Isotopic fractionation instrumental

Isotopic fractionation provides illustrative examples of first-order expansions of unknown functions. In general, the mass spectrometric measurement r/ of the ratio between two isotopes of mass m( and m, of the same element, differs from the natural value R/. Only a very small fraction of the original sample produces ions and different processes taking place in different parts of the mass spectrometer act differently on the sensitivity of each isotope. We assume that instrumental isotopic fractionation is mass-dependent. 

A third source of error is associated with the fragmentation pattern caused by dissociation of the molecular ions formed in the source region of the spectrometer. Under severe conditions these processes may proceed with substantial isotopic fractionation, and this obscures the measurements of isotopic composition at the collector. To some extent careful standardization of the instrumental conditions may ensure that errors from fragmentation are systematic, and thus cancel (at least to some extent). Alternatively, softer ionization methods can be used to prevent most or all of the fragmentation. The bottom spectrum in Fig. 7.7 illustrates this approach it shows the mass spectrum of chlorobenzene obtained by photoionization. Only the parent molecular ions are observed. It should be kept in mind, however, that softer ionization usually yields smaller ion currents and consequently statistical counting errors increase. 

Figure 1. Schematic representation of the calcium mass spectrum in (a) natural materials, (b) a Ca- Ca tracer solution used for separating natural mass dependent isotopic fractionation from mass discrimination caused by thermal ionization, and (c) a typical mixture of natiwal calcium and tocer calcium used for analysis. The tracer solution has roughly equal amounts of Ca and Ca. In (c) the relative isotopic abundances are shown with an expanded scale. Note that in the mixed sample, masses 42 and 48 are predominantly from the tracer solution, and masses 40 and 44 are almost entirely from natural calcium. This situation enables the instrumental fractionation to be gauged from the Ca/ Ca ratio, and the natural fractionation to be gauged from the sample Ca/ Ca ratio.

We have known for many years that large isotopic fractionations of heavy elements like Pb develop in the source regions of TIMS machines. Nonetheless, most of us held fast to the conventional wisdom that no significant mass-dependent isotopic fractionations were likely to occur in natural or laboratory systems for elements that are either heavy or engaged in bonds with a dominant ionic character. With the relatively recent appearance of new instrumentation like MC-ICP-MS and heroic methods development in TIMS analyses, it became possible to make very precise measurements of the isotopic ratios of some of these non-traditional elements, particularly if they comprise three or more isotopes. It was eminently reasonable to reexamine these systems in this new light. Perhaps atomic weights could be refined, or maybe there were some unexpected isotopic variations to discover. There were. 

More recently, enantiomer ratios have been used as evidence of adulteration in natural foods and essential oils. If the enantiomer distribution of achiral component of a natural food does not agree with that of a questionable sample, then adulteration can be suspected. Chiral GC analysis alone may not provide adequate evidence of adulteration, so it is often used in conjunction with other instrumental methods to completely authenticate the source of a natural food. These methods include isotope ratio mass spectrometry (IRMS), which determines an overall 13C/12C ratio (Mosandl, 1995), and site-specific natural isotope fractionation measured by nuclear magnetic resonance spectroscopy (SNIF-NMR), which determines a 2H/ H ratio at different sites in a molecule (Martin et al 1993), which have largely replaced more traditional analytical methods using GC, GC-MS, and HPLC. 

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

A number of research groups have used SIFT instruments for measurements directed toward IS chemistry. The Birmingham group of Adams and Smith, the inventors of the SIFT technique [16], was particularly active in this regard and a major focus of their SIFT measurements was the systematic study of reactions of hydrogenated ions, e.g. CH,, CjH,, NH , HnS+, HnCO+ etc., with numerous molecular species [18]. Further contributions by this group include detailed studies of isotope exchange in ion-neutral reactions, studies for which the SIFT is eminently suited, since the ion source gas and the reactant gas are not mixed. From these studies and detailed kinetic models of interstellar ionic reactions, it is now understood that the observed enhancement of the rare isotopes (e.g. D, 13C) in some IS molecules is due to the process of isotope fractionation in ion-neutral reactions [19]. 

An NMR spectrometer for SNIF-NMR (Fig. 6.17) (site-specific natural isotope fractionation NMR) measurements [226] must be specifically equipped and adapted, e.g. for deuterium analysis by a high field magnet (e.g. 9.4 T, corresponding to 400 MHz ( H) and 61.4 MHz ( H) resonance frequency, or 11.4 T, corresponding to 500 MHz ( H) and 76.8 MHz ( H) resonance frequency), a specifically adapted H-NMR probe with fluorine lock and proton decoupling, highly stable electronics and software for spectra acquisition and data processing/treatment. Instrumental details... 

In the determination of metals from biological and medical matrixes, thermal ionization mass spectrometry is seldom used. Disadvantages of thermal ionization MS are the great fluctuations in the results, caused by different instrumental requirements. Isotope fractionation resulting from vaporization of the sample and the dependence of this process on temperature are the main sources of error. However, the development of computer-controlled sample preparation and measurements have minimized these errors ... 

As indicated in the reaction sequences of equation 1, fractionation of the tritium does occur between the calciiom hydroxide and acetylene formed as products of the initial reaction of the water sample with the calcium carbide. However, the isotopic effect was found by Hohndorf and Oro (14) to be constant at 31.4 1.2 percent, thus indicating that the isotope fraction is independent of chemical yield for the reaction. For the system under consideration, the production of a constant volume of benzene was not attainable due to the restrictions of varying sample size, changes in the efficiency of trimerization due to catalyst depletion from previous samples, and the potential for varying rate of reaction of the water-carbide system. This fact, coupled with the lack of adequately documented tritiated benzene standards, led to the adoption of a system calibration rather than a yield determination for each sample in conjunction with a separate instrument calibration. 

In biological and clinical investigations the position is somewhat different. In both instances the enrichments measured are often large and it is normal practice to express the enrichment of a particular fraction as compared to the same fraction prior to the introduction of the labelled compound to the system. In such cases most workers express enrichment of isotopic content as either atom% or atom% excess which have been defined earlier (p. 5). Where is used at low enrichments and measured in an isotopic ratio instrument normally equipped to monitor the m/e 28 and m/e 29 ion beams, the following formula is applicable ... 

Although generally only the lighter elements are affected by isotopic fractionation, the increased precision of modern IRMS instrumentation has enabled the... 

It is widely accepted that the precision or reproducibility of isotope ratio measurements can be improved dramatically by using an MC instrument. However, further correction or careful procedural protocols are required to also improve the trueness of the resulting isotopic data. There are two major considerations for better quality isotope ratio measurements using LA-ICP-MS analysis the isotope fractionation during the LA and/or ionization process in the ICP, and the contribution of the matrix effect (non-spectral interference) to the mass bias effect. The latter is discussed in Section 4.6. First, the level of isotope fractionation during LA or ionization processes is discussed in this section. 

An ICP-MS instrument with an MC array system can easily improve the precision of the isotope ratio measurements. However, stability of the signal intensity profile, correction for isotope fractionation during LA and ionization, and correction of the mass bias effect using a matrix-matched calibration standard are also very important for improving the trueness of the isotopic data. 

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