Fractionation and mixing of Nd isotopes during thermal ionization mass spectrometry: implications for high precision Nd-142/Nd-144 analyses

High precision TIMS analyses of the La Jolla Nd standard show that although the internal precision (2 sigma(mean)) of most runs is better than 4 ppm for the Nd-142/Nd-144 and Nd-143/Nd-144 values, the external reproducibility (2 sigma) is a factor of two worse. This is because in several runs, the mass-dependent fractionation of the isotope ratios does not follow the exponential law for the whole run. Divergence from exponential law behavior starts at a different point in time for each run and is accompanied by a switch from normal to 'reverse' fractionation, where isotope ratios get lighter with time. Ratios measured thereafter display increased scatter, upward shifts in Nd-142/Nd-144, Nd-143/Nd-144, Nd-148/Nd-144, and Nd-150/Nd-144, and a downward shift in Nd-145/Nd-144. The deviations are to the concave side of the exponential law curves in Nd-X/Nd-144 vs. Nd-146/Nd-144 space, and hence can be explained by mixing of ions derived from isotopically heterogeneous sample reservoirs that fractionate independently along the exponential law curve. Such variably fractionated sample domains would be expected to develop on the filament in response to prevailing temperature gradients and heterogeneous sample distribution. Such an effect can also account for the apparent stalling or reversal of the mass fractionation. Isotope mixing during mass spectrometry is of great concern when measuring Nd-142 anomalies because an inevitable consequence of the phenomenon is that Nd-142/Nd-144 deviations will always be positive when using Nd-146/Nd-144 for mass fractionation correction. This might preclude the detection of potential negative Nd-142 anomalies, or produce spurious positive Nd-142 anomalies. Upward shifts in Nd-142/Nd-144 as high as 70 ppm can be observed during an analysis. In practice, this could result in measurement errors and uncertainties of about the same magnitude as the natural anomalies geochemists try to resolve, i.e., < 50 ppm. The ultimate attainable accuracy and reproducibility of isotope ratios will thus be constrained by ion-counting statistics as well as the extent of isotope mixing during the measurement on a mass spectrometer.