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Solid-phase extraction

Solid-phase extraction (SPE) is a sample preparation technique in analytical chemistry that isolates, concentrates, and purifies analytes from complex liquid or gaseous matrices by selectively retaining them on a solid sorbent through interactions such as adsorption, partitioning, or ion exchange, followed by elution with an appropriate solvent.^[1]^[2] Developed initially in the 1940s with early applications using activated carbon filters for recovering organic micropollutants from water samples, SPE evolved through the 1970s with the introduction of standardized sorbents like silica-based materials and pre-packed cartridges, marking its transition from rudimentary filtration to a versatile chromatographic method.^[3]^[1] The technique gained widespread adoption due to innovations such as reversed-phase sorbents (e.g., C18-modified silica) in the late 1970s and SPE disks in 1989, enabling efficient sample cleanup and preconcentration for downstream analyses like liquid chromatography (LC) and gas chromatography (GC).^[1] At its core, SPE operates on the principle of differential distribution of analytes between a mobile phase (the sample solution) and a stationary phase (the sorbent), typically involving three steps: conditioning the sorbent to activate its binding sites, loading the sample to allow analyte retention while interferences pass through, and elution to recover the purified analytes in a smaller volume for enhanced sensitivity.^[2]^[4] Common sorbent types include normal-phase (polar silica), reversed-phase (non-polar C8 or C18), ion-exchange (cationic or anionic), and advanced materials like molecularly imprinted polymers (MIPs) or magnetic nanoparticles, selected based on the analyte's polarity, charge, or size.^[1]^[5] SPE's primary advantages over traditional liquid-liquid extraction (LLE) include reduced solvent consumption (often by 50-90%), faster processing times (typically 10-30 minutes per sample), higher reproducibility, and recovery rates exceeding 80-100% for many analytes, making it indispensable for trace-level detection with limits of detection (LODs) as low as 0.005 μg/L in environmental and biological samples.^[1]^[2] It is extensively applied in fields such as pharmaceutical analysis for drug extraction from plasma, environmental monitoring for pesticides and heavy metals in water and soil, food safety assessments for contaminants like mycotoxins, and clinical diagnostics for biomarker isolation, often achieving preconcentration factors up to 60 to improve analytical precision.^[4]^[6] Recent advancements, including automated SPE systems, miniaturized formats like solid-phase microextraction (SPME), and integrations with nanomaterials for magnetic SPE (MSPE) and flow-based systems, further enhance its efficiency and green chemistry compliance by minimizing waste and enabling high-throughput processing as of 2025.^[7]^[8]^[9]

Fundamentals and History

Definition and Overview

Solid-phase extraction (SPE) is a chromatographic sample preparation technique that isolates, purifies, and concentrates specific analytes from complex liquid matrices, including extracts from solids such as biological fluids, environmental samples, and food extracts by selectively retaining them on a solid sorbent phase.^[10]^[1] This method operates on the principle of partitioning analytes between a liquid mobile phase (the sample solvent) and the solid sorbent, enabling efficient separation without requiring continuous flow like traditional column chromatography.^[10] The core components of SPE include the solid sorbent phase, typically silica-based materials or synthetic polymers packed into cartridges or disks, the liquid mobile phase carrying the sample, and solvents for conditioning, washing, and elution.^[1]^[10] The primary purpose of SPE is to perform sample cleanup by removing matrix interferences, preconcentrate trace-level analytes, and prepare samples for downstream analytical techniques such as high-performance liquid chromatography (HPLC), gas chromatography (GC), or mass spectrometry (MS).^[1]^[10] In a typical SPE process, the analyte is first adsorbed onto the conditioned sorbent during sample loading, followed by washing steps to eliminate unwanted interferences, and finally elution with a suitable solvent to recover the purified and concentrated analyte.^[1]^[10] This technique is commonly applied to diverse matrices, including biological fluids like urine and blood, environmental samples such as water and soil extracts, and food extracts.^[1] Compared to traditional liquid-liquid extraction (LLE), SPE offers greater selectivity and reduced solvent consumption, making it a more efficient alternative for routine sample preparation.^[1]

