Solid-phase microextraction
Solid-phase microextraction (SPME) is a solvent-free sample preparation technique that employs a thin polymer-coated fiber to extract and preconcentrate analytes from gaseous, liquid, or solid matrices, integrating sampling, extraction, and enrichment into a single step for subsequent analysis by methods such as gas chromatography (GC) or liquid chromatography-mass spectrometry (LC-MS).[1] Developed in 1990 by Catherine L. Arthur and Janusz Pawliszyn, SPME represents a major advancement in analytical chemistry by minimizing solvent use and simplifying workflows compared to traditional methods like liquid-liquid extraction.[1] The technique operates on the principle of analyte partitioning between the sample matrix and the stationary phase coated on a fused-silica fiber, typically 1 cm long and housed in a syringe-like device for easy insertion and retraction.[2] Extraction modes include direct immersion in the sample or headspace sampling above it, with analytes desorbed thermally for GC or via solvent for LC, achieving equilibrium-based, non-exhaustive extraction that is rapid and selective based on coating polarity (e.g., polydimethylsiloxane for nonpolar compounds or divinylbenzene for volatiles).[3] Common fiber coatings include polydimethylsiloxane (PDMS), polyacrylate, and mixed phases like PDMS/divinylbenzene (DVB), enabling versatility for diverse analyte classes from volatile organic compounds to pharmaceuticals.[4] SPME has found widespread applications in environmental monitoring (e.g., detecting pesticides in water), food safety (e.g., flavor profiling in beverages), bioanalysis (e.g., drug metabolites in urine), and forensics (e.g., alcohol in blood), offering high sensitivity with limits of detection as low as femtogram levels in specialized applications[5] and compatibility with automation for high-throughput analysis.[3] Its advantages include reduced sample manipulation, portability for in-field use, and eco-friendliness due to minimal waste, though limitations such as fiber fragility and matrix interferences have spurred innovations like coated blade spray and in-tube SPME variants.[2] Since its inception, SPME has evolved with advanced coatings (e.g., sol-gel or ionic liquids) and devices, solidifying its role as a cornerstone of green analytical chemistry.[4]History and Development
Invention and Early Milestones
Solid-phase microextraction (SPME) was invented by Janusz Pawliszyn, a professor in the Department of Chemistry at the University of Waterloo in Ontario, Canada, during the late 1980s as a solvent-free alternative to traditional sample preparation methods for trace-level analysis.[6] The technique emerged from efforts to simplify the extraction of volatile and semi-volatile organic compounds using a small sorbent coating on fused silica fibers, enabling direct interfacing with analytical instruments without the need for extensive cleanup.[7] Pawliszyn's initial work addressed limitations in existing methods like purge-and-trap and liquid-liquid extraction, focusing on portability and minimal sample manipulation for environmental and clinical applications.[8] The foundational publication introducing SPME appeared in 1990, co-authored by Catherine L. Arthur and Pawliszyn, detailing the use of thermal desorption from coated fused silica optical fibers for gas chromatography (GC) analysis of low-molecular-weight organics in aqueous samples.[7] This proof-of-concept demonstrated efficient extraction of compounds like benzene and toluene with detection limits in the parts-per-billion range, establishing SPME's viability for rapid, in-field sampling. A key milestone followed in 1991 with the publication of the international patent application WO 91/15745, which described the method and device for SPME and desorption.[9] Theoretical underpinnings were formalized in 1997 through Pawliszyn's comprehensive review in Critical Reviews in Analytical Chemistry, which outlined the kinetics and equilibrium models governing analyte partitioning between sample matrices and fiber coatings, providing a framework for optimizing extraction conditions.