Analytical characteristics of a multilayer dispersion element (2d = 60 A) in the determination of fluorine in minerals by electron microprobe

The analytical characteristics of a W/Si multilayer dispersion element (MLDE) with a nominal 2d spacing of 60,~ are assessed and compared with those of a conventional thallium acid phthalate (TAP) crystal for the determination of fluorine in silicate and phosphate minerals by electron microprobe. Measured count rates for fluorine were found to be up to fourteen times higher using the MLDE compared with TAP giving improved detection limits of 0.02 to 0.08 wt. % F (six sigma, 100s count time) compared with equivalent data of 0.06 to 0.12 wt. % F by TAP. At equivalent spectrometer angles, peak widths by MLDE were broader by a factor of 2 to 3. However, an important advantage of MLDE was found to be the absence of multiple order diffractions higher than the second order. In consequence, the troublesome third order P-Ka line interference on the F-Ks line, normally encountered in the analysis of apatites, is absent in determinations made using the MLDE. Conversely small interferences on the F-Ks line were detected from the first-order La line of Fe and second-order L a line of Mg. X-ray data and spectra of selected minerals are presented to demonstrate these interferences. Detection limits for elements that can also be determined using the MLDE were 0.07 to 0.15 wt. % oxygen and 0.34 wt. % carbon. KEVWORDS: electron microprobe analysis, fluorine, multilayer dispersion element.