What does fixed geometry refer to in XRF measurements?

• A. A fixed geometry has no impact on results.
• B. Fixed geometry refers to fixed instrument settings only.
• C. Fixed geometry means a constant sample-to-detector arrangement used to improve reproducibility.
• D. Fixed geometry means calibrating geometry with each measurement.

Answer: (C)

Fixed geometry means keeping the same physical arrangement between the sample, the X-ray source, and the detector from one measurement to the next. This includes a constant sample-to-detector distance and fixed angles, so the solid angle, attenuation, and scattering conditions stay the same. By fixing geometry, you minimize changes in detected peak intensities for a given element, which improves reproducibility and accuracy across measurements. If geometry varied, different path lengths and solid angles would alter how much signal you collect, making quantification less reliable.

The idea that geometry has no impact is incorrect because the setup directly influences detection efficiency and peak counts. It’s more than just instrument settings; it’s about the physical arrangement. And fixed geometry isn’t about calibrating geometry each time; it’s about keeping the geometry constant during measurements to avoid introducing variability.