Substrate fluorescence and the shape of the piece under measurement can all contribute to measurement error. The XRF instrument must be adjusted to account for materials found in the test piece which are not duplicated during calibration. This correction may be a normalization or other base correction, or a complete recalibration using the appropriate materials.

The need for this correction can be simply determined by measuring an uncoated piece with the intended application (calibration). Usually, no correction is necessary if the measurement is acceptably close to zero. When measurements must be made on curved surfaces, the piece must be placed in the instrument with the axis of the curve perpendicular to the axis of the detector.

The validity of measurements made by inspection personnel can be assured by providing a check standard manufactured from a representative part, marked with a thickness and an allowable measurement tolerance. Inspection personnel should be required to measure this check standard at the beginning and end of each measurement period and record the results in order to document a history of in-tolerance conditions.

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