Impact of High Sulfur Concentration on Measurement of Light Elements by XRF

ANN M. DILLNER (1), Xiaoya Cheng (1,2), Hege Indresand (1), Warren H. White (1)

(1) University of California, Davis (2) Zhejiang University, China

     Abstract Number: 396
     Last modified: May 11, 2010

Abstract
X-ray fluorescence (XRF) is used by the Interagency Monitoring of Protected Visual Environments (IMPROVE) network to measure the mass of specific elements in particulate matter (PM) samples collected at pristine sites. In XRF analysis, excitation x-rays cause elements in the samples to fluoresce at characteristic energies. Elements with low fluorescing energy, namely, silicon, aluminum, magnesium and sodium, are difficult to measure quantitatively because their fluorescence is attenuated in the PM matrix. Additionally, silicon and aluminum fluoresce at energies just below sulfur, a very abundant element, which makes their spectral peaks difficult to resolve, particularly at extreme relative concentrations. Correcting for matrix and spectral interference effects is challenging. IMPROVE attempts to account for matrix affects and for some spectral interferences. IMPROVE data provide empirical evidence that aluminum and silicon concentrations are incorrectly reported when their concentrations are low relative to sulfur. The objective of this research is to demonstrate this interference under controlled conditions and quantify its effect on ambient PM samples. To quantify this interference, we collected parallel ambient PM$_(2.5) samples with a wide range of soil-element concentrations and low sulfur concentrations. The samples were analyzed by XRF using both the custom-built Crocker Nuclear Laboratory instrument employed by IMPROVE, with a Cu-anode tube and Si-Li detector, and a commercial instrument, with a Ca secondary target and Ge detector. Pure ammonium sulfate was then sampled onto the same filters to produce a range of S/Si observed in the IMPROVE network. Re-analyses of the ‘spiked’ filters by both XRF instruments show that high relative concentrations of sulfur produce an over estimate of silicon from the Crocker instrument and an under estimate of silicon from the commercial instrument. Iron, which fluoresces at higher energy than sulfur, is not impacted by sulfur addition. Results for all concentrations of silicon and aluminum will be presented.