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7.S: Collecting and Preparing Samples (Summary)

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  • 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

    composite sample
    coning and quartering
    convenience sampling
    density gradient centrifugation
    distribution ratio
    extraction efficiency
    grab sample
    gross sample
    in situ sampling
    judgmental sampling
    laboratory sample
    masking agents
    Nyquist theorem
    partition coefficient
    random sampling
    sampling plan
    secondary equilibrium reaction
    selectivity coefficient
    separation factor
    size exclusion chromatography
    Soxhlet extractor
    stratified sampling
    supercritical fluid
    systematic–judgmental sampling
    systematic sampling
    target population


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