Choosing the thicknesses of the standards you need
The number of standards you need depends on:
- The thickness range you want to measure (maximum – minimum thicknesses).
- What are the requirements of your XRF calibration software? This can vary greatly among the XRF suppliers, especially for more complex applications (e.g., multilayer or alloy coatings). In this case, we suggest you consult with your XRF supplier about the number of standards you might need to satisfy the calibration requirements of your instrument’s software for any given application.
- As a general rule, we suggest obtaining one standard that is near or below the minimum thickness you must measure, one standard that is at or 20%-50% above the maximum thickness you need to measure and one standard near the mid-point of the thickness range you need to measure or near a thickness that your samples will have. Most calibrations require one or two standards as a minimum. In general, more standards can result in better accuracy when using calibration functions that apply statistical regression to develop a calibration curve. In these cases, there is a point of diminishing returns as you use more standards to do your calibration. We usually find 3 - 4 standards is sufficient for most applications in these cases. For calibrations using Fundamental Parameters (FP), the optimum number of standards for calibration depends on the quality of the FP algorithms employed by the XRF instrument you are using. We have found the quality of these FP calibrations varies greatly among the XRF suppliers. However, usually 2 or 3 standards are adequate to achieve a good, accurate FP calibration. Again, we refer you to your XRF supplier to guide you as to the optimum number of standards to use for your specific applications.
- Your own ideas and preferences with respect to calibrations. Some customers feel more comfortable ordering standards that are as close as possible in thickness and/or composition as the samples they plan to test. This approach is the least dependent on the accuracy of the calibration modeling software used by your XRF instrument. It a safe, practical approach and is most accurate if you plan to order only one standard to calibrate using Fundamental Parameters software.