SPME sorption time

The effect of SPME sorption (extraction) time on SPME efficiency was investigated by using three time intervals IS, 30, and 60 min. Table 3 lists the peak area ratios as a function of the sorption time. The SPME sorption time had no significant effect on SPME efficiency. In order to save analysis time, the 15-min sorption time was chosen for the remainder of this work. Since a typical GC run lasts approximately 20 min, shorter SPME extraction time is also more practical because both SPME and GC analysis can be performed simultaneously. 

Table 3. Effect of Sorption Time on SPME Efficiency ...

Figure 3.28D Plot of the sum of peak areas for AR 1242 against the sorption time for SPME a 100 /am thick nonbonded polydimethylsiloxane fused silica fiber was used.

Determination of the optimum time for which the SPME sorbent will be in direct contact with the sample is made by constructing an extraction-time profile of each analyte(s) of interest. The sorption and desorption times are greater for semi volatile compounds than for volatile compounds. To prepare the extraction-time profile, samples composed of a pure matrix spiked with the analyte(s) of interest are extracted for progressively longer times. Constant temperature and sample convection must be controlled. Stirring the... 

SPME is a modified SPE procedure based on the use of a coated fiber made of fused silica. After the extraction the fiber is directly introduced into the injector of the GC instmment to allow the direct transfer of the analytes into the chromatographic column, thus avoiding the use of organic solvents. Chromatographic stationary phases, such as poly(methylsiloxane), are used as chemically bonded coatings of the fiber. SPME is an inexpensive and easily automated technique, but its most important drawbacks are the poor detection limits compared to SPE and the time required for sorption on the fiber. 

SPME was developed by Pawlisz)m and coworkers in 1987 [161-163]. The reader may find further information on the historical evolution, principles, and commercially available devices of SPME in an excellent review by the pioneer of the technique [164]. SPME is based on a partitioning equilibrium of the solutes between the sorbent phase and the aqueous and/or gas matrix. A small amount of sorbent phase is dispersed on a solid support, which will be exposed to the sample for a predetermined time. Different implementations were developed such as suspended particles, coated-stirrer, vessel walls, disks, stirrers, or membranes, although the fiber and in-tube are explored theoretically and experimentally in depth. The former consists of a thin, fused-silica fiber-coated with sorbent on its surface and mounted in a modified GC syringe, which protects the fiber and allows handling. The latter in-tube implementation consists of an internally coated tube or capillary. The analytes are extracted by sorption when either coated fiber or tube are immersed in the water sample (direct SPME) or in the headspace above the sample (HS-SPME). 

The variables that affect SBSE extraction performance are similar to the ones affecting SPME, namely, extraction time, extraction temperature (mainly for headspace mode, it should be noted however that the sorption process on the PDMS coating is not favored at high temperatures), selection of modifiers to increase the extraction efficiency (sueh as small amounts of organic solvents or high contents of salts), pH, stirring, and sample volume. 

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