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Thompson et al.:

J

ournal of

AOAC I

nternational

V

ol.

98, N

o.

6, 2015 

1719

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).

MLT Study and ICP-AES Comparative Data

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/

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

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

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

6

6

6

6

6

7

7

7

7

Adult milk powder

6.6

6.9

7.6

3.9

5.1

3.6

4.7

3.0

5.8

Infant powder hydrolyzed milk

6.8

6.8

8.1

a

3.3

5.2

3.2

3.4

2.1

1.9

Adult powder low fat

6.4

6.5

8.3

a

3.6

5.7

3.0

4.8

2.4

5.3

Child powder

6.6

7.3

7.3

4.2

5.1

3.4

4.6

2.6

2.4

Infant elemental powder

6.2

6.5

4.4

4.0

5.5

3.5

4.8

2.3

5.9

Average of five matrixes

6.5

6.8

7.1

3.8

5.3

3.3

4.5

2.5

4.3

Adult RTF high protein

7.6

8.1

14.0

a

4.8

33.8

a

25.5

a

11.9

a

2.8

14.2

a

Adult RTF high fat

8.5

a

7.9

10.9

a

5.1

48.1

a

26.1

a

8.9

3.4

9.7

SRM 1849a

2.4

3.0

1.9

1.7

1.5

4.0

3.8

2.0

2.0

SMPR required RSD

R

8

10

8

8

8

10

10

10

10

a

 Would fail the SMPR criterion for reproducibility.

78