6. AOACSPIFANMethods-2018Awards

160

Pacquette & Thompson: J ournal of AOAC I nternational V ol. 98, N o. 6, 2015  1705

(c)  Various plasticware and pipets.

AOAC Official Method 2011.19 Chromium, Selenium, and Molybdenum in Infant Formula and Adult Nutritional Products

C. Reagents [ Caution : Use normal laboratory safety precautions (laboratory coats and safety glasses with side shields) when handling concentrated acids, bases, and organic solvents. Additional protections such as face shields, neoprene gloves, and aprons should be used where splashing may occur. Avoid breathing vapors by working in approved hoods.] (a)  Laboratory water.— Use 18 MΩ water throughout for dilution. (b)  Concentrated nitric acid (HNO 3 ).— 65–70% trace metal-grade HNO 3 (J.T. Baker, Phillipsburg, NJ) or equivalent throughout. (c)  Hydrogen peroxide.— 30%ACS reagent grade (J.T. Baker) or equivalent. (d)  Methanol.— 99.99% analytical reagent grade (EMD Chemicals Inc., Gibbstown, NJ) or equivalent for matrix matching. (e)  Potassium stock solution.— 10 000 mg/L K (High Purity Standards, Charleston, SC) or equivalent. D. Standards (a)  2 mg/L Cr and Mo and 1 mg/L Se multielement stock standard solution in HNO 3 . —High-Purity Standards or equivalent. (b)  5 mg/L Ge and Te multielement stock standard solution in HNO 3 . —High-Purity Standards or equivalent. (c)  SRM 1849a.— National Institute of Standards and Technology (NIST; Gaithersburg, MD) or other suitable SRM to serve as a control for this analysis. E. Procedure (a)  Standard preparation .—Prepare intermediate standards from commercial stock standards at 40 ng/mL Cr and Mo and 20 ng/mL Se. Custom-blended multielement stock standard in HNO 3 is acceptable. Prepare three multielement working standards containing 0.8, 4.0, and 20 ng/mL Cr and Mo and 0.4, 2.0, and 10 ng/mL Se, plus blank, with both 50 ng/mL Ge and Te ISs in HNO 3 . Ge is used as the IS for both Cr and Mo, and Te must be used for Se. (b)  Sample preparation .—Prepare powder samples by reconstituting approximately 25 g sample in 200 mL warm laboratory water (60°C). Accurately weigh approximately 1.8 g reconstituted test portion into the digestion vessel. This represents approximately 0.2 g original powder sample. SRM 1849a is weighed at 0.2 g directly into microwave vessel. Fluid samples may be prepared by accurately weighing approximately 1 g test portion weighed directly into the digestion vessel after mixing. For the recommended one-step digestion (two stages in microwave program), add 0.500 mL 5000 ng/mLGe and Te IS solution (with a micropipette calibrated at point-of-use to deliver the target volume with a tolerance of ±0.8%: do not add the ISs online) and 5 mL trace metal-grade HNO 3 followed by 2 mL H 2 O 2 to the microwave digestion vessels. Seal vessels according to manufacturer’s directions and place in microwave. Ramp temperature from ambient to 180°C in 20 min, and hold for 20 min in stage 1. In stage 2, the microwave

Inductively Coupled Plasma (ICP/MS)/Mass Spectrometry

First Action 2011 Final Action 2014

ISO-IDF–AOAC Method

A. Principle A test portion is heated with nitric acid in a closed vessel microwave digestion system at 200°C. Digested test solution, or an appropriate dilution, is analyzed with the ICP/MS instrument standardized with acid matched standard calibrant solutions. An ionization buffer (K) is used to minimize easily ionizable element (EIE) effects, methanol is added to normalize the C content, and Ge and Te are used as ISs. It is permissible to combine the analysis of Cr/Mo/Se with simultaneous determination of any or all of these elements: Na, K, P, Mg, Ca, Fe, Zn, Cu, and Mn. B. Apparatus (a)  Microwave oven system .—Commercial microwave oven system designed for laboratory use at 0–300°C, with closed vessel system and controlled temperature ramping capability. It is recommended that a vessel design be selected that will withstand the maximum possible pressure, since organic material, and carbonates if not given sufficient time to predigest, will generate significant pressure during digestion. Vent according to manufacturer’s recommendation. ( Caution : Microwave operation involves hot pressurized acid solution. Use appropriate face protection and laboratory clothing.) Additional instrument parameters are summarized in Table 2011.19A . (b)  ICP mass spectrometer.— With collision reaction cells. In the multilaboratory testing study, five different ICP/MS instrument models from three major vendors delivered equivalent performance.

Table 2011.19A. ICP/MS parameters

Typical operating conditions

RF power, W RF matching, V

1600

1.8

Sampling depth, mm Extract 1 lens, V Carrier gas, L/min Make-up gas, L/min

9 0

0.9 0.2

Nebulizer (glass concentric) Spray chamber temperature, °C

MicroMist

2

Interface cones

Ni

He cell gas flow rate, mL/min cell gas flow rate, mL/min Nebulizer pump rate, rps H 2

4.5 4.2

0.1 (0.5 mL/min)

52 Cr, 95 Mo/ 72 Ge in He mode 78 Se/ 130 Te in H 2 mode

Analyte/IS/gas mode