7: Obtaining and Preparing Samples for Analysis
- Page ID
- 127237
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)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
- This page discusses the importance of sampling in achieving accurate and precise chemical analyses, specifically related to ACS Reagent Grade specifications. A key point is that accurate sampling is crucial to minimize errors and meet standards, such as limiting iron in NaBr to under 5 ppm. The text also explains variance in sampling and analytical methods, illustrating the propagation of uncertainty through examples and exercises.
- 7.2: Designing a Sampling Plan
- This page discusses sampling plans for scientific analysis, highlighting their importance in achieving analytical goals. It distinguishes between qualitative and quantitative analysis needs, emphasizing random, judgmental, systematic, and convenience sampling methods, and the considerations for sampling heterogeneous populations.
- 7.3: Implementing the Sampling Plan
- The page discusses the steps involved in implementing a sampling plan, emphasizing the importance of preventing contamination and preserving samples to maintain their representativeness of the target population. It describes various sample types, including solutions, gases, and solids, and the methods used for their collection, preservation, and preparation.
- 7.4: Separating the Analyte From Interferents
- The text discusses the complexity of quantitative analysis when dealing with analytes and interferents in a sample. It describes a method's selectivity based on its sensitivity to the analyte versus the interferent, using specific equations. It highlights the selectivity coefficient as a crucial factor in characterizing method selectivity, emphasizing the need to account for an interferent???s contribution under certain conditions for accurate analysis.
- 7.5: General Theory of Separation Effiiciency
- The page explains the process of analytical separation, which aims to isolate an analyte or remove an interferent from a sample matrix. A distinction is made between the chemical or physical properties of these components to achieve separation. Key concepts include the analyte's and interferent's recoveries, defined by specific equations, and the separation factor.
- 7.6: Classifying Separation Techniques
- The document details various analytical separation techniques based on differences in chemical or physical properties between analytes and interferents. It includes methods like filtration, dialysis, and chromatography for size-based separations; centrifugation for mass or density differences; masking for complexation reactions; and techniques such as distillation, sublimation, and recrystallization for changes in physical or chemical states.
- 7.7: Liquid-Liquid Extractions
- The document discusses liquid-liquid extraction as a key method for separating compounds, used in environmental, clinical, and industrial labs. It highlights the importance of this technique in monitoring trihalomethanes in water supplies, often through gas chromatography after extraction with pentane. The text explains the concepts of partition coefficients and distribution ratios, emphasizing their roles in determining extraction efficiency.
- 7.8: Separation Versus Preconcentration
- The page discusses two common analytical issues: interference from matrix components and low analyte concentrations. Separation techniques can address both by isolating the analyte in a new phase and increasing its concentration through preconcentration. An example given is the gas chromatographic analysis for organophosphorous pesticides in water, where analytes are extracted and concentrated 67-fold using solid-phase extraction with ethyl acetate.
- 7.9: Problems
- The document addresses various analytical chemistry problems and exercises related to sampling, dissolution techniques, variance analysis, and liquid-liquid extraction efficiencies. It covers topics such as monitoring environmental samples, error calculation, and comparing digestion methods. The document is instructional, prompting the reader to analyze given data, calculate specific values, and develop sampling strategies.
- 7.10: Additional Resources
- The page provides a comprehensive list of references and experiments related to sampling techniques in analytical chemistry. It covers aspects such as the importance of sampling on analytical results, methods for extracting analytes, homemade sampling devices, general sampling terminology, sampling statistics, and sampling-related problems.
- 7.11: Chapter Summary and Key Terms
- This chapter summary focuses on the critical aspects of acquiring a representative sample for analysis. It emphasizes the importance of a well-structured sampling plan, including choices about sample types (e.g., random, systematic), collection methods (e.g., grab, composite), and considerations of the population's nature (homogeneous or heterogeneous).