[10] This publication built on early experimental validations and emphasized SPME's non-exhaustive nature, distinguishing it from exhaustive extraction techniques. The same year, Pawliszyn published the book Solid Phase Microextraction: Theory and Practice (Wiley-VCH), which synthesized initial principles, method development strategies, and beginner experiments, solidifying SPME's conceptual foundation. A pivotal expansion came in 1999 with Pawliszyn's edited volume Applications of Solid Phase Microextraction (Royal Society of Chemistry), which compiled early validations across diverse matrices and analytes, demonstrating SPME's versatility in environmental monitoring, food safety, and pharmaceutical analysis.[11] This work marked a key milestone by shifting focus from proof-of-concept to practical implementation, influencing subsequent adoptions in routine laboratory protocols. Early fiber coatings, such as polydimethylsiloxane, proved essential in these prototypes for selective analyte retention based on partitioning coefficients.[7]Evolution and Commercialization
Following the invention of solid-phase microextraction (SPME) by Janusz Pawliszyn in the late 1980s, the technique transitioned from a laboratory prototype to a commercially viable tool in the 1990s.[12] Supelco (now part of MilliporeSigma) introduced the first commercial SPME device in 1993, featuring fused-silica fibers coated for gas chromatography (GC) applications, which facilitated solvent-free extraction and direct injection into analytical instruments.[13] This commercialization marked a pivotal step, enabling broader adoption in environmental and analytical laboratories by providing standardized, user-friendly hardware that integrated sampling, extraction, and preconcentration.[14] By the early 2000s, SPME gained formal recognition through standardization efforts, enhancing its reliability for regulatory compliance. In 2000, the ASTM International published standard D6520-00, outlining procedures for SPME extraction of volatile and semi-volatile organic compounds from water and headspace, primarily for environmental monitoring of contaminants like hydrocarbons.[15] This standard, along with subsequent revisions, promoted consistent methodology across industries, underscoring SPME's role in reproducible trace-level analysis without solvents.[16] The technique's evolution extended to liquid chromatography (LC) interfaces during the early 2000s, addressing limitations of fiber-based SPME with aggressive mobile phases. In-tube SPME variants, developed around 1997 and refined by 2002, utilized internally coated capillary tubes for online coupling with high-performance LC, enabling automated extraction and desorption for polar analytes in complex matrices. This adaptation broadened SPME's utility beyond GC, supporting applications in bioanalysis and pharmaceuticals.[17] Parallel to these developments, patent activity surged in the early 2000s, reflecting innovation in automation to enhance throughput and precision. Notable filings included US Patent Application 2002/0098594 A1 in 2002 for automated SPME methods and collectors, which integrated robotic handling for high-volume sampling. This growth in intellectual property, building on foundational patents like WO 91/15745, facilitated the integration of SPME into commercial autosamplers and hyphenated systems by mid-decade.Principles and Theory
Extraction Mechanisms
Solid-phase microextraction (SPME) primarily operates through two distinct extraction mechanisms: adsorption and absorption, depending on the nature of the fiber coating. Adsorption involves the accumulation of analytes on the surface of solid sorbents, such as divinylbenzene (DVB) or carbon-based materials, where interactions are driven by physical forces like van der Waals or π-π bonding, making it suitable for capturing low-concentration volatile and semi-volatile compounds with high selectivity but limited capacity due to surface saturation.[18] In contrast, absorption occurs when analytes partition into the bulk of liquid-like polymeric coatings, such as polydimethylsiloxane (PDMS), through solubility-based diffusion, allowing for greater extraction capacity proportional to the coating volume and broader applicability to non-polar analytes, though it may exhibit narrower linear ranges compared to adsorption.[19] These mechanisms enable SPME to concentrate trace analytes efficiently without solvents, with the choice between them dictated by analyte properties and desired sensitivity.[4] The extraction process in SPME relies on diffusion, partitioning, and equilibrium dynamics, which vary significantly between direct immersion (DI-SPME) and headspace (HS-SPME) modes. In DI-SPME, the coated fiber is immersed directly in the sample matrix, where analytes diffuse across a stagnant boundary layer into the coating, governed by concentration gradients and partitioning coefficients that favor extraction of polar or semi-volatile compounds; agitation or sonication can enhance mass transfer by reducing this layer's thickness.