HIGH-PRECISION MULTICOLLECTOR ISOTOPIC ANALYSIS OF LOW-LEVELS OF ND AS OXIDE

30 ng Nd samples loaded with silica gel and phosphoric acid on single rhenium filaments reproducibly provide (NdO+)-Nd-144 ion beams of approximately 1.5-2.10(-11) A for several hours, without need of a poor source vacuum, or need to introduce oxygen into the mass spectrometer source. The loading technique discriminates strongly against CeO+, and accurate normalization for mass fractionation to Nd-142/Nd-144 is possible. A 5-collector procedure for analysis and interference correction (O-18, O-17 overlaps, CeO+, PrO+, SmO+) is described which permits analysis of 30 ng Nd loads to internal precision of +/- 0.000005 (2se) in approximately 2.5 hr. 17 loads (each 30 ng) of the laboratory Nd standard (analysed March-June 1990) yielded external precision of +/- 0.000007 (2sd) on Nd-143/Nd-144, and Nd-143/Nd-144 and Nd-145/Nd-144 values within 2se of the value determined on 500 ng Nd+ runs, and a further 17, analysed October 1990-April 1991, give identical results within error. Eight sample analyses also agree within 2se between Nd+ and NdO+ analyses, the latter being loaded from rinses of the disposable pipette tips used to load the Nd+ runs.