intensity I over path length l yields the exponential relation I = I_0 e^{-\epsilon c l}.^[6] Absorbance links to transmittance T = I / I_0 via the base-10 logarithmic form A = -\log_{10} T, which linearizes the exponential decay for practical plotting and analysis, ensuring A increases linearly with c or l under ideal conditions.^[6] The molar absorptivity \epsilon typically carries units of L mol⁻¹ cm⁻¹, with c in mol L⁻¹ and l in cm, rendering A dimensionless and facilitating calibration curves for concentration determination in analytical applications.^[11] The law relies on key assumptions, including monochromatic incident light, dilute solutions to avoid solute interactions, and negligible scattering, fluorescence, or reflection within the sample.^[6] Limitations arise when these are violated, such as in polychromatic illumination causing nonlinear deviations, or high concentrations inducing inner filter effects where emitted or scattered light is reabsorbed, distorting measured absorbance.^[12] Additionally, chemical equilibria in the sample can shift effective c, further compromising linearity beyond dilute regimes.^[6]

Absorption Spectra

Characteristics and Interpretation

An absorption spectrum is a graphical representation of the absorbance of light by a sample as a function of wavelength or frequency, typically plotted with absorbance on the y-axis and wavelength (in nanometers) or wavenumber (in cm⁻¹) on the x-axis.^[13]^[14] This plot reveals the specific wavelengths where the sample absorbs energy, corresponding to transitions between molecular energy levels, such as electronic excitations in ultraviolet-visible (UV-Vis) spectroscopy or vibrational modes in infrared (IR) spectroscopy.^[15] Absorbance values generally range from 0 (no absorption) to 2 (near-complete absorption at that wavelength), providing a quantitative measure of light interaction with the analyte.^[13] Key features of absorption spectra include prominent peaks, known as absorption bands, which indicate the wavelengths of maximum absorption due to specific molecular transitions.^[15] These bands arise from electronic, vibrational, or rotational excitations, with their positions serving as unique signatures for molecular structures.^[14] Bandwidths, or the width of these peaks, reflect the resolution of the transitions and often incorporate fine structure from vibrational or rotational sublevels, though they can broaden due to environmental influences.^[13] Baseline corrections are essential to establish a reference level for zero absorption, subtracting any background signal from the instrument or sample matrix to ensure accurate peak identification.^[13] Interpretation of absorption spectra relies on analyzing peak positions to identify functional groups and integrated peak intensities to quantify concentrations. For instance, in IR spectroscopy, a strong absorption band around 1700 cm⁻¹ typically indicates a carbonyl (C=O) stretch, allowing deduction of molecular composition through comparison with known spectral libraries.^[14] Peak positions provide qualitative insights into bonding environments, while the area under the curve or peak height, when combined with path length and molar absorptivity, enables concentration determination via the Beer-Lambert law.^[13] This approach prioritizes conceptual matching of spectral patterns to molecular motifs rather than exhaustive cataloging. Several factors influence the appearance of absorption spectra, necessitating controlled conditions for reliable interpretation. Solvent effects can shift peak positions (bathochromic or hypsochromic shifts) and alter bandwidths by interacting with the solute's electronic or vibrational states, such as through polarity or hydrogen bonding.^[13]^[16] Temperature broadening occurs as increased thermal energy populates higher vibrational levels, leading to wider peaks and reduced resolution of fine structure.^[13]^[17] Additionally, pH variations can modify protonation states of ionizable groups, thereby changing absorption characteristics, particularly in UV-Vis spectra of acids or bases.^[16] Absorption spectra are often presented in normalized forms, such as molar absorptivity (ε) versus wavelength, to facilitate comparison across samples by removing concentration dependencies.^[13] Difference spectra, obtained by subtracting a reference spectrum from the sample, highlight changes over time or conditions, aiding kinetic studies of reactions without baseline drift issues.^[16] These presentation methods enhance readability and emphasize subtle spectral variations for precise molecular analysis.