Historical Development

The origins of solid-phase extraction (SPE) trace back to the 1950s, when early experimental applications employed activated carbon filters for the analysis of trace organic compounds in water samples.^[3] Pioneering studies, such as those by H. Braus and colleagues in 1951 and A.A. Rosen and team in 1955 and 1959, demonstrated the potential of these sorbents to adsorb organics from aqueous matrices, laying the groundwork for preconcentration techniques rooted in liquid chromatography principles.^[3] These initial efforts were driven by the need for sensitive detection in environmental monitoring, marking SPE's emergence as a practical sample preparation method. In the 1970s, SPE advanced significantly with the introduction of bonded silica sorbents, which offered greater chemical stability and selectivity compared to earlier materials like activated carbon or polymeric resins.^[1] This period saw the commercialization of the first pre-packed SPE cartridges, notably the Sep-Pak silica-based products launched by Waters Corporation in 1978, which facilitated easier handling and reproducibility in laboratory settings.^[11] Key contributions from researchers like G.R. Harvey in 1973 and G.A. Junk in 1974 further refined sorbent applications for trace analysis, propelling SPE toward broader adoption.^[3] The 1980s and 1990s brought widespread commercialization of reversed-phase and ion-exchange SPE modes, with bonded phases like C18 silica enabling efficient hydrophobic interactions and cation/anion exchangers supporting charged analyte separations.^[1] A major milestone was the invention of solid-phase microextraction (SPME) in 1990 by C. Arthur and Janusz Pawliszyn, as detailed in their seminal paper, a solvent-free variant that miniaturized the technique for gas chromatography applications.^[12] Early work by chromatographers such as James S. Fritz, who advanced ion-exchange sorbents and analytical methodologies, played a crucial role in these developments.^[13] Growth was accelerated by environmental regulations, including U.S. EPA Method 525 in 1988, which standardized SPE for organic pollutant detection in drinking water.^[14] From the 2000s onward, SPE evolved toward automation and integration, with 96-well plate formats introduced in the late 1990s for high-throughput processing and fully automated systems emerging to enhance efficiency in bioanalysis.^[15] Online SPE coupled with liquid chromatography-mass spectrometry (LC-MS) became prominent, allowing direct sample injection and real-time analysis, as exemplified in methods developed throughout the decade for pharmaceutical and environmental screening.^[16] These innovations, building on decades of foundational research, solidified SPE as an indispensable tool in analytical chemistry.

Principles and Procedures

Retention Mechanisms

Solid-phase extraction (SPE) relies on specific retention mechanisms that facilitate the selective binding of analytes to a solid sorbent phase from a liquid sample, enabling their separation from matrix interferences. These interactions mimic the retention principles observed in liquid chromatography, where analytes partition or adsorb based on their physicochemical properties relative to the stationary phase.^[17] The primary mechanisms encompass hydrophobic, polar, ionic, and size exclusion interactions, each tailored to analyte polarity, charge, and size for optimal selectivity.^[1] Hydrophobic retention, also known as non-polar interaction, occurs through van der Waals forces and partitioning of non-polar analytes into the hydrophobic surface of sorbents such as octadecylsilane (C18) or octylsilane (C8) modified silica. This mechanism is particularly effective for extracting non-polar compounds, like pesticides or pharmaceuticals, from aqueous (polar) matrices, as the analyte's hydrophobic moieties associate with the non-polar sorbent chains, displacing water molecules.^[17]^[1] Polar retention involves adsorption via hydrogen bonding, dipole-dipole interactions, or π-π stacking between polar analytes and unmodified or polar-modified sorbents, such as bare silica, aminopropyl, or diol-functionalized silica. These interactions are suited for retaining polar compounds, including alcohols, amines, or carbohydrates, from non-polar solvents, where the sorbent's polar groups form reversible bonds with complementary sites on the analyte.^[17]^[1] Ionic retention is driven by electrostatic attractions between charged analytes and oppositely charged functional groups on the sorbent, such as sulfonic acid for cation exchange or quaternary ammonium for anion exchange. This mechanism targets ionizable compounds, like amino acids or organic acids, enabling strong retention under conditions where the analyte and sorbent bear opposite charges, often with capacities ranging from 0.8 to 1.3 meq/g.^[17]^[1] Size exclusion retention separates analytes based on molecular size, using porous sorbents like restricted access media (RAM) that allow small molecules to enter pores for retention while excluding larger macromolecules, such as proteins, from the internal structure. This passive mechanism is less dependent on chemical affinity and is commonly applied in biological sample cleanup.^[1]^[17] Mixed-mode retention combines two or more of the above mechanisms, such as hydrophobic partitioning with ionic exchange (e.g., C18-sulfonic acid sorbents), to enhance selectivity for complex samples containing analytes with multiple interaction sites. This approach allows dual retention—non-polar for initial capture and ionic for fine-tuned elution—improving recovery and reducing co-extraction of interferents.^[1]^[17] Several factors influence the strength and specificity of these retention mechanisms. The sample and buffer pH is crucial, particularly for ionic and mixed-mode SPE, as it controls analyte ionization; for instance, adjusting pH two units above or below the analyte's pKa ensures neutrality for reversed-phase retention or matching charges for ion exchange.^[17]^[1] Solvent polarity affects partitioning in hydrophobic and polar modes, with elution solvents selected based on their dielectric constant (e.g., methanol at ε = 32.6 for moderate polarity) to disrupt interactions efficiently.^[1] Ionic strength modulates electrostatic forces in ion exchange by screening charges, often increased with salts like NaCl to promote analyte desorption.^[1] Additionally, sorbent particle size, typically 40–60 μm for conventional cartridges, impacts surface area and flow dynamics, with smaller particles enhancing retention efficiency but potentially increasing backpressure.^[18]^[1]

