As we noted in Chapter 1, each area of chemistry brings a unique perspective to the broader discipline of chemistry. For analytical chemistry this perspective is as an approach to solving problem, one representation of which is shown in Figure 15.1.
Figure 15.1 Flow diagram showing one view of the analytical approach to solving problems. This diagram is modified after Atkinson, G. F. J. Chem. Educ. 1982, 59, 201–202.
If you examine the procedure for a standard method it appears, it often seems that its development was a straightforward process of moving from a problem to a solution. Unfortunately—or, perhaps, fortunately for those who consider themselves to be analytical chemists!—developing a standard method is seldom routine. Even a well-established standard method, carefully followed, can yield poor data.
An important feature of the analytical approach outlined in Figure 15.1 is the feedback loop involving steps 2, 3, and 4, in which the outcome of one step may lead us to reevaluate the other steps. For example, after standardizing a spectrophotometric method for the analysis of iron (step 3), we may find that its sensitivity does not meet the original design criteria (step 2). In response, we might choose a different method, change the original design criteria, or improve the sensitivity.
The feedback loop in Figure 15.1 is maintained by a quality assurance program, whose objective is to control systematic and random sources of error.1 The underlying assumption of a quality assurance program is that results obtained when an analysis is under statistical control are free of bias and are characterized by well-defined confidence intervals. When used properly, a quality assurance program identifies the practices necessary to bring a system into statistical control, allows us to determine if the system remains in statistical control, and suggests a course of corrective action if the system falls out of statistical control.
An analysis is in a state of statistical control when it is reproducible and free from bias.
The focus of this chapter is on the two principal components of a quality assurance program: quality control and quality assessment. In addition, considerable attention is given to the use of control charts for routinely monitoring the quality of analytical data.