A Look at Matrix Effects

Oral Presentation

Prepared by M. Delaney, C. Blodget
Massachusetts Water Resources Authority (MWRA), 190 Tafts Ave, Winthrop, MA, 02152, United States

Contact Information: mike.delaney@mwra.com; 617-660-7801


If you look closely enough, all quantitative analytical chemistry methods probably have matrix effects (aka matrix interferences) to some extent. However, the natural tendency for a testing laboratory is to “blame” the sample matrix when matrix spike/matrix spike duplicate (MS/MSD) recoveries are outside of limits and move on to the next sample (“It’s a matrix effect!”). However, the draft versions of EPA methods 608, 624, and 625 proposed by EPA in the 2015 Methods Update Rule (MUR) upped the ante on MS/MSD recoveries because they state that results for samples with out of specification MS/MSD recoveries or relative percent differences "may not be reported or used for permitting or regulatory compliance purposes." These methods do acknowledge that for multi-analyte methods, QC indicators will occasionally exceed statistically derived control limits just by chance, but even so, the lab is obliged to re-test the sample and qualify the results.

When the 2015 MUR was proposed and NELAC was working on changing its LOD and LOQ sections, we took at look at how we use MDLs and RLs. We generally quantitate down to our RLs and everything below that is just regarded as a non-detect. However, none of this involves sample matrix and, therefore, doesn’t explicitly take into account the potential for false-positives from matrix interferences.

The environmental analytical chemistry literature on matrix interferences/matrix effects is small and the topic doesn’t appear to have been addressed in a fundamental way. This presentation will examine the possible types of matrix interferences/matrix effects. It will then examine method performance using routine QC indicators from our laboratory to gauge which methods appear to have a tendency for matrix interference problems.