The Importance of Monitoring XRF Count Rates

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Published Date Written by Sharon Banks

As discussed in a previous article Overcoming the "Sell By" Date of an XRF Calibration (09 July 2009), drift correction is a handy tool in x-ray fluoresence (XRF) analysis, but must always be used with care and reason. Simply applying drift correction to a change in count rate without careful monitoring thereof and being aware of the cause of the change in counts could end up being more costly than the time saved drift correcting the problem.
An important consideration is that analytical error introduced during the drift correction procedure may become unacceptably excessive when increasingly large correction factors are applied to the raw count rates. Such a situation is fortunately recognised when a trained analytical eye is keeping a close watch over the quality standards used to monitor instrumental performance.
A good example of an increasing degradation in x-ray counts over time is the gradual deposition of sample material on the x-ray tube. In wavelength dispersive XRF instrumentation, X-ray tubes are situated either at an angle of 90 degrees above the analytical sample, or at an angle of around 45 degrees below the analytical position, the latter design obviously being more prone to sample deposition occuring on the tube window.
Tube deposition has been found in many-a-lab to occur when analysing loose powders or when introducing the analysis material as a powder briquette. Very interestingly and unexpected, the same is possible when analysing fusion beads (considered to be most suitable sample presentation method in XRF) should the edges of the glass discs "grind" during sample rotation.
The resultant deposition being evident as shown in the adjacent picture of an end-window x-ray tube.
Clearly, an XRF calibration and the count rate drift and correction factors applied thereto requires skillful monitoring to ensure prolonged application of the initial calibration.

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