What could be simpler than looking for a handful of elements in lubrication oil samples using your all powerful XRF?
On the surface, this is would appear to be a straight forward analysis, but in reality, the challenge of analyzing light matrix samples (low z elements of C and H with hetero atoms O, S , P, and N) by XRF is constrained by the wide variation in the mass absorbtion characteristics between sample types that you would want to analyze against calibration curves obtained from a set of mineral oil standards.
The data reduction algorithms capability of your XRF software can accurately model the interelement influence coefficients and then apply the necessary corrections. However this requires the use of many calibration standards with independently varying concentrations for the elements of interest since the mathematical model is contrained by a series of linear equations which in theory should be N+1 the number N of elements .
The use of fundamental parameter analysis software improves the estimation of the influence coefficients for additive elements by modeling the entire elemental composition of the sample including the unmeasured elements ( not detectable by XRF) in an acquisition or series of acquisitions conditions.
Two important TIPS
- 1) Use standards whose elemental concentrations have been randomized and thus varying independently from each other
- 2) Be sure to get standards that include the Oxygen content for each individual standard so that you can accurately model the Carbon, Hydrogen, and Oxygen contribution to the influence coefficients. or include them in the fundamental parameter calculation.
Given the physical nature of lubricating oil ranging from almost water like consistency to thick solid greases, the sample preparation requirements for XRF prove to be a good intersection for this type of material and the XRF analytical technique.
The lube oil product can almost always be measured on an “as received” basis and empirical calibrations for additives are readily created with a caveat. The added benefit of the fundamental parameter program in XRF allows for greater flexibility and a wider variety of lube oils with widely varying concentrations of elements used as additives to bestow a particular property to the lube oil product, are readily quantitatively characterized for the elemental concentrations of additive elements.
This is particularly important for new oil formulation to ensure the correct levels of additive are present in the oil and in wear metal detection in oil to be able to characterize wear debris with the view to estimating predictive maintenance scenarios
ASTM 7751 – attempts to address this and only a handful of vendors actually achieve accurate, precise analysis using their advanced software features.
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