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Analysis of Land, Solids and Waste
many pollutants (e.g. organophosphorus pesticides), but can be used for some organochlorine pollutants.
5.2.5 Analytical Determination
The instrumental methods of analysis generally follow the procedures outlined in the previous two chapters.
What did we find were the most common procedures for the analysis of organic compounds and for metals in aqueous samples?
What extra considerations would you think necessary for the analysis of solid extracts?
Most organic materials would be analysed by using gas chromatography. Atomic spectrometric methods (AA, ICP-OES and ICP-MS) are the most common techniques for metals.
Due consideration would have to be taken of the possible interferences from other components that may be extracted. For gas chromatographic analysis, this could take the form of careful assessment of the cleanup techniques prior to the chromatographic analysis (see Section 4.2.2 above). High-resolution columns may be a necessity. If atomic absorption techniques are used for metal analysis (you may find that flame atomic absorption often has sufficient sensitivity), then background corrections will be required (see Section 4.3.3 above).
5.2.6 Quality Assurance and Quality Control
In Section 4.2.3 above, we discussed the addition of standards prior to the pretreatment of water samples to determine the recovery efficiencies. A similar procedure could be used to determine extraction efficiencies from solid samples.
What is the problem with direct addition of a standard to a solid sample to determine the extraction efficiency?
There is no guarantee that the extraction efficiency of the standard will be the same as the analyte. The later may be so strongly bound within the solid structure that it would be incompletely extracted. The internal standard may be less loosely bound, particularly if the extraction takes place immediately after addition, and so would be more easily extractable.
Introduction to Environmental Analysis
There is no easy way around this problem, although allowing the standard to equilibrate with the sample for several hours would be a good practice to adopt. There is always the possibility that extraction may not be complete for the analyte even if the standard does indicate complete extraction. During the validation of new analytical techniques for solids, there is often a confirmatory analysis of certified reference materials. Such reference materials are chosen to correspond as closely as possible to the samples being determined.
Many of the applications of solid analysis involve sampling in areas of heavily contaminated land. In these circumstances, constant quality checks are needed to confirm that no sample contamination has taken place. This is carried out by including in the analysis scheme blank samples introduced at each stage of the sampling procedure (see SAQ 2.5 above). Any positive result from a blank would indicate contamination at that stage.
A monitoring exercise is planned for lead deposited on soil close to a busy roadway. What sampling positions would you select?
5.3 Specific Considerations for the Analysis of Biological Samples
We will discuss first the sampling and extraction of components from plant material, and later consider the differences in approach which may be necessary in the case of animal tissues.
5.3.1 Sampling and Storage of Plant Material
The sample may be foliage, roots, or the whole plant. A single species should be sampled, with each specimen, if possible, being at a similar stage of maturity. If foliage is being sampled, the minimum sampling height should be such that there is no possibility of contamination by upward splashing from the soil, assuming that the species is tall enough! The maximum height is often determined by practical considerations. A suitable sample size is often 500-1000 g. The sample may be stored under refrigeration for a few days if it cannot be analysed immediately.
Even a simple procedure such as washing may extract the analyte. Patting the sample dry by using a paper tissue can result in contamination of the sample
Analysis of Land, Solids and Waste
with trace metals. It is consequently often preferable to avoid washing altogether if suitable clean samples can be found. Soft brushing may be an alternative. The cleanliness of samples is particularly important for trace metal analysis where the concentration may be higher in the surrounding soil than in the plant specimen.
Some pollutants may have been deposited from the atmosphere on to the leaf surface. If you are studying uptake of the pollutant by the plant, then this would have to be removed by washing. If, however, you were studying transfer of the pollutant along the food chain, then this should be included or determined separately. Dioxins, for instance, are not taken up by plants, but can enter the food chain by deposition on leaves which are then eaten by herbivores.