Relation to Other Spectra

Absorption spectra are closely related to transmission spectra, as both describe the interaction of light with a sample but from complementary perspectives. The transmittance T, defined as the ratio of transmitted intensity to incident intensity (T = I/I_0), is directly related to absorbance A through the Beer-Lambert law, where T = 10^{-A}.^[18] This inverse relationship means that regions of high absorption correspond to low transmission, and the two spectra are mathematically equivalent in many contexts, though transmission spectra emphasize the light that passes through the sample while absorption focuses on the energy removed.^[19] In practice, the terms are sometimes used interchangeably for transparent media, but absorption measurements prioritize quantifying the absorbing species, whereas transmission highlights overall light propagation.^[18] Absorption spectra complement emission spectra, which arise from the relaxation of excited states back to the ground state. Absorption promotes molecules from the ground state to excited states by absorbing photons, while emission involves the subsequent release of energy as light, often resulting in a red-shifted emission spectrum relative to absorption due to vibrational relaxation in the excited state, known as the Stokes shift.^[20] This shift, typically measured as the difference between the absorption and emission maxima, ensures that emission wavelengths are longer than those absorbed, preventing overlap and enabling phenomena like fluorescence. Absorption spectra can predict possible emission lines by identifying allowed electronic transitions, providing a foundational understanding of the energy levels involved in both processes.^[20] Scattering spectra, such as those from Raman spectroscopy, probe similar molecular vibrations as infrared (IR) absorption but through different mechanisms. In IR absorption, vibrations are excited directly by light if they induce a change in the molecular dipole moment, limiting observations to polar bonds.^[21] Raman scattering, however, involves inelastic scattering of light where the incident photon exchanges energy with the molecule, requiring a change in polarizability rather than dipole moment, thus allowing detection of non-polar bonds that are IR-inactive.^[22] This complementarity enables Raman to access vibrational information akin to IR absorption without the selection rule restrictions, though both techniques map out molecular energy levels in the vibrational regime.^[21] Reflection spectra are linked to absorption through surface and bulk interactions, particularly in opaque or scattering media like solids. Absorption contributes to reflection by attenuating light that penetrates the material, and the Kubelka-Munk theory relates the diffuse reflectance R to the absorption coefficient K and scattering coefficient S via the function F(R) = (1 - R)^2 / (2R) \approx K/S, approximating bulk absorption from surface-reflected light. However, while absorption spectroscopy measures bulk properties throughout the sample depth, reflection spectra primarily capture interfacial effects and scattering at the surface, making the techniques distinct for heterogeneous materials.^[23] In practical applications, absorption spectroscopy excels in quantitative bulk analysis of concentrations via the Beer-Lambert law, offering high sensitivity for trace analytes in solutions or gases.^[18] Emission spectroscopy, by contrast, is often favored for qualitative identification of elements or species due to its bright, line-specific signals from excited atoms or molecules, though it may require higher sample excitation energies. These distinctions guide the choice of technique based on whether precise quantification or spectral fingerprinting is prioritized.^[24]

