All calibration standard solutions were prepared from 10 mg/L
multi-element standard solution (AnApure KRIAT Co, Ltd. Daejeon,
Korea) by dilution with 19.6% (w/w) HNO3 (the same percentage of
acid present in the samples) in ultrapure deionised water. Analytical
reagent grade concentrated HNO3 (70%) was obtained from
Dong Woo Fine-Chem Co., Ltd Iksan, Korea. The Certified Reference
Material (CRM), non-fat milk powder was obtained from National
Institute of Standards and Technology (NIST-1549), Gaithersburg
MD, USA. Ultrapure deionised water with a resistivity