5. Control cards

Control charts are tools used to check the stability of an analytical method over time. They are therefore not validation tools, but quality control tools. They are conceptually simple: a standard material, whose average content has been defined, is analyzed regularly and the responses obtained are plotted in chronological order on a graph. This material may be a CRM, but for economic reasons this is rarely the case. Instead, traceability to a CRM is ensured when the reference value of the standard material is estimated. Then, if the map shows that there are variations, other than those linked to chance, around the reference value, we can assume that the measurement system is disturbed. To make this decision easier, a graphical method is used, with control limits drawn on either side of the expected value. Responses must remain within these limits. In addition, we define the rules for deciding...