Experimental Techniques

Instrumentation and Setup

Absorption spectroscopy instrumentation typically consists of several core components that facilitate the measurement of light absorption by a sample across specific wavelength ranges. The light source provides the incident radiation; for ultraviolet-visible (UV-Vis) spectroscopy, common sources include deuterium arc lamps for the UV region (approximately 190–350 nm) and tungsten-halogen lamps for the visible region (approximately 350–900 nm). In infrared (IR) spectroscopy, thermal sources such as globars or Nernst glowers emit broadband radiation from about 400 nm to 50 μm. A monochromator or interferometer then selects or disperses wavelengths: in dispersive UV-Vis systems, holographic gratings or prisms isolate narrow bands, while IR setups often employ interferometers to enable simultaneous detection of all wavelengths via Fourier transform processing. The sample is held in a cuvette or cell for liquids, with fused silica preferred for UV transparency down to 190 nm and glass or quartz for visible ranges; path lengths are typically 1–10 cm to optimize signal according to the Beer-Lambert law, which underpins absorbance calculations. Detectors convert transmitted light to electrical signals, such as photomultiplier tubes for high-sensitivity UV-Vis detection or thermal detectors like pyroelectric devices for IR.^[16]^[6]^[25] Instrument configurations vary between single-beam and double-beam spectrophotometers to address practical challenges like source instability. In a single-beam setup, the light passes sequentially through the sample and reference, offering simplicity, lower cost, and higher light throughput for sensitive measurements, though it requires manual switching and is susceptible to drift. Double-beam systems split the beam using a chopper or beam splitter, directing one path through the sample and the other through a reference, which automatically compensates for fluctuations in source intensity or detector response, improving accuracy over long scans. Modern instruments incorporate digital controls for automated wavelength selection, data acquisition, and beam alignment, enhancing reproducibility.^[16]^[26] Sample preparation ensures uniform interaction with the light beam and minimizes scattering or interference. For liquid solutions, samples are diluted in solvents compatible with the wavelength range (e.g., avoiding UV-absorbing solvents like ethanol below 250 nm) and placed in matched cuvettes to control path length precisely. Gaseous samples require sealed cells with windows transparent to the spectral region, often with adjustable path lengths up to several meters for low-concentration detection. Solids are prepared as dispersions, such as potassium bromide (KBr) pellets for mid-IR analysis, where the sample (1–2% by weight) is ground finely with KBr and pressed under vacuum to form a transparent disk approximately 1 mm thick.^[27] Alternatively, attenuated total reflectance (ATR) is widely used for solids and liquids in IR spectroscopy, involving direct contact with a crystal (e.g., diamond or ZnSe) to measure surface absorption without extensive preparation, reducing issues like moisture sensitivity in KBr methods. Path length control is critical in all cases to maintain linearity in absorbance readings.^[16] The basic experimental setup involves initial alignment of optical components to maximize light throughput, followed by recording the incident intensity I_0 using a blank (solvent or empty cell) across the desired wavelength range. The sample is then inserted, and transmitted intensity I is measured by scanning wavelengths, typically from low to high to avoid heating effects in IR. Data processing computes absorbance as A = \log_{10}(I_0 / I), directly relating to concentration via the Beer-Lambert law.^[16] Safety protocols emphasize protection from intense light sources, such as UV-blocking eyewear for deuterium lamps to prevent eye damage, and proper ventilation for volatile solvents or IR sources that generate heat. Calibration ensures reliability: wavelength accuracy is verified using standards like holmium oxide solution, which exhibits sharp peaks at known positions (e.g., 241.5 nm, 287.2 nm), while stray light is minimized through clean optics and enclosure design to keep levels below 0.1% across the spectrum. Photometric linearity is checked with neutral density filters or potassium dichromate solutions, and modern digital interfaces allow real-time monitoring and automated corrections for baseline drift.^[16]^[28]

Specific Methods

Ultraviolet-visible (UV-Vis) absorption spectroscopy operates in the wavelength range of approximately 200-800 nm, where absorption arises from electronic transitions in molecules, such as π → π* and n → π* excitations in organic compounds.^[13] This method commonly employs diode array detectors to enable rapid scanning and simultaneous acquisition of full spectra across the range.^[16] Infrared (IR) absorption spectroscopy targets the mid-IR region from 4000 to 400 cm⁻¹, corresponding to vibrational modes of molecular bonds, including stretching and bending transitions that provide fingerprints for functional groups.^[29] Traditional dispersive IR uses a prism or grating to separate wavelengths, while Fourier transform IR (FTIR) employs a Michelson interferometer with a moving mirror and beam splitter to generate an interferogram, which is Fourier-transformed into a spectrum for higher resolution and speed.^[30] FTIR offers advantages in sensitivity and multiplex detection (Felgett advantage), but IR methods generally suffer from strong water interference due to O-H absorption bands overlapping with sample signals.^[31] Atomic absorption spectroscopy (AAS) focuses on elemental analysis by atomizing samples in a flame or graphite furnace to produce ground-state atoms, which absorb light from element-specific hollow cathode lamps emitting narrow atomic lines.^[32] In flame AAS, aspiration into an air-acetylene flame achieves rapid atomization for high-throughput measurements, whereas graphite furnace AAS provides lower detection limits through controlled heating stages for trace elements.^[33] Near-infrared (NIR) absorption spectroscopy covers 700-2500 nm, probing overtone and combination bands of fundamental vibrations, which are weaker but enable non-destructive analysis of solids, liquids, and tissues without extensive preparation.^[34] Emerging methods include terahertz absorption spectroscopy, which examines low-energy vibrational and rotational modes in the 0.1-10 THz range for applications in biomolecules and materials.^[35] Time-resolved absorption spectroscopy uses pump-probe configurations to study reaction kinetics by exciting samples with a pump pulse and monitoring transient absorption with a delayed probe.^[36]

