C. M. Stevens

The Isotopic Composition of Methane in Polar Ice Cores

Air bubbles in polar ice cores indicate that about 300 years ago the atmospheric mixing ratio of methane began to increase rapidly. Today the mixing ratio is about 1.7 parts per million by volume, and, having doubled once in the past several hundred years, it will double again in the next 60 years if current rates continue. Carbon isotope ratios in methane up to 350 years in age have been measured with as little as 25 kilograms of polar ice recovered in 4-meter-long ice-core segments. The data show that (i) in situ microbiology or chemistry has not altered the ice-core methane concentrations, and (ii) that the carbon-13 to carbon-12 ratio of atmospheric CH4 in ice from 100 years and 300 years ago was about 2 per mil lower than at present. Atmospheric methane has a rich spectrum of isotopic sources: the ice-core data indicate that anthropogenic burning of the earths biomass is the principal cause of the recent 13CH4 enrichment, although other factors may also contribute.

Solar Neutrinos: Proposal for a New Test

The predicted flux on the earth of solar neutrinos has eluded detection, confounding current ideas of solar energy production by nuclear fusion. The dominant low-energy component of that flux can be detected by mass-spectrometric assay of the induced tiny concentration of 1.6 x 107 year lead-205 in old thallium minerals. Comments are solicited from those in all relevant disciplines.

Method for the determination of the concentration and of the carbon and oxygen isotopic composition of atmosspheric carbon monoxide

Abstract A procedure is described by which carbon monoxide in the air at the lowest concentration levels is quatitatively removed from the air and converted to pure carbon dioxide. The original isotopic composition of both carbon and oxygen is preserved and determined by analysis with an isotope-ratio mass spectrometer. The problems associated with isotopic analysis of these samples is discussed.

A Vacuum Lock for Routine Solid Analyses with a Mass Spectrometer

A vacuum lock for rapid introduction of solid samples into a mass spectrometer is described. Analyses of the highest resolution requiring high vacuum can be made in five to ten minutes following the introduction of the sample. With an analysis time of about one hour using a surface ionization type of source, six to eight samples can be run in an eight‐hour day. The unit is incorporated in a 12‐in. 60° mass spectrometer.

Mass-spectrometric observations of uranium and plutonium monohydrides formed by ion—molecule reaction☆

Abstract The monohydride ions UH÷, UD+, PuH+ and PuD+ have been observed in the mass spectrum of U and Pu during the isotopic analysis of these metals by surface ionization mass spectrometry. By introducing hydrogen and vapors of hydrogen compounds into the ion source, the hydrides are found to be produced by endothermic ion—molecule chemical reactions of the metal-ion beam and residual gases. Hydride product ions have been obtained with measurable intensity for the systems: U÷+H2, U++D2, U++H2O, U÷+D2O, U++H2S, Pu++H2, Pu++D2. Analysis of relatively crude product-ion energy spectra permit the dispotential as a high-pass energy filter permits complete rejection of hydride ions in the mass spectrometer, due to the endothermic nature of the reactions involved.

236U/238U measurements in three terrestrial minerals and one processed ore☆

Abstract The 236 U/ 238 U ratio was measured mass spectrometrically in the following terrestrial uranium samples: uraninite with (2.3±0.8) × 10 −10 ; schroeckingerite with (2.0±2.0) × 10 −10 ; thucholite with (0.8±1.5) × 10 −10 ; and a processed ore with (6.2±2.2) × 10 −10 .

The isotopic abundance of molybdenum in terrestrial minerals

Abstract The isotopic abundance of molybdenum has been analyzed in molybdenite and wulfenite minerals from six locations all over the world and no anomalies have been found greater than 0.15%. No results were obtained with respect to differences in mass-dependent fractionation of the samples because of overriding instrumental mass discrimination.

Semiautomatic Data‐Collection Systems for Mass Spectrometers

Two programmed digital data‐collection systems have been developed and used with static‐field mass spectrometers to improve the precision of isotopic analyses of solid samples and to reduce the time required for collection and reduction of data. A large high‐sensitivity spectrometer using ion pulse counting detection has been equipped with a programmer and multiscaler so that up to four masses can be sequentially scanned and the resulting counts stored in different groups of the core memory. The simpler scheme of adding a programmed integrating digital voltmeter to sample the detector output of an analytical spectrometer is used where high sensitivity is not required. Both instruments use voltage switching between masses and magnetic sweeping of the mass peaks. Mass programming is done simply, but precisely and directly, by the use of operational amplifier techniques in voltage supplies.

Isotopic Abundances of Actinide Elements in Lunar Material

The abunldanzces of uranium and thorium were measured mass spectrometrically as 0.59 � 0.02 and 2.24 � 0.06 parts per million, respectively, in the fines of Apollo 11 builk sample. The ratio 235U/238U was 0.007258 � 0.000016, in agreement with terrestrial uranium. The following upper limits were set: 236U/238U≤3 x 10-9; parts per million of sample, 239Pu.≤1 x 10-9; 244Pu≤9 x 10-1l, and 247Cm ≤ 1.25 x 10-10. More alpha activity from 227Th and its decay products appears to be presenit than would be in equtilibrilum with the 235U in the sample. The search for other actinide and transactinide nuclides is continuing.