Dear Simon
Regarding your question
I would like to highly recommend Epsilon4 for measure your sample as chemical composition(low concentration Na, Fe) and thickness measurement.
Because Na is light element(Na, Mg, Si, Al, P, S and Cl) can be absorb in air, Epsilon4 has He system to flow He gas when measure light element.
The picture as below is shown about Helium flow is improve intensity (improve repeatability)