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Lecture 6

CHMB16Fall2012 Lecture 6 Notes .docx

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Kagan Kerman

CHMB16Fall2012 Lecture 6: Sampling o Choosing a good representative sample is the underlying basis for any good analysis o Sampling: process of choosing a representative bulk sample from some total material (lot) o Sample preparation: process of transforming a bulk sample into a sample ready for analysis (a lab sample) o To ensure that the sample being analyzed is representative of the lot, a chemist must carefully select procedures for both sample collection and sample preparation o A method for sample collection should address the following questions: how will sample contamination be avoided? How many samples should be collected for analysis? How big should each sample be? Do samples need to be collected from more than one location? Statistics of Sampling o In any experiment random errors are always a factor o One source of these random errors is variations in the analytical procedures used o Another source is variations in the sampling o The size of these random errors can be represented by variance 2 o Variance = (standard deviation) o The total variance = analytical variance + sampling variance o Origin of Sampling Variance: 31" o what is the change that a random sample will have the same composition as the bulk sample? o Consider a sample that is a mixture of 2 types of particles A and B o If the mixtures contains nA particles of A and nB p
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