Stable isotope ratio measurements gases

While the dual inlet technique is generally the most precise method for stable isotope ratio measurements, the continuous-flow technique offers online sample preparation, smaller sample size requirements, faster and simpler analyses, increased cost-effectiveness, and the possibility of interfacing with other preparation techniques, including elemental analysis, gas chromatography (GC), and liquid chromatography (LC). For these reasons, CF-IRMS is the subject of the majority of research currently being conducted in the field of forensic science [1],... 

Compound specific stable isotope analysis using gas chromatography combined with an isotope ratio mass spectrometer - GC-IRMS (see also Chapter 7) - is now a mature analytical technique in environmental science and technology, especially in the area of contaminant source attribution and in assessing the biodegradation of contaminants.108 Several studies have focused on 13C/12C, 180/160 and 170/160 isotope ratio measurements for volatile organic and metalor-ganic compounds to study isotope fractionation effects and to identify contamination in the environment.109... 

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

One technique to conquer the collection job of large sample volume (2.5 1) in glass containers in traditional measurements of the stable isotope of CO2 is by using a gas chromatograph isotope ratio mass spectrometer (GC/IRMS) to analyze and of CO2 at ambient CO2 concentration (of approximately 369 ppm). The sample volumes are reduced down to the level of ml and collection of large sample numbers in the field is allowable. Detection limit of this technique for CO2 is 10 ppm and meets the requirement for global monitoring of atmospheric CO2. 

As stated above, the isotope ratio measurements are made by comparing the ion beam intensities obtained from an unknown sample gas with those from a reference gas. In the case of nitrogen, which in the gaseous form exists as the diatomic molecule N2, and which has only two stable isotopes and N), the isotope ratio measurement is made simply by a comparison of sample and reference ion beams at miz 29 and 28 (i.e., and W N + ). In a... 

After chemical pretreatment, the samples are burnt to produce carbon dioxide and nitrogen. A small amount of this gas is bled into a mass spectrometer, where the stable isotope ratios of carbon and nitrogen are measured. These ratios provide useful information on the purity of the sample and clues about the diet and climatic conditions of the living organism. The carbon isotope ratio is also used to correct for isotopic fractionation in the radiocarbon measurement. The carbon dioxide is collected in a glass ampoule or converted to graphite for radiocarbon measurement on the AMS system. 

Coplen, T.B. (2011) Guidelines and recommended terms for expression of stable isotope-ratio and gas-ratio measurement results. Rapid Commun. Mass Spectrom., 25, 2538-2560. 

Stable-isotope dilution analysis is an analytical technique in which a known quantity of a stable-labelled isotope is added to a sample prior to extraction, in order to quantitate a particular compound. The ratio of the naturally abundant and the stable-labelled isotope is a measure of the naturally abundant compound and can be determined only by gas chromatography-mass spectrometry since the naturally abundant and the stable-labelled isotope cannot be completely separated gas chromatographically. 

A measure of the amount of a stable isotopic label that exceeds its natural abundance in unlabeled tracee. This is most directly accomplished using an ion ratio mass spectrometer to measure the ratio of ion currents for isotopomers such as C02 at mass 44 and 2 at mass 45. From the difference between the ion current ratio for a sample (Zsampie) and the ion current ratio for a reference gas (Zreference), the atom percent excess (APE) can be estimated. See Tracer/Tracee Ratio Compart-mental Analysis Isotope Exchange Kinetics... 

An instrument that measures the isotopic mass ratio of a gas by bombarding the sample in an electron beam, such that the molecular ions generated can be deflected in their trajectories through a magnetic field in accordance to their charge/mass ratios. These devices are extremely accurate and reliable, and many stable isotope experiments can be analyzed by converting the isotopi-caUy substituted metabolite into carbon dioxide, water, or molecular nitrogen prior to I RMS measurements. 

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