AVAILABILITY OF STABLE ISOTOPES

The availability of stable isotope-labeled PA makes an accurate quantitative determination of this imino acid possible. A short high-performance liquid chromatography (HPLC) run prior to the mass spectrometer inlet will result in a discrete peak of PA. For the definitive diagnosis of AASA dehydrogenase deficiency, a simultaneous determination of AASA would be preferred. The absence of a commercially available labeled standard leaves this analysis in the experimental stage. 

Depending on the scientific question and the availability of stable isotope precursors, this approach can also be applied to study the biosynthesis of many other plant volatiles such as monoterpenes, alcohols, and aldehydes. 

The commercial availability of stable isotope ( C, N, H)-labelled compounds and highly accurate mass spectrometers has made it possible to probe the details of metabolic pathways involved in macronutrient catabolism and neogenesis. This paper highlights aspects of animal nutrition and metabolism in which uniformly C-labelled [U- C] substrates and C-massisotopomer distribution approaches have been applied to investigations of amino acid and carbohydrate synthesis and catabolism. We will focus on the application of [U- C] substrates as tracers in chickens, fish, sheep, and cells in culture to quantify rates of macronutrient synthesis, identification of the sources of dietary nutrients that serve as substrates for neogenesis of macronutrients, and investigations of the intercormectivity of the pathways of macronutrient metabolism with those of the Krebs cycle to preserve metabolic flexibility via anaplerotic and cataplerotic sequences. 

Short reviews appeared on the various MS techniques for quantitation of stable isotopes and long-lived radioisotopes and the application of Mg stable isotopes as tracers in biology and medicine. The radioactive isotope Mg is not usually available and has a short half-life (21.3 h), hence its hmited usefulness as a tracer. The sensitivities and interference problems encountered in activation analysis for Al, Mg, Mo, P, Si and Zr were discussed. Much higher sensitivities were found for cyclotron-produced than for reactor-produced fast neutrons or 14 MeV neutrons. ... 

Over the last 30 years the study of the stable isotope composition of carbonates has been one of the more active areas of research in carbonate geochemistry. These studies have particular application to later discussion of carbonate diagenesis and historical geochemistry of carbonate rocks. Many of the same considerations involved in understanding elemental distribution coefficients apply to the fractionation of stable isotopes. Consequently, we have included a discussion of the chemical principals associated with isotope behavior in this chapter. Only a relatively brief summary of these basic chemical considerations will be presented here, because recent books and extensive reviews are available on this topic (e.g., Arthur et al., 1983 Hoefs, 1987). Also, our discussion will be restricted to carbon and oxygen isotopes, because these isotopes are by far the most important for the study of carbonate geochemistry. The principles, however, apply to other stable isotopes (e.g., sulfur). 

In practice, two possibilities exist the use of a stable isotope-labeled molecule or a radiolabeled IS. The most commonly used stable isotope is deuterium, which can be incorporated relatively easily into molecules. 15N, 13C, and 180 are used too, but to a lesser extent. Considerable efforts have been made to increase the availability and range of stable isotope-labeled compounds. In the United States, the National Stable Isotope Resource Center has been established at the Los Alamos Scientific Laboratories to help meet the need for stable isotope-labeled compounds. Funds have been made available for the synthesis of many labeled compounds in sufficient... 

These results are in good agreement with the average altitude of the slopes of the ends and the average altitude of the central part of the topographic profile above the tunnel (even better agreement could probably be obtained if local isotopic altitude equations were available). The stable isotopes thus indicate independently that recharged water descends in vertical fractures. 

For HPLC MS/MS assays the use of stable isotope labeled internal standards is by far the best method to overcome any potential matrix effects and random variation in the MS/MS detector. If for any reason this stable isotope internal standard is not available, an analog compound with a mass different from the analyte can also be used. The chromatographic retention time of the internal standard, however, should be as close as possible to the retention time of the analyte. This ensures, that time dependent random variation in the ionization chamber, or whereever else in the MS/MS detector, are compensated by the internal standard. In a toxicokinetic assay described by Chi et al. (2003), for example, an internal standard was used which showed the same retention time as the analyte. 

There are several disadvantages of using stable Isotopes as labels. They are more expensive than radioisotopes and the supply is limited. Analysis requires extensive, time-consuming sample preparation, and requires use of costly Instruments not readily available to all investigators. The cost and supply of stable Isotopes limits the quantities which can be used. One mg of 99.7% Zn costs 0.45. The same purity of Zn cost 155 per mg. The cost is expected to Increase in the near future, when a new batch of zinc Isotopes becomes available. Thus zinc Isotopes for use in a single human study may cost thousands of dollars. Specific Isotopes are sometimes out of stock and not available for several months or longer. 

