Some Further Sampling Considerations

All sampling efforts take representative samples. A representative sample is a sample of a whole (e.g., of a waste pile) which can be expected to exhibit the expected value of the sampled population. 

Characteristics of composite sampling are that the samples (or measurements) are taken following a random approach and then bulked. The best use is for relatively homogeneous material, unless stratified composites are created. A phase I ESA investigation often uses composite samples. 

A few cautions are needed (US EPA, 1995). Only a mean is obtained from composite sampling. There is thus no information on variability. Extremes may also influence the obtained mean. Thirdly, the level of confidence is unknown. If one needs more information, then one should use discrete sampling (grab samples). 

Composite sampling is often used to reduce the cost of analyzing a large number of samples. Experimental costs are substantially reduced when the frequency of individual samples containing the analytes of interest is low. However, composite sampling also has some limitations that must be considered. 

The number of subsamples required to make up a truly representative composite sample depends on the variability in results of the individual samples. The following expression quantifies the intuitive relationship  

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Some further consideration reveals that this analysis is also true if the sample consists of many crystals which disporportionate instead of just a single crystal. Hence, in a normal sample, the disproportionation can be made without a change in the total surface area, number of CS planes, or number of crystals. The net change in elastic-strain energy is then given by [(17s)" ... 

There are several points to think about when considering an on-line trial. Some of these considerations are generic to all analyzers, but, in addition, there are some specific safety aspects about using a Raman analyzer. The generic questions revolve around the sampling interface integrity and the analyzer enclosure. These generic questions are outside the scope of this chapter, but further information can be found in Chapter 3. The Raman-specific questions resolve around three areas of laser safety, namely with respect to the sample, the process environment, and the operator. These three areas can be broken down further ... 

Violet, P.V.50 is redder at greater depth of shade, while samples which are further reduced with TiOz appear bluer and considerably duller. P.V.50 is tinctorially weaker than P.V.23, which in some media is as much as twice as strong as the former. 

Another consideration when choosing a detector is whether it is important to preserve the separated analytes, either for use or for further analysis. Some methods, such as evaporative laser scattering detection and mass spectrometry, destroy the sample during the measurement. Other methods, such as fluorescence or radiochemical detection, may require chemical labeling of the analytes ... 

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