Solid Phase Extraction: Workflows, Sorbents, and Uses

Solid phase extraction, commonly abbreviated SPE, is a sample preparation technique that isolates target compounds from complex mixtures by passing a liquid sample through a sorbent material that selectively retains the analytes of interest. Since its earliest iterations using activated carbon filters roughly seven decades ago, SPE has evolved into one of the most widely used cleanup and preconcentration tools in analytical chemistry, showing up in environmental monitoring, clinical toxicology, food safety testing, and pharmaceutical development.¹ The technique sounds simple in principle, but its versatility comes from the enormous range of sorbent chemistries, physical formats, and workflow configurations now available.

The Basic Workflow

An SPE procedure follows four general steps, regardless of the specific sorbent or format being used. First, the sorbent is conditioned with a solvent to activate its surface and prepare the functional groups for interaction with the target analytes. Second, the sample is loaded onto the sorbent, during which the compounds of interest bind to the sorbent while many unwanted matrix components pass through. Third, the sorbent is washed with a carefully chosen solvent that removes remaining interferences without dislodging the target compounds. Fourth, an elution solvent breaks the interaction between the analyte and the sorbent, releasing the cleaned-up, concentrated extract for analysis by an instrument such as a mass spectrometer or chromatograph.

Each step requires optimization. The wrong wash solvent can strip away the analytes you want to keep; an elution solvent that is too weak leaves analytes stuck to the sorbent. These details are where much of the practical knowledge in SPE resides, and getting them right is often what separates a method that works beautifully from one that produces unreliable results.

Retention Mechanisms

The sorbent inside an SPE cartridge or disk is not a one-size-fits-all material. Different sorbents exploit different chemical interactions to grab analytes from solution, and choosing the right one depends on what you are trying to extract and what the surrounding matrix looks like.

Reversed-phase sorbents are the workhorses. These are hydrophobic materials, most commonly bonded silica with C18 or C8 chains, that retain nonpolar to moderately polar compounds from aqueous samples. They work through Van der Waals and hydrophobic interactions. Most general-purpose SPE methods for environmental water or biological fluids start here.

Ion-exchange sorbents target charged molecules. Strong cation exchangers (with sulfonic acid groups, for instance) retain positively charged species, while strong anion exchangers retain negatively charged ones. Combining strong anion and cation exchangers in tandem has been shown to pull acidic, neutral, and basic compounds out of water simultaneously, with recoveries reaching the low-to-mid nineties in pure water when neither cartridge’s pH was adjusted.²

Mixed-mode sorbents combine two retention mechanisms in a single material. A reversed-phase backbone functionalized with ion-exchange groups, for example, can retain both nonpolar and ionic analytes at the same time. This is especially useful in multi-residue methods where the target list includes compounds with very different chemical properties. Researchers have developed mesoporous silica functionalized with both C18 chains and sulfonic acid groups specifically for extracting veterinary drug residues from meat, taking advantage of both reversed-phase and strong cation-exchange interactions in one cartridge.³ More recently, amphoteric sorbents have appeared that incorporate both cation- and anion-exchange groups into the same polymeric network, allowing simultaneous extraction of acidic and basic analytes from a single cartridge.⁴

Molecularly Imprinted Polymers

Standard SPE sorbents are general-purpose by design. They retain broad classes of compounds based on polarity or charge, which is a strength when you need a wide net but a weakness when the sample matrix is so complex that interferences co-extract with your analytes. Molecularly imprinted polymers, or MIPs, offer a more targeted approach. These are synthetic polymers built around a template molecule, so the final material contains binding sites that are complementary in shape, size, and functional group arrangement to that specific analyte or its structural relatives.⁵

The practical payoff is selectivity. In a head-to-head comparison extracting amphetamines from wastewater, MIPs produced much cleaner extracts than a standard hydrophilic-balance sorbent, reducing the signal suppression caused by leftover matrix components from roughly 70% down to about 30%. That translated into lower detection limits and better accuracy.⁶ MIPs have found traction in food contaminant analysis, where complex matrices like meat, milk, or plant extracts make generic extraction difficult. Pesticides, drug residues, and mycotoxins are all targets where MIP-based SPE has shown real advantages.⁷

MIPs do have limitations. Each polymer must be designed for a particular analyte or analyte class, so they are not suitable for broad screening methods that need to catch dozens of unrelated compounds. The synthesis itself can be time-consuming, and template bleeding, where residual template molecule leaches out and contaminates the extract, remains an issue that requires careful washing protocols.

