Many workers have commented on the poor performance and poor reproducibility of atomically-light elements by portable X-ray fluorescence (pXRF), but seldom is the physics that is responsible for this discussed. 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 rarely discussed. This contribution addresses both of these short comings by presenting X-ray transmission modelling and empirical data to show 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. Here we document some of the key limitations X-ray physics place on the performance of atomically-light elements (particularly Na, Mg, Al, Si) by pXRF.

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