SPSFAM Heavy Metals ERP Book

K ubachka et al .: J ournal of AOAC I nternational V ol . 100, N o . 4, 2017  1081

among those obtained for As(III) and As(V) and assumes that, at a minimum, either As(III) or As(V) must be detected at a level equal to or greater than the iAs LOD. The average method LODs and LOQs for iAs in RTD juices and juice concentrates obtained are shown in Table 2. Based on these results, the typical analytical limits listed in AOAC First Action Official Method 2016.04 are estimated as follows: method LOD = 0.25 µg/kg and LOQ = 2.0 µg/kg for iAs, DMA, and MMA in RTD juices; method LOD = 1.5 µg/kg and LOQ = 12 µg/kg for iAs, DMA, and MMA in juice concentrates. Laboratories completed this study by running three to five analytical batches. Calibration curves were generated for each of these batches over the working range. Typically, four to five calibration levels over the range of 0.5–20 ng/g were used; R 2 values reported were >0.995 in all but one batch for two analytes from one laboratory. Calibration check standards, typically a 2 or 4 ng/g mix standard, were analyzed by all eight laboratories periodically throughout each analytical batch. The measured solution concentrations were monitored and compared to their prepared concentrations (reported as percent recoveries). For a total of 124 check standard determinations, the averages ±1 SD (and ranges) were as follows: iAs, 102 ± 4% (88–111%); DMA, 101 ± 5% (88–113%); and MMA 102 ± 5% (85–113%). Retention times ( t R ) for check standards varied within an analytical batch for As(III), DMA, MMA, and As(V) by an average (and maximum) of 0.03 min (0.07 min), 0.04 min (0.13 min), 0.08 min (0.27 min), and 0.19 min (0.35 min), respectively (data not shown). Over time and multiple batches analyzed, MMA and As(V), in particular, will show larger variability with t R , generally shortening as columns foul. Six to 10 method blanks were analyzed by each laboratory over the course of this study ( n = 66); and none of the four analytes was detected above the calculated LODs in 65 of 66 method blanks. A small amount of As(V) attributed to an autosampler carryover issue was detected in one method blank. These results were expected based on the relatively simple sample preparation process and the ability to easily control contamination in those steps. However, during preliminary work before the validation exercise, some laboratories reportedAs(V) contamination in method blanks. This contamination was found to originate from the (NH 4 ) 2 HPO 4 used to prepare the mobile phase used. The contamination was apparent when the baseline of the m/z 75 trace would noticeably dip at the void volume of the separation, near AsB elution, and As(V) would be detected. This is surmised to occur when As(V) collected on the column head or within the injector and was released periodically, thus resulting in a small As(V) peak. A protocol was developed to determine whether the mobile phase salt being used contained an unacceptable level of arsenic. This is described in FirstAction Official Method 2016.04 [section I(d) (1)–(5) ]. For the sake of efficiency, the central laboratory tested multiple (NH 4 ) 2 HPO 4 sources and supplied approximately 40 g from an acceptable source to each laboratory along with the validation samples. Laboratories were also instructed to run method blank fortifications at two spike levels, including one level near each laboratory’s LOQ for RTD juices (approximately 0.4 ng/g in the analyzed solution, equivalent to 2 µg/kg in the sample) Calibration and Method Blanks

and another level equivalent to 25 µg/kg. Seven laboratories prepared low-level method blank fortifications, with five laboratories using spike levels of 2 µg/kg and the other two laboratories using levels of 5 and 10 µg/kg. Each laboratory prepared between three and six replicates for a total of 31 replicates. The average recoveries ±1 SD of the low-level- fortified method blanks were 100 ± 8, 97 ± 9, and 101 ± 8% for iAs, DMA, and MMA, respectively. A total of three individual fortification recoveries [one each for iAs (79%), DMA (69%), and MMA (64%)] were outside the 80–120% acceptable range set forth in the QC criteria for the method. Both of the samples that yielded recoveries <70% were prepared by the same laboratory. Six laboratories prepared 25 µg/kg method blank fortifications, and one laboratory prepared their high-level method blank fortification at a concentration of 10 µg/kg. Each laboratory prepared between three and eight replicates for a total of 30 replicates. The average recoveries ±1 SD of the high- level-fortified method blanks were 102 ± 5, 100 ± 6, and 101 ± 6% for iAs, DMA, and MMA, respectively, and no recoveries were outside the 80–120% acceptable range. Reported concentrations of iAs, DMA, and MMA from each laboratory are summarized in Figure 1 and Table 1. Values between the method LOD and LOQ are reported as “trace.” As discussed earlier, the results from one laboratory for DMA and MMA were rejected for a failure to verify DMA and MMA standard concentrations as directed in the method. In addition, one replicate of grape juice sample (GJ2) from one laboratory was rejected as an outlier by the Cochran test. The values for iAs and DMA in the rejected replicate were approximately 50% of the overall average values. Repeatability SD (s r ), reproducibility SD (s R ), RSD r , RSD R , and Horwitz ratio (HorRat) values were calculated for all average results found to be above the method LOD, including those that were considered trace levels, and are presented in Table 1. Eleven of the 13 samples had average iAs concentrations that were above the method LOQs; for RTD juices, these results covered the concentration range of approximately 5–25 µg/kg. For these 11 samples, the RSD r ranged from 2.1 to 5.2%, the RSD R ranged from 3.9 to 12%, and the HorRat values ranged from 0.13 to 0.38. The two samples that were found to have average iAs concentrations