Isochronous mass spectrometry

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

---