General SPE Procedure

Solid-phase extraction (SPE) follows a standardized four-step procedure that is universally applicable across different retention modes, relying on the partitioning of analytes between the sample matrix and the solid sorbent based on their chemical affinities.^[19]^[20] The first step, conditioning, activates the sorbent surface to ensure reproducible retention by passing a solvent through the cartridge, typically starting with a strong solvent like methanol (5-20 mL) to wet the sorbent, followed by an equilibration solvent matching the sample matrix (e.g., 15-50 mL of water or buffer) to establish the proper solvation environment; the sorbent must remain wet to avoid deactivation.^[19]^[21]^[20] In the second step, loading, the pretreated sample is introduced to the conditioned sorbent at a controlled flow rate, allowing analytes to partition and bind while matrix components pass through; sample volumes can range from microliters to 1 liter depending on the format, but the amount of analyte should not exceed 5% of the sorbent mass to prevent breakthrough.^[19]^[20]^[21] The third step, washing, removes unwanted matrix interferences and weakly retained species using a selective solvent (typically 5-10 mL) that disrupts non-specific bindings without eluting the target analytes, thereby improving selectivity; the wash volume is often equivalent to one cartridge volume to maintain efficiency.^[19]^[20]^[21] Finally, elution desorbs the retained analytes using a strong solvent (e.g., an organic modifier like acetonitrile or methylene chloride, 0.2-20 mL) that overcomes the retention forces, concentrating the analytes in a small volume for subsequent analysis; multiple small aliquots may be used to maximize recovery while minimizing dilution.^[19]^[20]^[21] Optimization of the procedure involves key parameters such as flow rate, typically 0.5-2 mL/min for cartridges to ensure adequate contact time and prevent channeling, sample volume limited to avoid overload (e.g., up to 1 L for disk formats), and sorbent mass ranging from 50-500 mg to match analyte load (analyte mass ≤5% of sorbent).^[19]^[20] Common issues include clogging from particulate matter, which can be mitigated by pre-filtering or centrifuging the sample, and poor recovery due to suboptimal conditions, addressed by adjusting pH, solvent strength, or sorbent quantity.^[19]^[20]^[21] Manual operations often employ vacuum or positive pressure manifolds to process multiple samples simultaneously, while automation through robotic systems enables high-throughput handling of large volumes with consistent flow control.^[19]^[20]

Modes of SPE

Normal Phase SPE

Normal phase solid-phase extraction (SPE) utilizes polar stationary phases to retain polar analytes from non-polar sample matrices, relying on adsorption mechanisms driven by differences in molecular polarity. This mode is particularly suited for samples dissolved in organic solvents where the analytes exhibit moderate to high polarity, allowing selective isolation through interactions such as hydrogen bonding, dipole-dipole forces, and π–π interactions between the analyte and the sorbent surface. Unlike other SPE variants, normal phase emphasizes the use of non-aqueous environments to maximize retention efficiency.^[1]^[17] Common sorbents in normal phase SPE include unmodified silica (with silanol groups, -SiOH), alumina (Al₂O₃), and Florisil (magnesium silicate, Mg₂SiO₃), all of which provide a highly polar surface for analyte adsorption. These materials are selected for their ability to strongly bind polar functional groups like hydroxyl, carbonyl, or amino moieties present in the target compounds. Functionalized variants, such as amino (-NH₂), diol, or cyano (-CN) bonded silica, can offer tuned selectivity but unmodified forms remain foundational for broad polar retention.^[1]^[22] The procedure for normal phase SPE adapts the general SPE workflow to polarity gradients, beginning with conditioning the sorbent using a non-polar solvent like hexane or chloroform to activate the polar sites without introducing interference. The sample, prepared in a low-polarity solvent such as hexane or dichloromethane, is then loaded to allow polar analytes to adsorb onto the sorbent while non-polar matrix components pass through. Washing follows with a medium-polarity solvent (e.g., diethyl ether) to remove weakly retained impurities, and elution is achieved with a strong polar solvent like methanol or isopropanol to disrupt the polar interactions and recover the analytes. This stepwise solvent progression ensures high recovery rates, often exceeding 90% for polar targets in optimized conditions.^[1]^[17]^[22] Applications of normal phase SPE are prominent in the isolation of polar pesticides from organic extracts or lipids from non-aqueous food samples, where it facilitates cleanup and preconcentration prior to chromatographic analysis. For instance, it has been effectively used to extract organophosphorus pesticides from olive oil, achieving recoveries of 81.5–103.2% and detection limits in the low ppb range. The mode's advantages include excellent selectivity for polar compounds in non-polar media and straightforward integration with normal phase chromatography, but it is disadvantaged by high sensitivity to water contamination, which can deactivate sorbents and reduce retention capacity, necessitating rigorous anhydrous conditions.^[1]^[23]^[17]

