Page 196 - Modern Analytical Chemistry
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1400-CH07 9/8/99 4:02 PM Page 179
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Obtaining and Preparing
Samples for Analysis
When 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.
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