Drift plot, replicated

FIGURE 2.16. Drift plot ( ) replicated responses measured as a function of time. Design responses ( ) and corresponding corrected design responses ( ) are also shown. The horizontal line (-) represents the initial replicated response valne. 

Various calibration schemes similar to those given in Section 2.2.8 were simulated. The major differences were (1) the assumption of an additional 100% calibration sample after every fifth determination (including replications) to detect instrument drift, and (2) the cost structure outlined in Table 4.6, which is sununarized in Eq. (4.2) below. The results are depicted graphically in Figure 4.5, where the total cost per batch is plotted against the estimated confidence interval CI(X). This allows a compromise involving acceptable costs and error levels to be found. 

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

Apart from the standard Shewart charts, the analyst can also apply X-charts, on which the mean of several replicate measurements is plotted, or R-charts, where the difference between two replicate measurements is plotted. X- and R-charts give an indication of the reproducibility of the method. Drift in analytical procedure, for example, slows changes in the system caused by the aging of parts of instruments, decalibration in wavelength, or the aging of calibration stock solutions, can be detected early when a Cusum chart (cumulative sum) is applied. In Cusum charts, the analyst reports the cumulative sum of the differences between delivered and reference values. If this reference value is certified (CRM), the Cusum chart allows the accuracy of the determination to be monitored. 

The replicate measurements (at least two each time) are made over a prolonged period of time (at least 40 times), on a check-standard sample which must remain constant and homogeneous during the measurement cycle. Software packages are available to assist in the evaluation of results and in plotting the control charts, and any measurements outside the control limits are automatically highlighted. Visual inspection of the charts may reveal systematic trends on drifts. The variability is then analysed, its causes identified and brought under control and reduced. 

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