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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.
Candidates for 2016 Method of the Year
83