Rice is one of the top three most produced and consumed crops worldwide, after wheat and coarse grains. It is the staple for around 50% of the world’s population. As a natural product, rice contains many nutrients, but it may also contain certain amounts of toxic elements including arsenic and cadmium, which are two of the "ten chemicals of major public health concern" identified by the World Health Organization (WHO). Arsenic is a metalloid that exist in two forms, inorganic and organic. In general, inorganic arsenic is more toxic to human than organic arsenic. The Codex Alimentarius Commission (CAC) have set maximum levels of inorganic arsenic and cadmium in rice. For milled rice, the maximum level of inorganic arsenic and cadmium is 0.2 mg/kg and 0.4 mg/kg respectively; for husked rice, the maximum level of inorganic arsenic is 0.35 mg/kg. The use of reliable methods for the measurement of toxic metal/metalloid contaminants is important in safeguarding the quality of these products and protection of public health.

The reference values provided by GLHK were used as the assigned values for evaluating the performance of participants in this PT programme. The relevant Calibration and Measurement Capabilities (CMCs) of GLHK on metal contaminants in food are registered in the BIPM Key Comparison Database (KCDB). The standard deviation for proficiency assessment was derived from the Horwitz Equation.