Tracer isotopic abundances

Following the movement of airborne pollutants requires a natural or artificial tracer (a species specific to the source of the airborne pollutants) that can be experimentally measured at sites distant from the source. Limitations placed on the tracer, therefore, governed the design of the experimental procedure. These limitations included cost, the need to detect small quantities of the tracer, and the absence of the tracer from other natural sources. In addition, aerosols are emitted from high-temperature combustion sources that produce an abundance of very reactive species. The tracer, therefore, had to be both thermally and chemically stable. On the basis of these criteria, rare earth isotopes, such as those of Nd, were selected as tracers. The choice of tracer, in turn, dictated the analytical method (thermal ionization mass spectrometry, or TIMS) for measuring the isotopic abundances of... 

Once geological samples are dissolved, a mixed Ca- Ca tracer is added to the sample ( Ca/ Ca 1). The isotopic abundances in the mixed sample-tracer solution are illustrated in Figure 1. When the mixed solution has roughly equal amounts of the isotopes Ca, Ca, and Ca, then a near-optimal situation is achieved with regard to the corrections for the presence of the tracer (cf Johnson and Beard 1999 for a detailed analysis of spike-sample ratios). The... 

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.

Isotope ratio measurements are performed whenever the exact ratio, or abundance, of two or more isotopes of an element must be known. For example, the isotopic ratios of lead are known to vary around the world, so it is possible to determine the source of lead in paint, bullets and petrol by knowing the isotopic abundances of the four lead isotopes 204, 206, 207, 208. Another example is the use of stable isotopes as metabolic tracers, where an animal is both fed and injected with an element having artificially enriched isotopes and the fractional absorption of the element can be accurately determined. 

The narrow range of lead isotopic abundances in the Laurion area as shown in this study and the fact that major amounts of lead from this area were mined and used in ancient times indicate that these ratios should serve as a reasonable tracer. The data also justify the assumption previously made, but until now unproved, that the lead output of the Laurion mines was essentially of one uniform and recognizable type. 

The half-life (Eq. 9-4) determines the isotopic abundance needed to achieve a given radiation rate, a practical matter in providing a sufficient rate of decay to permit counting with an acceptably low statistical error. Even very short-lived isotopes such as 13N, have proved useful as tracers.13 The amount of an isotope giving 3.7 x 1010 disintegrations per second (this is 1 g of pure radium, 0.3 mg of 3H, or 0.22 g of 14C) is known as the curie (Ci). One milli-curie (mCi) provides 2.22 x 109 disintegrations / min... 

Isotope ratio measmements were carried out by LA-ICP-MS on protein spots of a brain sample with Alzheimer s disease in two dimensional (2D) gels after tracer experiments with highly emiched Cu and Fe isotope tracers." In Figme 8.3 part of the 2D gel and the transient signals for Cu and Cu are shown. Isotope ratio measmements on protein spots did not reveal any changes of isotope abundances of Cu, whereas an enrichment of Fe was found in several protein spots in the 2D gel. The " Fe/ Fe isotope ratios measmed in selected protein spots of an Alzheimer protein sample after tracer experiments are summarized in Figme 8.4." ... 

Isotopic studies relating to nutrition and diet have originated from two diverse fields bio-medical research and archaeology Numerous studies have been reported by researchers in the areas of biochemistry and medicine using either isotopically enriched compounds or the natural variations in isotopic abundances Such studies usually involve a specific chemical as a tracer of biochemical pathways, and in these studies soft tissues or body fluids are analyzed Recent work in nutrition has begun to examine the isotopic composition of carbon in macronutrients in diets and their disposition in body tissues (1 2) The isotopic composition of hard tissue (i e bone) however has been largely ignored in biochemical studies ... 

Precollapse cloud cores are composed of cold molecular gas with ternperamres in the range 7 -15 K, and with gas densities —10 -10 mol cm (Figure 1). Some clouds may be denser yet, but this is hard to determine because of the limited density ranges for which suitable molecular tracers are abundant (typically isotopes of carbon monoxide and ammonia). Masses of these clouds range from roughly a solar mass to thousands of solar masses, with the distribution of clump masses fitting a power-law such that most of the clumps are of low mass, as is also true of stars in general. The cloud properties described below are used to constrain the initial conditions for hydrodynamic models of the collapse of cloud cores. 

