The fundamentals of X-ray physics are well understood, and many workers have commented on the poor performance and poor reproducibility of atomically-light elements by portable X-ray fluorescence (pXRF). However, the physics responsible for this is rarely discussed in the pXRF literature. Furthermore, it is well known that analysing samples through calico (cotton fabric) or paper bags results in lower concentrations reported by the pXRF instrument but, again, the physics of this is seldom discussed. This paper addresses these shortcomings by presenting X-ray transmission modelling and empirical data to show that the effects reported by many workers are entirely consistent with the physics of X-rays passing through matter, and thus that they are entirely predictable.

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