Absolute isotopic abundance and the atomic weight of a reference sample of thallium

Abstract

T he accepted atom ic we ig ht of thalli um has remaine d at a valu e of 204.37 ± 0.03 s ince 1962.At thi s le ve l of uncertainty, howeve r , t he atomic weight becomes a limiting fa ctor to hi g h accu racy analys is.Th e ne w mass spectr ometric determination of th e atomic weight of th allium has bee n co m pleted.A hi g h precis ion assay tec hnique was developed so that accura te ly known quan titi es of the 'OJTI and '•'TI separated iso topes co uld be mixed to produ ce st andards for calibrat io n of the mass spec trometer.This assay tec hnique involved th e gravim e tri c deter m in atio n o f 99.3 per cent of th e th allium as TI ,C rO •.The so lubl e thallium was th en aliquoted and det ermined by iso tope dilution mass spec tr ome try.Befor e making up the fin al soluti ons from which the assay and calibr atio n sampl es would be withdr aw n , the separated iso topes wer e purified by so lvent ex tr ac tio n and electrodeposi tio n.A t un gs te n filament s urface ionization tec hniqu e was deve lo ped for the determinati o n of pr ec ise iso topic abundance meas ure me nts fo r thal li um .Thi s techn ique allowed isotopi c analysis of the se pal ated iso topes, ca li brat ion standards, and a natural th alli um re ference st andard with prec isio ns of be tt er than 0. 1 perce nt.T he '•'TI/ 'OJTI abso lute iso to pi c abu nd ance ra t io of the re ference sampl e was found to be 2.387 14 ± 0.00 I 0 1, yielding an atom ic weight of 204.38333 + 0.000 18.

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