Development of an accreditation process for analytical methods to measure microplastics in drinking water for regulatory monitoring.

Development of a laboratory accreditation program to ensure competency of laboratories performing analytical measurements is a key step in adopting new analytical measurement methods for regulatory decision-making. Here, we describe California's three-part accreditation process for spectroscopically measuring microplastics in drinking water, and show how data from a multi-laboratory method comparison study informed development of accreditation programs for the resulting methods, which can inform analogous future work for other analytes. The first part is periodic performance evaluation (PE) samples, in which laboratories are provided blind samples of known composition to quantify within acceptable performance limits. The second is inspection, or audit, assessing whether the laboratory has the proper equipment to conduct the work and whether it is correctly employing proper procedures. The third is implementation of a quality management system providing documentation that protocols demonstrated during inspection are continuously maintained. These fell into three broad categories: instrument maintenance; laboratory cleanliness, especially important for microplastics and one that must be accompanied by a blanks measurement and correction process; and training so samples are being processed by qualified analysts. An intercomparison exercise among 22 laboratories was necessary to determine what parameter permutations were important for PE samples, and expected accuracy from competent laboratories. The recommended PE sample composition was two size categories (20-50um and 500um-1mm), two polymer types, and two morphologies (fibers and non-fibers). Discussions among intercomparison exercise participants were key in determining the factors that most contributed to laboratory variability, and the focus for both on-site inspections and quality management systems.