Sampling procedures are extremely important in the analysis of soils, sediments and sludges. It is therefore essential to ensure before the analysis is commenced that correct statistically validated sampling procedures are used to ensure as far as is possible that the portion of the sample being analysed is representative of the bulk of material from which the sample was taken. The most complex problem occurs whenever the trace component to be determined has properties which are very similar to those of a major component of the sample to be analysed. To set up an efficient extraction process when using these techniques, it is necessary to resort once again to a knowledge of basic chemistry. Houba described a different device for the automatic subsampling of sediments for proficiency testing. Thoms showed that freeze-sampling collects representative sediment samples, whereas grab-sampling introduces a bias in the textural composition of the 120 mesh fraction, due to washout and elutriation of the finer fractions.