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Thompson et al.:
J
ournal of
AOAC I
nternational
V
ol.
98, N
o.
6, 2015
Specificity and linearity studies were conducted with pure
analyte (no matrix). For the specificity studies, individual
standards of each analyte were prepared at 50 mg/L. For
linearity, standards were prepared at nine concentrations of each
analyte, spanning a range from 50% of the lowest calibration
standard to 50% above the highest calibration standard. For
precision studies, the 11 SPIFAN matrixes (infant powder, milk;
infant powder, soy; infant powder, milk partially hydrolyzed;
infant powder, soy partially hydrolyzed; adult powder, low
fat; adult powder, milk; pediatric formula powder; infant
elemental powder; infant RTF, milk; adult RTF, high protein;
and adult RTF, high fat) were tested for all 12 elements on
8 days in duplicate each day, by two analysts using two Agilent
ICP/MS units. For accuracy/recovery studies, each SPIFAN
matrix was spiked with various concentrations of each analyte
and analyzed by the method in triplicate over 3 days. Finally,
Standard Reference Material (SRM) 1849a [National Institute
of Standards and Technologies (NIST), Gaitherburg, MD] was
used to verify accuracy. The SRM is a milk-based hybrid infant/
adult nutritional powder with certified values for Ca, Cu, Cr, Fe,
Mg, Mn, Mo, P, K, Se, Na, and Zn and was included as a QC
sample (QCS) in each run of the method.
AOAC Official Method 2015.06
Minerals and Trace Elements in Infant Formula
and Adult/Pediatric Nutritional Formula
ICP/MS Method
First Action 2015
(Applicable for determination of Na, Mg, P, K, Ca, Cr, Mn,
Fe, Cu, Zn, Se, and Mo in infant formula and adult/pediatric
nutritional formula.)
Caution:
Chemicals employed are common-use solvents and
reagents, harmful if inhaled, swallowed, or absorbed
through the skin. Refer to adequate manuals or
Materials Safety Data Sheets (MSDSs) to ensure
that the safety guidelines are applied before using
chemicals. Microwave operation involves hot
pressurized acid solution. Use appropriate personal
protective equipment such as a laboratory coat,
safety glasses, rubber gloves, and a fume hood.
Dispose of all materials according to federal, state,
and local regulations.
A. Principle
This method is an extension of AOAC Final Action Method
2011.19
to determine nine additional elements. Nitric acid, ISTD,
and hydrogen peroxide are added to the sample in microwave
vessels, and the samples are digested using preprogrammed
temperature control. The addition of hydrogen peroxide helps
reduce carbon and nitrous oxide levels in the digestate. The
presence of carbon in the samples causes signal enhancement of
Se. Therefore, to matrix match the samples, carbon in the form of
methanol is added to both the standard solutions and the digestate
before analysis. Ge (for 11 elements) and Te (just for Se) are
used as ISTDs. Analysis is performed by ICP/MS. Polyatomic
interferences with the lowmass elements are reduced or eliminated
by analyzing in the He collision mode using kinetic energy
discrimination (KED). For Se measurements, the H
2
gas mode
is preferred for increased sensitivity. Quantitation of 12 elements
Table 1. Summary of SLV acceptance criteria from SMPR 2011.009 and SMPR 2014.004
SMPR 2014.004
SMPR 2011.009
Parameter
Mn
Cu
Fe
Zn
Mg
P
Na
Ca
K
Cr
Mo
Se
Analytical range
a
,
b
0.001–1.0
0.001–1.2 0.01–20 0.1–18
3–110
15–800
10–850
20–1280
10–2000
20–1600 20–1000 10–500
LOD
a
,
b
NA
c
NA
NA
NA
NA
NA
NA
NA
NA
7
7
4
LOQ
a
,
b
0.001
0.001
0.01
0.1
3
15
10
20
10
20
20
10
Repeatability
≤5% at >0.005
b
≤8% at <0.005
b
≤5%
Recovery
90–110% at >0.005
b
80–115% at <0.005
b
90–110% of mean spiked recovery over the range of the assay
Reproducibility (over
the analytical range)
≤10%
≤10% ≤10% ≤10% ≤10%
≤8%
≤8%
≤8%
≤8%
≤15% ≤15% ≤15%
a
Concentrations apply to a) RTF liquids “as-is”; b) reconstituted powders (25 g into 200 g of water); and c) liquid concentrated diluted 1:1 by weight.
b
mg/100 g reconstituted final product for Mn, Cu, Fe, Zn, Mg, P, Na, Ca, and K; µg/100 g for Cr, Mo, Se.
c
NA = Not applicable.
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