Application of capability indices and control charts in the analytical method control strategy

In this study, we assessed the usefulness of control charts in combination with the process capability indices, Cpm and Cpk, in the control strategy of an analytical method. The traditional X‐chart and moving range chart were used to monitor the analytical method over a 2‐year period. The results confirmed that the analytical method is in‐control and stable. Different criteria were used to establish the specifications limits (i.e. analyst requirements) for fixed method performance (i.e. method requirements). If the specification limits and control limits are equal in breadth, the method can be considered “capable” (Cpm = 1), but it does not satisfy the minimum method capability requirements proposed by Pearn and Shu (2003). Similar results were obtained using the Cpk index. The method capability was also assessed as a function of method performance for fixed analyst requirements. The results indicate that the method does not meet the requirements of the analytical target approach. A real‐example data of a SEC with light‐scattering detection method was used as a model whereas previously published data were used to illustrate the applicability of the proposed approach.