History. Use of stable Isotopes as research tools begin in the 1930 s with use of deuterium, and N (9). In the 1940 s radioisotopes became available. The advantages radioi-... 

Further research is needed if efficient labeling of both animal and plant food models with stable isotopes of zinc is to be achieved. This represents an Important challenge in the development of stable Isotope methodology for measurement of dietary zinc availability in human studies. 

Having discussed above some features of the conceptual and analytical framework concerned with stable isotope approaches for the study of zinc absorption, we present in this section summaries of a number of experiments involving application of these concepts and methods of stable isotope approaches for examination of zinc availability from human foods. 

In principle, the applications of ICP-MS resemble those listed for OES. This technique however is required for samples containing sub-part per billion concentrations of elements. Quantitative information of nonmetals such as P, S, I, B, Br can be obtained. Since atomic mass spectra are much simpler and easier to interpret compared to optical emission spectra, ICP-MS affords superior resolution in the determination of rare earth elements. It is widely used for the control of high-purity materials in semiconductor and electronics industries. The applications also cover the analysis of clinical samples, the use of stable isotopes for metabolic studies, and the determination of radioactive and transuranic elements. In addition to outstanding analytical features for one or a few elements, this technique provides quantitative information on more than 70 elements present from low part-per-trillion to part-per-million concentration range in a single run and within less than 3 min (after sample preparation and calibration). Comprehensive reviews on ICP-MS applications in total element determinations are available. " ... 

Stable Isotope Techniques. Although the availability of radioiron isotopes has facilitated our understanding of iron nutrition, their utilization is becoming restricted for safety and ethical reasons, especially when infants, children, and women are involved. The availability of enriched stable iron isotopes (Table I) and methodologies for quantifying them make stable isotopes a feasible alternative to radioisotopes as biological tracers. Neutron activation analysis and mass spectrometry are currently available to nutritionists for quantifying stable isotopes of minerals. 

Although the precision of El-MS abundance measurements is not comparable to that of TI-MS, the El mass spectrometer is much more widely available. More stable iron chelates that would permit GC sample introduction and reduce the memory effect problem would significantly improve the method. A potentially serious drawback of stable isotope tracers is that the amounts used are more in the range of substrate than tracer levels. A related question pertains to the validity of the extrinsic labeling technique when stable isotope tracers are used. In an ongoing study in our laboratory, we are attempting to compare the absorption of intrinsic and extrinsic stable iron isotope tracers. 

The Achilles Heel In the use of stable Isotopes for human nutritional studies Is the methodology available for the detection of the enriched stable Isotope Unlike the radioactive studies where one can use a relatively cheap and very accessible technique such as scintillation counting, there appear to be only two methods for the analysis of metal Isotopes mass spectrometry (10-18) and neutron activation analysis (7-9) To date, both methods have proven to be less than desirable because of the time-Intensive nature of the analysis ... 

At present there is no standard method for measurement of stable isotope enrichment in biological samples The diversity of analytical approaches reflects the varied backgrounds of the investigators involved and the instrumentation which has been available to them Nutrition studies tend to generate many samples and a dedicated mass spectrometer or other facility is advantageous ... 

The term carrier-free is often used to indicate the absence of stable isotopes or longer-lived radioisotopes of the radionuclide considered. However, due to the omnipresence of most stable elements, carrier-free radioisotopes of stable elements are, in general, not available. The presence of stable isotopes or longer-lived radioisotopes has to be taken into account, and the specific activity is smaller than calculated by eq. (13.2). As long as the presence of such other isotopes cannot be excluded, it is more correct to distinguish no-carrier-added (n.c.a.) and carrier-added radionuclides. On the other hand, radioisotopes of radioelements are carrier-free if longer-lived radioisotopes are absent. 

Although some measurements were available as early as the 1970s and before (see Francey et al., 1999 for a recent summary of stable isotope air... 

The common stable isotopes used in biosynthetic studies are H, N, and 0. Stable isotope-labeled precursors have replaced radiolabeled precursors in many biosynthetic studies for the following reasons 1) no appropriate radiolabeled isotope is available (e.g., N and O) 2) the detection methods frequently permit location of the label in the product directly and 3) radiocontamination and safety issues are reduced. The negative aspects of stable isotope studies... 

The previous examples are a minute sampling of the many different types of stable isotope studies now being applied to inorganic and organic marine samples. Solid-source mass spectrometers are available for highly sensitive and precise determinations of the stable... 

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