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1708 Pacquette & Thompson: J ournal of AOAC I nternational V ol. 98, N o. 6, 2015

, and HorRat values for 2011.19 collaborative study a

Table 6. RSD r

, RSD R

Cr, µg/kg

RSD r

,

RSD R

, % HorRat

Mo, µg/kg

RSD r, %

RSD R,

Se, µg/kg

RSD r

,

RSD R

, % HorRat

Matrix

RTF Labs

%

RTF Labs

% HorRat

RTF Labs

%

20

8 3.3 6.7 0.33

27

8 2.4 2.5 0.13

Infant powdered milk partially hydrolyzed Adult powder low fat Adult powdered milk Child formula powdered

48 16 30

8 4.7 7.1 0.39 7 3.4 12.1 0.57 7 5.5 9.2 0.48 7 3.8 13.4 0.67

63 33 30 18

8 1.6 3.1 0.18 7 1.0 7.9 0.42 8 3.3 4.6 0.24 8 1.7 7.9 0.38 7 1.0 3.0 0.14

30 24 24 23 93

8 5.9 7.2 0.37 8 6.1 6.1 0.31 8 3.8 7.3 0.37 8 6.4 9.3 0.46 8 2.3 8.1 0.36 8 4.7 5.0 0.23

Infant elemental powdered 24

Adult RTF high protein

130 8 7.0 8.1 0.37 140 8 2.1 5.8 0.27

150 190

Adult RTF high fat

8 1.2 3.8 0.19 133

Average 4.5 6.5 0.32 a  RSDs are shown for means that are at or slightly below PLOQ of 20 µg/kg RTF for Cr and Mo because they are indicative of method performance. No results are above the SMPR required 15% RSD R . For the five cases in which the number of participating laboratories is listed as n = 7, the eighth laboratory’s data could be included and still meet SMPR reproducibility requirements ( see footnotes for Tables 7–9). 4.4 9.3 0.46 1.9 5.3 0.27

intermediate precision was mostly in the range of 2–5% RSD, with the highest at 6.5% RSD. These results are consistent with the subsequent collaborative study, for which reproducibility was about 2–3% higher than the intermediate precision. Repeatability was not probed extensively in the SLV because the short-term precision is very good and the method, after all, does require the duplicate precision to be below the 5% RSD required by the SMPR. Accuracy was checked via overspike recoveries in the SLV. Table 4 shows the mean recovery of triplicate overspikes on each of 3 days. Spike levels were at 50–200% of the native levels. All recoveries were within 90–110%, meeting the SMPR.

Table 5 shows the SRM 1849a results during the SLV. This SRM was tested 16 times with excellent precision. The mean results for Cr and Se were within the certified range, whereas Mo was just barely low. The subsequent collaborative study means (one analysis in duplicate from each of the eight laboratories using five different ICP/MS instrument models) were almost identical to those from the Abbott SLV, including the Mo value obtained. Data for linearity and LOQ were obtained during the SLV, but these figures of merit have to be proven for a given instrument setup/model in the same way that the participating laboratories did the prework for the MLT.

Table 7. Collaborative study individual results for Cr (mg/kg, as is) a

Adult milk powdered

Infant powdered hydrolyzed milk

Adult powdered low fat

Infant elemental powdered

Adult RTF high protein

Adult RTF high fat

Lab No.

Child powder

1 a

0.025 0.018

0.121 b

0.237 c

0.254 c

0.220 d

0.150 d

0.143 0.126 0.112 e

0.108 e

2 3 4 5 8 9

0.130 0.145

0.154 0.156

0.250 0.128 0.129 0.130 0.135

0.121 0.124

results are the blind duplicates tested during the study. b  Rejected by Cochran’s test; if not excluded RSD R is 12.8%. c  Rejected by Cochran’s test; if not excluded RSD R is 9.3%. d  Rejected by Cochran’s test; if not excluded RSD R sentative data), RSD R tative data), RSD R

rises from 12.1 to 22.4%; if just the 0.237 value is rejected (so that eight laboratories have repre-

rises from 9.2 to 11.7%; if just the 0.371 value is rejected (so that eight laboratories have represen-

rises from 13.4 to 15.0%; if just the 0.150 value is rejected (so that eight laboratories have repre-

sentative data), RSD R is 12.7%. e  Although rejected by Single Grubbs’ test these data points were kept in the final statistical analysis, as the data appear extraordinarily tight; if ex- cluded, the RSD R falls to 2.9% and the HorRat to 0.13 for this product.

f  Rejected, 23% duplicate RSD; no retest result supplied. g  Rejected, 13% duplicate precision; no retest result supplied. h  Rejected, 15% duplicate RSD; no retest result supplied.