AOAC Final Action Methods in 2017

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

shaker is appropriate), the container opened and the contents dumped into a plastic container into which a magnetic stir bar is placed. While stirring, remove the 1 g sample with a disposable pipet for weighing directly into the tared microwave vessel. For ingredients such as whole milk powder, whey powder, or whey protein concentrate, use a direct weight of 0.3 g. For powder products, the test portion size is net 0.20 g powder sample, which should be taken from a 25 g powder + 200 g warm (60°C) laboratory water reconstitution (i.e., 1.8 g of the 11.1% reconstitution). For butter or processed cheese (take a mold- free portion) use a direct weight of 0.3 g. After weighing the sample, add 0.500 mL ISTD stock using a calibrated digital pipet, 5 mL nitric acid (ultrapure reagent grade), and 2 mL of 30% 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 the 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 an RSD of 0.2%). (b) Seal the vessels and place into the 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 the vessels in a fume hood, unscrew the cap/venting nut slowly to gradually release pressure, and then completely remove the cap. (d) Slowly 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 final volume does not need to be quantitative because ISTDs were added prior to digestion; therefore, the analyte – ISTD ratios will be constant. (a) Using the appropriate tuning solutions, tune the instrument for optimal sensitivity in KED mode and/or reaction mode according to the instrument design. Also, tune the instrument to find the P/A calibration factors needed for those calibration curves that will extend above roughly 100 µg/L (depending 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 F. Determination

dissolved, fill the bottle to approximately 1000 mL with laboratory water. Transfer to a 1 L plastic bottle fitted with a PTFE-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 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 the 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 a bottle dispenser or pipet) 5 mL nitric acid (ultrapure reagent grade) into the same volumetric flask. Pipette (using a 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, whereas the PB is used as the 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 a Class A glass pipet) 0.00, 1.00, 5.00, 20.00, and 40.00 mL, respectively, multielement standard stock solution into separate 50 mL volumetric flasks or sample tubes. Add 0.500 mL ISTD stock (using a Class A pipet or digital pipet calibrated at point-of-use to 0.8% accuracy), 5 mL (using repipetter or PTFE bottle dispenser) nitric acid (ultrapure reagent grade), and 0.5 mL methanol to each flask. Dilute 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 .

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E. Sample Preparation

(a) Make a single sample preparation. In sample vessels, weigh test portions to the nearest 0.0001 g. For liquid products (includes milk), the test portion size is 1.0 g. Liquid samples shall be thoroughly shaken (5 min in a mechanical

Table 2015.06A. Concentrations of standards and ISTDs in calibration standard solutions

Standards and calibration standard solutions

Na, mg/L Mg, mg/L P, mg/L K, mg/L Ca, mg/L Cr, µg/L Mn, mg/L Fe, mg/L Cu, mg/L Zn, mg/L Se, µg/L Mo, µg/L

Cal Blk

0

0

0

0

0

0

0

0

0

0

0

0

Cal Std 1 Cal Std 2 Cal Std 3 Cal Std 4

0.500 0.200 0.500 1.00 1.00 0.800 0.00500 0.0500 0.00500 0.0200 0.400 0.800 1.00 2.50 5.00 5.00 4.00 0.0250 0.250 0.0250 0.100 2.00 4.00 4.00 10.0 20.0 20.0 16.0 0.100 1.00 0.100 0.400 8.00 16.0 8.00 20.0 40.0 40.0 32.0 0.200 2.00 0.200 0.800 16.0 32.0 2.50 10.0 20.0

ISTD (at 50 µg/L)

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Te

Ge