In the context of XRF measurements, which statement is true?

• A. Increasing the number of counts reduces uncertainty.
• B. Increasing the number of counts increases uncertainty.
• C. Calibration duration has no effect on uncertainty.
• D. Sample moisture content determines uncertainty.

Answer: (A)

In XRF measurements, the randomness in the number of detected X-ray counts follows Poisson statistics: the standard deviation of counts is the square root of the total counts collected, N. This means the precision improves as N grows, because the relative uncertainty is 1 divided by the square root of N. So, more counts—or a longer counting time to accumulate them—reduces the uncertainty.

The other statements aren’t correct for this primary source of uncertainty. Calibration duration does affect how well calibration coefficients are known and drift corrections are applied, so claiming it has no effect isn’t accurate. Moisture content can influence attenuation and matrix effects, which can shift counts and bias results, but that’s about systematic effects rather than the fundamental counting uncertainty.