6. AOACSPIFANMethods-2018Awards

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Thompson et al.: J ournal of AOAC I nternational V ol. 98, N o. 6, 2015  1719

Table 7. Straight % RSDs of six or seven laboratory results from the MLT (no outliers removed; includes SLV means)

Na

Mg

P

K

Ca

Mn

Fe

Cu

Zn

No. of laboratories Adult milk powder

6

6

6

6

6

7

7

7

7

6.6 6.8 6.4 6.6 6.2 6.5 7.6

6.9 6.8 6.5 7.3 6.5 6.8 8.1 7.9 3.0

7.6

3.9 3.3 3.6 4.2 4.0 3.8 4.8 5.1 1.7

5.1 5.2 5.7 5.1 5.5 5.3

3.6 3.2 3.0 3.4 3.5 3.3

4.7 3.4 4.8 4.6 4.8 4.5

3.0 2.1 2.4 2.6 2.3 2.5 2.8 3.4 2.0

5.8 1.9 5.3 2.4 5.9 4.3

8.1 a 8.3 a

Infant powder hydrolyzed milk

Adult powder low fat

Child powder

7.3 4.4 7.1

Infant elemental powder Average of five matrixes Adult RTF high protein

14.0 a 10.9 a

33.8 a 48.1 a

25.5 a 26.1 a

11.9 a

14.2 a

8.5 a

Adult RTF high fat

8.9 3.8

9.7 2.0

SRM 1849a

2.4

1.9

1.5

4.0

8

10

8

8

8

10

10

10

10

SMPR required RSD R

a  Would fail the SMPR criterion for reproducibility.

matrix because they were provided blind duplicates of each material. The data from the SLV described above provided another point and were averaged in at equal weighting with the other laboratories’ data, so that data were collected from 6 to 7 different laboratories in total. Table 7 shows the straight RSDs of the mean results from either 11 or 13 results for each matrix (five laboratories × 2 + SLV, or six laboratories × 2 + SLV). Given the unequal weighting of the source data, the borderline number of laboratories participating, and the fact that no outliers were removed (other than those from failing system suitability), these RSDs are not exactly the reproducibility parameter (RSD R ) but should be a very good estimation of it. The RSDs in Table 7 were very consistent except for the Adult RTF products, which had many disparate results. It is widely believed that these two RTFs were too far past the end of shelf life and were no longer viable to test. With removal of these two products, the RSDs in Table 7 all pass the required reproducibility of the SMPR shown at the bottom of the table with the exception of P, for which two product matrixes were just above the required 8.0% RSD. It can be hypothesized that the RSDs for the low mass, high concentration elements are a little higher than for the trace elements at higher masses (on the right side of Table 7) because of slight differences in how these instruments handled collision/reaction interference removal and how well they performed P/A crossover calibrations. There were four different models of ICP/MS instruments contributing to the data in Table 7: an Agilent

before taking the ( n = 3) statistics shown in Table 6. All elements in all matrixes had average spike recoveries in the 90–110% range (with rounding), and so the SMPR was met for recovery. Again, the method was not challenged for the low levels of Cu, Fe, or Mn in this regard. Table 5 shows the accuracy of average values from nine determinations for each element in SRM 1849a. Accuracies ranged from 96.2% (Mo) to 107.7% (P), in agreement with the spike recovery results. Only Ca, P, Cu, and Mo produced results outside of the certified range, but the results were consistent with the MLT results from other laboratories and with ICP-AES results ( see below). It should be noted that during these studies Ni was shown to be an acceptable alternative to Ge as an ISTD (data not shown), but due to the significant concentration of Ni in cocoa products, Ge was chosen as the ISTD for the method (except for the use of Te for Se determinations in the H 2 gas mode). The same laboratories that participated in the MLT study of Cr, Mo, and Se (OMA 2011.19 ; 6) were asked to provide data for the other nine elements of this present study. Five laboratories provided results for Na, Mg, P, K, and Ca, while six laboratories provided results for Fe, Zn, Cu, and Mn. These laboratories provided two results/ MLT Study and ICP-AES Comparative Data

Table 8. Percentage difference of six or seven MLT laboratory mean relative to Abbott 6-day SLV using microwave digestion-ICP-AES Product type Na Mg P K Ca Mn Fe Cu Zn SRM 1849a –0.4 –1.4 2.8 0.3 –1.6 –0.8 1.1 0.1 2.9 Adult milk protein powder 5.5 2.8 3.8 2.7 4.0 4.5 7.2 3.9 8.0 Infant powder hydrolyzed milk 4.4 2.6 2.0 0.7 1.8 6.6 5.8 3.5 4.7 Adult powder low fat 2.3 0.4 1.5 –0.5 1.4 0.5 3.8 0.5 3.8 Child powder 4.7 0.5 5.2 0.8 2.6 3.1 7.0 2.5 5.7 Infant elemental powder 6.0 4.2 6.0 3.0 4.7 4.5 6.5 5.6 5.9 Adult RTF high protein 5.2 2.4 –0.5 1.1 –7.0 –13.8 3.6 –1.7 –2.6 Adult RTF high fat 4.6 1.3 –11.5 1.2 –34.7 –24.5 13.0 0.5 0.4