At Texaco's Research Center, X-ray Fluorescence (XRF) is our workhorse method for the determination of additive elements in lubricating materials. Other analysis techniques are used to assist in method development by confirming compositions of standards and quality control samples, or by verifying analysis results from newly developed XRF methods.
Often it is impractical to make a determination of all analytes in a sample by XRF due to inherent limitations of X-ray fluorescence measurements and the complexity of calibrating for many analytes in a single method. Analyses of high volumes of samples can be made more efficient by omitting difficult or infrequently encountered elements. When a unique sample comes along, alternative analytical techniques are used to determine elements for which the X-ray spectrometer is not sensitive or not calibrated. These concentrations can be combined with theoretical influence coefficients to correct existing XRF calibrations for the effects of unanalyzed elements. A working example, sodium in research samples, is given. Another example, barium in oils, shows the serious shortcomings of this approach. Alternative techniques such as inductively coupled plasma atomic emission spectroscopy, atomic absorption spectrometry, sulfur determination by high temperature combustion with infrared detection, and wet chemical methods can be used to complete an analysis or to confirm the accuracy of XRF methods for determinations of additive elements in lubricants. Examples of our elemental analysis methods, how they relate to ASTM procedures, and how we use our techniques are described.