Use of experimental designs to evaluate the ruggedness of a quantitative HPLC method

The ruggedness of an analytical method is defined by its capacity to remain unaffected (e.g. to maintain its performances) when small changes in experimental conditions happen. It can be studied by applying changes within the same order of magnitude of those which could occur when running routinely the method. In the case of a L.C. method, given the high number of factors (temperature, flow rare, mobile phase composition, detection wavelength, injected sample quantity...) susceptible to vary and the possible interactions between them. varying each factor independently would be awkward. The experimental design method enables to model the effects and interactions of a high number of factors varying simultaneously with a limited number of runs. After having estimated the order of magnitude of the potential changes in the factors level, a central composite design, allowing independent estimations of model coefficients, must be carried out around the working point defined during the method optimisation. Such a design allows the use of a second order model and gives the possibility to have an extremum inside the experimental domain. So it is possible to predict the consequences of small variations in operating conditions on the responses. A global function is proposed to account for ruggedness in terms of capacity for the method to deliver steady and reproducible assays for a product, which represents the aim of quantitative analysis. A statistical treatment of the design results reveals that the percentage of acetonitrile in the mobile phase, the column temperature and the pH of the mobile phase are the factors that have a significant influence on the method ruggedness and are critical. Hence their identification leads to recommendations in the operating protocol; the ruggedness of the analytical method will be considered as maintained only when the tolerances defined for those parameters judged critical are respected.