QUALITY CONTROL OF PXRF SPECTROMETRY

A portable X-Ray Fluorescence spectrometer (pXRF) has been utilized in multiple projects to characterize near-total geochemistry and provides results similar to fusion and multi-acid methods. PXRF analyses can provide near-instantaneous elemental composition of sediments, with minimal sample preparation and at low cost. However, questions persist regarding data accuracy and precision.

To provide a normalized sample grain-size, volume, and to minimize nugget effects in glacially-derived sediments, samples are dried and sieved to <0.063 mm (silt + clay) prior to analysis. To ensure quality control, a number of Standard Reference Materials (SRM) and Certified Reference Materials (CRM) are analyzed with each project, and comparisons made with previously published results. For further quality control, a sub-suit of sediment samples are analyzed by ICP-MS/ES using lithium borate fusion, multi-acid, and aqua regia digestions.

Bivariate plots display a high degree of linearity (r² > 0.8) for Ca, Fe, K, Mn, Rb, Sr, V, Zn, Zr, and to a lesser degree for Ba, Cu, Cr, Ti, and Pb. These 14 elements return precise and generally accurate results within each project; however, continued analyses of CRM’s and SRM’s may display drift in accuracy between projects. Concentration levels also play a significant factor in the accuracy of the pXRF data. At low concentration levels near the detection limit of the pXRF, there can be greater scatter in results. At high concentration levels, data needs to be adjusted using post-data calibration to obtain accurate results.

To date, this research supports the application of pXRF for the characterization of major and trace elements in glacial-derived sediments. PXRF provides reliable data for 14 elements at standard whole rock geochemistry levels, and can be used as a screening tool to select subsets of samples for traditional laboratory methods. While routine collection of geochemical data by traditional laboratory methods can be beyond the scope and budget of many programs, pXRF analyses can be used as an alternative analytical tool.