Statistical intervals to validate an autoanalyzer for monitoring the exhaustion of alkaline degreasing baths

We describe how to use the statistical intervals for validating a qualitative method for determining the alkaline degreasing baths exhaustion. A homemade autoanalyzer based on flow-injection-evaporative light-scattering detector (FI-ELSD) coupling measures two instrumental responses related to the contents of surfactant and mineral oil. These two responses are necessary to decide whether the degreasing bath is exhausted. The instrumental responses ri are compared to their corresponding decision values i.e. cut-off response (rcut-off) and screening response (rscreening). These decision values are calculated by defining the one-side prediction bound around the specification limit (SL) of both analytes. The prediction bound of each analyte must be defined differently according to their corresponding specification limit. Performance parameters, such as sensitivity, specificity, false response rates and the unreliability region, are established. The performance of this qualitative method of analysis is checked by analyzing a set of 10 real samples. Our results show that the method is accurate as far as mineral oil content is concerned.