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18.104.22.168 Derivatization Procedure
The AE was derivatized into the corresponding alkyl bromide using HBr fission, by employing the following procedure. The SPE eluent, or standard, in 500 ^l of
Methods for Environmental Trace Analysis
ethyl acetate was dispensed into a ‘Chromacol’ screw-capped vial. An internal standard (undecanol) was then added. Then, HBr (33% in glacial acetic acid (0.5 ml)) was added to the mixture and the vial was capped tightly. (Note that if acetonitrite (ACN) is present in the solvent, a white precipitate, consistent with CH3CNHBr, is formed - this does not appear to affect the reaction.) The sample vial (‘Chromacol’ screw-capped vial) was placed in a heating block, which had been pre-heated to 100-105°C, for 4 h. The vial was removed from the heating block and left to cool to ambient temperature, or in a refrigerator, before opening. The vial was then uncapped and 1,1,1-trichloroethane (500 ^l) added. The vial was then recapped and vortex-mixed for 5 s. NaOH (2 M, 4 ml) was then added and the vial vortex-mixed for 10 s, the lower organic layer was allowed to settle, and then removed to a glass vial. The resultant sample was then extracted twice more with 500 ^l aliquots of 1,1,1-trichloroethane. The combined extracts were made up to 1.5 ml for analysis.
22.214.171.124 Analysis by GC-MS
Separation and identification of the alcohol ethoxylate was carried out on an HP 5890 Series II Plus gas chromatograph, fitted to an HP 5972A mass spectrometer. A 30 m x 0.25 mm id x 0.25 ^m film thickness DB-5ms capillary column was used, with temperature programming from an initial temperature held at 70°C for 2 min before commencing a 10°Cmin-1 rise to 250°C, with a final time of 20 min. The split/splitless injector was held at 230°C and operated in the splitless mode. The mass spectrometer transfer line was maintained at 240°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode. The EI MS detector scan range was 45-425 m/z. The key identifier ions used for the alkyl bromide were m/z 135 and 137.
126.96.36.199 Typical Results
These are shown in Figure 7.19 .
150 -100 -50 0
Figure 7.19 Results obtained from the matrix solid-phase dispersion of an alcohol ethoxylate from spiked fish tissue, showing the influence of different elution solvent systems: 1, 20 ml of methanol followed by 20 ml of acetonitrile; 2, 20 ml of dichloromethane/acetonitrile (1:1 (vol/vol)) followed by 20 ml of methanol; 3, 20 ml of dichloromethane/acetone (1:1 (vol/vol))  (cf. DQ 7.14).
Comment on the results obtained in this study (see Figure 7.19).
It is found that the recovery of alcohol ethoxylate is influenced by the elution solvent. Elution system ‘3’ offers the highest recovery, along with good precision.
This chapter has identified the main extraction techniques used for the extraction of organic analytes from solid matrices, e.g. soil. The main purpose of each technique has been to remove the analyte from the matrix as effectively as possible. In some instances, e.g. supercritical fluid extraction, some attempt is made to achieve selectivity of extraction by altering the operating conditions, such as temperature and pressure, or by the addition of an organic modifier. In other situations, the sole purpose of the extraction technique is to remove the analyte from the matrix under the ‘strongest’ possible conditions without any concern for selectivity. The selectivity in these circumstances results exclusively from the method of analysis which follows. It should also be borne in mind, depending on the level of contamination of the sample, that further extract pre-concentration and/or clean-up may be required prior to analysis in order to achieve trace level analytical results. The methods used for effective pre-concentration are described later in Chapter 10.
While an attempt has been made to place some context on each extraction technique, it should also be noted that most of the techniques described have been validated by the United States Environmental Protection Agency (USEPA) in terms of standard methods. The current USEPA methods for the extraction of pollutants from solid matrices are shown in Table 7.3. It should be noted that the supercritical fluid extraction technique has three specific methods for a range of compounds of environmental importance, whereas other techniques have more general, non-specific methods.
Based on knowledge of the extraction techniques discussed in this chapter, make a comparison of each technique. As a suggestion, the following headings could be used for comparison purposes: brief description of technique; sample mass; extraction time per sample; solvent type; solvent consumption; sequential or simultaneous extraction; relative cost of equipment; level of automation; approval of methods (USEPA).