Stir bar sorptive extraction

In SBSE a glass-lined magnetic bar is covered with a thick layer of PDMS [77,78]. Through magnetic stirring of the bar in the sample solution the components are enriched in the PDMS phase. The typical manipulations in SBSE are (1) the stir bar is introduced in the liquid and stirred for a given time at a selected stirring speed and temperature, (2) removed with tweezers, (3) rinsed with distilled water to remove matrix compounds such as sugars, proteins, etc., (4) dried on lint-free tissue and finally (5) introduced in desorber tubes for thermal extraction (for capillary GC analysis) or in a mini vial for liquid desorption (for LC analysis). Advantages of this technique are ease-of-use, high sensitivity, high accuracy of analysis and reduced risks of contamination compared with other sample preparation techniques. Like all sorptive extraction methods SBSE is an equilibrium technique. Enrichment factors at equilibrium can be predicted from the log P values [79,80]. In practice, it is not necessary to work at this point and extractions are commonly performed for a much shorter defined period of time. Stir bars have been commercialized under the name TWISTER and are, together with the analytical tools, available from Gerstel (Mülheim a/d Ruhr, Germany). SBSE has been applied for quality control, and trace (ppb) and ultra-trace (ppt) analysis of water samples, beverages, diary products, biological fluids, etc. A recent review by F. David and P. Sandra describes the applications of SBSE for environmental analysis, food analysis, and biomedical and life science applications [81]. The performance of SBSE in combination with liquid extraction and LC-MS analysis has been compared to SPME for the determination of organophosporus insecticides in honey [82] and to SPE for fungicide residues in grapes [83].

Two typical examples are discussed, namely elucidation of endocrine disrupting chemicals (EDCs) in cheap red wines and the analysis of benzoic acid in yoghurt.

In a cheap red wine sample some EDCs were detected by SBSE-CGC-MS. The question was from where did these EDCs originate? The red wine sample was closed by a plastic stopper and 10 mg of the stopper was directly analysed by thermal desorption-CGC-MS analysis (Figure 10). The main peak is diisobu-tylphthalate (DiBP) (peak 5). The other compounds are the nonylphenols (peak group 4), benzoic acid (peak 2) and the typical wine components ethyl

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Time (min)

Figure 10 TD-CGC-MS analysis of 50 mg of a plastic stopper. A 10-mg sample of the stopper was thermally desorbed at 250°C and the released volatiles were analysed by capillary GC-MS on a 30-m L x 0.25-mm ID x 0.25 mm df HP5-MS. The MS was operated in the scan mode and ion m/z 149 was selected.

Abundance

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Time (min)

Figure 11 SBSE analysis of a yoghurt sample on the presence of benzoic acid. Extraction time was 60 min at 1,400 rpm and analysis was performed by capillary GC-MS on a 30-m L x 0.25-mm ID x 0.25 mm df HP-FFAP (Agilent) column.

octanoate (peak 1) and ethyl decanoate (peak 3) that were adsorbed on the stopper.

A yoghurt sample, claimed to be free of preservatives, was diluted 1:3 with distilled water and acidified to pH 2 with 1N hydrochloric acid. SBSE extraction was performed followed by CGC-MS analysis (Figure 11). The concentration of benzoic acid in the yoghurt sample was 28ppm. Because of a log K(o/w) for benzoic acid of 1.87 maximum recovery is 40% at equilibrium. For quantification, the standard addition method was applied. The limit of quantification (LOQ)

(at signal-to-noise level S/N10) was 8ppb in the scan mode and 0.5 ppb in the ion-monitoring mode.

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