Contributing OfficeCenter for Food Safety and Applied Nutrition
The current AOAC method to measure nutrient elements is over thirty years old. The method lacks several key features for accurate analysis of elements in foods, including internal standardization, closed vessel microwave assisted digestion, and a calibration curve with a linear dynamic range spanning several orders of magnitude. Additionally, the method is not in line with the current FDA validation requirements, and modifications are required to produce acceptable results in foods. There is a clear need for a modern method to measure nutrient elements in a variety of food matrices including infant formula.
We describe a new method that measures twenty-three elements (As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Se, Sr, Tl, V, Zn) in foods using closed vessel microwave assisted digestion followed by inductively coupled plasma optical emission spectrometry. This method harmonizes sample preparation with other FDA food methods, and includes internal standardization and a calibration curve spanning four orders of magnitude. A 0.5 g sample portion is decomposed in the presence of nitric acid, microwave irradiance, and high pressure. The sample solution is then converted to a fine aerosol and introduced into an argon plasma where analyte species are atomized and/or ionized and emit light at distinct wavelengths corresponding to their elemental identity. A spectrometer separates the light according to wavelength and measures the amount of light, which is directly proportional to the concentration of the element in the sample. Modern instrumentation has automated most of this process, drastically improving sample throughput. The proposed method takes advantage of these innovations by measuring every element in the sample simultaneously.
Analytical recoveries in the range of 80-120 % are shown for eleven certified reference materials for elements with concentrations above the limit of quantification. In this study, we present final optimization of the method and an FDA level two single laboratory method validation. Upon completion of this portion of the study, an FDA level three multi-lab validation will be performed with participation from a diverse set of regulatory labs.