When Matrix Matters: A Case Study in Dirty Water and Clean Method Detection Limits

Regulatory fines, unwarranted public scrutiny, and needless corrective actions can occur when laboratory method detection limits (MDLs) and quantitation limits are applied to samples with significant matrix. This is because the standard MDL procedure is based on laboratory reagents only and does not include the effects of matrix. At a federal level, there is some recognition of this problem, and the 40 CFR MDL includes a matrix-based procedure At the state and local level, however, the lack of federal guidance regarding the appropriate application of a matrix-based MDL results in confusion and uncertainty about how to consider the impact of a sample matrix to obtain technically defensible monitoring data to demonstrate regulatory compliance. Problems ensue when laboratories indicate that they can meet the compliance limits (or agencies use stated or published MDLs and RLs in setting permit limits) without regard for the matrix. When laboratories report to levels based on matrix-free MDLs in samples with elevated matrix, false positive, false negative, and biased results are common, potentially including dubious results that are above permit compliance limits. The laboratory generally has some knowledge that the matrix is affecting results, but it does not know to what degree and is not empowered by the regulatory authorities to technically justify a change in reporting levels. Additional guidance is needed as to how agencies and laboratories should support matrix-based MDLs, reporting limits, and defensible permit limits. Doing so ensures that local regulators can suitably protect human health and the environment without being arbitrary or capricious with regard to regulated parties. The authors present a case study of analysis of polychlorinated biphenyl Aroclor 1242 in a challenging water matrix using Method 608 to comply with a low wastewater discharge limit.