Use of 1012 ohm current amplifiers in Sr and Nd isotope analyses by TIMS for application to sub-nanogram samples

Abstract

We have investigated the use of current amplifiers equipped with 1012 ohm feedback resistors in thermal ionisation mass spectrometry (TIMS) analyses of sub-nanogram sample aliquots for Nd and Sr isotope ratios. The results of analyses using the 1012 ohm resistors were compared to those obtained with the conventional 1011 ohm amplifiers. We adopted a method where long baselines were measured before and after data acquisition to save time during sample analysis. The noise level on the new design 1012 ohm resistors is 3–5 fold lower compared to the 1011 ohm resistors, which results in a two fold better analytical precision on 143Nd/144Nd and 87Sr/86Sr analyses for beam intensities of 10 mV for the 143Nd and 87Sr masses, respectively. Reproducibility of 87Sr/86Sr on repeat analyses of 1 ng Sr sample loads is better using 1012 ohm resistors compared to 1011 ohm resistors and is a factor of 2 better compared to previous studies. The external reproducibility of 143Nd/144Nd for 1 ng Nd sample loads is a factor of 1.4 better compared to previously published Nd+ results and is similar to those obtained using the more time-consuming NdO+ techniques. Reproducibility tests on 100 pg Nd and Sr samples of reference materials yield 176 ppm of 143Nd/144Nd and 92 ppm of 87Sr/86Sr. These data suggest that we can resolve variability in the fourth decimal place of Nd and Sr isotope ratios in geological samples as small as 100 pg.