AOAC Final Action Methods in 2017

P ACQUETTE & T HOMPSON : J OURNAL OF AOAC I NTERNATIONAL V OL . 101, N O . 2, 2018 539

(d) Magnetic stir plate. (e) polytetrafluoroethylene (PTFE)-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. — PTFE; Adjustable volume 0.5 – 5 mL. (k) Volumetric pipets. — Class A, assorted sizes. (l) Digital pipets. — 1 mL adjustable, to deliver 500 µL with accuracy tolerance of better than 0.8% and precision of better than 0.2% RSD. (a) Multielement standard stock solution. — National Institute of Standards and Technology (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 (HF) acid. 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 acid. This stock standard solution expires on the date given by the manufacturer. (c) Tuning and pulse/analog (P/A) factor tuning stock solutions. — 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, so that calibration residuals are <4% ( see section F ). (d) QC sample (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. (g) Nitric acid. — Concentrated trace metal grade. (h) Hydrogen peroxide, 30%. — ACS reagent grade. (i) Laboratory water. — metal-free, organic-free, pyrogen-free, filtered 18 M V cm quality. (j) Tergitol ™ . — Type 15-S-9, Sigma, or equivalent surfactant (optional). C. Reagents

nutritional formula. Also applicable to the determination of Na, Mg, P, K, Ca, Mn, Fe, Cu, Zn, Se and Mo in milk and milk products. See Table 2015.06D for analytical ranges.) Caution : The chemicals used are common-use solvents and reagents that are harmful if inhaled, swallowed, or absorbed through the skin. Refer to the adequate manuals or Material Safety Data Sheets to ensure that the safety guidelines are applied before using chemicals. Microwave operation involves a hot pressurized acid solution. Use appropriate personal protective equipment, such as a laboratory coat, safety glasses, rubber gloves, and a fume hood. Dispose of all materials according to federal, state, and local regulations. This method is an extension of AOAC Official Method 2011.19 to determine nine additional elements. The method is also extended to support analysis of dairy products (milk, milk powder, whey powder, whey protein concentrate, butter and cheese) for eleven of the twelve elements (Cr is excluded because none of the dairy products contained Cr above the LOQ, and so reproducibility could not be determined.). Nitric acid, internal standard (ISTD), and hydrogen peroxide are added to the sample in microwave vessels, and the samples are digested using preprogrammed temperature control. The addition of hydrogen peroxide helps reduce carbon and nitrous oxide levels in the digestate. The presence of carbon in the samples causes signal enhancement of Se. Therefore, to matrix-match the samples, carbon in the form of methanol is added to both standard solutions and the digestate before analysis. Ge (for 11 elements) and Te (only for Se) are used as the ISTDs. Analysis is performed by inductively coupled plasma (ICP)-MS. Polyatomic interferences with low mass elements are reduced or eliminated by analysis in He collision mode using kinetic energy discrimination (KED). For Se measurements, the H 2 gas mode is preferred for increased sensitivity. Quantitation of the 12 elements is achieved essentially simultaneously by comparing the analyte – ISTD response ratios in the unknown samples with a standard curve constructed from the response ratios of calibration standards. A. Principle (a) ICP MS. — With quartz spray chamber, quartz torch, Ni/Pt sample cone, Ni/Pt skimmer cone, autosampler, and printer. The ICP-MS must have collision/reaction cells. In two independent multilaboratory testing (MLT) studies four different ICP-MS instrument models from three major vendors delivered equivalent performance. (b) Microwave oven. — A commercial microwave designed for laboratory use at 0 – 300°C, with a closed-vessel system and controlled temperature ramping capability. Use manufacturer- recommended vessels. In the MLT studies, five different microwave designs delivered equivalent performance. ( Caution : Microwave operation involves a hot pressurized acid solution. Use appropriate face protection and laboratory clothing.) (c) Hydrogen generator (hydrogen is recommended for better Se sensitivity). — On-demand supply of >99.999% pure hydrogen at >150 mL/min. Alternatively, a high-pressure cylinder (99.999% purity) may be used. B. Apparatus

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(k) Argon gas. — ≥ 99.996% purity. (l) Helium gas. — ≥ 99.9999% purity.

D. Preparation of Standards and Solutions

(a) Tergitol solution (optional, approximately 5%, v/v). — Add about 700 mL laboratory water to a 1 L plastic bottle containing a PTFE-coated stirring bar. Place the bottle on a magnetic stirrer and begin stirring at a moderate speed. Slowly add 50 mL Tergitol from a graduated cylinder. When the Tergitol is