Comparative study of different approaches to the determination of robustness for a sensitive-stacking capillary electrophoresis method.: Estimation of system suitability test limits from the robustness test

A robustness study for a sensitive-stacking capillary electrophoresis method based on "acetonitrile-stacking" was carried out. Ten variables (pH, acetonitrile and triethanolamine in the buffer, injection time, injection pressure, acetonitrile and NaCl in the sample, capillary and tray temperature and separation voltage), whose levels were varied by 10% around the nominal level, were examined by a Plackett-Burman design (two-level design). The effects on corrected peak area and resolution (responses) were calculated and interpreted using three statistical approaches: dummy variables, distribution effects (Dong's algorithm) and calibration curve. Dong's method was found to be the most suitable to evaluate the robustness, since it considers qualitative (resolution) and quantitative (corrected peak area) responses and does not need a minimum number of dummy variables in the experimental design. From these studies, we can deduce that the first four variables were significant at 10% around the nominal level, and therefore a new design was made with those four variables at 5% nominal level. Then, only two variables proved to be significant for the resolution between some peaks, so the system suitability test limits were defined for these resolutions.