Isotope Ratio Measurements by Gas Source Mass Spectrometry

3 Isotope Ratio Measurements by Gas Source Mass Spectrometry 

The majority of elements analyzed with respect to isotope ratios in order to study isotope variations in nature are light gaseous elements such as H, and N and gas forming elements, e.g., C and 

Gas source mass spectrometry (GSMS) with electron impact (El) ion source produces nearly mono-energetic ions (similar to TIMS) and is an excellent tool for the high precision isotope analysis of light elements such as H, C, N and O, but also for S or Si.7,100,101 Precise and accurate measurements of isotope ratios have been carried out by gas source mass spectrometers with multiple ion collectors by a sample/standard comparison and the 8 values of isotope ratios were determined (see Equation 8.4). Electron impact ionization combined with mass spectrometry has been applied for elements which readily form gaseous compounds (e.g., C02 or S02) for the isotope analysis of carbon and sulfur, respectively). 

Further applications of mass spectrometry for the analysis of gases are discussed in Chapter 7. 

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This chapter focuses on selected developments and applications of mass spectrometry for the analysis of gases and volatile compounds relevant in inorganic mass spectrometry. A brief introduction to isotope ratio measurements by gas source mass spectrometry is presented in Section 8.3. 

The steady-state operation of the ICP source is beneficial for the correction of instrumental mass bias by standard-sample bracketing, where the raw (measured and uncorrected) isotope ratio data of a sample are referenced to the results obtained for an isotopic standard, which is preferentially analyzed before and after each sample [27, 35]. This technique is similar to the standardization method commonly used in gas source isotope ratio mass spectrometry. To account best for drifts in instrumental mass bias, which can be particularly severe for light elements such as Li and B, data collection often utilizes multiple but short analytical measurements for samples that are each bracketed by standard analyses. Switching between samples and standards can be very rapid, if long washout protocols are not required, and mass spectrometric measurements of about 5 min or less have been used to optimize the precision of Li and Mg isotope ratio measurements by MC-ICP-MS [111, 112]. 

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)." ... 

Gas-isotope-ratio mass spectrometry is used to measure the and O abundances in the H2 and CO2 samples, respectively. The instrumentation is known as gas-isotope-ratio mass spectrometry because all samples entering the ion source of the mass spectrometer must be in gaseous forms such as H2 for abundance measurements and CO2 for abundance measurements. Upon entry into the ion source of the mass spectrometer, the H2 or the CO2 gas is ionized by electrons to form positively charged ions of and for H2 or and C 0 for CO2. Because of the difference in ionic masses between these positively charged ions, they are separated into two ion beams through a magnetic field. The amounts of H and in the H2 or and in the CO2 are directly proportional to the amplified ion beam intensities of the and or C 02 ... 

The extent to which isotope fractionation is observed for a given element is determined by both the relative mass difference between its isotopes and the extent to which the element participates in physical processes and/or chemical reactions. For the light elements H, C, N, O, and S, variations in their isotopic composition caused by mass-dependent isotope fractionation have been extensively studied using gas source isotope ratio mass spectrometry. For most of the metallic and metalloid elements (except Li and B), the relative mass difference between the isotopes is more limited, such that the variation in isotopic composition thus created is considerably more limited. The high precision with which isotope ratios can be measured nowadays however, not only allows the small isotope fractionations to be revealed, but also quantified. Even for the heaviest naturally occurring element U, variation in its isotopic composition due to the occurrence of isotope fractionation has been demonstrated [13]. As a general rule, elements that can occur in the environment in several oxidation states tend to show more pronounced isotope fractionation. 

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