BS ISO 20649:2015 pdf download – Infant formula and adult nutritionals — Determination of chromium, selenium and molybdenum — Inductively coupled plasma mass spectrometry (ICP-MS).
3.6 Standards 3.6.1 Cr/Mo/Se multi-element stock standard solution in nitric acid, ρ Cr = 2 mg/l, ρ Mo = 2 mg/l and ρ Se = 1 mg/l. High-Purity™ Standards 1) , or equivalent. 3.6.2 Ge/Te multi-element stock standard solution in nitric acid, ρ Ge = 5 mg/l and ρ Te = 5 mg/l. High-Purity™ Standards 1) , or equivalent. 3.6.3 Standard reference material SRM 1849a, from the National Institute of Standards and Technology (NIST) or other suitable standard reference material to serve as a control for this analysis. 3.7 Preparation of standard solutions Prepare intermediate standards from commercial stock standards at 40 ng/ml for Cr and Mo and at 20 ng/ml for Se. Custom blended multi-element stock standard in HNO 3 is acceptable. Prepare three multi-element working standards containing 0,8 ng/ml, 4,0 ng/ml and 20 ng/ml Cr and Mo and 0,4 ng/ml, 2,0 ng/ml and 10 ng/ml Se, plus a blank, with both 50 ng/ml Ge and Te internal standards, in HNO 3 . Ge is used as the internal standard for both Cr and Mo, and Te shall be used for Se. 4 Apparatus Usual laboratory glassware and equipment and, in particular, the following. 4.1 Microwave oven. Use a commercial microwave designed for laboratory use at 0 °C to 300 °C, with a closed vessel system and a controlled temperature ramping capability. Select the vessel design 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. Vent according to the manufacturer’s recommendation. CAUTION — Microwave operation involves hot pressurized acid solution. Use appropriate face protection and laboratory clothing. 4.2 ICP-MS. ICP mass spectrometer with collision reaction cells and hydrogen and helium source. In the collaborative study, five different mode ICP-MS instruments from three major vendors delivered equivalent performance.
Analyse a 4 ng/ml Cr and Mo working standard solution and 2 ng/ml Se working standard or other suitable quality control solution every 10 test portions to monitor for instrument drift and linearity. The result shall be within 4 % of the standard’s nominal concentration. The inclusion of a method blank (run as a sample, its measured concentration shall be < half of the lowest calibration standard), a duplicate sample (relative percent difference (RPD) ≤ within 10 % for Cr, 7 % for Se and 5 % for Mo) and known reference materials serving as control samples (recovery check within control limits) are mandatory for good method performance. If any of these quality control checks fails, results should be considered invalid. The order of analysis should be calibration standards, followed by rinse, blank check, check standard, control sample, sample, sample duplicate (up to 10 samples), and finally check standard.