XRF coating measurement instruments are only as accurate as the standards used during calibration. While the Community Bureau of Reference in Brussels is beginning to offer material suitable for calibration of these devices, most standards used in the United States offer traceability to the National Institute of Standards and Technology (NIST).

NIST only offers two Standard Reference Materials of use in XRF measurement: gold on nickel (SRM 1399b) and tin-lead composition (SRM 1131). Industry vendors manufacture other standards using NIST test methods usually involving measurement of weight gain of a known density coating over a known area of substrate material using NIST-traceable weights. NIST and most industry vendors offer a standard accuracy of ±5%.

Non-destructive coating measurement should be considered a process because counting errors are incorporated into the measurement result. In XRF measurements, variations can be attributed to random changes in x-ray tube output and conversion of count rate to thickness, including variations in the coating standards. Fortunately, these counting errors are subject to normal distribution statistics and can be minimized by increasing calibration and measurement time and sample exposure area. This will narrow the distribution of measurements about their mean. XRF measurement uncertainty can usually be kept to better than 2%.

Currently, the maximum precision obtainable from XRF instrumentation is approximately 0.30%. In any case, it is appropriate to examine the mean and standard deviation of several measurements made without moving the sample in order to determine the validity of any one given measurement. Variation due to x-ray counting statistics is usually insignificant when compared to variations caused by the coating process.

Abstract
Accuracy and Precision
Principle of Measurement
Gold and Nickel Measurements on Copper
Gold and Nickel Measurements on Phosphor Bronze
Gold and Nickel Measurements on Kovar and Other Glass-Sealing Alloys
Gold and Nickel Measurements on FR-4 Circuit Board Substrates
Tin-Lead Solder on Copper, FR-4 Circuit Board Substrates, and Kovar
Tin-Lead Solder on Phosphor Bronze
Adjustments for Part Geometry
Measurement Assurance
Conclusion