Applications

Chemical Analysis

Absorption spectroscopy plays a central role in quantitative chemical analysis by enabling precise determination of analyte concentrations through adherence to the Beer-Lambert law, which states that absorbance is directly proportional to concentration under conditions of constant path length and molar absorptivity. In pharmaceutical applications, this principle is routinely applied to assess drug purity; for instance, UV-Vis spectrophotometry measures the absorbance of active pharmaceutical ingredients at characteristic wavelengths, allowing calculation of concentrations. The limit of detection (LOD) in such analyses is governed by the signal-to-noise ratio, typically defined as three times the standard deviation of the blank divided by the slope of the calibration curve, for photometric methods in routine laboratory settings. For qualitative identification, absorption spectra act as molecular fingerprints, facilitating compound verification by comparing observed λ_max and spectral shapes to reference data. In UV-Vis spectroscopy, this is particularly effective for conjugated systems, where the extent of π-conjugation shifts absorption maxima to longer wavelengths (bathochromic shift), enabling distinction of structural isomers or functional groups without prior separation. Such spectral matching confirms identity in organic synthesis or impurity profiling, with high specificity derived from the unique electronic transitions involved. Absorption spectroscopy also supports the study of reaction kinetics and mechanisms by monitoring temporal changes in absorbance, which correlate directly with reactant or product concentrations via Beer's law. This time-resolved approach yields rate constants and reaction orders; for example, in homogeneous reactions, absorbance at a fixed wavelength tracks species evolution, revealing pseudo-first-order kinetics with rate constants determined from exponential decay fits.^[37] Practical examples include spectrophotometric titrations, where continuous absorbance measurements during reagent addition identify equivalence points through curve inflections, applicable to systems where traditional indicators fail, such as titrations of absorbing analytes or products alone. In complexometric analysis, metallochromic indicators like Eriochrome Black T form colored complexes with metal ions, and absorbance shifts upon titration with EDTA signal endpoint detection, enhancing selectivity for divalent cations in aqueous solutions.^[38] Modern advances in chemical analysis have integrated absorption spectroscopy with chromatographic separations in hyphenated techniques, such as high-performance liquid chromatography coupled to UV detection (HPLC-UV), which resolves complex mixtures before spectral quantification. These developments have enhanced sensitivity and reduced analysis times for pharmaceutical formulations, enabling simultaneous identification and purity assessment in multi-component samples.

