Solid‐phase microextraction (SPME) is a modern sampling/sample preparation method, used for isolation and preconcentration of organic molecules from a variety of matrices. SPME uses a short piece of a fused silica fiber coated with a polymeric stationary phase. The fiber is mounted in a device resembling a syringe. During transport, storage and manipulation, the fiber is retracted into the needle of the device. During extraction and desorption of the analytes, the fiber is exposed. Analytes present in a sample partition into or onto the coating, depending on its type. The process continues until equilibrium is reached between the coating and the sample. From then on, longer extraction times do not result in larger amounts of analyte extracted. Once the extraction is finished, the fiber is retracted back into the needle, and the device is transferred to a gas or liquid chromatograph for analyte separation and determination. When gas chromatography (GC) is used, the analytes are thermally desorbed from the fiber in a GC injector. Coupling of SPME with high‐performance liquid chromatography (HPLC) requires a special interface.
Two distinct SPME coating types are available commercially. Coatings of the first type, including poly(dimethylsiloxane) (PDMS) and poly(acrylate) (PA), extract analytes by absorption. This process is non‐competitive, therefore in most cases the amount of an analyte extracted by such coatings from a sample is independent of the matrix composition. No saturation or displacement effects occur. The amount of an analyte extracted from a sample is linearly dependent on its initial concentration, provided that several important variables, including (but not limited to) temperature, extraction time and mass transfer conditions, are kept constant. Coatings of this type usually perform very well for compounds of medium to low volatility.
Coatings of the second type, including poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB), Carbowax™/divinylbenzene (CW/DVB) and Carboxen™/poly(dimethylsiloxane) (CX/PDMS), extract analytes by adsorption. This process is limited to the surface of the coating. It is competitive, which means that a molecule with higher affinity to the coating can displace a molecule with lower affinity. Since the number of active sites on the surface of any coating is limited, a linear response for those coatings can be expected only when the concentrations of all the compounds that can be extracted by the coating from a sample are low. Adsorption‐type coatings are particularly suitable for volatile analytes, for which they offer much better sensitivity than PDMS or PA.
SPME is very well suited for field applications, especially when the analysis is carried out on site. The fiber can be exposed directly to the medium analyzed, for example lake water or ambient air, without the need to collect a sample and without knowing the exact volume of the sample the fiber is exposed to. Analysis can then be performed using field portable instrumentation. Manual operation of the device is very simple. The fibers are reusable, which makes the cost of analysis low. Separation of volatile components sampled by SPME can be very fast when a dedicated system (available commercially from SRI Instruments) is used.
Alternatively, SPME can be used to sample in the field, and then transported for the analysis to the laboratory. Modified devices have to be used for this purpose to avoid analyte losses during transport and storage, as well as contamination of the samples. Transporting fibers is much easier than transporting glass or metal containers with water or air samples.
The main disadvantage of SPME in the field is its lack of robustness. The needle can be easily bent, and the fiber can be broken when handled without sufficient care. New designs of field portable SPME devices address those issues. Also, it might be difficult to accurately control in the field all the experimental parameters that affect the amount of analyte extracted.
