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The analytical technique for PCDDs and PCDFs from solids includes extraction, followed by clean-up and concentration of the extract, and then GC-MS analysis. Why is an isotopic standard better than, say, a compound which is structurally similar to PCDDs and PCDFs and is not found in the analytical mixture?
Using GC-MS with selected ion monitoring, how would you set about confirming the identity of a low-intensity chromatographic peak as a particular dioxin?
8.3 A Typical Analytical Scheme
The pretreatment is summarized in Figure 8.8 and the subsequent chromatographic separation in Table 8.2. The scheme is an example of a method where the emphasis is placed on sample clean-up and separation combined with low-resolution mass spectrometry, rather than relying on high-resolution MS techniques. I wish to use this scheme as an exercise to test your understanding of the principles of sample pretreatment and the subsequent analytical determination.
EXTRACTION Air-dry and sieve 250 g of sample Add 13C standards Extract with hexane/acetone Water wash Remove acetone
CLEAN-UP Add to multi-layer column - Anhydrous Na2SO4 Conc. H2SO4 on celite Silica gel Anhydrous H2SO4 Elute with petroleum ether
Add to Florisil Column
CH2Cl2 (PCDDs, PCDFs)
CONCENTRATION Add 20 |l dodecane to PCDD/PCDF extract Remove CH2Cl2
FURTHER TREATMENT FOR PROBLEM SAMPLES HPLC clean-up using graphitized carbon column
Figure 8.8 A typical pretreatment scheme for soil. From ‘Determination of polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans in UK soils’, Technical Report, Her Majesty’s Inspectorate of Pollution, HMSO, London, 1989.
Introduction to Environmental Analysis
Table 8.2 Chromatographic conditions used in the analytical
determination of soils“ (cf. Figure 8.8)
Tetra- and pentachlordibenzo-p-dioxins and furans:
• 5 m x 0.2 mm i.d. BP-5 (medium polarity) capillary column, plus
50 m x 0.2 mm i.d. RSL-950 or CP-Sil 88 (highly polar) capillary column
• Oven temperature gradient, 170-240° C
• Elution time, 20 min
Hexa-, hepta- and octachlorodibenzo-p-dioxins and furans:
• 50 m x 0.2 mm i.d. BP-5 capillary column
• Oven temperature gradient, 170-290° C
• Elution time, 20 min
“ ‘Determination of polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins
and polychlorinated dibenzofurans in UK soils’, Technical Report, Her Majesty’s
Inspectorate of Pollution, HMSO, London, 1989.
This will form a suitable conclusion to this open learning material on environmental analysis.
First of all, let us compare the scheme with the analysis of DDT as discussed earlier in Section 4.2. You should note the overall similarity of the individual
What are the major differences between the two analytical schemes?
1. The initial chromatographic clean-up for the dioxin analysis uses more than one stationary phase, thus reflecting the complexity of the extract. The number and type of phases used for dioxin extract cleanup vary considerably between literature methods. Acidic and basic silica and alumina columns are in common use. Note here the use of a multi-layer column rather than individual columns, hence saving analytical time and minimizing the possibility of sample contamination or loss.
2. Additional pretreatment for ‘problem’ dioxin samples. HPLC separation using graphitized carbon is used. This form ofcarbon has been found to be highly selective towards planar molecules. It is a straightforward, although time-consuming, operation to determine which sample types need further pretreatment, by comparison of the GC-MS chromatogram of one sample with and without the additional step.
Note, for both the steps given above, the necessity to minimize pretreatment time while still maintaining the efficiency of the clean-up procedure.
Why do you think there is a change of solvent composition between extraction and clean-up for the dioxin analysis?
The extraction stage uses a hexane/acetone mixture. Acetone is often used as a solvent modifier in extractions from solids to increase the polarity of the solvent and to assist in the penetration of the solvent into the samples (see Section 5.3 earlier). The first clean-up procedure involves application of the extract on to a chromatography column and subsequent elution of the non-polar components with a non-polar solvent (petroleum ether). The presence of a polar solvent in the extract would lower the efficiency of the chromatographic separation.
In both of the DDT and dioxin analytical procedures, a second column is required to separate chlorinated species (pesticides, PCBs, etc.).
Why do you think these components were not removed by the first column?
The chlorinated compounds have similar chemical structures. They are all neutral, non-polar, high-molecular-mass compounds and will have similar chromatographic retention properties. The first column in any clean-up is generally to remove interference from compounds with widely different chromatographic properties. The non-polar eluent used will elute the chlorinated species together. Separation of these closely related species will require a second and more selective column with sequential elution of the compounds by a series of solvents of increasing polarity.