The most common method of determining the U-Pb date of an accessory mineral is the determination of uranium and lead isotopic abundances via isotope dilution and thermal ionization mass spectrometry (ID-TIMS). Most measurements in the 1970s and 1980s required that an aliquot of the dissolved mineral solution be made, with one portion being spiked with an enriched U and ° Pb tracer solution, and the... 

Modern thermal ionization mass spectrometry (TIMS) is now sufficiently sensitive and precise to measure individual selenium-isotope abundances (e.g., Se/ Se) in solid samples or residues so that it can be used to study environmental cycling/distri-butions (Johnson era/., 1999). Microbial reduction of selenate leads to isotopically lighter selenite, i.e., the reduction has a Se/ Se fractionation factor, , of about — 5.5%o (Johnson et al., 1999). INAA has been used to determine different selenium isotopes, especially Se in plant tracer studies. 

Analysis of the isotope distribution in target compounds can be, and frequently been, limited to measuring bulk specific activity (in the case of radioisotopes) or the overall abundance (in the case of stable isotopes) of the tracer isotope. Alternatively, the topological distribution of the isotope label can be narrowed down by chemical degradation in the case of radioactive tracers. In case of stable isotopes, the topology of isotope distribution in the target molecule can be assessed in considerable detail by NMR spectrometry, mass spectrometry, or a combination of these methods. 

Dilution with stable isotopes offers the possibility of performing tracer experiments but also of circumventing systematic errors. The principle [518] can be applied for every element which has at least two stable or longlife isotopes. For its application the analyte with a known isotopic composition but which differs from that of the sample is added to a known amount of sample, and mixed thoroughly. The isotopic abundance ratio R then is given by ... 

Figure 2 Top Hg isotope abundance in nature (A) and tracer solution (B). Bottom GC-ICP-MS chromatogram of a sediment extract after incubation with Hg(ii) showing the relative increase in the CH3 iHg signal.

Whereas ETAAS lacks the capability to perform isotope measurements, NAA has been used in the past and precision and accuracy ranging from 1% to 10% have been reported. In many cases, NAA can not provide data on all the isotope ratios due to the lack of a suitable radioactive daughter nuclide. For example, Mg, Ca, Ca, Fe, and are very useful tracers but they cannot be measured by NAA. Mass spectrometry occupies a unique position for determining not only the concentration of trace elements by isotope dilution but also for measuring the isotope abundances of the elements. 

For each element, signals from its different isotopes are obtained. Their intensity ratios correspond to the isotopic abundances in the sample. This fact can be used in calibration by isotopic dilution with stable isotopes, and in tracer experiments. Isotopic abundance is also useful for the recognition of spectral interference. The spectra in ICP-MS are simpler than in ICP-AES,... 

Radiotracers of an element differ physically from its stable isotopes in the number of neutrons and, as they are unstable, can be quantified directly via monitoring of the decay process through detection of emitted particles and/or gamma rays. Stable isotopic tracers, in contrast, are composed of the same isotopes as the natural element. The two differ only in the relative abundances of their isotopes with tracers being commonly enriched in the isotope(s) that is/are least abundant in nature. This means that stable isotopic tracers can only be quantified indirectly via the changes in the isotopic abundances of the natural element introduced by tracer addition, which is measurable by mass spectrometric techniques. 

The isotopic abundances of the tracee or the tracer can be obtained from measured isotope ratios using Eqs. (16.2) and (16.3), while atomic masses can be taken from the literature (10]. Based on IDMS principles, measured isotope ratios can be converted into tracer to tracee ratios in the sample analyzed. The amount of tracee can then be calculated if the amount of tracer in the sample is known. The latter can be determined either by IDMS using an additional tracer or by non-IDMS techniques for quantitative analysis employing external calibration or standard addition techniques. 

Advances in instrumentation may also fill a niche in isotope tracer studies of lead exposure and metabolism in organisms. In those studies, the inherent isotopic abundances are altered by 50% to > 200% with the addition of a lead tracer enriched in one of the four stable lead isotopes, and those changes may be readily detected with ICPMS. It is also especially suited for such studies because it can be used to analyze samples relatively rapidly. 

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

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