Chemistry: analytical and immunological testing – Hydrogen – per se
Patent
1996-04-26
1998-06-16
Soderquist, Arlen
Chemistry: analytical and immunological testing
Hydrogen, per se
436155, 436158, 436159, 436161, 436173, 436181, 436182, G01N 3300
Patent
active
057669546
DESCRIPTION:
BRIEF SUMMARY
This is a National Stage Application of PCT application No. PCT/GB95/02192 filed Sep. 15, 1995.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of determining the quantitative isotopic composition of hydrogen comprised in a compound, and to apparatus for carrying out the method.
2. Description of the Prior Art
The isotopic analysis of the hydrogen contained in an organic compound is a matter of wide interest in fields varying from petrochemicals to space science. However, present methods of determining the isotopic composition of hydrogen are laborious, time-consuming and suffer from many problems such as isotopic fractionation, memory effects and the fact that many materials are permeable to hydrogen gas. Typically, the time taken for a single isotopic analysis is of the order of tens of minutes, and after one analysis has been performed, a longer period of time elapses before it is possible to perform another analysis. Further, present methods do not allow separate isotopic analysis of the hydrogen contained in several compounds in a mixture of compounds.
One method commonly used to isotopically analyse the hydrogen contained in an organic sample involves the combustion of the organic matter to carbon dioxide and water. The water is cryogenically separated from the CO.sub.2 and passed over hot uranium or zinc shavings in order to reduce it to hydrogen gas, which is then mass-spectrometrically analyzed to give the isotopic composition. This procedure is slow, and is liable to the problem of isotope fractionation, so that an accurate isotopic analysis is not obtained.
A solution to these problems was suggested by Zvi Sofer (Anal. Chem. Volume 58, 1986, pp 2029-2032 "Chemistry of Hydrogen Gas Preparation by Pyrolysis for the Measurement of Isotope Ratios in Hydrocarbons".) This method involves the direct pyrolysis of the hydrocarbon to hydrogen gas at elevated temperatures, (>900.degree. C.). The gas formed by pyrolysis is then isotopically analyzed using a mass spectrometer. The result is corrected by a factor that takes into account the isotope fractionation effect. This factor is calculated from the rate constants of the reactions taking place, and is only accurate if there are no other elements present apart from carbon and hydrogen, so that this method is only suitable for hydrocarbons of the type C.sub.x H.sub.y, and must take place inside a sealed system,. which adds to its complexity.
Harting (Isotopenpraxis, 1989 vol 25 (8) pp 347-348) teaches another batch process for the conversion of hydrocarbons to hydrogen. The analyte is pyrolyzed at a temperature of 1000.degree.-1300.degree. K. in an evacuated quartz furnace packed with a Chromium catalyst for between 5 and 30 minutes. The hydrogen so produced may then be manually transferred into the inlet system of a mass spectrometer using a Toepler pump. Complete transfer of the sample is necessary to avoid errors due to fractionation so that the process is very time-consuming. Harting's experiments suggest that the catalytic reaction produces substantially complete conversion in 5 minutes at 1300.degree. K. and that no significant errors are introduced by diffusion of the hydrogen through the walls of the quartz furnace.
A much faster general method for the isotopic analysers analysis of organic compounds is taught in EP 0419167. A sample comprising a mixture of analytes is injected into a flow of an inert carrier gas (e.g. helium) substantially at atmospheric pressure and the resulting flow passed through a gas-chromatographic column in order to separate the analytes. The effluent from the column is passed, again at atmospheric pressure, through a catalytic conversion furnace, typically comprising a Pt/Rh catalyst maintained at 1200.degree. C., so that hydrocarbon analytes are converted to carbon monoxide, molecular hydrogen and molecular nitrogen. These gases, and the inert carrier gas are then introduced into the source of an isotopic-ratio mass spectrometer via a pressure-reduction stage. It has been fo
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Brockwell Timothy Graham
Freedman Philip Antony
Micromass Limited
Soderquist Arlen
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