Reversed Phase SPE

Reversed phase solid-phase extraction (SPE) employs non-polar sorbents to isolate non-polar or moderately polar analytes from polar, typically aqueous, sample matrices through hydrophobic interactions. This mode is particularly suited for extracting hydrophobic compounds, where the sorbent's non-polar surface partitions analytes away from the aqueous phase, enhancing selectivity and preconcentration. Common sorbent types include silica-based materials modified with octadecyl (C18) or octyl (C8) alkyl chains, which provide moderate to strong hydrophobic retention, and polymer-based options such as styrene-divinylbenzene (SDB) copolymers that offer robust chemical stability and capacity for a wide range of organics.^[21]^[24] The retention mechanism relies on hydrophobic partitioning, where non-polar analytes are attracted to the sorbent's alkyl chains or aromatic polymer backbone, displacing water molecules and enabling efficient capture from aqueous mobile phases. This process is governed by the analytes' logP values, with higher hydrophobicity leading to stronger retention. In practice, the procedure begins with conditioning the sorbent using methanol to solvate the stationary phase, followed by equilibration with water or aqueous buffer to create a polar environment compatible with the sample. The sample is then loaded in an aqueous buffer, allowing analytes to adsorb onto the sorbent; a washing step with water or a dilute organic solvent removes polar interferences without eluting targets. Elution is achieved with a stronger organic solvent like methanol or acetonitrile, typically in small volumes to concentrate the analytes.^[21]^[24]^[25] pH adjustment plays a crucial role in optimizing retention, often maintained between 2 and 7 to protonate ionizable analytes (e.g., weak acids or bases), reducing their polarity and enhancing hydrophobic interactions with the sorbent. For instance, in environmental water analysis, pH is commonly set near neutral (7.0 ± 1.0) to ensure stability of non-ionized forms. This mode is widely applied for sample cleanup in biological fluids like plasma or urine, where reversed phase sorbents such as C18 or hydrophilic-lipophilic balanced polymers remove matrix components while recovering drugs and metabolites with high efficiency (79-106%). In water samples, it facilitates the extraction of polycyclic aromatic hydrocarbons (PAHs), achieving recoveries of 81-99% for compounds like naphthalene and phenanthrene from river water, aiding in environmental monitoring.^[26]^[27]^[24]

Ion Exchange SPE

Anion Exchange

Anion exchange in solid-phase extraction (SPE) employs positively charged sorbents to selectively capture negatively charged analytes via electrostatic interactions, distinguishing it from other retention mechanisms like hydrophobicity. This mode is particularly suited for isolating anions from complex matrices, where the sorbent's fixed positive charges attract oppositely charged species while repelling similarly charged interferents.^[28] Common sorbents for anion exchange include strong anion exchangers featuring quaternary amine groups (e.g., -N(CH₃)₃⁺) bonded to silica or polymeric supports, which maintain a permanent positive charge across a broad pH range (pKa > 13). Weak anion exchangers utilize amino groups (primary or secondary amines, e.g., -NH₂ or -NHCH₃), which become protonated and positively charged at acidic pH values below their pKa (approximately 9-10). Retention relies on the electrostatic attraction of anions, such as carboxylates from organic acids or sulfonates, under conditions where the analytes are deprotonated (pH typically 2 units above the analyte pKa for weak acids) and the sorbent is charged, often at low pH (<7) for weak types to ensure protonation. These sorbents exhibit capacities of 0.1-1 meq/g, enabling efficient binding of targeted anions like phosphates without excessive non-specific interactions.^[20]^[28]^[17] The adapted SPE procedure begins with conditioning the cartridge using a water-miscible organic solvent like methanol to solvate the sorbent, followed by an aqueous buffer at pH <7 to impart the positive charge. The sample is acidified (e.g., pH 4-6) to promote analyte ionization and sorbent activation before loading, ensuring maximal retention. Washing employs a neutral buffer (e.g., 25 mM ammonium acetate at pH 6-7) to remove unbound matrix components. Elution disrupts the ionic bonds using a high pH buffer like 5% NH₄OH for weak sorbents (neutralizing the charge) or a high-ionic-strength salt solution (e.g., 1 M NaCl or 5% NH₄OH in methanol) for strong exchangers, effectively releasing the anions. To mitigate interference from cations, which may compete for binding sites, competing anions like chloride (via dilute HCl in the wash) are introduced to mask or displace them, enhancing selectivity for the target anions.^[28]^[20]

