A Low-cost Continuous-flow Gas Interface for Coupling an Elemental Analyzer with a Micadas AMS: gas flow Mathematical Model and first results

ABSTRACTA fully automatic continuous-flow gas injection interface was built to couple an elemental analyzer with a MICADAS accelerator mass spectrometer (AMS) as a low-cost option that does not require an absorber trap for CO2 injection. The complication of the variable ion current during gas injection can be overcome by understanding and controlling the mass flow-dependent ionization yield. The time-varying CO2 concentrations and carbon mass flows are estimated with a mathematical model in order to investigate their relationship with the abundant isotope (12C–) signal. This model is based on a complete CO2 diffusion equation and instantaneous mass flow. It shows a good agreement between model calculations and the measurements. A reversible suppression of the formation of ions occurs, if the carbon mass flow exceeds 2.0–2.3 µg C/min. This result repeats for different injection capillaries and for different carrier volumetric flow rates.

Stable isotopes in clinical research: safety reaffirmed

Approaching half a century of stable-isotope usage in human metabolic studies has been without documented significant adverse effect. Side-effects with acute D dosing are transitory with no demonstrated evidence of permanent deleterious action. The threshold of D toxicity has been defined in animals and is far in excess of concentrations conceivably used in human studies. The possibility that D may have additional beneficial pharmacological applications cannot be excluded. For isotopes other than D, evidence of observed toxicity remains to be produced even at dosages far in excess of the range used in metabolic studies. Absence of adverse effect may be attributable to small mass differences and the similar properties of tracer and predominantly abundant isotope. Absolute determination of stable isotope toxicity in humans is rendered impossible by ethical considerations. Also, the precision of extrapolating toxicity thresholds from animal studies remains unknown. However, should perturbation of the delicate homoeostatic characteristic of living organisms occur with use of stable isotopes, it is almost undoubtedly at some level of administration greatly in excess of those administered currently in biomedical research.

How to Calculate Isotope Ratios

The calculation of isotope ratios requires special consideration because isotope ratios, unlike matter or energy, are not conserved. In this chapter I shall show how extra terms arise in the equations for the rates of change of isotope ratios. The equations developed here are quite general and can be applied to most of the isotope systems used in geochemistry. As an example of the application of these new equations, I shall demonstrate a simulation of the carbon isotopic composition of ocean and atmosphere and then use this simulation to examine the influence on carbon isotopes of the combustion of fossil fuels. As an alternative application I shall simulate the carbon isotopic composition of the water in an evaporating lagoon and show how the composition and other properties of this water might be affected by seasonal changes in evaporation rate, water temperature, and biological productivity. Equations for the rates of change of individual isotopes in a reservoir are not essentially different from the equations for the rates of change of chemical species. Isotopic abundances, however, are generally expressed as ratios of one isotope to another and, moreover, not just as the ratio but also as the departure of the ratio from a standard. This circumstance introduces some algebra into the derivation of an isotopic conservation equation. It is convenient to pursue this algebra just once, as I shall in this section, after which all isotope simulations can be formulated in the same way. I shall use the carbon isotopes to illustrate this derivation, but the same approach can be used for the isotopes of other elements, such as sulfur, oxygen, nitrogen, hydrogen, or strontium. The most abundant isotope of carbon has a mass of 12 atomic mass units, 12C. A less abundant stable isotope is 13C. And much less abundant is the radioactive isotope 14C, also called radiocarbon. It is convenient to express the abundances of these rare isotopes in terms of ratios of the number of atoms of the rare isotope in a sample to the number of atoms of the abundant isotope. We call this ratio r, generally a very small number.

Introductory remarks

The fast-neutron breeder reactor is the principal means now envisaged of exploiting the very large resource of energy residing in the naturally abundant isotope of uranium , 238 U. Extensive research and development programmes are being carried out in a num ber of countries to realize this potential. There are about a dozen substantial reactors operating; and wide-ranging supporting programmes include fuel processing and development, and safety and environmental issues. The purpose of this Discussion Meeting is to present the principal recent scientific and engineering results of these world-wide program m es; and, in the final session, to discuss the future trends in this research and its utilization.

THE APPROACH TO NUCLEAR STATISTICAL EQUILIBRIUM

The transformation of a region composed initially of 28Si to nuclei in the vicinity of the iron peak, which is thought to take place in the late stages of evolution of some stars, is considered in detail. In order to follow these nuclear transformations, a nuclear reaction network is established providing suitable reaction links connecting neighboring nuclei. A method of solution of the network equations is outlined. Thermonuclear reaction rates for all neutron, proton, and alpha-particle reactions involving the nuclei in this network have been determined from a consideration of the statistical properties of nuclei. The evolution of this silicon region has been followed in time for two cases: T = 3 × 109 °K, ρ = 106 g cm−3 and T = 5 × 109 °K, ρ = 107 g cm−3. While both the observed solar and meteoritic abundances display a broad peak in the vicinity of iron, centered on 56Fe, in these calculations 54Fe is found to be the most abundant isotope in this mass range. Beta decays required to change the peak to 56Fe are very slow. As the transformation 2 28Si → 54Fe + 2p is endothermic by ~1.3 MeV, these results suggest that the silicon-to-iron conversion may not comprise an exothermic nuclear burning stage of stellar evolution.