Isotope ratio mass spectrometry advantages

Deuterium nmr spectroscopy has been utilized for the last decade to determine large (primary deuterium) KIEs in reactions with isotopes present at the natural abundance level (Pascal et al., 1984,1986 Zhang, 1988). A great advantage of this approach is that labelled materials do not have to be synthesized. Neither is there any need for selective degradation procedures, which are often necessary to produce the molecules of low mass, e.g. C02, acceptable for isotope ratio mass spectrometry. Moreover, the KIEs for several positions can be determined from one sample. However, until quite recently the relatively low precision of the nmr integrations that are used for the quantitative assessment of the amount of deuterium at specific molecular sites has limited the applicability of this technique for determining small (secondary deuterium) KIEs. 

This chapter provides an overview of mass spectrometer function and operation. It describes specific instrument types with demonstrated or potential application for measuring radionuclides and surveys the application of these instruments to radionuclide detection. Finally, it discusses the circumstances under which use of mass spectrometers is advantageous, the type of mass spectrometer used for each purpose, and the conditions of sample preparation, introduction and analysis. Its perspective is from a national laboratory active in environmental and non-proliferation monitoring. It emphasizes isotope ratio measurements, but mass spectrometric measurements also provide isotope mass information. Several recent books describe elemental and isotope ratio mass spectrometry in far greater detail than is presented here (Barshick et al., 2000 De Laeter, 2001 Montaser, 1998 Nelms, 2005 Platzner, 1997 Tuniz et al., 1998). High-resolution mass spectrometry forms the basis of the mass scale used for elemental and isotopic masses (Coplen, 2001), but this application of MS falls outside the scope of this chapter. 

Isotope dilution mass spectrometry (ID-MS) is widely accepted as a quantification procedure of proven accuracy in elemental analysis and isotope ratio measurements [4]. Several areas of research in nuclear science, geochronology, medicinal chemistry, environmental science, and agricultural science have benefited from this technique. ID-MS is applicable to all elements that have at least two stable isotopes. Monoisotopic elements can be analyzed only if they have a long-lived natural or artificial radioisotope. For example, iodine and thorium have been determined with spikes of the long-lived isotopes 29i and 25 Th, respectively [44]. TI-MS and ICP-MS are the methods of choice for accurate ID-MS analysis. ICP-MS has the advantage that several elements can be analyzed simnltaneously under the same experimental conditions. Other ionization techniqnes discussed in this chapter have also been coupled with ID-MS. 

In addition, one of the main features of mass spectrometry is, and this is the major advantage in comparison to other atomic and molecular non-mass spectrometric techniques, that it offers the possibility of determining isotope ratios and abundances of isotopes with high precision and accuracy in all types of samples (in solid, liquid and gaseous materials as well). Isotope ratio measurements have applied increasingly for stable isotopes in nature, especially for investigating... 

The isotopic ratios, altered by spike additions, are then measured on a portion of the sample by mass spectrometry. Even though the method is limited to elements having two or more naturally occurring or long lived isotopes, it is very sensitive and accurate and relatively free from interference effects. Thus, it has a great advantage over other analytical techniques. Thermal source and electron bombardment mass spectrom-... 

The principle of the isotope dilution analysis (IDA) is described in Section 6.4. Due to its advantages as a definitive and accurate analytical method for the determination of element concentration via isotope ratio measurements, IDA is being increasingly applied in mass spectrometry, especially in ICP-MS and LA-ICP-MS as one of the most frequently used techniques. For example, the isotope dilution technique is employed in species analysis in biological systems, " e.g., for the determination of mercury species in tuna material,or in aquatic systems. Further applications of the isotope dilution technique are the determination of selenomethionine in human blood serum by capillary HPLC-ICP (ORC) MS ° or sulfur speciation in gas oil, diesel or heating fuel by LA-ICP-MS. Evans and co-workers have reported on the high accuracy analysis of sulfur in diesel fuel by IDA. ICP-SFMS has been employed for Si species analysis in biological or clinical samples and... 

Electron ionization usually gives some molecular ions and sometimes these are abundant. Often it is difficult to be certain which ion, if any, is the molecular ion. For this rea.son, it is advantageous to obtain both El and Cl spectra, the first giving good structural information and the second good molecular mass information. g Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS),... 

The best mass spectrometry precision for measurement of a ratio is usually achieved with isotope amount ratio measurements close to unity. Hence silver was used by Begley and Sharps for their fundamental studies in order to exploit the advantages of a natural 1 1 system. However, it is generally acknowledged that modest deviations, by a factor of 3 or 4, from this optimum are unlikely to degrade the overall performance by a significant degree. 

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