Analytical Chem istry - DePauw University

340 Analytical Chemistry 2.0grab sample gross sample heterogeneoushomogeneous in situ sampling judgmental samplinglaboratory sample masking masking agentsNyquist theorem partition coefficient preconcentrationpurge-and-trap random sampling recoveryrecrystallization retentate sampling plansecondary equilibriumreactionsize exclusionchromatographyselectivity coefficientSoxhlet extractorseparation factorstratified samplingsublimation subsamples supercritical fluidsystematic–judgmentalsamplingsystematic samplingtarget population7J Chapter SummaryAn analysis requires a sample. How we acquire that sample is critical. Thesamples we collect must accurately represent their target population, andour sampling plan must provide a sufficient number of samples of appropriatesize so that the uncertainty in sampling does not limit the precisionof 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, compositesamples, or in situ samples; whether the population is homogeneous orheterogeneous; the appropriate size for each sample; and, the number ofsamples to collect.Removing a sample from its population may induce a change in itscomposition due to a chemical or physical process. For this reason, we collectsamples in inert containers and we often preserve them at the time ofcollection.When an analytical method’s selectivity is insufficient, we may needto separate the analyte from potential interferents. Such separations cantake advantage of physical properties—such as size, mass or density—orchemical properties. Important examples of chemical separations includemasking, distillation, and extractions.7KProblemsAnswers, but not worked solutions, tomost end-of-chapter problems are availablehere.1. Because of the risk of lead poisoning, the exposure of children to leadbasedpaint is a significant public health concern. The first step in thequantitative analysis of lead in dried paint chips is to dissolve the sample.Corl has evaluated several dissolution techniques. 22 In this study,samples of paint were collected and pulverized with a Pyrex mortar andpestle. Replicate portions of the powdered paint were then taken for22 Corl, W. E. Spectroscopy 1991, 6(8), 40–43.