PREPARATION AND CHARACTERIZATION OF SYNTHETIC MIXTURES OF LITHIUM ISOTOPES

Re-certification of the absolute isotopic com position of the natural lithium isotopic reference material (IRM), IRMM-016, requires measurem ents calibrated by means of synthetic mixtures of highly enriched lith ium isotopes. Ten such mixtures were prepared by weighing and mixing o f two well characterised, isotopically enriched, Li2CO3 compounds. The starting materials, 99.9981% enriched Li-6, and 99.9937% enriched Li- 7, were purified by ion exchange, and the purified materials converted from LiOH to Li2CO3 by reaction with CO2. Ten new mixtures were prepa red by mixing different weighed amounts of these dissolved Li2CO3 carr ier compounds. The compounds had an estimated level of impurities of 1 00 +/- 100 mu g.g(-1) (expanded uncertainty with a coverage factor of 2). In the ten mixtures, the n(Li-6)/n(Li-7) ratio varies from 0.025 t o 14 and the achieved expanded relative uncertainty on the amount rati o prepared is typically 2.10(-4). These mixtures were then used to det ermine the correction factor, K, for mass discrimination of the measur ement procedure and instrument concerned.