OMB Meeting Book_9-11-14

50.1.41 AOAC Official Method 2011.19 Chromium, Selenium, and Molybdenum in Infant Formula and Adult Nutritional Products Inductively Coupled Plasma-Mass Spectrometry First Action 2011

A. Principle

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 presented to the inductively coupled plasma-mass spectrometer (ICP-MS) instrument standardized with acid matched standard calibrant solutions. An ionization buffer (potassium) is used to minimize easily ionizable element (EIE) effects, methanol is added to normalize the carbon content, and nickel germanium and tellurium are used as internal standards. 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, Mn. ( a ) Microwave .—Commercial microwave designed for laboratory use at 0–300°C, with closed vessel system and controlled temperature ramping capability. It is recommended that the vessel design be selected that will withstand the maximum possible pressure, since organic material, and also carbonates if not given sufficient time to predigest, will generate significant pressure during digestion. (Vessels can reach 700 psi or more on occasion.) Vessels must be designed to operate with only 6 mL solution volume, or the volume must be adjusted accordingly. 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. Apparatus

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 He cell gas flow rate, mL/min H 2 cell gas flow rate, mL/min Nebulizer pump rate, rps Analyte/internal standard/ gas mode Interface cones

MicroMist

2

Ni

4.5 4.2

0.1 (0.5 mL/min)

Cr, 95

Mo/ 60

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

2

( b ) ICP-mass spectrometer.— With collision reaction cells (CRCs). In the multilab testing study, five different model ICP-MS instruments from three major vendors delivered equivalent performance. ( c ) Various plasticware and pipets.

C. Chemicals and 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 throughout. ( c ) Hydrogen peroxide.— 30% ACS reagent grade. ( d ) Methanol.— 99.99% analytical reagent grade for matrix matching. ( e ) Potassium.— 10 000 mg/L in nitric acid for matrix matching. (may be replaced by multielement standards that contain K, if major minerals are determined simultaneously)

D. Standards

( a ) 2 mg/L Cr and Mo and 1 mg/L Se multielement stock standard solution in nitric acid.— High-Purity Standards (Charleston, SC), or equivalent. ( b ) 5 mg/L Ni Ge and Te multielement stock standard solution in nitric acid.— High-Purity Standards, or equivalent. (c ) SRM 1849a (NIST) or other suitable standard reference material to serve as a control for this analysis