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Pacquette & Thompson:
J
ournal of
AOAC I
nternational
V
ol.
98, N
o.
6, 2015
1707
solutions at concentrations that are representative of nutritional
products in samples for ICP/MS analysis. The typical H
2
gas
mode for Se, and He gas mode for Cr and Mo, were used.
(f)
Accuracy
.—Accuracywas demonstratedby analyzing three
NIST SRMs on 2 independent days, measuring spike recoveries
in 10 nutritional products on 3 different days, and comparing
results for 10 nutritional products obtained by this method to
results obtained by other in-house validated ICP-atomic emission
spectrometry and atomic fluorescence spectrometry methods. The
spike levels of the analytes added to the products were between
50 and 200% of the analyte concentrations in each product.
(g)
Precision
.—Both within-day and between-day RSD
values were determined by analyzing two in-house laboratory
control samples. Within-day precision was determined by
analyzing the laboratory control samples in duplicate on each
day, and between-day precision was measured by using the mean
results of the duplicate samples analyzed on each day on 10
different days.
(h)
Ruggedness and robustness
.—To determine the
ruggedness of the method, laboratory control samples were
analyzed by two analysts on 10 different days. Also, NIST SRM
1849 was analyzed in triplicate with varying sample weights and
with different ISs.
(i)
Reproducibility
.—Eight laboratories completed a
multilaboratory testing protocol with this method on seven
samples submitted as blind duplicates (14 total samples analyzed
plus the SRM1849a control, which was not blinded). Represented
were four countries and five ICP/MS instrument models from
three major vendors. Results showed an average RSD
R
of 9.3%
for Cr, 5.3% for Mo, and 6.5% for Se, with an average HorRat
ratio of 0.35 across all three analytes and samples.
SLV Data
The SLV data were not published with the method above; they
will be briefly summarized here. Table 3 shows the intermediate
precision for each of the SPIFANmaterials across 8 different days
of results using two different analysts and two Agilent ICP/MS
units, a model 7500 and a model 7700. As noted previously,
the infant formulas contained no added Cr or Mo, so they were
mostly not included in the MLT. For levels above the PLOQ,
Table 4. SLV overspike recoveries (mean from 3 days, in
triplicate each day) from SPIFAN matrixes at 50–200% of
native levels
a
Product
Cr
RSD Mo RSD Se RSD
Infant powder milk
109 10.0 92.5 2.5 106 4.7
Infant powder soy
108 1.5 93.1 1.2 103 6.0
Infant powdered milk
partially hydrolyzed
90.1 1.1 95.2 5.7 99.5 2.3
Infant powdered soy
partially hydrolyzed
91.0 3.0 108 1.0 101 2.3
Adult powder low fat
101 4.8 95.4 8.1 99.3 3.3
Adult powdered milk
104 1.6 97.4 4.6 103 4.1
Child formula powdered 102 2.5 96.2 2.1 104 4.8
Infant elemental
powdered
102 8.8 96.9 2.7 106 4.6
Infant RTF milk
103 6.1 92.3 5.4 101 4.9
Adult RTF high protein 98.0 2.5 92.0 2.7 104 4.9
Adult RTF high fat
98.0 2.8 93.3 3.1 105 7.0
a
Native levels that are below the PLOQ and other concentrations can
be found in Table 3.
Table 5. SRM 1849a results during SLV (
n
= 16, two
analysts, two Agilent instruments) and collaborative
study (run once in duplicate as a known sample by each
laboratory, together with other blinded samples)
Cr
Mo
Se
Mean SLV, µg/kg,
n
= 16
105
166
82.7
RSD SLV, %
1.4
1.1
1.8
Certified interval, µg/kg
104.0–110.4 166.7–174.7 78.3–84.1
Mean MLT (not including
SLV), µg/kg
106
167
82.6
RSD MLT, % (eight labs)
2.4
2.8
1.5
Table 3. SLV intermediate precision data for Cr, Mo, and Se (from 8 independent days of testing in duplicate, 4 days per
each analyst using two different Agilent ICP/MS units)
a
Sample type
Cr Mean
RSD
IP
Mo Mean
RSD
IP
Se Mean
RSD
IP
Infant powder milk
4.9 (<PLOQ)
7.7
17.7 (<PLOQ)
8.2
25.6 (<PLOQ)
6.5
Infant powder soy
8.1 (<PLOQ)
5.2
36.5 (<PLOQ)
4.5
24.8 (<PLOQ)
3.3
Infant powdered milk partially hydrolyzed 2.4 (<PLOQ)
20
20.4 (<PLOQ)
4.0
26.6 (<PLOQ)
2.7
Infant powdered soy partially hydrolyzed 6.0 (<PLOQ)
9.2
33.0 (<PLOQ)
3.6
26.8 (<PLOQ)
4.1
Adult powder low fat
47.6
1.3
62.7
3.0
29.7
3.2
Adult powdered milk
15.8 (<PLOQ)
5.0
32.6 (<PLOQ)
4.9
24.3 (<PLOQ)
2.8
Child formula powdered
30.2
1.3
30.0
5.1
23.7
2.9
Infant elemental powdered
23.9
6.3
17.8
7.5
23.2
2.8
Infant RTF milk
7.6 (<PLOQ)
53
16.9 (<PLOQ)
10
29.6 (<PLOQ)
2.9
Adult RTF high protein
130
2.9
154
3.2
92.6
3.0
Adult RTF high fat
141
1.8
193
2.6
133
3.6
a
Units of concentration are µg/kg as RTF with powders reconstituted 11.1% (w/w). Note that several of the means are designated as <PLOQ and their
RSDs are correspondingly higher. PLOQ = practical limit of quantitation.
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