Statistical method comparison: short- and long-column liquid chromatography assays of ketoconazole and formaldehyde in shampoo

When an analytical method is modified or a new one is developed in order to attain, for instance, shorter analysis times, lower cost or more convenience of use, the new method should be validated before being accepted for routine use. It is also necessary to demonstrate that the new or alternative method does not have worse performance characteristics than the existing or reference method. Therefore, a comparison of the performances between alternative and reference method should be made. In this comparison, precision and bias are the evaluated performance parameters. With the current tendency to use more frequently short-column methods in HPLC, such method comparisons will become increasingly important. In this paper, two alternative short column HPLC methods to assay ketoconazole and formaldehyde in an anti-dandruff shampoo are compared with the reference method on a long column. The assays were initially developed on such a long column (25 cm). The comparison procedure consists of data visualisation and statistical analysis. It was found that, in the linear concentration ranges of ketoconazole (from 0.05 to 0.15 mg/ml) and formaldehyde (from 3.5×10^-5 to 3.5×10^-4% (m/v)), the alternative methods do not possess neither a statistically worse precision nor an unacceptable bias compared to the reference assays on the long column. Therefore, they can be used as alternatives to the conventional method.