Trace Element Analyses by EMP: Pb-in-Monazite and New Multipoint Background Method

Precision in electron microprobe analysis is primarily a matter of counting statistics, and involves voltage, beam current, counting time, and geometric efficiency. Besides potential issues with dead-time correction and beam damage, counting statistics do not significantly affect the accuracy of major and minor element analyses. Accuracy chiefly depends on standards and matrix correction. Additional challenges with both accuracy and high precision appear when measuring trace elements [1]: 1) Analytical precision requires the use of high intensity monochromators, optimized PHA settings, high beam current, higher voltage, and/or lengthy count time. These latter requirements can lead to beam damage, surface contamination, and internal charge effects. 2) Characterization and measurement of the background become crucial for accurate results when the peak-to-background ratio approaches 1. 3) Accurate results require correction or avoidance of peak and background interferences, even for some 2 nd or 3 rd order peaks. Careful characterization of the analyzed material to evaluate beam exposure effects is recommended prior to analysis. In addition, it is valuable to acquire (a) qualitative WDS scans to measure background and to evaluate peak and background interferences, and (b) element mapping to characterize major element zoning.