IMPROVING PRECISION AND ACCURACY FOR TRACE ELEMENTS IN EPMA

Trace element characterization in electron probe micro analysis (EPMA) is limited by intrinsic random variation in the x-ray continuum (precision) and the accuracy of continuum measurement. Traditionally we characterize background intensity by measuring on either side of the emission line and interpolating the intensity underneath the peak to obtain the net intensity. Alternatively, we can measure the background intensity at the on-peak spectrometer position using a number of standard materials that do not contain the element of interest. This so-called mean atomic number (MAN) background calibration (Donovan, et al., 2016) uses a set of standard measurements, covering an appropriate range of average atomic number, to iteratively estimate the continuum intensity for the unknown composition (and hence average atomic number). Measurements demonstrate improved precision in less than ½ the time compared to off-peak measurements. Combined with a “blank” correction calibration on a matrix matched standard, we can also attain accuracy equal to our precision.

The continuum can also be characterized by multiple (more than two) off-peak measurements, also referred to as the multi-point background (MPB) method (Allaz, et al.) By directly characterizing the curvature of the continuum and automatically optimizing the background fit based on regression statistics, we can improve our background interpolation accuracy, even in complex materials for which we do not have a suitably matrix matched trace element blank standard.

Donovan, Singer and Armstrong, A New EPMA Method for Fast Trace Element Analysis in Simple Matrices ", American Mineralogist, v101, p1839-1853, 2016

Allaz, J., Jercinovic, M. J., Williams, M. L., & Donovan, J. J. (2014). “Trace element analyses by EMP: Pb-in-monazite and new multipoint background method”, Microscopy and Microanalysis, 20 (3), 2014