6. AOACSPIFANMethods-2018Awards

73

1714 Thompson et al.: J ournal of AOAC I nternational V ol. 98, N o. 6, 2015

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 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 0.2% RSD). (b)  Seal the vessels, and place into 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 vessels in a fume hood. Unscrew the cap/venting nut slowly to gradually release the pressure. Then, completely remove the cap. (d)  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 the final volume does not need to be quantitative because ISTDs were added prior to digestion; therefore, the analyte/ISTD ratios will be constant. F. Determination (a)  Using the appropriate tuning solutions, tune the instrument for optimal sensitivity in the KED mode and/or reaction mode according to the instrument design. Also, tune the instrument to find the P/Acalibration factors that are needed for those calibration curves that will extend above roughly 100 µg/L (depends 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 determined in the He collision mode, using KED. Te must be used as the ISTD for Se determinations, and we recommend that Se be determined in H 2 mode, i.e., reaction mode. Analyze Cal Std 3, or other suitable QC solution, every 10 test portions to monitor for instrument drift and linearity (result must be within 4% of Table 2015.06C. Typical ICP/MS parameters for Agilent 7700x RF power, W a 1600 RF matching, V 1.8 Sampling depth, mm 9 Extract 1 lens, V 0 Carrier gas, L/min 0.9 Make-up gas, L/min 0.2 Nebulizer (glass concentric) MicroMist Spray chamber temp., °C 2 Interface cones Ni He cell gas flow rate, mL/min 4.5 H 2 cell gas flow rate, mL/min 4.2 Nebulizer pump rate, rps 0.1 (0.5 mL/min) Peristaltic pump tubing White/white, 1.02 mm id Drain tubing Blue/yellow, 1.52 mm id a  RF = Radio frequency.

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 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 bottle dispenser or pipet) 5 mL nitric acid (ultrapure reagent grade) into the same volumetric flask. Pipette (using 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, while the PB is used as a 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 Class A glass pipet) 0.00, 1.00, 5.00, 20.00, and 40.00 mL, respectively, of the multielement standard stock solution into separate 50 mL volumetric flasks or sample tubes. Add 0.500 mL ISTD stock (using Class A pipet or digital pipet), 5 mL (using repipetter or Teflon bottle dispenser) nitric acid (ultrapure reagent grade), and 0.500 mL methanol to each flask. Fill 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 . E. Sample Preparation (a)  Prepare samples in duplicate. In sample vessels, weigh test portions to the nearest 0.0001 g. For liquid products, the test portion size is 1.0 g. For powdered products, the test portion size is net 0.20 g of a powder sample, which should be taken from a 10% (w/w) reconstitution in warm (60°C) water (i.e., 2.0 g of the 10% reconstitution). Add 0.500 mL ISTD stock using a calibrated digital pipet, 5 mL nitric acid (ultrapure reagent grade), and 2 mL 10% hydrogen peroxide. ( Note : the PB/Cal Blk Table 2015.06B. Microwave operating parameters: Stages 1 and 2 are operated sequentially, without removing vessels from the oven Stage 1 sample digestion 1 Power 100% (1600 W) 2 Ramp to temp., min 20 3 Hold time 20 4 Temp., °C 180 5 Cool down, min 20 Stage 2 sample digestion 1 Power 100% (1600 W) 2 Ramp to temp., min 20 3 Hold time, min 20 4 Temp., °C 200 5 Cool down, min 20 Total, h 2