Interlaboratory study on quantitative methods of analysis of C10- C13 polychloro-n-alkanes

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Analytical Chemistry





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Seven laboratories participated in an international interlaboratory comparison exercise to compare the quantitative methods used for measuring C10-C13 polychloro-n-alkanes (PCAs). Participants were supplied with two solutions (PCA-1, PCA-70) containing PCAs of 'known' but unstated concentrations, and two real world samples (fish extracts FE1 and FE2) each consisting of a cleaned up extract (lipid-free) of a fish tissue known to contain PCAs. A well-characterized commercial formulation, PCA-60, of stated concentration was also supplied and was used as the external standard for the exercise. Participants having other commercially available C10-C13 PCA mixtures were encouraged to use them as external standards for the study, and the choice of the quantitative method employed was left to participants, though all were based on high-resolution gas chromatography with detection by electron capture negative ion mass spectrometry (plus electron capture detection in one case). The results of the study met with mixed success. For measurements on the PCA-1 sample, whose composition and gas chromatographic profile were quite different from the PCA-60 sample, the determined concentration was 99.3 ± 19.5 ng/μL (mean ± the standard deviation of the laboratory means); the true concentration of this mixture was 74 ng/μL. For the PCA-70 sample, which has a composition and gas chromatographic profile similar to that of PCA-60, the result was 297 ± 132 ng/μL, compared to the true concentration of i 18 ng/μL. It is still unclear why the larger discrepancy arises for the latter sample; this observation implies that different commercial formulations used as standards would provide quite different estimates of PCA concentrations. The interlaboratory precision for measurements on the FE1 sample (coefficient of variation (CV) of 27%) was better than that for the FE2 sample (CV of 47%). An explanation for the larger variation is that some of the quantitative procedures used in measuring PCA levels in the FE2 sample did not take into account the effects of coeluting interferences, which are observed at nominal mass spectral resolution, thus making some of the values too high.