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.
Microwaveoperationinvolveshot pressurizedacidsolution. 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 low mass 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 is achieved essentially simultaneously by comparing the analyte/ISTD response ratios
in the unknown samples to a standard curve constructed from response ratios of calibration
standards.
B. Apparatus
(
a
)
ICP mass spectrometer.—
With quartz spray chamber, quartz torch, Ni/Pt sample cone, Ni/Pt
skimmer cone, autosampler, and printer. The ICP mass spectrometer must have collision
reaction cells (CRCs). In a limited multilaboratory testing study, four different ICP/MS instrument
models from three major vendors delivered equivalent performance.
(
b
)
Microwave oven.—
Commercial microwave designed for laboratory use at 0–300°C, with
closed vessel system and controlled temperature ramping capability. Use manufacturers’
recommended vessels. (
Caution
: microwave operation involves hot pressurized acid solution.
Use appropriate face protection and laboratory clothing.)
(
c
)
Hydrogen generator (hydrogen is recommended for better Se sensitivity).—
Parker Balston
(Haverhill, MA) Model H2PD-150, or equivalent. Alternatively, a high pressure cylinder (99.999%
purity) may be used.
Candidates for 2016 Method of the Year
68