# 7: Obtaining and Preparing Samples for Analysis

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When we use an analytical method to solve a problem, there is no guarantee that will obtain accurate or precise results. In designing an analytical method we consider potential sources of determinate error and indeterminate error, and we take appropriate steps—such as reagent blanks and the calibration of instruments—to minimize their effect. Why might a carefully designed analytical method give poor results? One possible reason is that we may have failed to account for errors associated with the sample. If we collect the wrong sample, or if we lose analyte when we prepare the sample for analysis, then we introduce a determinate source of error. If we fail to collect enough samples, or if we collect samples of the wrong size, then the precision of our analysis may suffer. In this chapter we consider how to collect samples and how to prepare them for analysis.

• 7.1: The Importance of Sampling
If the individual samples do not represent accurately the population from which they are drawn—a population that we call the target population—then even a careful analysis will yield an inaccurate result. Extrapolating a result from a sample to its target population always introduces a determinate sampling error. To minimize this determinate sampling error, we must collect the right sample.
• 7.2: Designing a Sampling Plan
A sampling plan must support the goals of an analysis. A material scientist interested in characterizing a metal’s surface chemistry is more likely to choose a freshly exposed surface, created by cleaving the sample under vacuum, than a surface previously exposed to the atmosphere. In a qualitative analysis, a sample need not be identical to the original substance if there is sufficient analyte present to ensure its detection.
• 7.3: Implementing the Sampling Plan
Implementing a sampling plan usually involves three steps: physically removing the sample from its target population, preserving the sample, and preparing the sample for analysis.
• 7.4: Separating the Analyte From Interferents
When an analytical method is selective for the analyte, analyzing a sample is a relatively simple task. For example, a quantitative analysis for glucose in honey is relatively easy to accomplish if the method is selective for glucose, even in the presence of other reducing sugars, such as fructose. Unfortunately, few analytical methods are selective toward a single species; thus, we must separate analytes from interferents.
• 7.5: General Theory of Separation Effiiciency
The goal of an analytical separation is to remove either the analyte or the interferent from the sample’s matrix. To achieve this separation we must identify at least one significant difference between the analyte’s and the interferent’s chemical or physical properties. A significant difference in properties, however, is not sufficient to effect a separation if the conditions that favor the extraction of interferent from the sample also removes a small amount of analyte.
• 7.6: Classifying Separation Techniques
We can separate an analyte and an interferent if there is a significant difference in at least one of their chemical or physical properties, such as size, mass or density, the ability to form complexes, a change in physical state, a change in chemical state, or the ability to partition between phases.
• 7.7: Liquid-Liquid Extractions
A liquid–liquid extraction is an important separation technique for environmental, clinical, and industrial laboratories. In a simple liquid–liquid extraction the solute partitions itself between two immiscible phases. One phase usually is an aqueous solvent and the other phase is an organic solvent, such as the pentane used to extract trihalomethanes from water.
• 7.8: Separation Versus Preconcentration
Two common analytical problems are matrix components that interfere with an analyte’s analysis and an analyte with a concentration that is too small to analyze accurately. As we have learned in this chapter, we can use a separation to solve the first problem. Interestingly, we often can use a separation to solve the second problem as well.
• 7.9: Problems
End-of-chapter problems to test your understanding ot topics in this chapter.