2.4. In-house reference material
White jasmine rice (a representative of rice) and soybeans (a representative of legume) were prepared as in-house reference materials for in-house quality control (QC). The QC samples were prepared as mentioned earlier for the studied samples except the packing step. After well-mixing in a humidity-controlled air condition room, the homogeneous samples were packed in plastic bags, about 10 g each and kept in a freezer at −20 °C. The bags were labelled with specific sample code numbers. The assigned values of nutrient and toxic elements were developed from duplicate analysis of 10 single samples of in-house QC samples on 10 different days. The QC sample was then analyzed along with the unknown samples in each run to monitor the precision of the nutrient and toxic element measurements over time.