Methods to be Reviewed by OMB in 2016

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Thompson et al.: Journal of AOAC International Vol. 98, No. 6, 2015 1713
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.
(d) Magnetic stir plate.
(e) Teflon-coated magnetic stir bars.
(f) Analytical balance.— Capable of weighing to 0.0001 g.
(g) Fume hood.
(h) Common laboratory glassware/plasticware.
(i) Repipetter.—50 mL.
(j) Bottle top dispenser.— Teflon; adjustable volume
0.5–5 mL (BDH Aristar, Radnor, PA), or equivalent.
(k) Volumetric pipets.—Class A, assorted sizes.
(l) Digital pipets.—1 mL (Rainin EDP-Plus, Oakland, CA)
or equivalent.
C. Reagents
(a) Multielement standard stock solution.—NIST or
NIST-traceable containing Se at 20 µg/L; Cr and Mo at 40 µg/L;
Mn and Cu at 0.25 mg/L; Zn at 1 mg/L; Fe at 2.5 mg/L; Mg at
10 mg/L; P at 25 mg/L; Ca and K at 50 mg/L; and Na at 25 mg/L
in 2% HNO3 + trace hydrofluoric acid (HF). This stock standard
solution expires on the date given by the manufacturer.
(b) Multielement ISTD stock solution.—NIST or NIST-
traceable containing Ge and Te at 5 mg/L in 2% HNO3 + trace
HF. This stock standard solution expires on the date given by
the manufacturer.
(c) Tuning and pulse/analog (P/A) factor tuning stock
solutions (High-Purity Standards, Charleston, SC, or
equivalent).—NIST or NIST-traceable containing various
elements at concentration levels recommended by the
manufacturer. Because this ICP/MS method determines the
major elements at relatively high concentrations, it is important
to understand the solutions needed and the procedure to obtain
high quality calibration curves in which the detector is used in
both pulse counting and analog modes. A properly calibrated
instrument will deliver the linearity requirements of the method,
for example, that calibration residuals are <4% (see section F).
(d) QCS.—Standard Reference Material (SRM) 1849a
(NIST) milk-based hybrid infant/adult nutritional powder with
certified values for Ca, Cu, Cr, Fe, Mg, Mn, Mo, P, K, Se,
Na, and Zn. Supplied as a unit of 10 packets each containing
approximately 10 g material. This is the recommended control
material for this analysis, but other suitable SRMs could be
substituted.
(e) Methanol.—99.99%, analytical reagent grade.
(f) Nitric acid.—Concentrated, ultrapure reagent grade (J.T.
Baker, Phillipsburg, NJ; Ultrex II or equivalent).
(g) Nitric acid.—Concentrated (65–70%, w/v), trace metal
grade (BDH Aristar Plus, West Chester, PA, or equivalent).
(h) Hydrogen peroxide, 30%.—ACS reagent grade.
(i) Laboratory water.—Millipore treated, 18 MΩ cm, or
equivalent.
(j) Tergitol®.—Type 15-S-9, Sigma or equivalent surfactant.
(k) Argon gas.—≥99.996% purity.
(l) Helium gas.—≥99.9999% purity.
(m) Hydrogen gas.—≥99.9995% purity, for Se analyses
(recommended).
D. Preparation of Standards and Solutions
(a) Tergitol solution (approximately 5% v/v).—Add about
700 mL laboratory water to a 1 L plastic bottle containing a
Teflon-coated stirring bar. Place the bottle on a magnetic stirrer
and begin stirring at a moderate speed. Slowly add 50mLTergitol
from a graduated cylinder. When the Tergitol is dissolved, fill the
bottle to approximately 1000 mLwith laboratory water. Transfer
to a 1 L plastic bottle fitted with a Teflon-constructed dispenser
with adjustable volume from 0.5 to 5 mL. This solution is added
to the autosampler rinse solution to minimize residue buildup
in the spray chamber. It does not otherwise affect the analysis.
Expiration: 6 months; store at room temperature.
(b) Nitric acid rinse solution (2% v/v) for autosampler
rinse port with Tergitol added.—Mix 20 mL concentrated nitric
Table 2015.06A. Concentrations of standards and ISTD in calibration standard solutions, and corresponding practical LOQ
(PLOQ)
Na, mg/L Mg, mg/L P, mg/L K, mg/L Ca, mg/L Cr, μg/L Mn, mg/L Fe, mg/L Cu, mg/L Zn, mg/L Se, μg/L Mo, μ g/L
Cal Blk 0 0 0 0 0 0 0 0 0 0 0 0
Cal Std 1a 0.500 0.200 0.500 1.00 1.00 0.800 0.00500 0.0500 0.00500 0.0200 0.400 0.800
Cal Std 2 2.50 1.00 2.50 5.00 5.00 4.00 0.0250 0.250 0.0250 0.100 2.00 4.00
Cal Std 3 10.0 4.00 10.0 20.0 20.0 16.0 0.100 1.00 0.100 0.400 8.00 16.0
Cal Std 4 20.0 8.00 20.0 40.0 40.0 32.0 0.200 2.00 0.200 0.800 16.0 32.0
ISTD (at 50 µg/L) Ge Ge Ge Ge Ge Ge Ge Ge Ge Ge Te Ge
PLOQ 0.25 0.10 0.25 0.50 0.50 0.40 0.0025 0.025 0.0025 0.010 0.20 0.40
aCalibration standard.
Candidates for 2016 Method of the Year
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