David T. Adams

EPMA and Quantitative EDS of Rare Earth Elements in Geochronological Reference Materials

Microanalysis of rare earth elements (REE) in geologic materials is traditionally done with electron microprobes with wavelength dispersive spectrometers (EPMA) with software algorithms to correct for peak interferences and choice of background models because of its superior spectral resolution and detection limits [1]. However, the number of elements that require standardization, often more than 20, and required counts times to achieve desired detection limits may result in analysis times of 15 minutes or more per analysis spot. Scanning electron microscopy with energy dispersive spectroscopy (SEMEDS) may provide faster acquisition times as all elements are analyzed simultaneously and peak interferences are deconvoluted using linear least squares modeling. REE concentrations in geochronological reference materials, monazite (NAM) and xenotime (BS-1) determined using EPMA, SEM-EDS, and solution ICP-MS [2] are compared to asses the accuracy and precision of quantitative EDS and suitability of these materials as microanalytical reference materials.

Apatite Revisited: The Role of Anisotropy in Apatite Analysis

Damage due to specimen-beam interaction is a well known phenomenon in many different materials including glasses and certain minerals. Apatite is one such mineral found in geological and biological specimens, but its behavior under a focused electron beam is notoriously problematic [1] [2]. One example of the effects of electron bombardment intensity in apatite is the change in fluorine X-ray intensity. Regardless of orientation, F X-ray intensity initially increases then decreases during the analysis. However, the rate of this change is dependent on crystallographic orientation as well as operating conditions. Time dependent intensity (TDI) effects are not isolated to F. Changes in X-ray intensity are also observed in Ca, P, Na, S, and Cl in apatite. Clear concise methods to obtain accurate electron beam analyses (EDS and WDS) particularly with respect to long analysis time, fine spatial resolution, and relatively high beam currents necessary to obtain low detection limits are lacking. Advances in correction techniques in modern day electron microprobe software and electron microbeam techniques can help in correcting the change in intensity of X-rays during a relatively long WDS analysis and for knowing the precise crystallographic orientation of the apatite grain(s) with respect to the electron beam (e.g. EBSD analysis).