SPSFAM Heavy Metals ERP Book

1084 K ubachka et al .: J ournal of AOAC I nternational V ol . 100, N o . 4, 2017 showed the largest time shift [As(III) and DMA changes were insignificant, data not shown]. The maximum differences observed within a batch were 0.9 min for MMA and 0.6 min for As(V). These shifts were most likely reflective of the pH difference between the standard and sample solutions. The MMA deviation was more pronounced because the pKa 2 of MMA is 8.7 (20), near the pH of the mobile phase (8.2–8.3).

Analysis of Certified Reference Materials

It is unfortunate that Certified Reference Materials for arsenic species in fruit juice were not available at the time of this study. NIST SRM 1643e Trace Elements in Water, which is certified for total arsenic concentration at 58.98 ± 0.70 µg/kg, was analyzed by seven laboratories. In addition, two laboratories analyzed NIST SRM 1640a Trace Elements in Natural Water, which is certified for total arsenic concentration at 8.010 ± 0.067 µg/kg. Each laboratory analyzed a minimum of one preparation of SRM with each batch of samples analyzed. One concern regarding the use of either of these reference materials was the presence of nitric acid (5%, v/v) in the standards and its potential effect on the chromatography. Dilution of NIST SRM 1643e using water by a factor of 7.5 results in a nitric acid concentration of <1%, which did not significantly affect the chromatography. Smaller dilutions resulting in higher concentrations of nitric acid noticeably decreased the t R of the As(V) peak (data not shown). Results for the analysis of reference materials are shown in Table 5. The overall average concentration of iAs determined in SRM 1643e was 56.7 ± 5.1 µg/kg ( n = 33), which accounts for 96% of the certified value for total arsenic. For both MMA and DMA, 32 of 33 replicates were found to be

Mass Balance

Total arsenic values of all 13 juice samples were determined by two participating laboratories using the FDA method EAM 4.7 (21). These values were used to calculate the respective mass balances between the sum of the arsenic species determined and the total arsenic determined in the samples. Good mass balance indicates that the majority of the total arsenic in the samples is accounted for during the speciation analysis. Total arsenic values were not provided to the laboratories before starting the exercise. As shown in Table 4, the mass balances obtained for all 13 samples were within the method’s acceptable range of 65–115%. Two RTD juice samples with total arsenic ≤10 µg/kg and two apple juice concentrate samples with total arsenic <15 µg/kg had mass balances <70%, whereas all other samples had mass balances ranging from 84 to 110%. The speciation results for the two juice concentrate samples with lower mass balances were

Table 4. Summary of mass balance data for RTD apple, grape, and pear juice and apple juice concentrate samples

Sum of As species concn, µg/kg a

Analyte added, µg/kg

Total As value, µg/kg b

Mass balance, %

Sample

NA c

AJ1 AJ2

11.5 ± 0.8 47.3 ± 4.4

13.4 ± 1.9 51.9 ± 5.5

86 91

25 As(V), 25 MMA

AJ3 GJ1 GJ2 GJ3 GJ4 PJ1 PJ2 PJ3

NA NA

6.4 ± 0.6 6.9 ± 0.5 43.2 ± 1.6 13.5 ± 0.8 33.1 ± 1.7 33.9 ± 2.0 15.1 ± 1.2 5.8 ± 0.8 87.0 ± 5.2 9.9 ± 3.3 4.9 ± 3.2

7.6 ± 0.88 10.2 ± 1.7 46.9 ± 2.6 12.3 ± 2.5 37.3 ± 3.2 34.1 ± 2.9 16.6 ± 0.6 8.8 ± 0.28 97.5 ± 3.5 14.7 ± 2.3 7.2 ± 1.68

84 68 92

30 DMA

NA

110

10 As(V), 10 MMA 30 MMA 10 As(III)

89

99 91 66 89 67

NA NA NA NA

AJC1 AJC2 AJC3

Acknowledgments

68 a  Sum of As species = iAs + DMA + MMA; mean ± 1 σ with seven laboratories reporting ( n = 21). b  Total As value from ICP-MS analysis by two laboratories; mean ± 1 σ. c  NA =Not applicable.

We thank Kathleen Rudokas, Michael Seibert, and Nohora Shockey (U.S. Food and Drug Administration, Forensic Chemistry Center) for their roles in this study.