Solid Phase Microextraction

Figure 1 (A) Design of a commercial SPME device. (B) SPME-HPLC interface: (a) stainless steel (SS) 1/16" tee; (b) 1/16" SS tubing; (c) 1 /16" polyetheretherketone (PEEK) tubing (0.02" i.d.); (d) two-piece finger-tight PEEK union; (e) PEEK tubing (0.005" i.d.) with a one-piece PEEK union. (C) SPME-GC interface.

Figure 1 (A) Design of a commercial SPME device. (B) SPME-HPLC interface: (a) stainless steel (SS) 1/16" tee; (b) 1/16" SS tubing; (c) 1 /16" polyetheretherketone (PEEK) tubing (0.02" i.d.); (d) two-piece finger-tight PEEK union; (e) PEEK tubing (0.005" i.d.) with a one-piece PEEK union. (C) SPME-GC interface.

be used to extract analytes from very small samples. For example, SPME has been used to probe for substances emitted by a single flower bloom during its lifespan.

Figure 1A illustrates the commercial SPME device manufactured by Supelco, Inc. (Bellefonte, PA, USA).

The fibre, glued into a piece of stainless steel tubing, is mounted in a special holder. The holder is equipped with an adjustable depth gauge, which makes it possible to control repeatably how far the needle of the device is allowed to penetrate the sample container (if any) or the injector. This is important, as the fibre can be easily broken when it hits an obstacle. The movement of the plunger is limited by a small screw moving in the z-shaped slot of the device. For protection during storage or septum piercing, the fibre is withdrawn into the needle of the device, with the screw in the uppermost position. During extraction or desorption, the fibre is exposed by depressing the plunger, which can be locked in the lowered (middle) position by turning it clockwise (the position depicted in Figure 1A). The plunger is moved to its lowermost position only for replacement of the fibre assembly. Each type of fibre has a hub of a different colour. The hub-viewing window permits a quick check to be made of the type of fibre mounted in the device.

If the sample is placed in a vial, the septum of the vial is first pierced with the needle (with the fibre in the retracted position) and the plunger is lowered, which exposes the fibre to the sample. The analytes are allowed to partition into the coating for a predetermined time, and the fibre is then retracted back into the needle. When gas chromatography (GC) is used for analyte separation and quantitation, the fibre is inserted into a hot injector, where thermal desorption of the trapped analytes takes place (Figure 1C). All extracted compounds are introduced to the analytical instrument facilitating high sensitivity of determinations. The fibre desorption process can be automated by using an appropriately modified, commercially available syringe autosampler. For high performance liquid chromatography (HPLC) applications, a simple interface mounted in place of the injection loop can be used to re-extract analytes into the desorption solvent (Figure 1B). The extraction phase can also coat the inner wall of the capillary. This approach to microextraction can be automated using a number of commercially available autosamplers, but it is limited to extraction of relatively clean samples, which do not plug capillaries.

The SPME device is suitable for both spot and time-averaged sampling. As described above, for spot sampling, the fibre is exposed to a sample matrix until equilibrium is reached between the sample matrix and the coating material on the fibre. In the time-average approach, on the other hand, the fibre remains in the needle during the exposure of the SPME device to the sample. The coating works as a trap for analytes that diffuse into the needle, resulting in the integration of concentration over given time.

SPME sampling can be performed in three basic modes: direct extraction, headspace extraction and extraction with membrane protection. Figure 2 illustrates the differences between these modes. In direct extraction mode (Figure 2A), the coated fibre is inserted into the sample and the analytes are transported directly from the sample matrix to the extracting phase. To facilitate rapid extraction, some level of agitation is required to transport the analytes from the bulk of the sample to the vicinity of the fibre. For gaseous samples, natural flow (e.g. convection) is frequently sufficient to facilitate rapid equilibration, but for aqueous matrices, more efficient agitation techniques, such as fast sample flow, rapid fibre or vial movement, stirring or sonication are required to reduce the effect of the depletion zone produced close to the fibre as a result of slow diffusional analyte

Figure 2 Modes of SPME operation: (A) direct extraction, (B) headspace extraction and (C) membrane-protected SPME.

transport through the otherwise static layer of liquid surrounding the fibre.

In the headspace mode (Figure 2B), the analytes are extracted from the gas phase equilibrated with the sample. The primary reason for this modification is to protect the fibre from adverse effects caused by nonvolatile, high molecular weight substances present in the sample matrix (e.g. humic acids or proteins). The headspace mode also allows matrix modifications, including pH adjustment, without affecting the fibre. In a closed system consisting of a liquid sample and its headspace, the amount of an analyte extracted by the fibre coating does not depend on the location of the fibre, therefore the sensitivity of headspace sampling is the same as the sensitivity of direct sampling as long as the volumes of the two phases are the same in both sampling modes. Even when headspace is not used in direct extraction, a significant sensitivity difference between direct and headspace sampling can occur only for very volatile analytes. However, the choice of sampling mode has a significant impact on the extraction kinetics. When the fibre is in the headspace, the analytes are removed from the headspace first, followed by indirect extraction from the matrix. Therefore, volatile analytes are extracted faster than semivolatiles. Temperature has a significant effect on the kinetics of the process, since it determines the vapour pressure of analytes. In general, the equilibration times for volatile compounds are shorter for headspace SPME extraction than for direct extraction under similar agitation conditions, for the following reasons: (i) a substantial portion of the analytes is present in the headspace before the extraction process begins; (ii) there is typically a large interface between sample matrix and headspace; and (iii) the diffusion coefficients in the gas phase are typically higher by four orders of magnitude than in liquids. The concentration of semivolatile compounds in the gaseous phase at room temperature is small, consequently headspace extraction rates for those compounds are substantially lower. These rates can be improved by using efficient agitation or by increasing the extraction temperature.

In the third mode (SPME extraction with membrane protection, Figure 2C), the fibre is separated from the sample by a selective membrane, which lets the analytes through while blocking the interferences. The main purpose for the use of the membrane barrier is to protect the fibre against adverse effects caused by high molecular weight compounds when very dirty samples are analysed. While extraction from headspace serves the same purpose, membrane protection allows the analysis of less volatile compounds. The extraction process is substantially slower than direct extraction because the analytes have to diffuse through the membrane before they can reach the coating. Use of thin membranes and increase in extraction temperature, applied to analysis of polyaromatic hydrocarbons (PAHs) in matrices containing humic matter, result in shorter extraction times.

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