Interlaboratory study on quantitative methods of analysis of C-10-C-13 polychloro-n-alkanes

Seven laboratories participated in an international interlaboratory compari son exercise to compare the quantitative methods used for measuring C-10-C- 13 polychloro-n-alkanes (PCAs), Participants were supplied with two solutio ns (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. P articipants having other commercially available C10-C13 PCA mixtures were e ncouraged to use them as external standards for the study, and the choice o f the quantitative method employed was left to participants, though all wer e based on high-resolution gas chromatography with detection by electron ca pture negative ion mass spectrometry (plus electron capture detection in on e 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 9 9.3 +/- 19.5 ng/mu L (mean +/- the standard deviation of the laboratory mea ns); the true concentration of this mixture was 74 ng/mu L. 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/mu L, compared to the true co ncentration of 118 ng/mu L. It is still unclear why the larger discrepancy arises for the latter sample; this observation implies that different comme rcial formulations used as standards would provide quite different estimate s 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 th e FE2 sample did not take into account the effects of coeluting interferenc es, which are observed at nominal mass spectral resolution, thus making som e of the values too high.