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(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

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.

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.