Sample size and the limits to precision in Pb-isotopic dating by ID-TIMS

Abstract

Using isotope analysis of ancient (4555 Ma) radiogenic Pb by ID-TIMS as an example, I evaluate the sample size required to achieve the target precision of the 207Pb/206Pb ratio 0.007% (2σ), corresponding to the uncertainty of the 207Pb*/206Pb* age of 0.1 million years, considering various analytical uncertainties. In a hypothetical perfect analytical setup that would enable measuring isotopic ratios without noise, losses and biases, this precision can be achieved by analysis of a sample containing 2.9 picograms of Pb. Assessment of the sources of noise, loss and bias introduced by sample preparation and mass spectrometry shows that incomplete ionization of Pb during evaporation from the filament and baseline noise of a mass spectrometer make the greatest contributions to the additional uncertainty. Subtraction of analytical blank and minor spike isotopes can also substantially increase the uncertainty under some analytical conditions. The contributions from the other sources are smaller, but can become significant if a higher precision level is sought.