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Modern Analytical Cheymistry - Chapter 7

Obtaining and Preparing Samples for Analysis Sau đó, chúng tôi lần đầu tiên sử dụng một phương pháp phân tích để giải quyết một vấn đề, nó không phải là bất thường để tìm thấy rằng kết quả của chúng tôi có độ chính xác vấn đề hoặc không chính xác như là vô nghĩa. Nhìn lại chúng ta có thể thấy rằng không có gì trong phương pháp này có vẻ sai lệch. Trong thiết kế phương pháp, chúng ta xem xét nguồn của lỗi xác định và không xác định và bước đi thích hợp, chẳng hạn như. | Chapter UỂ 7 Obtaining and Preparing Samples for Analysis W .__ hen we first use an analytical method to solve a problem it is not unusual to find that our results are of questionable accuracy or so imprecise as to be meaningless. Looking back we may find that nothing in the method seems amiss. In designing the method we considered sources of determinate and indeterminate error and took appropriate steps such as including a reagent blank and calibrating our instruments to minimize their effect. Why then might a carefully designed method give such poor results One explanation is that we may not have accounted for errors associated with the sample. When we collect the wrong sample or lose analyte while preparing the sample for analysis we introduce a determinate source of error. If we do not collect enough samples or collect samples of the wrong size the precision of the analysis may suffer. In this chapter we consider how collecting samples and preparing them for analysis can affect the accuracy and precision of our results. 179 l8o Modern Analytical Chemistry 7A The Importance of Sampling When a manufacturer produces a chemical they wish to list as ACS Reagent Grade they must demonstrate that it conforms to specifications established by the American Chemical Society ACS . For example ACS specifications for NaHCO3 require that the concentration of iron be less than or equal to 0.001 w w. To verify that a production lot meets this standard the manufacturer performs a quantitative analysis reporting the result on the product s label. Because it is impractical to analyze the entire production lot its properties are estimated from a limited sampling. Several samples are collected and analyzed and the resulting mean X and standard deviation s are used to establish a confidence interval for the production lot s true mean p p X 7.1 Figure 7.1 Percent of overall variance s2 due to the method as a function of the relative magnitudes of the standard deviation of the method .