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8.4.2 Example 8.3: Solid-Phase Microextraction of BTEX from Water^
188.8.131.52 Extraction Conditions These were as follows:
• Sample volume: 10 ml
• Fibre: 100 ^m polydimethylsiloxane
Comments SPME fibre inserted into either the sample or headspace above the sample (with/without stirring; with/without salt) for varying amounts of time.
184.108.40.206 Analysis by GC
Separation and identification of the BTEX mixture was carried out on a Carlo Erba HRGC 5300 Mega Series gas chromatograph, with split/splitless injection
-SPME fibre holder
Cap with septum Vial
SPME fibre holder
Cap with septum Coated fibre Vial
5 10 15
? o c
CD S' — CO CO 1=
Figure 8.13 Analysis of o-xylene and BTEX (in water) using solid-phase microextraction: (a) direct SPME fibre mode; (b) headspace SPME fibre mode; (c) results obtained for o-xylene using mode (a); (d) results obtained for BTEX using mode (b): ?, no stirring; H, with stirring; ?, with stirring, plus salt: ?, benzene; ?, toluene; a, ethylbenzene; •, m-, p-xylene(s); x, o-xylene  (cf. DQ 8.11).
fThe acronymn ‘BTEX’ is commonly used to refer to a low-boiling-point mixture of benzene, toluene, ethylbenzene and xylene(s).
and flame ionization detection. A 30 m x 0.25 mm id x 0.1 pm film thickness DB-5 capillary-column was used, with temperature programming from an initial temperature held at 50°C for 3 min before commencing a 16°C min-1 rise to 120°C, with a final hold time of 7 min. The detector temperature was set at 250°C.
220.127.116.11 Typical results
These are shown in Figure 8.13 .
Comment on the results obtained in this study (see Figure 8.13).
In the ‘direct mode’ (Figures 8.13(a, c)) the SPMEfibre has been exposed to o-xylene for increasing amounts of time in three different sequences. These are unaided (‘no stirring’), ‘with stirring’, and finally ‘with stirring and salt’. It is noted that ‘no stirring’ results in the smallest signal obtained, after GC(FID) analysis, in all cases. Stirring provides a greater signal which can be improved by the addition of salt (‘salting out’). What should also be noted is the time-scale (minutes) in the ‘direct mode’ needed to achieve an appropriate signal response. In the ‘headspace mode’ (Figures 8.13(b, d)), a range of BTEX compounds have been exposed to the fibre prior to GC(FID) analysis. In this case, it should be observed that (i) each compound results in a different response, (ii) the response increases with respect to time, and (iii) the time-domain is in seconds (not minutes, as in the ‘direct mode’).
Based on knowledge of the extraction techniques discussed in this chapter, make a comparison of each technique. As a suggestion, the following headings can be used for comparison purposes: brief description of technique; sample volume; extraction time per sample; solvent consumption; relative cost of equipment; ‘acceptability’; approval of methods (USEPA).
The presence of trace organics in natural and waste waters can often cause a problem in terms of the selected analytical technique. In order to be able to quantify the concentrations of trace organics in aqueous samples, appropriate methods of pre-concentration therefore need to be selected. This present chapter has summarized the main methods available for such preconcentration procedures. The traditional approach has utilized liquid-liquid
Methods for Environmental Trace Analysis
extraction. However, since the 1970s solid-phase extraction (SPE) has become increasingly popular, particularly as it is possible to automate the procedure. Most recently (the 1990s), the use of solid-phase microextraction (SPME) has offered an alternative approach to pre-concentration. However, it is not foreseen that SPME will replace SPE, but rather offer an alternative method which is ‘portable’ and hence can be applied outside of the laboratory.
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2. Madier, C. ‘Extraction of phenols from river water’, BSc Project, Northumbria University, Newcastle, UK, 1997.
3. Louch, D., Motlagh, S. and Pawliszyn, J., Anal. Chem., 64, 1187-1199 (1992).
4. Ahmed, H. K., ‘Separation of common herbicides from water samples using SPE, followed by HPLC-UV’, MSc Dissertation, Northumbria University, Newcastle, UK, 1996.
5. Biziuk, M., Namiesnik, J., Czerwinski, J., Gorlo, D., Makuch, B., Janicki, W., Polkowska, Z. and Wolska, L., J. Chromatogr., A, 733, 171-183 (1996).
6. Honing, M., Riu, J., Barcelo, D., van Baar, B. L. M. and Brinkman, U. A. Th., J. Chromatogr., A, 733, 283-294 (1996).
7. Vink, M. and van der Poll, J. M., J. Chromatogr., A, 733, 361-366 (1996).
Methods for Environmental Trace Analysis. John R. Dean