Astronomy and Remote Sensing

Absorption spectroscopy plays a pivotal role in astronomy by enabling the analysis of stellar atmospheres through the observation of Fraunhofer lines, which are dark absorption features in the continuous spectrum of stars caused by cooler gases absorbing specific wavelengths. These lines, first systematically cataloged by Joseph von Fraunhofer in 1814, reveal the chemical composition, temperature, and velocity of stellar atmospheres by identifying atomic and molecular species such as hydrogen, helium, and metals. For instance, the Fraunhofer lines in the solar spectrum allow astronomers to determine the abundance of elements like iron and calcium, providing insights into the star's physical conditions.^[39] In the interstellar medium (ISM), absorption spectroscopy detects molecular hydrogen (H₂) through its ultraviolet absorption lines, which trace the distribution and properties of diffuse gas clouds along lines of sight to background stars. These H₂ lines, observed primarily in the far-UV range (around 912–1108 Å), indicate the presence of cold, dense regions where molecules form, helping to map the ISM's structure and dynamics. Additionally, during exoplanet transits, when a planet passes in front of its host star, absorption spectroscopy measures the planet's atmospheric transmission spectrum, revealing molecular signatures like water vapor, carbon dioxide, and methane through depth variations in the stellar light. This technique has identified atmospheric compositions in hot Jupiters, such as enhanced water absorption in WASP-12b.^[40]^[41]^[42] Remote sensing applications extend absorption spectroscopy to Earth's atmosphere, where techniques like Differential Optical Absorption Spectroscopy (DOAS) measure trace gases such as ozone by analyzing narrow-band UV absorptions in scattered sunlight or direct beams over long paths. DOAS exploits the distinct absorption fingerprints of ozone in the 280–340 nm range to quantify column densities, aiding in monitoring stratospheric depletion. Satellite-based instruments, including the Ozone Monitoring Instrument (OMI) launched in 2004 aboard NASA's Aura satellite, use nadir-viewing UV spectroscopy to map global ozone distributions with resolutions of 13×24 km, supporting climate and air quality assessments.^[43]^[44] Key challenges in these applications include Doppler broadening in stellar atmospheres, arising from thermal motions of atoms that widen absorption lines and complicate abundance determinations, requiring high spectral resolutions exceeding 10,000 to resolve profiles. In atmospheric remote sensing, long path lengths (up to thousands of kilometers for satellite views) introduce multiple scattering and variable optical depths, necessitating corrections for aerosol interference and photon path distributions to achieve accurate retrievals. Data analysis often relies on equivalent width measurements, defined as the integrated area of an absorption line normalized by the continuum flux, to quantify line strengths and derive elemental abundances via curve-of-growth methods, as applied in high-resolution stellar spectra.^[45]^[46]^[47] Seminal discoveries underscore the technique's impact: in 1868, Pierre Janssen and Norman Lockyer independently detected helium's yellow emission line (587.6 nm) in the Sun's chromosphere during a total solar eclipse, marking the first identification of an element unknown on Earth.^[48] More recently, since 2022, the James Webb Space Telescope (JWST) has advanced molecular mapping through mid-infrared transmission spectroscopy, detecting carbon dioxide and methane in exoplanet atmospheres like WASP-39b, revealing diverse chemical environments at unprecedented sensitivity. As of 2025, JWST observations have provided new constraints on potential biosignatures, such as dimethyl sulfide (DMS) in the atmosphere of K2-18 b.^[49]^[50]

Biological and Medical Uses

Absorption spectroscopy plays a central role in quantifying biomolecules, particularly proteins and nucleic acids, through their characteristic ultraviolet (UV) absorbance. Proteins exhibit strong absorption at 280 nm primarily due to aromatic amino acids such as tryptophan and tyrosine, enabling rapid and non-destructive concentration measurements in biological samples.^[51] Similarly, DNA and RNA absorb maximally at 260 nm owing to the π-π* transitions in their nucleotide bases, allowing for straightforward nucleic acid quantification without the need for dyes or labels.^[51] The ratio of absorbance at 260 nm to 280 nm (A260/A280) serves as a purity indicator, with values around 1.8 for DNA and 2.0 for RNA signifying minimal protein contamination.^[52] In medical diagnostics, absorption spectroscopy underpins non-invasive techniques for monitoring vital parameters. Pulse oximetry employs near-infrared (NIR) and red light (typically 660 nm and 940 nm) to differentiate oxygenated and deoxygenated hemoglobin based on their distinct absorption spectra, providing real-time peripheral oxygen saturation (SpO2) measurements essential for respiratory assessment.^[53] For jaundice diagnosis, particularly in neonates, transcutaneous bilirubinometry uses visible light absorption around 450-460 nm, where bilirubin exhibits peak absorbance, to estimate serum levels and guide phototherapy decisions without invasive blood draws.^[54] These methods highlight absorption spectroscopy's utility in point-of-care settings, reducing patient discomfort while maintaining clinical accuracy. Advanced applications extend to imaging and therapeutic monitoring, leveraging absorption contrasts in vivo. Photoacoustic imaging combines optical absorption with ultrasonic detection, where NIR laser pulses induce thermoelastic expansion in absorbing tissues (e.g., hemoglobin-rich vasculature), generating acoustic waves for high-resolution anatomical and functional imaging in oncology and cardiology.^[55] In pharmacokinetics, UV-Vis absorption tracks drug metabolites by monitoring changes in absorbance spectra during in vitro and ex vivo assays, aiding in the assessment of metabolic stability and bioavailability.^[56] Enzyme activity assays, such as those measuring NADH production at 340 nm in dehydrogenase reactions, exemplify real-time kinetic monitoring critical for drug development and biochemical research.^[57] Recent advancements in the 2020s focus on portable NIR devices for non-invasive glucose monitoring, addressing diabetes management challenges. These systems analyze overtone absorptions in the 1000-1700 nm range from glucose's C-H and O-H bonds in interstitial fluid, with machine learning algorithms improving prediction accuracy despite inter-individual variability.^[58] Clinical implementation requires rigorous calibration to account for biological factors like skin pigmentation, hydration, and motion artifacts, ensuring equitable performance across diverse populations and minimizing diagnostic biases.^[58] Infrared absorption methods briefly reference vibrational modes for biomolecular structural analysis, complementing UV-Vis in comprehensive profiling.^[59]