[20] Conversely, HS-SPME exposes the fiber to the vapor phase above the sample, allowing volatile analytes to partition first into the headspace before diffusing into the coating, which minimizes matrix interferences and accelerates equilibrium for gaseous or highly volatile species due to faster gas-phase diffusion rates.[18] Equilibrium is achieved when the analyte concentration in the coating stabilizes relative to the sample and headspace, with partitioning influenced by factors like temperature and analyte volatility, ensuring reproducible extraction in both modes.[21] The sample matrix—whether gas, liquid, or solid—profoundly affects mass transfer and extraction efficiency in SPME by altering diffusion paths and analyte availability. In gaseous matrices, high diffusion coefficients facilitate rapid mass transfer to the coating, enabling efficient extraction of volatile organic compounds (VOCs) but limiting applicability to semi-volatiles due to slower kinetics.[22] Liquid matrices, such as aqueous solutions, present slower diffusion and higher resistance from viscous boundary layers, reducing efficiency for non-polar analytes unless enhanced by salting-out agents like NaCl, which decrease solubility and boost partitioning into the headspace or coating.[23] Solid matrices, including soils or tissues, pose the greatest challenges, as analytes are often bound within pores, impeding release and mass transfer; techniques like thermal desorption or matrix-compatible coatings improve efficiency by promoting analyte volatilization into the headspace.[22] Overall, matrix complexity can introduce competitive binding or fouling, necessitating mode selection—HS for dirty samples and DI for cleaner ones—to optimize recovery. SPME's non-exhaustive nature is particularly leveraged in pre-equilibrium extraction kinetics, where sampling occurs before full equilibrium, allowing rapid, diffusion-controlled uptake for time-sensitive analyses. In this regime, analyte accumulation is linear with extraction time, determined by the sampling rate influenced by diffusion coefficients and boundary layer thickness, making it ideal for non-depletive sampling in dynamic environments like biofluids or field monitoring without significantly altering original concentrations.[24] This kinetic approach shortens analysis times—often to under 2 minutes for volatiles—while maintaining sensitivity at low ng/mL levels, as seen in applications for pharmaceuticals in urine, and is enhanced by thin-film or vacuum-assisted configurations to accelerate mass transfer without compromising accuracy.[25] Pre-equilibrium thus supports quantitative profiling in complex matrices by focusing on free analyte fractions, bypassing the need for exhaustive equilibrium.[26]Theoretical Models and Equations
The partition coefficient in solid-phase microextraction (SPME), denoted as K_{fs}, is defined as the ratio of the analyte concentration in the fiber coating (C_f) to that in the sample matrix (C_s) at equilibrium: K_{fs} = \frac{C_f}{C_s}. This coefficient quantifies the distribution of the analyte between the extraction phase and the sample, serving as a fundamental parameter for predicting extraction efficiency.[27] At equilibrium, the amount of analyte extracted (n) onto the fiber is governed by the equation n = \frac{K_{fs} V_f V_s C_0}{K_{fs} V_f + V_s}, where V_f is the volume of the fiber coating, V_s is the sample volume, and C_0 is the initial analyte concentration in the sample. This model assumes complete equilibrium and negligible analyte loss, allowing for the calculation of extraction recovery based on phase volumes and partitioning behavior. For cases where K_{fs} V_f \ll V_s, the equation simplifies to n \approx K_{fs} V_f C_0, indicating that extraction is proportional to the initial concentration.[27] For non-equilibrium conditions, the time-dependent extraction is described by a kinetic model that accounts for diffusion limitations, particularly through the boundary layer adjacent to the fiber surface. The amount extracted at time t, n(t), can be approximated as n(t) = K_{fs} V_f C_0 \left(1 - \exp\left(-\frac{D A t}{K_{fs} V_f \delta}\right)\right), where D is the analyte diffusion coefficient in the sample matrix, A is the fiber surface area, and \delta is the thickness of the stagnant boundary layer. This exponential form arises from Fickian diffusion principles, with the rate constant \frac{D A}{K_{fs} V_f \delta} reflecting mass transfer controlled by convection and diffusion in the sample; it is valid when diffusion within the fiber coating is not rate-limiting and extraction is non-depletive.