Cation Exchange

Cation exchange solid-phase extraction (SPE) is a technique that selectively retains positively charged analytes, known as cations, through electrostatic interactions with negatively charged functional groups on the sorbent surface.^[1] Common sorbents include strong cation exchangers (SCX) featuring sulfonic acid groups (-SO₃H) and weak cation exchangers (WCX) with carboxylic acid groups (-COOH), both typically bonded to silica or polymeric supports such as polystyrene-divinylbenzene.^[20] These functional groups provide a negatively charged surface when deprotonated, enabling the attraction of cations like protonated amines, metal ions, or quaternary ammonium compounds.^[29] Retention in cation exchange SPE relies on ionic interactions, where the sorbent's anionic sites bind to positively charged analytes under conditions that promote analyte protonation.^[1] For optimal retention, the sample pH is adjusted to approximately two units below the analyte's pKa, ensuring the analyte exists primarily in its protonated (cationic) form, while the sorbent remains deprotonated at pH values greater than 4 for both SCX and WCX materials.^[29] SCX sorbents maintain their charge across a wide pH range (1–14), making them suitable for strongly acidic conditions, whereas WCX sorbents are effective only above pH 4–5 when the carboxylic groups ionize.^[20] This pH-dependent protonation enhances selectivity for basic analytes, such as pharmaceuticals or environmental pollutants, by minimizing interference from neutral or anionic species.^[1] The procedure for cation exchange SPE follows the general SPE workflow but is tailored to ionic conditions for effective retention and elution.^[29] Conditioning begins with a water-miscible organic solvent (e.g., methanol) to solvate the sorbent, followed by an aqueous buffer at pH >4 to deprotonate the functional groups and establish the desired counterion, such as H⁺.^[20] The sample, adjusted to an acidic pH (approximately two units below the analyte's pKa) to protonate analytes, is then loaded at a controlled flow rate (typically <1 mL/min for 100-mg beds) to prevent breakthrough.^[29] Washing employs a neutral or low-ionic-strength buffer to remove unbound matrix components without disrupting ionic bonds.^[1] Elution is achieved using a low-pH acidic solution (e.g., 5% HCl) to protonate the sorbent and deprotonate the analyte, or a high-ionic-strength salt solution to displace cations via competition.^[20] This mode offers high selectivity for basic drugs, such as amphetamines or protonated alkaloids, and quaternary ammonium surfactants, distinguishing them from non-ionic interferents in complex matrices like biological fluids or wastewater.^[1] Ion-exchange capacities typically range from 0.2 to 2 meq/g, depending on the sorbent's functional group density and support material, with SCX silica-based sorbents often achieving around 0.2 meq/g.^[20] These properties make cation exchange SPE particularly valuable for preconcentration and purification in analytical applications requiring clean separation of charged species.^[29]

Formats and Configurations

Cartridges

Solid-phase extraction (SPE) cartridges represent the most prevalent format for processing small to medium sample volumes, typically ranging from 0.5 mL to 50 mL. These devices consist of medical-grade polypropylene tubes with volumes of 1 to 30 mL, packed with 50 to 1000 mg of sorbent material, such as silica-based or polymer-based phases, held in place between two polyethylene frits featuring 10 to 20 μm pore sizes to prevent sorbent loss while allowing efficient flow.^[30]^[1] The design ensures mechanical stability and chemical inertness, with the sorbent bed typically occupying 0.1 to 3 mL, enabling retention of analytes up to approximately 5% of the sorbent mass without breakthrough under optimized conditions.^[1] In usage, SPE cartridges are compatible with both manual vacuum manifolds and automated robotic systems, facilitating parallel processing of 1 to 100 samples. Samples are loaded, washed, and eluted at controlled flow rates of 1 to 5 mL/min, which balances extraction efficiency and minimizes channeling or incomplete retention.^[31]^[32] This format aligns with the general SPE procedure by allowing sequential solvent applications through gravity, positive pressure, or vacuum assistance, often yielding 100 to 500 μL of concentrated eluate suitable for downstream analysis.^[1] Key advantages of cartridge-based SPE include ease of handling due to their disposable nature, which reduces cross-contamination risks, and scalability for batch operations in laboratory settings.^[1] They offer cost-effective implementation with high reproducibility, particularly when using vacuum manifolds to standardize flow across multiple units.^[30] Variations in cartridge design include pre-packed formats, such as Sep-Pak C18 cartridges, which arrive ready-to-use with consistent sorbent distribution, versus bulk-filled options that allow custom packing for specialized applications.^[1] Additionally, end-capped sorbents, particularly in silica-based phases like C18, incorporate trimethylsilyl groups to minimize residual silanol activity, thereby reducing unwanted secondary interactions with polar analytes and improving recovery yields.^[33]^[34]

