Quantification of Total Arsenic in South African Wheat Flour by High Resolution – Inductively Coupled Plasma – Mass Spectrometry (HR-ICP-MS)

A method for the determination of the total As concentration in wheat flour samples from South African crops was developed based on a microwave-assisted digestion procedure. Different aspects of sample preparation, as well as optimization of the instrumental technique, such as strategies for signal enhancement, were investigated. The optimum digestion procedure was achieved using 6 mL nitric acid (HNO3) with 40 µL hydrogen peroxide (H2O2) to facilitate the dissolution of the organic matter in the samples with gentle heating up to 95 °C over a period of 55 min. The analysis was performed using high resolution inductively coupled plasma-mass spectrometry (HR-ICP-MS) for optimum interference control for As. The measurement trueness of the method was confirmed with an En-value = 0.01 calculated for the NIST SRM 1568b (Rice Flour) and an En-value = 0.9 calculated for the NCS ZC 80002b (Wheat) CRMs, respectively. The repeatability of the method was calculated to give a relative standard deviation (RSD) of −1 for the samples in solution and the limit of quantification (LOQ) was 0.069 ng g−1. This method was successfully applied to the analysis of 10 wheat flour samples with concentrations in the range of 5.5 to 12.3 ng As g−1 dry sample with a relative uncertainty of 11% (k=2). The concentrations of As in wheat flour samples obtained in this study were below the permissible levels of As allowed in food and food products.