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Sep 27, 2013 · Other Doppler broadenings involve stellar rotation, pulsation, and mass loss (P Cygni profile). 3. Pressure (and collisional) broadening. The ...Missing: spectroscopy | Show results with:spectroscopy
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[PDF] Ground-based photon path measurements from solar absorption ...
Retrieval of photon path lengths with sub-percentage precision from atmospheric spectra is always a challenge. For the A-band, the high optical depths mean ...
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[PDF] Stellar Populations - Lecture III - UMD Astronomy
ABUNDANCES FROM HIGH-RESOLUTION SPECTROSCOPY. With high-resolution spectra of individual stars, we can measure equivalent widths of absorption lines from ...
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Helium first discovered during 1868 eclipse; the element later ...
Mar 27, 2024 · The chemical element helium was first observed during an 1868 solar eclipse in India. French astronomer Pierre Janssen focused a spectroscope on the solar ...
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Webb's Impact on Exoplanet Research - NASA Science
Sep 2, 2025 · The James Webb Space Telescope has ushered in a new era in exoplanet research. To learn about the diversity of exoplanets and their ...
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A Simple and General Method for Determining the Protein and ... - NIH
Here, we consider the absorbance spectrum to be a sum of protein and nucleic acid components, determined algebraically from the UV absorption at 260nm and 280nm ...
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Ultraviolet absorbance at 260 and 280 nm in RNA measurement is ...
Aug 10, 2025 · The ratio of absorbance at 260 and 280 nm (the A260/280 ratio) is frequently used to assess the purity of RNA and DNA preparations. A low A260/ ...
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Pulse oximetry - PMC - NIH
Pulse oximetry is based on two physical principles: (a) the presenceof a pulsatile signal generated by arterial blood, which is relativelyindependent of non- ...Missing: spectroscopy | Show results with:spectroscopy
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Point-of-care device to diagnose and monitor neonatal jaundice in ...
As bilirubin concentration increased, measured spectra showed increasing absorbance in the blue with little to no change in absorption due to hemoglobin (Fig.
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Advances in Clinical and Biomedical Applications of Photoacoustic ...
Photoacoustic imaging is an imaging modality that derives image contrast from the optical absorption coefficient of the tissue being imaged.
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UV-Vis Spectroscopy: Principle, Strengths and Limitations and ...
Dec 18, 2023 · Samples contaminated with proteins will lower the 260/280 ratio due to higher absorbance at 280 nm. Wavelength used in absorbance analysis.<|control11|><|separator|>
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Kinetic modelling: an integrated approach to analyze enzyme ... - NIH
Aug 25, 2017 · For example, the spectrophotometric data of coupled reactions like the PDC assay initially showed a lag phase before reaching a linear phase ( ...
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A new generation of sensors for non-invasive blood glucose ... - NIH
This review compares the progress and challenges of electrochemical, optical, and electromagnetic/microwave non-invasive blood glucose monitoring techniquesElectrochemical Technology · Flexible Biosensors Based On... · Optical TechnologyMissing: portable 2020s
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Optical spectroscopy for in vivo medical diagnosis—a review of the ...
Aug 26, 2020 · In this review, several types of optical spectroscopy techniques (elastic optical scattering spectroscopy, fluorescence spectroscopy, Raman spectroscopy, and ...