Disks and Other Formats

Solid-phase extraction disks, such as the widely used Empore format, feature a planar design with diameters typically ranging from 47 mm to 90 mm, incorporating sorbent particles embedded in an inert polytetrafluoroethylene (PTFE) matrix to form a mechanically stable structure suitable for processing sample volumes of 100 mL to 2 L.^[35] The thin bed thickness of approximately 0.5 mm enables rapid flow rates, facilitating efficient extraction while minimizing backpressure.^[35] These disks are particularly effective for high-volume aqueous samples, where the embedded sorbent provides a large surface area for analyte retention.^[1] In usage, SPE disks are commonly employed with vacuum filtration holders or manifolds, allowing samples to pass through the disk under controlled vacuum, which simultaneously filters particulates and extracts target analytes from water matrices without requiring prior phase separation.^[21]^[36] This configuration supports processing of dirty or particulate-laden samples, such as environmental water, by retaining solids on the disk surface while capturing dissolved compounds within the sorbent bed.^[35] Alternative formats include 96-well plates, which enable high-throughput parallel processing of up to 96 samples in a standard microtiter plate configuration, ideal for bioanalytical workflows requiring simultaneous extractions.^[37] Pipette tips adapted for micro-SPE handle small volumes of 10–200 μL, incorporating sorbent beds directly into the tip for automated or manual dispersive extraction in low-volume applications like therapeutic drug monitoring.^[38] Disk formats offer advantages such as reduced channeling due to the uniform, dense particle packing in the PTFE matrix, which enhances reproducibility and flow consistency, along with higher capacity for handling dirty samples containing suspended solids.^[35] However, they incur higher costs per use compared to traditional cartridges, primarily due to the specialized manufacturing of the embedded sorbent disks.^[39] Membrane variants, such as stacked disks, allow for multi-mode extractions by layering different sorbent types in a single assembly, enabling sequential retention mechanisms for complex sample fractionation in proteomics and multidimensional separations.^[40] These configurations maintain compatibility with standard vacuum-based procedures while expanding versatility for targeted analyte isolation.^[35]

Variants and Advances

Solid-Phase Microextraction

Solid-phase microextraction (SPME) represents a miniaturized, solvent-free adaptation of solid-phase extraction principles, utilizing a small volume of sorbent to isolate and concentrate analytes directly from complex matrices. Invented in 1989 by Robert P. Belardi and Janusz Pawliszyn, SPME integrates sampling, extraction, and sample introduction into a single step, enabling efficient preconcentration without the need for exhaustive extraction or large solvent volumes.^[41] This technique has evolved into a versatile tool, particularly valued for its simplicity and compatibility with chromatographic analyses. The core design of SPME features a thin fused silica fiber, typically 1 cm long and 110-170 μm in diameter, coated with a sorbent phase such as polydimethylsiloxane (PDMS) or other polymers and adsorbents, with coating thicknesses ranging from 7 to 100 μm to suit different analyte polarities and volatilities. The fiber is housed within a syringe-like assembly, allowing the protective needle to shield the coating during handling, while the plunger extends the fiber for exposure to the sample and retracts it for transfer to the analytical instrument. This configuration facilitates direct injection into gas chromatography-mass spectrometry (GC-MS) systems, minimizing carryover and contamination risks. The extraction procedure in SPME relies on the establishment of a partitioning equilibrium between the sample matrix (or headspace) and the fiber coating, where analytes diffuse into the sorbent phase over a defined time, often 10-60 minutes depending on the system. Following extraction, analytes are desorbed either thermally—by rapid heating in the GC injector at 200-300°C for 1-5 minutes—or via solvent extraction for liquid chromatography compatibility, directly introducing the enriched extract into the separation column. SPME operates in two primary modes: direct immersion, where the fiber contacts the liquid or solid sample, and headspace mode, which exposes the fiber to the vapor phase above the sample for reduced matrix interference. Additionally, thin-film SPME employs flat or blade-like supports with higher surface-to-volume ratios, enhancing mass transfer and achieving faster extraction kinetics compared to traditional fiber formats. SPME provides key advantages, including complete elimination of organic solvents for greener analysis, high portability for on-site sampling, and exceptional sensitivity for volatile and semi-volatile compounds due to efficient preconcentration.^[42] Since its inception, it has become a standard method for trace-level detection of environmental volatiles, offering automation compatibility and minimal sample alteration.^[42] Nonetheless, limitations include the inherently low sorption capacity from the small coating volume (typically 0.2-1 μL), which restricts its use for high-concentration analytes or those with low partition coefficients, as well as matrix effects that can compete for adsorption sites and skew quantification. Fiber durability is another constraint, with typical lifetimes of 50-100 uses before degradation from mechanical stress or chemical exposure necessitates replacement.^[42]