Metal-Organic Frameworks and Other Emerging Sorbents

Metal-organic frameworks, or MOFs, are crystalline materials built from metal ions connected by organic linker molecules. They have extraordinarily high surface areas and pore structures that can be tuned by changing the metal or linker chemistry, which makes them attractive as SPE sorbents.⁸ Research groups have applied MOFs to extract everything from pesticides in river water to synthetic cannabinoids in oral fluid samples. In the cannabinoid work, an amino-functionalized UiO-66 MOF outperformed other MOF families tested under SPE conditions.⁹

A variant called magnetic solid phase extraction, or MSPE, embeds magnetic nanoparticles into the sorbent material. Instead of packing the sorbent into a cartridge, you disperse it directly into the sample, let the analytes adsorb, and then collect the sorbent with an external magnet. This approach works well for large-volume or particulate-heavy samples where a traditional cartridge might clog. Magnetic MOFs have been used for pesticide preconcentration in environmental water, with recoveries in the range of about 80% to 107% across reservoir, river, and seawater samples.¹⁰ MSPE has also gained ground for mycotoxin extraction from food matrices, where advanced magnetic materials built from imprinted polymers, MOFs, or covalent organic frameworks can be surface-modified to match the target analyte’s chemistry.¹¹

Physical Formats

The classic SPE format is a small plastic cartridge, typically a syringe barrel packed with loose sorbent, through which the sample is pushed or pulled. But the same extraction principle has been adapted into several other physical configurations to suit different throughput needs and sample volumes.

SPE disks are membranes loaded with sorbent particles. Because the sorbent bed is very thin, disks allow higher flow rates and process larger sample volumes faster than cartridges, making them popular for environmental water analysis where you might need to push a liter or more of water through the extraction device. Disks also come in smaller diameters for lower-volume applications, fitting into syringe barrels, 96-well microtiter plates, or even pipette tips for high-throughput work in pharmaceutical and clinical labs.

The 96-well plate format deserves a separate mention because it transformed SPE’s role in high-throughput settings. Each well contains a small amount of sorbent, and an entire plate of 96 samples can be processed simultaneously using a vacuum manifold or positive-pressure unit. This format is standard in bioanalytical laboratories running pharmacokinetic studies, where hundreds of plasma samples per day is routine.

How SPE Compares to Liquid-Liquid Extraction

Before SPE became widespread, liquid-liquid extraction, or LLE, was the dominant sample preparation method. LLE relies on partitioning analytes between two immiscible liquid layers, typically an aqueous biological sample and an organic solvent. It still works fine for many applications, but SPE has carved out clear advantages in several areas.

Speed is one. A comparison using forensic toxicology samples found that an optimized SPE protocol cut extraction time by about 75% compared to LLE, while delivering better recoveries and lower matrix effects across a broad panel of drugs without additional method development.¹² Another study comparing SPE and LLE for alkaline drugs in blood found that SPE had a lower detection limit for about 39% of the drugs tested, while LLE won on only about 20%. SPE also doubled the number of specimens a single analyst could process in a given timeframe and detected several drugs that LLE missed entirely, including morphine and benzoylecgonine.¹³

SPE also uses far less organic solvent. An LLE procedure can easily require tens of milliliters of dichloromethane or similar solvents per sample, while SPE elution volumes are typically in the single milliliters. That matters both for cost and for chemical waste. On the other hand, SPE cartridges themselves are consumables, and at scale those costs add up. The choice between the two often comes down to the specific analytes, the matrix, and the laboratory’s throughput needs.

Online Automation

One of the more significant developments in SPE over the past two decades is online coupling, where the extraction cartridge is plumbed directly into the analytical instrument. A column-switching valve directs sample through the SPE sorbent for loading and washing, then switches flow so that the elution solvent carries analytes straight onto the chromatographic column and into the detector. The entire sequence, from raw sample to data point, runs without anyone pipetting or transferring vials.

Fully automated online SPE coupled to liquid chromatography and tandem mass spectrometry has been applied to environmental monitoring of antibiotics and pesticides in surface water¹⁴ and to detecting illicit drugs and their metabolites in wastewater.¹⁵ Online setups minimize sample handling, which reduces the risk of contamination and analyte loss during manual transfer steps. They also improve reproducibility, since every sample goes through exactly the same extraction timing and solvent volumes. The tradeoff is cost and complexity. An online SPE system requires additional pumps, valves, and software control compared to a standard LC instrument, and troubleshooting plumbing issues in a multi-valve configuration can be tedious.

Where SPE Gets Used

The breadth of SPE applications is genuinely vast, but a few areas dominate.

Environmental monitoring is probably the single largest application area. Regulatory agencies worldwide require measurement of pesticides, pharmaceuticals, and industrial chemicals in drinking water, surface water, and wastewater, often at parts-per-trillion concentrations. SPE is the standard preconcentration step for these analyses. Online SPE methods have been validated for simultaneous determination of biocides, pesticides, and pharmaceuticals in wastewater and surface water.¹⁶ Similar approaches now target pesticides and veterinary pharmaceuticals in groundwater near agricultural areas.¹⁷

