OPTIMIZATION OF X-RAY FLUORESCENCE SPECTROMETRY FOR ENVIRONMENTAL ANALYSIS OF ARSENIC AT LOW CONCENTRATIONS IN SEDIMENT AND SOIL MATERIALS

The metal arsenic (As) is a health threat to people living in regions where As-bearing minerals are present in rocks and soils. According to U.S. EPA, consumption of As at concentrations greater than 60 parts per million (ppm) can cause death. Lower doses can cause health problems ranging from cardiovascular ailments to skin diseases. As is widely distributed throughout Earth's crust in sulfide minerals. When eroded from rock, those minerals become concentrated in soils and stream sediments. People living in underdeveloped countries where agricultural and domestic activities are more likely to result in consumption of As-bearing soil and sediment are at particular risk for As poisoning. This research focuses on optimization of x-ray fluorescence spectrometry (XRF) methods for measurement of As at concentrations less than 20 ppm. Measurement of As by XRF at low concentrations is difficult because the most intense As XRF energy level (As Kα) is very close to two XRF energy levels of lead (Pb Kα, Pb Kβ), a metal commonly present with As in minerals. The proximity and intensities of the Pb energy levels to that of As Kα effectively ‘absorb' the As XRF energy, making its measurement at low concentrations unreliable. A secondary As Kβ energy level with no Pb interference can instead be measured, but with decreased analytical sensitivity. For environmental applications, it is important that the analytical sensitivity be high. This research shows that sensitivity of As analysis is optimized by independent measurement of both Pb energy levels and the primary As Kα energy level, followed by mathematical corrections for the Pb intensity absorption effects. This model was calibrated and tested using 10 USGS and NIST standards containing between 0.12 and 18.9 ppm As. Results of this research are applicable to a wide range of environmental studies focusing on the origin, concentration, and distribution of As in soils and sediment.