Precise isotopic analysis of lead in picomole and subpicomole quantities

A microanalytical procedure for the separation of Pb and U and their determination by isotope dilution-mass spectrometry is described. It is shown that high precision isotopic analysis of as little as 8 x 1011 Pb atoms is readily achievable. The level of Pb contamination is reduced to as little as 4 x 1010 atoms and the blank fluctuation is controllable. In the special case where the chemical separation could be omitted, 4 x 1010 Pb atoms were isotopically analyzed with a blank correction of 6 x 109 Pb atoms. This procedure was applied to a 0.2-mg lunar sample containing 2.5 ppM Pb and 1 ppM U and the U--Pb isotopic measurements obtained are fully compatible with measurements on large sample. In addition, Pb on the surface of individual lunar glass balls of 150-300 μm diamter was measured. Extension of this approach to other heavy metals may be possible. (auth)