Which statement correctly describes the effect of sample thickness on XRF results?

• A. Thick samples do not affect XRF results due to detector shielding.
• B. Thick samples can cause self-absorption and poor signal from deeper layers; thin films may not represent bulk composition; thickness influences absorption.
• C. Thin films always represent bulk composition more accurately than thick samples.
• D. Sample thickness only affects measurement time, not signal quality.

Answer: (B)

In XRF analysis, how thick a sample is determines how many of the emitted X-rays can escape the material and reach the detector. Thick samples experience self-absorption: X-rays generated deeper inside the material are partly absorbed as they travel toward the surface, so the signal you detect mainly comes from shallower regions. This reduces the overall signal and can bias the measured concentrations if the analysis assumes uniform composition with depth. For very thin films, the signal may be dominated by the film itself and may not represent the bulk material beneath; the substrate or underlying layers can influence the spectrum depending on thickness and composition. In short, thickness affects absorption and the depth from which X-rays contribute to the signal, not just how long the measurement takes. Shielding of the detector doesn’t eliminate absorption inside the sample, and a thin film doesn’t automatically represent bulk material.