Trace element determination in soy sauce: A novel total reflection X-ray fluorescence procedure and comparison with inductively coupled plasma–mass spectrometry

Soy sauce is a widespread food commodity very common in East and Southeast Asia. It features a complex matrix, including a highly saline matrix (NaCl up to 15%) and a relevant organic component, typically around 15%. Methods for trace element determination in this matrix have been scarcely reported and no procedure has been proposed including Total Reflection X-Ray Fluorescence (TXRF). Aim of the present paper is to setup a TXRF method requiring minimum sample treatment and suitable for process control and risk assessment related to soy sauce consumption. Accordingly, a digestion (HNO3/H2O2) plus dilution (1:5 w/w) procedure was developed, applied to six soy samples from the Chinese market and the results compared to Inductively Coupled Plasma–Mass Spectrometry data. As a result, the procedure was fully validated for the six elements that could be compared: Cu, Fe, Mn, Rb, Sr and Zn. A dilution only procedure was also assessed showing, on average, a −5% bias only. Accordingly, sample digestion yields highly accurate data, whereas a simple 1:5 dilution may be perfectly suited for most purposes. Regarding detection capabilities, the limits of detection are typically below 0.5 mg/kg for both digested + diluted and diluted only samples. The reported procedures are accordingly fit for purpose in quality assurance/quality control procedures and risk assessment related to soy sauce consumption.