Evaluation of performance characteristics of multistep analytical methods from collaborative study of linked samples

Following unexpectedly variable results from an international comparison study of the determination of selected polychlorinated biphenyl (PCB) congeners in shellfish tissue, a group of national metrology institutes collaboratively explored the analytical characteristics of their measurement systems using a designed study with four sample materials. This "Uncertainty Suite" consisted of a 10-congener mixture of PCBs in relatively nonvolatile isooctane, a 5-congener mixture in relatively volatile methylene chloride, a methylene chloride extract of freeze-dried mussel (Mytilus edulis) tissue, and the (homogenized) mussel tissue itself. These related-but-different samples presented the participants' measurement processes with a linked series of analytical challenges. Data evaluation tools were developed to combine and visualize measurement results for the different congeners of interest for each material and, exploiting the linkages among the samples, to help identify causes for observed changes in performance. In addition to characterizing individual measurement processes, (1) the limiting sources of measurement uncertainty were found to be chromatographic separation and signal quantification in a natural matrix, (2) the achievable among-participant total measurement uncertainty for PCB calibration solutions is approximately 1.9% over the mass fraction range from 40 to 500 ng/g, and (3) the achievable among-participant measurement precision for the determination of PCB congeners in mussel tissue at levels above 0.5 ng/g mass fraction is approximately 5.4%.