Silicate rock analysis by energy-dispersive X-ray fluorescence using a cobalt anode X-ray tube. Part I. Optimisation of excitation conditions for chromium, vanadium, barium and the major elements

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Abstract
A cobalt anode X-ray tube has overcome many of the deficiencies of other tubes in exciting the trace elements Cr, V and Ba in silicate rocks when analysed by energy-dispersive X-ray fluorescence (XRF). The optimum excitation conditions derived for the determination of chromium were an operating potential of 20 kV using a 12.5-µm iron foil filter in the primary X-ray beam. Under these conditions the fluorescence of iron K lines in samples is selectively suppressed. This has the effect of reducing both the proportion of analysis time expended unnecessarily in accumulating iron X-rays and the magnitude of the spectral background caused by incomplete charge collection of Fe Kα/Kβ events in the Si(Li) detector. Detection limits (6σ) for Cr, V and Ba of 6, 9 and 25 p.p.m., respectively, approach those values expected by routine wavelength-dispersive XRF in geological applications. A comparison was also made of the relative efficiency of cobalt and silver anode tubes to excite major elements in geological samples prepared as glass fusion discs. These tubes were fitted with beryllium windows of thickness 50 and 125 µm, respectively. The results indicated that the higher proportion of low-energy continuum radiation transmitted through the thinner beryllium window of the cobalt tube was at least as important in exciting the lightest elements Na, Mg and Al as the presence of the characteristic L-line X-rays of the silver anode tube.

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