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Thompson et al.:

J

ournal of

AOAC I

nternational

V

ol.

98, N

o.

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% HNO

3

+ 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% HNO

3

+ 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,

μ

g/L Mn, mg/L Fe, mg/L Cu, mg/L Zn, mg/L Se,

μ

g/L Mo,

μ

g/L

Cal Blk

0

0

0

0

0

0

0

0

0

0

0

0

Cal Std 1

a

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

a

 Calibration standard.

72