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Stable isotope ratio measurements - d 13C, d 18O, d 15N, d 34S, d D |
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Prior to stable isotope ratio measurements different samples has to be brought in gaseous form. A host of preparation methods have been developed and improved to convert different sample compounds to an appropriate gas including H2, CO2, N2, and SO2. δ18O measurement The d18O value of the water sample is derived from that of the CO2, with which there is an equilibrium. Therefore CO2-H2O isotope exchange method is used by equilibrating the water first with CO2 and then analyzing the CO2. δ D measurement Deuterium in water is measured by reducing the water to elemental hydrogen, using Zn. 250 mg BDH ANALAR Zn is introduced into an ampoule and heated, while the system is evacuated. Cooling the Zn at temperature of -196°C, the water is trapped on its surface. After that the ampoule is evacuated and heated. The hydrogen obtained by reduction can be introduced directly into the mass spectrometer. δ 13C measurement The measurement of isotopes in carbonate minerals is done on CO2 gas that normally is produced by acidification. The sample is prepared by means of phosphoric acid. The products of the reaction are water, which is trapped cryogenically at -60 oC, and carbon dioxide, which is trapped at liquid nitrogen temperature. After pressure adjustment carbon dioxide can be analyzed by the mass spectrometer. δ 15N measurement (NO3-, NH4+) During sample distillation the dissolved nitrate is reduced to NH3. The dissolved ammonium is liberated with MgO and the evolved NH3 is trapped in diluted H2SO4. Oxidation of ammonium sulphate is made by sodium hypobromite. The evolved N2 gas is then analyzed directly on the mass spectrometer. δ 34S measurement Sulphate sample and NaPO3 are added into a fused quartz tube, covered with quartz wool and heated to 610°C. The copper added into the colder part of the tube will reduce the formed SO3 to SO2. The temperature of the furnace is increased to 1100oC. The resulted gases (SO2, O2, H2O) are trapped into liquid nitrogen. The purification of SO2 is achieved by replacing the liquid nitrogen with a gas-trap at -40oC, in which contaminants are trapped and SO2 can be transferred into the measurement ampoule and connected to the mass spectrometer. |
