Uses of Isotope Ratios

We have aln ady discussed in Chapter 1 / how mass spectrometry is used by chemists for the identification and quantitative determination of one or more elements in a sample of matter. This chapter is devoted to describing how mass spedt ometry is used to obtain the type of information listed in items (2) and (3) in the previous paragraph. Chapter Jl describes how mass spectrometry is emphy edfor elucidating the structure and composition of surfaces. Finally, ir Section 321), the use of isotopic ratios determined by mass spectnrmetty is discussed. 

Aromatic nitro compormds, primarily 2,4,6-TNT, have long use as military explosives and commercial application in cast boosters to initiate insensitive blasting agents. In the laboratory, TNT maybe encountered as prills, flake, or a cast solid ranging in color from light yellow to brown. Acetone solutions of TNT slowly develop a pink color and red is produced with ethanolic KOH. Numerous TLC systems have been described for the identification of TNT, usually with visualization by alcoholic KOH. TLC or GC-MS identification of minor products, other isomers of TNT or DNT, provides information to establish a potential common source. The use of isotope ratio MS is now being explored for this purpose. GC or LC-TEA readily identifies TNT. Combinations of TNT and NH4NO3, known as Amatols , have been used as military explosives to conserve TNT and could be encountered in old military ordnance. 

Buchanan, H.A.S., Daeid, N.N., Meier-Augenstein, W, Kemp, H.F., Kerr, W.J., Middleditch, M. (2008) Emerging use of isotope ratio mass spectrometry as a tool for discrimination of 3,4-methylenedioxymethamphetamine by synthetic route. Anal. Chem., 80(9), 3350-3356. 

The use of isotope ratio mass spectrometry (IRMS) to compare the ratios of light atom isotopes in samples of forensic interest is finding increased importance. A recent report by Benson et al. details the use of IRMS to differentiate between samples of TATP that were prepared under differing conditions and from different precursors [61]. A three-dimensional plot of the carbon, hydrogen, and oxygen data clearly showed four clusters corresponding to the different sample sets. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

Two further expressions are used in discussions on isotope ratios. These are the atom% and the atom% excess, which are defined in Figure 48.6 and are related to abundance ratios R. It has been recommended that these definitions and some similar ones should be used routinely so as to conform with the system of international units (SI). While these proposals will almost certainly be accepted by mass spectrometrists, their adoption will still leave important data in the present format. Therefore, in this chapter, the current widely used methods for comparison of isotope ratios are fully described. The recommended Sl-compatible units such as atom% excess are introduced where necessary. 

Thus, the ratios of lead isotopes 204,206,207 and 208 can vary markedly depending on the source of the lead. One use of these ratios lies in determination of the ages of rocks from the abundances of the various isotopes and the half-lives of their precursor radioactive isotopes. 

The small differences in physical properties of substances containing elements with isotopes are manifested through mea.surement of isotope ratios. When water evaporates, the vapor is richer in its lighter isotopes ( Hj O) than the heavier one ( Hj O). Such differences in vapor pressures vary with temperature and have been used, for example, to estimate sea temperatures of 10,000 years ago (see Chapter 47). 

Thermal ionization has three distinct advantages the ability to produce mass spectra free from background interference, the ability to regulate the flow of ions by altering the filament temperature, and the possibility of changing the filament material to obtain a work function matching ionization energies. This flexibility makes thermal ionization a useful technique for the precise measurement of isotope ratios in a variety of substrates. 

Thermal or surface emission of ions is one of the oldest ionization techniques used for isotope ratio measurements. 

Isotope ratios are very useful for (a) identifying elements from their pattern of isotopes in a spectrum obtained on an ordinary mass spectrometer or (b) obtaining detailed information after accurate measurement of isotope ratios from special isotope ratio instruments. 

Dual viscous-flow reservoir inlet. An inlet having two reservoirs, used alternately, each having a leak that provides viscous flow. This inlet is used to obtain precise comparisons of isotope ratios in two samples. 

The Mattauch-Herzoggeometry (Fig. 3.20) enables detection of several masses simultaneously and is, therefore, ideal for scanning instruments [3.49]. Up to five detectors are adjusted mechanically to locations in the detection plane, and thus to masses of interest. Because of this it is possible to detect, e. g., all isotopes of one element simultaneously in a certain mass range. Also fast, sensitive, and precise measurements of the distributions of different isotopes are feasible. This enables calculation of isotope ratios of small particles visible in the image. The only commercial instrument of this type (Cameca Nanosims 50) uses an ion gun of coaxial optical design, and secondary ion extraction the lateral resolution is 50 nm. 

Richter S, Goldberg SA, Mason PB, Traina AJ, Schwieters JB (2001) Linearity tests for secondary electron multipliers used in isotope ratio mass spectrometry. Inti J Mass Spectrom 206 105-127 Rihs S, Condomines M, Sigmarsson O (2000) U, Ra, and Ba incorporation dining precipitation of hydrothermal carbonates imphcations for Ra-Ba dating of impure travertines. Geochim Cosmochim Acta 64 661-671... 

Isotope dilution (ID) is a technique for the quantitative determination of element concentrations in a sample, on the basis of isotope ratios [382]. An important prerequisite for isotope dilution is the availability of two stable isotopes, although in some cases the use of long-lived radionuclides allows the application range to be further extended [420]. 

In this chapter I explained how isotope ratios may be calculated from equations that are closely related, but not identical, to the equations for the bulk species. Extra terms arise in the isotope equations because isotopic composition is most conveniently expressed in terms of ratios of concentrations. I illustrated the use of these equations in a calculation of the carbon isotopic composition of atmosphere, surface ocean, and deep ocean and in the response of isotope ratios to the combustion of fossil fuels. As an alternative application, I simulated the response of the carbon system in an evaporating lagoon to seasonal changes in biological productivity, temperature, and evaporation rate. With a simulation like the one presented here it is quite easy to explore the effects of various perturbations. Although not done here, it would be easy also to examine the sensitivity of the results to such parameters as water depth and salinity. 

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