Precise quantitative determination of niobium at low level concentrations in geological samples by WD-XRF

We show in this paper that wavelength dispersive X-ray fluorescence spectrometry (WD-XRF) can provide high quality determinations (in terms of accuracy and precision) of Nb in rocks at low level concentrations comparable with data from much more sensitive analytical techniques. Two different sample preparation methods were investigated and combined with a long count time XRF strategy. We first developed a standard additions procedure for rock powders in conjunction with a careful evaluation of the position of analytical lines (peak and background). The precision (2s) varied from within 0.03 mu g g(-1) to 0.2 mu g g(-1) in the 0-5 mu g g(-1) concentration range, and the detection limit was 0.2 mu g g(-1). We then separated Nb from the geological matrix using the chemical procedure described by Rehkamper (1994); after separation, Nb was collected on an anion-exchange membrane which was directly excited by X-rays. This method yielded a precision (2s) of 0.2 mu g g(-1) at the mu g g(-1) level if 100 mg of sample was used, the detection limit being dependent on the mass of sample studied. Derailed descriptions of analytical procedures are given, particularly of innovative aspects of Nb collection on membranes. XRF results obtained by the two sample preparation methods were compared with instrumental neutron activation (INAA) To analysis. We found that the ion-exchange method tended to provide Nb concentrations which were about 10-15% lower than concentrations found by the standard additions method. We report data for fifteen international reference materials (NIM-N, AL-1, AN-G, BE-N, MRG-1, JA-I, JB-2, JB-3, JA-2, JF-1, JGB-1, BIR-I, DNC-1, W-l, W-2), for which our XRF results are in good agreement with literature data.