High-precision multicollector isotopic analysis of low levels of Nd as oxide

Abstract

30 ng Nd samples loaded with silica gel and phosphoric acid on single rhenium filaments reproducibly provide ¹⁴⁴NdO⁺ ion beams of ∼ 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 ${}^{142}\text{Nd}{}^{144}\text{Nd}$ is possible. A 5-collector procedure for analysis and interference correction (¹⁸O, ¹⁷O overlaps, CeO⁺, PrO⁺, SmO⁺ ) is described which permits analysis of 30 ng Nd loads to internal precision of ±0.000005 (2se) in ∼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 ${}^{143}\text{Nd}{}^{144}\text{Nd}$, and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ and ${}^{145}\text{Nd}{}^{144}\text{Nd}$ 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.