Portable X-Ray Fluorescence calibration – Workflow to Optimize the Analysis of Carbonates.

Portable X-Ray fluorescence (PXRF) instruments have become more and more common tools in the last few years for chemical analysis (major, minor and some traces elements) in carbonates and sedimentary rocks. PXRF is a relatively cheap and fast technique, non-destructive and prevents tedious sample preparation. Manufacturers calibrations are usually made for a very large range of rock types or materials but specific empirical calibrations are essential to improve the quantification of multi-elemental concentrations.

We present a general workflow for building an empirical calibration model for a PXRF spectrometer from choosing reference materials to validating calibration’s efficiency and quantifying its limit of quantification. We also compare the quantified results on a sedimentary rock suite obtained from sixteen calibrations that were built using several software available on the market, including a simple homemade spreadsheet, the Bruker calibration software named EasyCal, a free and open-source calibration app named CloudCal, and the built-in GeoMining and GeoExplorer calibrations provided by Bruker manufacturer on the Tracer 5 PXRF. This decoupled approach allows (i) to explore the pros and cons for each calibration software solutions, (ii) to investigate and weight the effect of each correction parameters (i.e. regression line geometry, peaks interferences, spectra normalization, influence coefficients, multi-phases excitation conditions) on the quantified results and (iii) to propose an original and empirical approach to estimate the lower limit of quantification (LOQ) of a PXRF instrument and its calibration.

For quality check, each of these newly built calibrations was then validated on carbonate samples of known composition, corresponding to a series of Devonian limestones (Belgium) showing a large CaCO3 content ranging between 20 and 99 wt% and for which powder press pellets were formerly measured on a laboratory WD-XRF. These matrix-match empirical calibrations are systematically bringing improvement of the quantification efficiency, compared to the manufacturer calibrations.