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 spectro metry (WD-XRF) can provide high quality determinations (in terms of accurac y 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 X RF strategy. We first developed a standard additions procedure for rock pow ders 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 t he detection limit was 0.2 mu g g(-1). We then separated Nb from the geolog ical matrix using the chemical procedure described by Rehkamper (1994); aft er separation, Nb was collected on an anion-exchange membrane which was dir ectly 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 li mit 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 analy sis. We found that the ion-exchange method tended to provide Nb concentrati ons which were about 10-15% lower than concentrations found by the standard additions method. We report data for fifteen international reference mater ials (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 wi th literature data.