Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples301Because the method variance is smaller than the sampling variance, weobtain a smaller relative error if we collect more samples, analyzing eachfewer times.Practice Exercise 7.5An analytical method has a percent relative sampling variance of 0.10%and a percent relative method variance of 0.20%. The cost of collecting asample is $20 and the cost of analyzing a sample is $50. Propose a samplingstrategy that provides a maximum percent relative error of ±0.50%(a = 0.05) and a maximum cost of $700.Click here to review your answer to this exercise.7CImplementing the Sampling PlanImplementing a sampling plan normally involves three steps: physicallyremoving the sample from its target population, preserving the sample, andpreparing the sample for analysis. Except for in situ sampling, we analyzea sample after removing it from its target population. Because samplingexposes the target population to potential contamination, the samplingdevice must be inert and clean.After removing a sample from its target population, there is a dangerthat it will undergo a chemical or physical change before we can completeits analysis. This is a serious problem because the sample’s properties nolonger are representative of the target population. To prevent this problem,we often preserve samples before transporting them to the laboratory foranalysis. Even when analyzing samples in the field, preservation may stillbe necessary.The initial sample is called the primary or gross sample, and may be asingle increment drawn from the target population, or a composite of severalincrements. In many cases we cannot analyze the gross sample withoutfirst reducing the sample’s particle size, converting the sample into a morereadily analyzable form, or improving its homogeneity.7C.1 SolutionsTypical examples of solution samples include those drawn from containersof commercial solvents; beverages, such as milk or fruit juice; natural waters,including lakes, streams, seawater and rain; bodily fluids, such as bloodand urine; and, suspensions; such as those found in many oral medications.Let’s use the sampling of natural waters and wastewaters as a case study inhow to sample solutions.Although you may never work with thespecific samples highlighted in this section,the case studies presented here mayhelp you in envisioning potential problemsassociated with your samples.Sa m p l e Co l l e c t i o nThe chemical composition of a surface water—such as a stream, river, lake,estuary, or ocean—is influenced by flow rate and depth. Rapidly flow-