New developments in monochromatic energy dispersive X-ray fluorescence instrumentation for monitoring toxic elements in food matrices: Advantages and limitations

Abstract

Monochromatic excitation (M-) based on a low-power X-ray (20–50W) source with doubly curved crystal (DCC) optics has been used successfully for X-ray fluorescence (XRF) analysis of many different sample matrices. Previously, a “first-generation” M-XRF analyzer, the HD Mobile® (X-ray Optical Systems, (XOS) East Greenbush NY), was used in a field-based public health study. The HD Mobile® was optimized to detect Pb, Cd, Hg and As in consumer products, food, and cultural medications, with limits of detection (LOD) on the order of a single digit μg/g (ppm) mass fraction. In this current study, two “second generation” DCC-enabled XRF analyzers were evaluated for detecting key toxic elements in food matrices. The two instruments evaluated included: the Zmax™ XRF Analyzer and the Emax™ Analyzer, which are both manufactured by Z-Spec (East Greenbush, NY, US). The Zmax™ uses an X-ray tube operated at 35W with a single DCC optic for a 17.5-keV excitation beam optimized to detect Pb, As, Hg, Ni and Cr. A fast silicon drift detector (SDD) is used to detect fluorescence x-rays. The Emax™ uses an X-ray tube operated at 40 W with DCC optics for a 30-keV excitation beam, and a thicker 1-mm SDD optimized to detect Cd, Mo, Sb, and Sn, along with many other elements. While the primary focus of the current study was the Emax™ and Zmax™, several other XRF analyzers were available and were included for comparison purposes. They included the HD Mobile® (XOS), and two ‘conventional’ portable XRF analyzers: the Niton™ XL3t GOLDD XRF, and the Niton™ XL5 Plus, both manufactured by ThermoFisher Scientific (Tewksbury, MA). Z-Spec used certified reference materials (CRMs) to optimize the Fundamental Parameters algorithm for the Zmax™ and Emax™ analyzers to achieve acceptable performance for key toxic elements in biological matrices. For the Zmax™, difference plots show good agreement between found values and CRM assigned values for the elements studied (Pb, As, Cu, Mn, Ni, Hg and Zn). For the Emax™, there was good agreement for values obtained for Cd in CRMs. Archived food-based proficiency test (PT) samples were analyzed to assess the performance of the Zmax™ and Emax™ as screening tools to detect key elements. Some of the PT samples were also analyzed using the two Niton™ XRF instruments as well as the HD Mobile® for comparison purposes. These PT samples were spiked with toxic elements and circulated by the FDA to US-based public health laboratories funded under the Food Emergency Response Network (FERN), with target values assigned based on consensus data obtained from analyses based on Inductively Coupled Plasma Mass Spectrometry (ICP-MS). For the FERN 2022 PT samples (sausage meat), which were spiked with Tl, Cd and Pb, Zmax™ values were within the tolerance range required for ICP-MS analysis. Estimated LODs in μg/g (ppm) were: Pb, 0.06; Tl, 0.06; Hg, 0.06; Cd, 0.04; and Sn, 0.11. Several ‘real-world’ samples were analyzed using the Zmax™ and Emax™ devices, including medicinal herbs such as moringa and pot marigold, as well as various spices like cloves, coriander, and curry.