Recent Developments

Recent developments in solid-phase extraction (SPE) have focused on acceleration techniques to enhance throughput and reduce analysis times. High-pressure online SPE coupled with ultra-high-performance liquid chromatography (UHPLC) has enabled faster sample processing for analytes like plant growth regulators. Similarly, 3D-printed cartridges incorporating custom sorbents, such as porous polymer monoliths, have streamlined workflows for environmental contaminants. Advancements in green chemistry have introduced nanomaterial-based sorbents, including graphene oxide composites, which improve adsorption efficiency while significantly minimizing organic solvent consumption compared to traditional methods.^[43] Magnetic SPE variants further promote sustainability by enabling rapid sorbent recovery through external magnets, eliminating the need for centrifugation and reducing waste in biomedical and environmental applications.^[44] Miniaturization has progressed with microfluidic chips that handle sample volumes below 1 μL, often at nL scales, integrating SPE directly with electrophoretic or chromatographic detection for trace-level analysis.^[45] High-throughput formats, such as 384-well automated plates, facilitate parallel extraction of hundreds of samples, accelerating screening in pharmaceutical and toxicological studies.^[46] Integration efforts have yielded online SPE-LC-MS systems for seamless, real-time monitoring of pollutants like pharmaceuticals in wastewater, with robotic on-flow configurations processing up to 16 analytes simultaneously. AI-optimized protocols, implemented via self-driving laboratories, autonomously refine extraction parameters—such as buffer compositions—achieving up to 96.7% reductions in chemical usage while maintaining high purity in nucleic acid purification.^[47] From 2023 to 2025, notable milestones include the launch of novel mixed-mode sorbents that combine ion-exchange and hydrophobic interactions for enhanced selectivity toward diverse analytes in complex matrices.^[46] Eco-friendly polymers, particularly water-based molecularly imprinted variants, have enabled reusable SPE columns with imprinting factors exceeding 4.5, supporting sustainable monitoring of antibiotics like gentamicin.^[48] As of 2025, further advances include Solid-Phase Extraction Capture (SPEC) workflows for nanoliter-scale protein processing and applications of poly(ionic liquids) in solid-phase microextraction.^[49]^[50]

Applications

Environmental and Food Analysis

Solid-phase extraction (SPE) plays a pivotal role in environmental analysis by enabling the isolation and concentration of pesticides from water samples, as demonstrated in the US Environmental Protection Agency (EPA) Method 525.3, which employs C18 disks to extract semivolatile organic compounds, including pesticides, from drinking water prior to gas chromatography-mass spectrometry analysis.^[51] This method achieves detection limits in the low parts-per-billion (ppb) range, supporting monitoring of contaminants at trace levels.^[14] Similarly, SPE facilitates the extraction of polycyclic aromatic hydrocarbons (PAHs) from soil matrices, where reversed-phase sorbents provide efficient recovery compared to liquid-liquid extraction, with correlation coefficients exceeding 0.99 for multiple PAH congeners.^[52] For pharmaceuticals in environmental water, SPE using styrene-divinylbenzene-based cartridges concentrates emerging contaminants like antibiotics and hormones, enabling compliance with regulatory thresholds through liquid chromatography-tandem mass spectrometry.^[53] SPE is also employed for preconcentration of heavy metals such as lead, cadmium, and mercury from water and soil using chelating resins or ion-exchange sorbents, achieving detection limits in the ng/L range for compliance with environmental regulations.^[54] In food analysis, SPE serves as a cleanup technique for mycotoxins in cereals and nuts, where specialized sorbents like Supel™ Tox cartridges remove matrix interferences from aflatoxins and ochratoxins, improving chromatographic resolution and achieving recoveries above 85% for multiple analytes.^[55] For glyphosate and its metabolite AMPA in red wine, molecularly imprinted polymer (MIP)-based SPE cartridges provide high selectivity, with recoveries exceeding 90%, minimizing matrix effects from polar interferents like phenolics.^[56] This approach supports residue analysis in complex oily matrices, such as vegetable oils, by selectively retaining additives and contaminants for subsequent quantification.^[57] Reversed-phase SPE has been widely adopted for emerging contaminants like per- and polyfluoroalkyl substances (PFAS) in environmental and food samples, with weak anion-exchange variants yielding recovery rates greater than 90% at ppb concentrations in water and soil.^[58] These methods align with multi-residue protocols that simultaneously target dozens of analytes, ensuring compliance with EU maximum residue limits (MRLs) and USEPA tolerances for pesticides and related pollutants in food and environmental matrices.^[59] Despite these advances, challenges persist in handling complex matrices, such as those in food and soil, where high levels of humic substances or lipids cause analyte suppression; mixed-mode SPE addresses this by integrating hydrophobic and ionic interactions for enhanced selectivity and cleaner extracts.^[60]

