instrumental techniques applied to mineralogy and geochemistry

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86 Clemente Recio collector gas source mass spectrometer. Since then, the IRMS has become the measurement technique of choice for most of the light elements (certainly so for H, C, N, O and S). Main differences between IRMS currently derive from the inlet method and the online preparation systems available. Originally, gases were generated off-line, and admitted into the sample bellows of the dual inlet (DI) system. A second bellow would have the reference gas, and sample and reference would be alternatively admitted into the ion source by means of capillaries and a 4-way change-over valve. The variable bellows allow matching the pressures of sample and reference gases, to prevent fractionation. Recently, continuous flow (CF) machines have become common. In CF instruments the dual inlet system is replaced by a carrier gas flow, commonly He. This type of design is particularly suitable for the use of on-line preparation systems, such as elemental analyzers, gas chromatographs or equilibrium devices. Regardless of inlet method (DI or CF), on reaching the ion source, the gas is ionized by electrons emitted by a heated filament, that strip an electron off the molecule, creating positive ions. Ionization efficiency is very variable, commonly in the range 0.01 to 0.1% (850 to 2000 molecules required per ion formed). A high voltage accelerates these ions through a set of exit slits that focus the beam into the mass analyzer, where ions are separated as a function of mass. The resolved ion beam is collected on ion detectors (commonly Faraday Cups), the signal is electronically amplified, and the isotopic ratios are reported by the data system. Sample gases fed to the DI systems are produced from the sample offline the mass spectrometer, in purpose built extraction lines. This is time consuming, and requires relatively large samples and specialized knowledge, but results in turn in highly precise measurements. The two most important advantages of the continuous flow systems are the increase in sample throughput and the decrease in the amount of sample required, such that measurements in the nanomol range (10 -9 M) are routine now. All CF methods use a He flow to introduce the sample gas into the mass spectrometer. The sample gas is produced in a range of automated preparation systems, that can lead to “bulk” (BSIA) or “compound-specific” (CSIA) isotopic analysis.
Alike as two water drops: distinguishing one source of the same substance from another 87 Elemental Analyzers An elemental analyzer coupled to a magnetic sector mass spectrometer (EA-IRMS) gives the bulk isotopic composition of the sample. Depending on the specific set up, it is possible to measure isotopic ratios of H, C, N, O and S in a range of solid and liquid matrices. In combustion mode, C-, N- and S-containing materials are loaded into Sn capsules, which are dropped into a reactor packed with suitable catalists. O 2 is admitted into the reactor, resulting in flash combustion of the sample to a mixture of N 2 , NO, CO 2 , O 2 , SO 2 and H 2 O, depending on the nature of the substances combusted. This gas mix is carried by the He flow to a reduction reactor, where NO x are reduced to N 2 , and excess O 2 is eliminated. Water is eliminated in a chemical trap, and the gases of interest separated on a suitable chromatography column. The pyrolysis mode is employed for the analysis of O and/or H. For oxygen, samples are loaded into Ag capsules and dropped into a ceramic reactor lined by a glassy carbon tube in the inside, and packed with nickelized carbon, where the sample is pyrolyzed to CO. As before, water is chemically trapped, and contaminant gases separated by chromatography. H from water samples is obtained by injecting 0.2-0.4 μl via a liquid autosampler onto a reactor packed with Cr. GC-C-IRMS CSIA is possible by coupling a GC to the mass spectrometer via a combustion interface. Initially, only C could be determined. Later on, however, modifications were made that allow measurement of N, O, and H isotope ratios. In its simplest form, the interface is an oven packed with an oxidant (CuO, NiO, Pt, … depending on manufacturer). The compounds eluted from the chromatography column are carried by the He flow; combusted in the interface, and carried into the source of the mass spectrometer. H 2 O generated during combustion is physically eliminated by membrane diffusion. If N is to be determined, an additional reduction oven is required. Precise 15 N measurements depend on quantitative exclusion of CO 2 (since this fragments in the source into CO + , which isobarically interferes with N 2 at masses 28 and 29). To this end, a cryogenic trap is placed before the inlet of the mass spectrometer. This requirement prevents C and N being determined on the same sample.
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