XRF analysis is a well-established, trusted, and reliable technique for measuring the thickness of the plating layer on a wide range of components – from jewelry to engineering equipment. As a key part of the quality control process, it’s likely you’ll be relying on your XRF output to control your processes and test the quality of your finished components before shipping. However, while XRF is a reliable technique, it’s also very easy to get incorrect results – especially for plating thickness measurements. In this article, we’ll discuss how errors can creep into your XRF measurements and what you can do to prevent this at your site.
Where XRF errors come from
Broadly speaking, errors creep into your measurements two ways: either there’s a problem with your instrument, or there’s a problem with your measurement method. We’ll take a look at instrument-level issues first.
How to ensure your XRF is performing properly.
An XRF analyzer is a precision instrument and will need taking care of during its lifetime – from routine checks completed by you to annual services by the manufacturer. This is what you need to do to make sure your XRF is delivering the right results.
Routine instrument checks Instrument checks are carried out by the operator at intervals recommended by the manufacturer. These checks monitor the internal performance of the instrument, such as X-ray detector intensity, detector resolution and detector gain. If minor changes are found, the instrument automatically adjusts for them. And if the changes are more than expected, your analyzer will raise an alert to contact the manufacturer. Adhering to the manufacturers recommended check intervals is important here – too infrequently and minor changes will be missed, too frequently and the analyzer can become overcorrected, and errors could be introduced.
Calibration After running your routine instrument checks, you’ll then need to verify your calibrations before using the XRF analyzer for quality control. You can use known production parts or use reference materials (also known as standards). It’s important that these reference materials are purchased from an accredited supplier and it’s good practice to have them recertified by an ISO 17025 accredited lab on a regular basis.
Instrument certification As part of the annual service of your instrument, it’s recommended that you have your XRF analyzer re-certified by the original manufacturer to ensure the sensitive components are behaving as they should. This is normally carried out by a trained engineer who will run a full diagnostic and make recommendations accordingly.
Analytical mistakes that can cause errors
Even with a perfectly setup instrument, there are other factors that can cause errors when you make a measurement.
Not using sample focusing Ensuring the sample is focused correctly is a crucial step in the measurement process. It ensures the X-ray tube, sample and detector are a known distance from each other and is essential for accurate coating thickness results. If the distance from the tube to sample to detector is too large, the coatings will appear thinner and vice versa. And if you’re measuring multi-layer coatings the errors all add up.
Changes in the substrate Unexpected elements within the substrate can interfere with the X-ray interactions on the coated layers. This means that the resulting X-ray signal will not correlate to the correct coating thickness. The good news is that these effects are well understood, so if you are calibrating using materials that are very similar to your actual substrate, these effects can be compensated for. Essentially, your base material needs to be very similar to your calibration references.
Measuring outside the range of the calibration The calibrations of your instrument are optimized for a finite range of coating thickness and composition. Over large ranges, the relationship between coating thickness and X-ray intensity is not linear, so you can’t assume that you’ll get a reliable result outside the calibration range of your instrument. Your XRF manufacturer will be able to tell you the working limits of your calibrations and you can set up warnings on your software that will alert the operator if the measurement falls outside the ‘safe’ range, so you can be sure that you’re always measuring with the calibration limit.