Isotope ratio mass spectrometry natural variation

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. 

Oxygen isotope ratios The use of oxygen isotope ratios is largely restricted to the measurement of water in fruit juices. Water in fruit is assimilated from groundwater and reflects the isotope ratios present in it variations between juices are therefore naturally derived from variations in the groundwater. Analysis for carbon and oxygen isotopes is carried out using mass spectrometry however, analysis is not simple. 

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

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, O and N and gas forming elements, e.g., C and S. 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 Precise and accurate... 

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