Absolute values have been obtained for the isotopic abundance ratios of a reference sample of nickel (Standard Reference Material 986), using thermal ionization mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated nickel isotopes, were used to calibrate the mass spectrometers. The resulting absolute isotopic ratios are: 58Ni/60Ni=2.596061±0.000728, 61Ni/60Ni=0.043469±0.000015,62Ni/60Ni=0.138600±0.000045, and 64Ni/60Ni=0.035295±0.000024, which yield atom percents of 58Ni=68.076886 ±0.005919, 60Ni = 26.223146±0.005144,61Ni=1.139894±0.000433, 62Ni =3.634528±0.001142, and 64Ni =0.925546±0.000599. The atomic weight calculated from this isotopic composition is 58.693353 ±0.000147. The indicated uncertainties are overall limits of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error.

,An absolute value has been obtained for the isotopic abundance ratio of a reference sample of rhenium, using surface emission mass spectrometry. Samples of known isotopic composition , prepared from nearly isotopically pure se parat ed rhenium isotopes , were used to calibrate the mass spectrometers. The resulting absolute '85 Re/'·7 Re ratio is 0.59738 ± 0.00039, which yields atom per· cents of ' 8s Re=37 .398±0.016 and '87Re=62.602±0.016. The atomi c weight calculated from this isotopi c c omposition is 186.20679 ± 0.00031. The indi cated uncertainties are overall limits of e rror based on 95 perce nt confidence limits for th e mean and allowances for the effe cts of known sources of possible systematic error.

August 8. 1979T 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.Key words: Absolu te rat ios; atom ic weight; iso topic a bundance; referen ce standard; thallium; tha lliu m ch ro mate .

The isotopic composition of cadmium, calcium and magnesium in the Brownfield chondrite have been measured. The measurements on cadmium show that this element is isotopically fractionated with the heavier isotopes relatively enriched to the extent of 2.7%0 per mass unit. This confirms earlier reports by DE LAETER (1976, 1978). Calcium and magnesium show no evidence of isotope fractionation, indicating that the process responsible for fractionating cadmium does not seem to have affected these more refractory elements.

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