Clinical and forensic toxicology is another stronghold. When a patient arrives at the emergency room with an unknown overdose or when a forensic lab processes postmortem blood, the analytical challenge is extracting a broad range of drugs from a messy biological matrix. SPE methods have been developed for simultaneous detection of opiates, cocaine, amphetamines, benzodiazepines, tricyclic antidepressants, and other drug classes from blood, urine, bile, and gastric contents.¹⁸ Pharmaceutical companies rely on SPE for bioanalytical assays during pharmacokinetic studies, cleaning up plasma samples before quantifying drug concentrations. These methods are validated to stringent FDA and EMA guidelines for linearity, precision, accuracy, and matrix effects.¹⁹

Food safety testing rounds out the trio. Pesticide residues, veterinary drug residues, and mycotoxins in food are regulated in most countries, and the sample matrices involved, think ground beef, milk, honey, or grain extracts, are notoriously difficult. Magnetic SPE using green nanosorbents synthesized from bovine serum albumin and curcumin has been applied to extract fungicides and pesticides from food and water, achieving extraction recoveries ranging from about 71% to 93%.²⁰

A less well-known application is the isolation of natural products. An SPE-guided workflow has been described for isolating secondary metabolites from plant extracts, using gradient chromatography data to calculate the solvent composition needed for stepwise elution on SPE cartridges. Researchers used this approach to isolate compounds from the crude extract of a tropical plant, obtaining purified fractions with a relatively simple protocol.²¹

The Matrix Effect Problem

One of the main reasons SPE exists is to deal with matrix effects, the way co-extracted sample components interfere with your instrument’s ability to accurately measure the analyte. In mass spectrometry, matrix effects usually show up as ion suppression or enhancement, where leftover phospholipids, salts, proteins, or other gunk from the sample change the analyte’s signal intensity even though the actual concentration has not changed. This can wreck the accuracy of a method if left unchecked.

An optimized SPE cleanup protocol for marine toxins reduced matrix effects to less than 15% for a panel of toxins that suffered heavily from signal distortion in crude extracts, particularly okadaic acid and pectenotoxin-2.²² In bioanalytical work with plasma, dispersive SPE using a titanium dioxide and carbon nanotube composite significantly reduced the response of dozens of interfering lipid species, improving accuracy for low-concentration drug measurements compared to a simple protein precipitation step.²³

The critical point for anyone developing an SPE method is that matrix effects do not disappear just because you added an extraction step. The choice of sorbent, the wash solvent composition, and the elution conditions all determine how much cleanup you actually achieve. A poorly optimized SPE method can produce extracts that are barely cleaner than a dilute-and-shoot approach. Internal standards, especially isotope-labeled ones, remain essential for correcting whatever residual matrix effects make it through.

Greener Approaches to SPE

Traditional SPE methods use petroleum-derived polymers and organic solvents, and cartridges are almost always single-use plastic devices. As the broader analytical chemistry community has moved toward greener practices, SPE has followed. One significant trend is the use of deep eutectic solvents as extraction media or sorbent modifiers. These are mixtures of two or more compounds that form a liquid at temperatures well below the melting point of either component alone. They are often made from cheap, biodegradable, low-toxicity starting materials.

A green micro-SPE method was developed using a syringe filter coated with electrospun nanofibers impregnated with a hydrophobic deep eutectic solvent made from thymol and menthol, combined with curcumin as a functional additive. The setup was applied to extract anti-diabetic drugs from wastewater, serum, and plasma, using nontoxic and biodegradable materials throughout.²⁴ Deep eutectic solvents have also been used as thin-film coatings for solid-phase microextraction of parabens in surface water²⁵ and as additives to magnetic chitosan-graphene oxide nanoparticles for flavonoid extraction from juice and tea samples, where hydrophilic and pi-pi interactions between the solvent and the analytes improved extraction performance.²⁶

Whether these greener sorbents and solvents will displace the established commercial products in routine labs is still an open question. Most regulatory methods specify particular cartridge types and solvents, and switching requires revalidation. But for research labs and for methods not yet locked into regulatory frameworks, the shift toward lower-toxicity, lower-waste SPE workflows is real and accelerating.

Method Validation in Regulated Settings

SPE does not just need to work in the lab; in many settings it needs to meet formal regulatory criteria. Pharmaceutical bioanalytical methods, for example, must comply with guidelines issued by bodies such as the US Food and Drug Administration and the European Medicines Agency. These guidelines lay out requirements for selectivity, sensitivity, linearity, precision, accuracy, recovery, matrix effects, dilution integrity, and stability. An SPE method that produces beautiful chromatograms but cannot meet the acceptance criteria for inter-day precision or demonstrate adequate stability of the analyte in processed extracts will not survive an audit.

Validated SPE-based methods have been published for a range of pharmaceutical compounds, with validation parameters explicitly evaluated against FDA and EMA requirements.²⁷ In forensic and clinical toxicology, similar validation expectations apply, though the frameworks may come from different organizations such as SWGTOX or ISO 17025 accreditation bodies. The practical takeaway is that SPE is not just a research curiosity; it is embedded in the regulatory infrastructure that governs drug development, food safety, and environmental compliance around the world. Its longevity as a technique owes as much to this institutional trust as to its chemistry.