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Chemistry LibreTexts

7: Collecting and Preparing Samples

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  • When we use an analytical method to solve a problem, there is no guarantee that our results will be accurate or precise. In designing an analytical method we consider potential sources of determinate error and indeterminate error, and take appropriate steps to minimize their effect, such as including reagent blanks and calibrating instruments. Why might a carefully designed analytical method give poor results? One possibility 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 while preparing 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 our precision may suffer. In this chapter we consider how collecting samples and preparing them for analysis affects the accuracy and precision of our results.

    • 7.1: The Importance of Sampling
      If the individual samples do not accurately represent the population from which they are drawn—what we call the target population—then even a careful analysis must yield an inaccurate result. Extrapolating this result from a sample to its target population introduces a determinate sampling error. To minimize this determinate sampling error, we must collect the right sample.  Even if we collect the right sample, indeterminate sampling errors may limit the usefulness of our analysis.
    • 7.2: Designing a Sampling Plan
      A sampling plan must support the goals of an analysis. In a qualitative analysis, a sample does not need to be identical to the original substance, provided that there is sufficient analyte to ensure its detection. In fact, if the goal of an analysis is to identify a trace-level component, it may be desirable to discriminate against major components when collecting samples.
    • 7.3: Implementing the Sampling Plan
      Implementing a sampling plan normally involves three steps: physically removing the sample from its target population, preserving the sample, and preparing the sample for analysis. Except for in situ sampling, we analyze a sample after removing it from its target population. Because sampling exposes the target population to potential contamination, the sampling device must be inert and clean.  The initial sample is called the primary or gross sample.
    • 7.4: Separating the Analyte from Interferents
      When an analytical method is selective for the analyte, analyzing samples 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.
    • 7.5: General Theory of Separation Efficiency
      The goal of an analytical separation is to remove either the analyte or the interferent from the sample’s matrix. To achieve this separation there must be at least one significant difference between the analyte’s and the interferent’s chemical or physical properties. A separation that completely removes the interferent may also remove a small amount of analyte. Altering the separation to minimize the analyte’s loss may prevent us from completely removing the interferent.
    • 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. This section provides a partial list of separation techniques, classified by the chemical or physical property being exploited.
    • 7.7: Liquid–Liquid Extractions
      A liquid–liquid extraction is an important separation technique for environmental, clinical, and industrial laboratories. A standard environmental analytical method illustrates the importance of liquid–liquid extractions. Municipal water departments routinely monitor public water supplies for trihalomethanes because they are known or suspected carcinogens. Before their analysis by gas chromatography, trihalomethanes are separated from their aqueous matrix by
    • 7.8: Separation Versus Preconcentration
      Two common analytical problems are: (1) matrix components that interfere with an analyte’s analysis; and (2) an analyte with a concentration that is too small to analyze accurately. We showed a separation can solve the first problem. We often can use a separation to solve the second problem as well. For a separation in which we recover the analyte in a new phase, it may be possible to increase the analyte’s concentration. This step in an analytical procedure is known as a preconcentration.
    • 7.E: Collecting and Preparing Samples (Exercises)
      These are homework exercises to accompany "Chapter 7: Collecting and Preparing Samples" from Harvey's "Analytical Chemistry 2.0" Textmap.
    • 7.S: Collecting and Preparing Samples (Summary)
      This is a summary to accompany "Chapter 7: Collecting and Preparing Samples" from Harvey's "Analytical Chemistry 2.0" Textmap.

    Thumbnail: An example of pipettes and microplates manipulated by an anthropomorphic robot (Andrew Alliance). Image used with permission (Cc BY-SA 3.0; Pzucchel).