7.S: Collecting and Preparing Samples (Summary)

An analysis requires a sample. How we acquire that sample is critical. The samples we collect must accurately represent their target population, and our sampling plan must provide a sufficient number of samples of appropriate size so that the uncertainty in sampling does not limit the precision of our analysis.

A complete sampling plan requires several considerations, including: the type of sample (random, judgmental, systematic, systematic–judgmental, stratified, or convenience); whether to collect grab samples, composite samples, or in situ samples; whether the population is homogeneous or heterogeneous; the appropriate size for each sample; and, the number of samples to collect.

Removing a sample from its population may induce a change in its composition due to a chemical or physical process. For this reason, we collect samples in inert containers and we often preserve them at the time of collection. When an analytical method’s selectivity is insufficient, we may need to separate the analyte from potential interferents. Such separations can take advantage of physical properties—such as size, mass or density—or chemical properties. Important examples of chemical separations include masking, distillation, and extractions.

7.9.1 Key Terms

centrifugation composite sample coning and quartering convenience sampling density gradient centrifugation dialysis distillation distribution ratio extraction extraction efficiency filtrate filtration grab sample gross sample

heterogeneous homogeneous in situ sampling judgmental sampling laboratory sample masking masking agents Nyquist theorem partition coefficient preconcentration purge-and-trap random sampling recovery recrystallization

retentate sampling plan secondary equilibrium reaction selectivity coefficient separation factor size exclusion chromatography Soxhlet extractor stratified sampling sublimation subsamples supercritical fluid systematic–judgmental sampling systematic sampling target population

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Contributors

David Harvey (DePauw University)