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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
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n = KV2C0V1/KV2 + V1 (8.1)
where n is the number of moles of the analyte absorbed by the stationary phase, K is the partition coefficient of an analyte between the stationary phase and the aqueous phase, C0 is the initial concentration of analyte in the aqueous phase, V1 is the volume of the aqueous sample, and V2 is the volume of the stationary phase.
As was stated earlier, the polymeric stationary phases used for SPME have a high affinity for organic molecules, and hence the values of K are large. These large values of K lead to good pre-concentration of the target analytes in the aqueous sample and a corresponding high sensitivity in terms of the analysis. However, it is unlikely that the values of K are large enough for exhaustive extraction of analytes from the sample. Therefore, SPME is an equilibrium method, but provided that proper calibration strategies are followed it can provide quantitative data.
Louch et al. [3] went onto show that in the case where V1 is very large (i.e. V1 » KV2) the amount of analyte extracted by the stationary phase
Continued on page 159 m
? Continued from page
could be simplified to the following:
n = KV2C0 (8.2)
and hence is not related to the sample volume. This feature can be most effectively exploited in field sampling. In this situation, analytes present in natural waters, e.g. lakes and rivers, can be effectively sampled, preconcentrated and then transported back to the laboratory for subsequent analysis. The dynamics of extraction are controlled by the mass transport of the analytes from the sample to the stationary phase of the silica-coated fibre. The dynamics of the absorption process have been mathematically modelled [3]. In this work, it was assumed that the extraction process is diffusion-limited. Therefore, the amount of sample absorbed, when plotted as a function of time, can be derived by solving Fick’s Second Law of Diffusion. A plot of the amount of sample absorbed versus time is termed the extraction profile. The dynamics of extraction can be increased by stirring the aqueous sample.
8.4.1 Experimental
The most common approach for SPME is its use for GC, although its coupling to HPLC has also been reported. The SPME device consists of a fused-silica fibre, coated with a gas chromatographic stationary phase, e.g. polydimethylsiloxane. In addition, various other stationary phases are available for SPME (Table 8.3). The small size and cylindrical geometry allow the fibre to be incorporated into a syringe-type device (Figure 8.11). This allows the SPME device to be effectively used in the normal unmodified injector of a gas chromatograph. As can be seen in Figure 8.11, the fused-silica fibre (approximately 1 cm) is connected to a stainless-steel tube for mechanical strength. This assembly is mounted
Table 8.3 Some commercially available SPME fibre coatings
7 ^m polydimethylsiloxane (bonded)
30 ^m polydimethylsiloxane (non-bonded)
100 ^m polydimethylsiloxane (non-bonded)
85 ^m polyacrylate (partially cross-linked)
60 ^m polydimethylsiloxane/divinylbenzene (partially cross-linked)
65 ^m polydimethylsiloxane/divinylbenzene (partially cross-linked)
75 ^m polydimethylsiloxane/Carboxen (partially cross-linked)
65 ^m Carbowax/divinylbenzene (partially cross-linked)
50 ^m Carbowax/Template resin (partially cross-linked)
Methods for Environmental Trace Analysis
- Plunger
- Barrel
- Z-slot
- Hub-viewing window
Septum-piercing needle
Fibre-attachment tubing
Fused-silica fibre
Figure 8.11 Schematic of a solid-phase microextraction device. Reprinted with permission from Zhang, Z., Yang, M. and Pawliszyn, J., Anal. Chem., 66, 844A-853A (1994). Copyright (1994) American Chemical Society.
within the syringe barrel for protection when not in use. For SPME, the fibre is withdrawn into the syringe barrel, and then inserted into the sample-containing vial for either solution or air analysis. At this point, the fibre is exposed to the analyte(s) by pressing down the plunger for a pre-specified time. After this predetermined time-interval, the fibre is withdrawn back into its protective syringe barrel and withdrawn from the sample vial. The SPME device is then inserted into the hot injector of the gas chromatograph and the fibre exposed for a prespecified time. The heat of the injector desorbs the analyte(s) from the fibre prior to GC separation and detection. SPME can carried out either manually or by an autosampler. As the exposed fibre is an active site for adsorption of not only the analytes of interest but also air-borne contaminants, it is essential that the SPME fibre is placed in the hot injector of the chromatograph prior to adsorption/desorption of the analytes of interest, in order to remove potential interferents.
A typical procedure used for SPME is described in Figure 8.12.
Figure 8.12 Typical procedure used for the solid-phase microextraction of liquids.
Methods for Environmental Trace Analysis
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