It is possible for the interference filters to go bad.
Filters function to block certain wavelengths, allow others to pass, and to transmit a specified % of incident light. If the filters have deteriorated significantly there is typically a gradual decrease in peak areas. Since the Lachat systems are calibrated frequently, the decline is not always readily apparent. If one is having difficulty with a method and it is determined that the areas obtained for standards are significantly less than the Lachat method support data, the filter is a possible cause. In some cases, if a user goes back and looks at calibration areas over the last year or several months they may see that the areas have gradually decreased over time. If another reason for smaller areas cannot be determined filter replacement should be considered.