Pharmaceutical and Biomedical Analysis

In pharmaceutical drug discovery, solid-phase extraction (SPE) plays a crucial role in purifying target compounds from complex reaction mixtures, often employing mixed-mode sorbents that integrate reversed-phase retention with ion-exchange capabilities to effectively isolate both basic and acidic analytes.^[61] This approach enhances selectivity by leveraging multiple interaction mechanisms, such as hydrophobic and electrostatic forces, to remove impurities like salts and unreacted reagents while concentrating the desired pharmaceuticals.^[62] For example, mixed-mode cation-exchange SPE has been utilized to fractionate basic drugs from plasma, demonstrating superior clean-up efficiency compared to single-mode methods.^[63] In biomedical analysis, SPE enables the precise isolation of drugs and metabolites from biological fluids like plasma and urine, supporting pharmacokinetic (PK) studies and therapeutic monitoring. Cation-exchange SPE, in particular, excels at extracting charged analytes such as amphetamines from urine, with reported recoveries often exceeding 90% under optimized conditions.^[64] When coupled with mass spectrometry (SPE-MS), this technique facilitates the quantification of low-dose drugs at ng/mL levels in PK investigations, minimizing matrix interferences and enabling sensitive detection in complex samples.^[65] High-throughput configurations, such as 96-well plate formats, streamline sample preparation for absorption, distribution, metabolism, and excretion (ADME) screening, processing hundreds of compounds daily with recoveries typically above 80-85%.^[66]^[67] Affinity-based SPE variants offer enhanced specificity for biologics, including peptides, by incorporating molecular recognition elements like antibodies or ligands onto the sorbent surface to selectively bind target molecules from biological matrices.^[68] This mode is particularly valuable for purifying synthetic peptides in pharmaceutical workflows, achieving high yields through tailored interactions that discriminate against non-specific binders.^[69] Integration of SPE with liquid chromatography-mass spectrometry (LC-MS) is standard for therapeutic drug monitoring, providing robust quantification of analytes in plasma with limits of detection as low as 5 ng/mL, e.g., for baclofen, and improved assay reproducibility.^[70]^[71] Such hyphenated systems support real-time clinical decisions by delivering clean extracts directly to the analytical column, reducing analysis time while maintaining analytical precision.^[72]

Advantages and Limitations

Advantages

Solid-phase extraction (SPE) offers several key advantages over traditional liquid-liquid extraction (LLE), particularly in terms of efficiency, environmental impact, and analytical performance.^[1] One primary benefit is the significant reduction in solvent usage; SPE requires substantially less organic solvent than LLE, often by 50-90%, making it a more cost-effective and greener alternative that minimizes waste generation and operational expenses.^[1] This solvent efficiency stems from the localized retention of analytes on a solid sorbent, which concentrates targets without the need for large volumes of immiscible phases as in LLE.^[1] SPE provides enhanced selectivity through mode-specific sorbent-analyte interactions, such as reversed-phase or ion-exchange mechanisms, which effectively minimize matrix interferences and enable cleaner extracts.^[1] This targeted retention allows for improved detection limits, often reaching parts per trillion (ppt) levels (e.g., 0.02–8.18 ng/L for pharmaceuticals in environmental samples), far surpassing the capabilities of LLE due to reduced background noise.^[1] Consequently, SPE facilitates higher sensitivity in downstream analyses like chromatography-mass spectrometry. The technique is notably faster, with extraction processes typically completing in minutes compared to the hours required for LLE's mixing and phase separation steps.^[1] This speed is amplified by SPE's compatibility with automation, including robotic systems and vacuum manifolds, enabling high-throughput processing of hundreds of samples per day (e.g., up to 160 samples in optimized setups).^[73] Automated SPE protocols enhance reproducibility and reduce manual labor, supporting large-scale laboratory workflows. SPE demonstrates versatility across diverse sample matrices, avoiding common LLE issues like emulsion formation and enabling consistent handling of complex samples such as biological fluids or environmental waters.^[1] It achieves high analyte recoveries, often in the 90–110% range for labile compounds, due to shorter exposure times and milder conditions that preserve analyte integrity better than LLE.^[1] Additionally, the use of disposable cartridges and disks in SPE reduces operator exposure to volatile organic solvents, enhancing laboratory safety.^[1]

Limitations

Solid-phase extraction (SPE) cartridges are susceptible to sorbent clogging caused by particulate matter in samples, which reduces flow rates and compromises extraction efficiency; this issue can be mitigated through pre-filtration of the sample prior to loading.^[1] Channeling, where the sample flows unevenly through the sorbent bed due to inconsistent packing, leads to poor analyte retention and incomplete extraction, often requiring uniform sorbent packing to ensure even distribution and optimal performance.^[1]^[74] Method optimization in SPE frequently involves trial-and-error adjustments to parameters such as sample pH (typically 2 units above or below the analyte's pKa for reversed-phase modes) and solvent selection based on eluotropic strength, which can be time-intensive and resource-demanding.^[1] Additionally, the cost of SPE sorbents adds to the expense, particularly for high-throughput analyses requiring multiple units.^[75] SPE sorbents have limited capacity, typically retaining only 1-10% of their mass in analytes before breakthrough occurs, and overloading can significantly reduce recovery rates by causing incomplete binding or elution inefficiencies.^[1] In complex matrices, such as environmental or biological samples, matrix effects like ion suppression or co-extraction of interferents (e.g., humic acids) further diminish accuracy and require additional cleanup steps to achieve reliable results.^[1]^[74] The predominantly disposable nature of traditional SPE cartridges generates substantial plastic and sorbent waste, raising environmental concerns about accumulation in landfills; recent advancements include shifts toward reusable magnetic sorbents, which allow for easier recovery and regeneration to minimize waste.^[1] Batch-to-batch variability in sorbent composition and performance, often stemming from manufacturing inconsistencies, can lead to reproducibility issues in recovery and extraction efficiency, a problem addressed by using certified reference standards to ensure consistency across lots.^[74]^[76]

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