Solid phase microextraction, commonly called SPME, is a sample preparation technique that pulls chemical compounds out of a liquid or gas and concentrates them onto a tiny coated fiber for analysis. Invented by Janusz Pawliszyn at the University of Waterloo and first published in 1990, it replaced the need for large volumes of chemical solvents traditionally used to extract compounds from samples. Today it is used across food science, environmental monitoring, and forensic analysis, and has been accepted as an official method by the US EPA, ISO, and ASTM.
How SPME Works
The core of an SPME device is a thin fiber, typically housed inside a syringe-like holder, coated with a material that attracts specific types of chemical compounds. When the fiber is exposed to a sample, target compounds migrate from the sample onto the coating. Depending on the coating type, compounds are either absorbed (dissolving into the coating like sugar into water) or adsorbed (sticking to its surface). This process continues until the system reaches equilibrium, the point where compounds move onto the fiber at the same rate they leave it.
The amount of any given compound that ends up on the fiber depends on the partition coefficient, which is essentially a measure of how strongly a compound prefers the fiber coating over the sample it came from. A high partition coefficient means more of that compound will concentrate on the fiber, making it easier to detect even at very low levels.
The Two-Step Process
An SPME analysis has two phases: extraction and desorption. During extraction, the fiber extends from its protective needle into the sample. After enough time for compounds to accumulate on the coating, the fiber retracts back into the needle and is removed from the sample container.
In the desorption step, the needle is inserted into an analytical instrument, most often a gas chromatograph. The fiber extends again, and heat causes the concentrated compounds to release from the coating and enter the instrument for identification and measurement. The entire workflow is simple enough to automate, which is one reason SPME has become popular in laboratories that process many samples.
Direct Immersion vs. Headspace Modes
SPME can be performed in two ways depending on the nature of the sample. In direct immersion mode, the fiber dips straight into a liquid sample. This works well for detecting polar compounds (those that mix readily with water) and avoids some artifacts that can occur when the coating becomes overloaded. Direct immersion is the preferred approach for metabolomics studies, where the goal is to capture as wide a range of compounds as possible, and it is also suitable for applications inside living organisms.
In headspace mode, the fiber sits in the air above the sample rather than touching the liquid itself. Volatile compounds evaporate from the sample and are captured by the fiber in the gas phase. This protects the fiber from damage by dirty or complex samples like soil, blood, or food. The tradeoff is that the coating can become saturated more easily in headspace mode, which may distort results for some compounds.
Fiber Coatings and What They Target
Choosing the right fiber coating is one of the most important decisions in an SPME experiment, because different coatings attract different types of compounds. The most commonly used coatings include polydimethylsiloxane (PDMS), polyacrylate, and combination coatings that layer two or three materials together.
- PDMS is a general-purpose, nonpolar coating that works well for compounds like hydrocarbons, solvents, and many flavor chemicals.
- Polyacrylate is more polar, making it better suited for compounds that dissolve in water, such as phenols and certain pesticides.
- Combination coatings like PDMS-divinylbenzene or carboxen-PDMS expand the range of compounds a single fiber can capture. A triple-layer fiber combining divinylbenzene, carboxen, and PDMS is commonly used in food flavor analysis paired with gas chromatography-mass spectrometry.
Carbon-based coatings represent another option. Active carbon materials interact nonspecifically with a broad range of substances and are particularly effective at trapping very volatile gases, aromatic hydrocarbons, and halogenated compounds. Newer sol-gel carbon nanotube fibers have shown significantly improved extraction for both polar compounds like phenols and nonpolar compounds like benzene and toluene compared to standard PDMS fibers.
Factors That Affect Sensitivity
Several variables influence how much of a target compound the fiber captures, and optimizing them can make the difference between detecting a substance and missing it entirely. Temperature affects how readily compounds leave the sample and interact with the coating. Higher temperatures push more volatile compounds into the headspace but can also cause some compounds to leave the fiber prematurely.
Extraction time matters because the fiber needs enough exposure to approach equilibrium. Agitating or stirring the sample speeds up this process by constantly moving fresh sample past the fiber. Adjusting the pH of a liquid sample can convert certain compounds into forms that are more easily captured. Adding salt to a water-based sample, a technique called salting out, reduces how well compounds dissolve in the water and forces more of them onto the fiber, boosting sensitivity.
Why It Replaced Older Techniques
Before SPME, the standard approach for pulling compounds out of samples was liquid-liquid extraction, which involved shaking a sample with a large volume of organic solvent, then evaporating the solvent to concentrate the compounds. This process was slow, used significant amounts of toxic chemicals, and generated waste that required careful disposal.
SPME eliminates organic solvents almost entirely. The fiber coating does the concentrating in a single step, with no solvent evaporation needed. This translates to faster preparation times, simpler procedures, and more efficient sample cleanup. Despite these advantages, many contract analytical laboratories still rely on liquid-liquid extraction or solid phase extraction cartridges for routine regulatory work, largely because switching official methods in established workflows takes time.
The SPME Arrow: A More Robust Design
A newer variant called the SPME Arrow addresses some limitations of the traditional fiber design. The Arrow uses an inner metal rod coated with sorbent material and protected by an outer metal tube, making it physically sturdier and less prone to breaking. More importantly, SPME Arrows carry 6 to 20 times more coating material than traditional fibers. This larger coating volume means higher sample capacity, better reproducibility, and the ability to extract the same amount of volatile compounds in roughly half the time. For food analysis in particular, the Arrow design captures a wider range of volatile compounds per extraction.
Common Applications
In food science, SPME is routinely used to profile the volatile compounds responsible for flavor and aroma. One well-known application is detecting cork taint in wine. Contaminated corks release a musty-smelling compound that ruins wine quality, and headspace SPME can identify it at extremely low concentrations. The technique also helps food manufacturers analyze off-flavors, determine the aroma profiles of fruits and vegetables, and separate biologically active substances in food products.
Environmental scientists use SPME to detect pesticides, herbicides, phenols, and other contaminants in water and soil. The technique is sensitive enough to pick up pollutants at trace levels that would be difficult to capture with traditional methods, and its acceptance by the US EPA and ISO as an official method reflects its reliability for regulatory monitoring.
Forensic laboratories apply SPME to measure alcohol concentration in blood and sugar levels in urine. Its small sample requirements and solvent-free workflow make it practical for cases where only limited biological material is available. Pharmaceutical researchers also use it to study how drugs and their byproducts behave in biological fluids, taking advantage of direct immersion SPME’s compatibility with living systems for in vivo sampling.