An improved HPLC–ICPMS method for determining inorganic arsenic in food: Application to rice, wheat and tuna fish

Because arsenic occurs in food both as toxic inorganic forms and as (presumed) non-toxic organic forms, there is a pressing need for robust analytical methods to selectively and quantitatively determine inorganic arsenic in food products. We report an HPLC–ICPMS method based on sample extraction with trifluoroacetic acid/H2O2, and measurement of arsenate by anion-exchange HPLC–ICPMS using aqueous malonic acid as mobile phase. The method showed good extraction efficiencies (generally >90%) and column recoveries (>95%) for samples of rice, tuna fish and wheat. Moreover, the use of 5 or 10 mM malonic acid at pH 5.6 gave sharp well-resolved HPLC peaks and a conveniently short retention time of 5 min for arsenate, which together contributed to a low limit of detection based on signal to noise ratio of three of 0.05 μg As L−1 (for 20 μL injection) corresponding to an LOD of 1 μg As·kg−1 dry mass in the samples. The inorganic arsenic and total arsenic content (dry mass basis) of 10 rice samples was 25–171 μg As·kg−1 and 36–218 μg As·kg−1, respectively, whereas the 24 tuna fish samples contained much lower levels of inorganic arsenic (<2–18 μg As·kg−1), although the total arsenic content was much higher (730–5030 μg As·kg−1). The single wheat sample contained 166 μg As·kg−1, which was present mainly as inorganic arsenic (152 μg As·kg−1). The combination of low acid concentration for extraction, high buffering capacity of malonic acid at pH 5.6, and good peak shape results in a highly reproducible and sensitive method for the determination of inorganic arsenic in food.