[27] Selectivity in SPME, particularly for headspace sampling (HS-SPME), is influenced by factors such as the Henry's law constant, which governs the partitioning of volatile analytes between the sample and gas phase. The dimensionless Henry's constant K_H = \frac{C_g}{C_s} relates the gas-phase concentration (C_g) to the sample concentration (C_s), enabling prediction of analyte availability in the headspace for extraction by the fiber. Higher K_H values favor selective extraction of more volatile compounds, enhancing method specificity for gaseous or semi-volatile analytes.[27] SPME extraction mechanisms can involve either absorption into a liquid-like coating or adsorption onto a solid sorbent, with theoretical models adapting the partition coefficient accordingly to describe phase interactions.Methodology
Fiber Coatings and Selection
Solid-phase microextraction (SPME) relies on specialized fiber coatings to selectively extract analytes from complex matrices, with the coating material and thickness dictating extraction efficiency and selectivity. The most widely adopted coating is polydimethylsiloxane (PDMS), a non-polar liquid polymer film used for absorbing non-polar semivolatile compounds such as polycyclic aromatic hydrocarbons and pesticides; it is commercially available in thicknesses of 7 μm (for fast extraction of volatiles), 30 μm (general-purpose), and 100 μm (higher capacity for lower concentrations).[18] Divinylbenzene (DVB), a solid sorbent, is bipolar and excels at adsorbing polar volatiles like amines and alcohols, often combined with PDMS in a 65 μm PDMS/DVB configuration for broader applicability. Carboxen/polydimethylsiloxane (CAR/PDMS), incorporating porous carbon particles in a 75 μm PDMS matrix, is optimized for trace-level gases and low-molecular-weight volatiles such as volatile organic compounds (VOCs) in air or headspace sampling. Selection of the appropriate fiber coating is guided by the physicochemical properties of the target analytes, including polarity, volatility, and molecular weight, to maximize partition coefficients and minimize extraction time. Non-polar PDMS is typically chosen for hydrophobic analytes, such as BTEX compounds, while polar coatings like polyacrylate (PA, 85 μm) suit hydrophilic semivolatiles; bipolar options like DVB or CAR/PDMS bridge these for mixed-polarity or highly volatile species (molecular weights < 100 Da). Thicker coatings enhance capacity for low-concentration analytes but prolong equilibration, whereas thinner films favor rapid analysis of abundant volatiles.[18] These choices play a key role in adsorption or absorption mechanisms during extraction. Fiber coatings are prepared using methods that ensure uniform deposition and adhesion to the fused silica or metal core. Physical deposition involves dipping the fiber in a polymer solution or slurry, followed by solvent evaporation, which is simple but prone to swelling or stripping in aggressive matrices. Sol-gel immobilization, developed for enhanced durability, chemically bonds the coating via hydrolysis and condensation reactions, incorporating silanes to create robust, high-surface-area films resistant to high temperatures and organic solvents; this technique has been pivotal in extending fiber performance for repeated use. Commercial SPME fibers, while effective, exhibit limitations such as mechanical fragility from the brittle fused silica core, which can break during handling or insertion, and a finite operational lifespan of 50–100 extractions before coating degradation reduces sensitivity. Alternative cores like nitinol improve robustness but may alter extraction kinetics slightly.[18]| Coating Type | Polarity | Typical Analytes | Thickness (μm) | Example Applications |
|---|---|---|---|---|
| PDMS | Non-polar | Semivolatiles (e.g., PAHs) | 7, 30, 100 | Environmental monitoring of organics[18] |
| PDMS/DVB | Bipolar | Polar volatiles (e.g., phenols) | 65 | Food and beverage analysis |
| CAR/PDMS | Bipolar | Trace gases (e.g., VOCs) | 75 | Air quality assessment |
| PA | Polar | Polar semivolatiles (e.g., alcohols) | 85 | Biomedical metabolite extraction |