Analysis of fuels, lubricants, and greases using X-ray fluorescence spectrometry

X-ray spectrometry (XRF) is a versatile instrumental method for elemental analysis in a wide variety of materials. The performance of three different XRF systems will be compared: a high power wavelength dispersive x-ray spectrometer (WDXRF), a low-power WDXRF, and a bench-top energy dispersive instrument (EDXRF). The data obtained show quite clearly the large difference in sensitivities between instruments. This translates into varying lower limits of detection (LLD) or quantification and sample throughput. For quantitative analysis well above the LLD, the accuracy of the results obtained on the different instruments is similar, or - for practical purposes of quality control in many applications - the differences in accuracy between the different instruments can be considered insignificant. Most of the applications and methods developed by a variety of laboratories and regulatory organizations are applicable to specimens of a specified nature and matrix. Some of the more versatile procedures include a method to account for differences in matrix between standard reference materials and unknowns. One of the effects that is usually neglected in this case is the wedge effect, which is caused by the fact that the volume that is excited by the beam and the volume from which characteristic radiation can be observed are not the same and vary (for a given spectrometer system) depending upon the density and the quantity of the sample. This effect will be illustrated and discussed. Copyright © 2005 by ASTM International.