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 8C. 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.
2.5. Statistical analysis
The homogeneity testing of representative minerals in each QC
sample was evaluated by one-way ANOVA using Data Analysis of
Microsoft Office Excel 2003. The calculated F value should be less
than the critical F value which implied that samples are
homogeneous.
Ordinary statistical methods were used to calculate means and
standard deviations (SD) using Data Analysis of Microsoft Office
Excel 2003.