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1714

Thompson et al.:

J

ournal of

AOAC I

nternational

V

ol.

98, N

o.

6, 2015

acid (ultrapure reagent grade) with 20 mL Tergitol solution (

a

)

and laboratory water to prepare a total volume of 1000 mL.

Expiration: 3 months; store at room temperature.

(c) 

P/A factor tuning working solution.—

Dilute and/or

combine P/A factor tuning stock solutions (or equivalent) to

manufacturer’s recommended dilution level with laboratory

water for use with the instrument. Expiration: 6 months; store

at room temperature.

(d) 

Calibration blank (Cal Blk) and preparation blank (PB)

solution.—

Add approximately 15 mL laboratory water to a

50 mL volumetric flask. Dispense (using bottle dispenser or

pipet) 5 mL nitric acid (ultrapure reagent grade) into the same

volumetric flask. Pipette (using digital pipet) 0.500 mL ISTD

stock and 0.500 mL methanol into the flask. Dilute to volume

with laboratory water. This solution serves as both the Cal Blk

and PB. The Cal Blk is used as the initial calibration point, while

the PB is used as a QCS (

see

below). Use the same lots of reagent

for samples. Expiration: 2 days; store at room temperature.

(e) 

Calibration standard solution set.—

Prepare Cal Blk, Cal

Std 1, Cal Std 2, Cal Std 3, and Cal Std 4 standard solutions

by pipetting (with Class A glass pipet) 0.00, 1.00, 5.00, 20.00,

and 40.00 mL, respectively, of the multielement standard stock

solution into separate 50 mL volumetric flasks or sample tubes.

Add 0.500 mL ISTD stock (using Class A pipet or digital pipet),

5 mL (using repipetter or Teflon bottle dispenser) nitric acid

(ultrapure reagent grade), and 0.500 mL methanol to each flask.

Fill the flasks to volume with laboratory water. Expiration:

2 days; store at room temperature. The analyte and ISTD

concentrations in the calibration standard solutions are shown

in Table

2015.06A

.

E. Sample Preparation

(a) 

Prepare samples in duplicate. In sample vessels, weigh

test portions to the nearest 0.0001 g. For liquid products, the test

portion size is 1.0 g. For powdered products, the test portion size

is net 0.20 g of a powder sample, which should be taken from

a 10% (w/w) reconstitution in warm (60°C) water (i.e., 2.0 g

of the 10% reconstitution). Add 0.500 mL ISTD stock using

a calibrated digital pipet, 5 mL nitric acid (ultrapure reagent

grade), and 2 mL 10% hydrogen peroxide. (

Note

: the PB/Cal Blk

solution prepared with the standards is the correct sample blank

for this method. Specifically, do not microwave digest the sample

blank, which can subject the blank to contamination. Also note

that the digital pipet used for the addition of ISTD solution must

be calibrated at point of use to ensure that it delivers a nominal

volume of 0.500 mL within a tolerance of ±0.8% and precision

better than 0.2% RSD).

(b) 

Seal the vessels, and place into microwave oven. Execute

a heating program equivalent to that shown in Table

2015.06B

,

suitable for total digestion of the sample.

(c) 

After digestion, place vessels in a fume hood. Unscrew

the cap/venting nut slowly to gradually release the pressure.

Then, completely remove the cap.

(d) 

Add approximately 20 mL laboratory water to the

contents of the vessel, swirl to mix, and transfer contents to a

50 mL sample vial. Add 0.5 mL methanol to the sample vial

and dilute to approximately 50 mL with laboratory water.

Shake briefly. The transfer or the final volume does not need to

be quantitative because ISTDs were added prior to digestion;

therefore, the analyte/ISTD ratios will be constant.

F. Determination

(a) 

Using the appropriate tuning solutions, tune the

instrument for optimal sensitivity in the KED mode and/or

reaction mode according to the instrument design. Also, tune

the instrument to find the P/Acalibration factors that are needed

for those calibration curves that will extend above roughly

100 µg/L (depends on instrument type). Table 

2015.06C

summarizes typical instrument parameters for analysis.

(b) 

Analyze test solutions using an ICP/MS instrument

standardized with the indicated standard solutions

(Table 

2015.06A

). Ge is used as the ISTD for the 11 elements

not including Se. Those 11 elements are determined in the He

collision mode, using KED. Te must be used as the ISTD for

Se determinations, and we recommend that Se be determined

in H

2

mode, i.e., reaction mode. Analyze Cal Std 3, or other

suitable QC solution, every 10 test portions to monitor for

instrument drift and linearity (result must be within 4% of

Table 2015.06B. Microwave operating parameters:

Stages 1 and 2 are operated sequentially, without removing

vessels from the oven

Stage 1 sample digestion

1

Power

100% (1600 W)

2

Ramp to temp., min

20

3

Hold time

20

4

Temp., °C

180

5

Cool down, min

20

Stage 2 sample digestion

1

Power

100% (1600 W)

2

Ramp to temp., min

20

3

Hold time, min

20

4

Temp., °C

200

5

Cool down, min

20

Total, h

2

Table 2015.06C. Typical ICP/MS parameters for Agilent

7700x

RF power, W

a

1600

RF matching, V

1.8

Sampling depth, mm

9

Extract 1 lens, V

0

Carrier gas, L/min

0.9

Make-up gas, L/min

0.2

Nebulizer (glass concentric)

MicroMist

Spray chamber temp., °C

2

Interface cones

Ni

He cell gas flow rate, mL/min

4.5

H

2

cell gas flow rate, mL/min

4.2

Nebulizer pump rate, rps

0.1 (0.5 mL/min)

Peristaltic pump tubing

White/white, 1.02 mm id

Drain tubing

Blue/yellow, 1.52 mm id

a

 RF = Radio frequency.

73