6. AOACSPIFANMethods-2018Awards

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1704 Pacquette & Thompson: J ournal of AOAC I nternational V ol. 98, N o. 6, 2015 factor with which to work. The Study Director allowed this small variation to the method as it will certainly not affect the validity of results if the correct dilution factor is used.

but analysis of these MLT data indicated too many rejections at that level, and so a 7% RSD requirement is now set for Se. The requirement for Mo is still 5% duplicate precision. Table 2 shows the number of failures in analyzing the 14 MLT samples using the original criteria (i.e., 5% RSD duplicate precision for Se). There are an inordinate number of failures (4/14 or almost 30%) for Laboratory 1 Cr results; this is the laboratory whose data were later entirely rejected from the study. However, it appears that only Laboratory 1 had this problem with Cr determination. In contrast, five laboratories had more than one sample rejected for Se when the duplicate RSD criterion were set to 5%. If the duplicate precision criterion was set to 7% RSD, only eight total failures occurred instead of the 19 shown in Table 2, out of a total of nine laboratories × 15 samples = 135 determinations, or about a 6% rejection rate. This may be higher than the <1% rejection rate for Mo because Se concentrations are routinely low, about 2–3x above the PLOQ in all the samples tested. Also, the laboratory that had the most Se data rejected, Laboratory 9, was also the one that had a compromised PLOQ as shown in Table 1. This underscores the importance of having optimal sensitivity for Se analyses in infant/pediatric formulas. Generally, we have observed that ICP/MS units that are not fitted with hydrogen gas for collisional reaction/reduction of argon interference cannot readily obtain the 0.2 ppb PLOQ in solution. As can be seen, setting a single criterion for duplicate precision to cover all concentration levels encountered and for all matrixes is difficult, but this does not mean it should not be done. This is perhaps the best way to avoid out-of-specification results due to systematic errors and rejecting the data before any unnecessary retesting or regulatory action begins, and this kind of suitability criterion should be strongly considered for any dispute resolution method, even chromatography-based methods in which it may take much more time to get the duplicate result. Upon completion of the sample analyses, participating laboratories were asked to send all of their data to the Study Director. An Excel spreadsheet was supplied by the Director, with a template for adding the sample weights, duplicate results, and spaces for all the calibration and QC results. Participants were also asked to report any deviations to the method and any relevant comments based on their experiences with the method. All data were statistically analyzed in a spreadsheet (5) using AOAC INTERNATIONAL guidelines to determine overall mean, repeatability SD (s r ), RSD r , reproducibility SD (s R ), RSD R , and Horwitz ratio (HorRat). Cochran’s ( P = 0.025, one-tail) and Grubbs’ (single and double, P = 0.025, two-tail) tests were used to determine statistical outliers. SPIFAN SMPRs for repeatability were ≤5% RSD and requirements for reproducibility were ≤15% RSD in products above a concentration of 10 ng/g Se and 20 ng/g Cr/Mo on an RTF basis.

Participants were reminded more than once that the written (now official) method mandates the use of several QC/system suitability solutions including a blank check (must be less than the PLOQ), a calibration verification standard (must be within 4% of its nominal concentration before and after samples are run), and a control sample (the concurrent analysis of SRM 1849a). The criterion for results was not explicitly stated because these laboratories do not have working control charts for such; however, the SRMexpected results were known from the prework phase, and the laboratories could see the SRM results concurrent with their samples. No laboratories indicated they discarded any data because of these QC solutions failing. A final key suitability requirement was the analysis of duplicates that had to agree within 10% for Cr, 7% for Se, and 5% for Mo. These duplicates are not to be confused with the blinded duplicates supplied in the collaborative study. The duplicate requirement is present not so much to improve the confidence interval by using the mean of two results (although that can be useful for concentrations near the PLOQ), but rather to indicate the presence of substantial indeterminate errors before the data are allowed to enter the pool of multilaboratory data. A 10% agreement between duplicates is a common, if somewhat arbitrary, criterion used in many GB (China) official methods, and others. In this case, the SLV and MLT data indicate that a 10% criterion is well above the excellent repeatability or intermediate precision expected of this method ( see SLV data later in this report), but this level is maintained for Cr because small levels of Cr contamination were sometimes unavoidable and usually irrelevant to results since most adult products had relatively high levels of Cr (infant formulas are not fortified with Cr or Mo). With the inclusion of the other QC tests in this method, especially the use of the control sample, the possibilities of other sources of indeterminate errors are small [e.g., pipetting the wrong amount of internal standard (IS) or a poorly made set of standards], and so Se and Mo have duplicate RSD requirements closer to 3× the typical short term precision of about 1.5–2.0%. Indeed, the expected duplicate precision for Se was originally set at 5% RSD for this method, which was optimal for the authors’ laboratories, Table 2. MLT duplicate samples failing to meet the original duplicate precision criterion of 10% RSD for Cr and 5% RSD for Se and Mo Lab No. Cr failures No. Mo failures No. Se failures a 1 4 0 1 (0) 2 0 0 0 3 0 0 3 (1) 4 1 0 2 (0) 5 1 1 4 (2)

6 7 8 9

Method

The Final Action method, as now published (6) and given below, is the updated version the participants used for this study. In particular, Ge was substituted as the IS for Ni, Cr, and Mo, and there is an option to analyze more elements concurrently. The QC/system suitability was more explicitly stated, and the revised 7% duplicate criterion for Se added.

0 0 1 0

0 0 0 0

0

5 (4) 4 (1)

10 11

0 a  Failures under 7% RSD criterion shown for Se in parenthesis.