Stable isotope natural abundance measurements

T. Preston (1992) The measurement of stable isotope natural abundance variations. Plant Cell Environ. 15, 1091-1097... 

In particular cases it is necessary to determine the origins or fate of oxygen in biosynthesis. For this only the stable isotope (natural abundance 0.2%) is available and it is analysed for by mass spectrometry, a single label giving rise to M + 2 peaks which are measured with comparative sensitivity. The stable... 

Stott, A.W., Davies, E., Evershed, R.E. and Tuross, N. 1997 Monitoring the routing of dietary and biosynthesized lipids through compound-specific stable isotope (delta C) measurements at natural abundance. Naturwissenschcften 84(2) 82-86. 

The foregoing review is centered on natural-abundance measurements of stable isotopes. In many cases, however, several potential foods or prey items are indistinguishable in their natural-abundance isotopic ratios, and thus isotopic analyses provide poor resolution. One strategy for overcoming this problem is to purposefully manipulate the isotopic ratios in one or more... 

The technique of stable Isotope dilution permits one to determine very accurately the trace element content of a particular sample as well as using stable Isotopes as metabolic tracers The concept of Isotope dilution for trace element determinations Is both simple and elegant A known quantity of an enriched stable Isotope Is added to the sample to be analyzed By measuring the amount of that Isotope (added) relative to another Isotope (not added) of the element, one can easily calculate the amount of analyte present originally In the sample Thus, the normal relative amounts of the two Isotopes (natural abundance) have been altered, or "diluted" ... 

The increasing availabihty of mass spectrometers and on-Hne interfaces for sample preparation has made natural abundance measurements accessible to a broad community of biological oceanographers, and stable isotope measurements are becoming a routine tool in studies of marine ecosystems. In general, measurements are now carried out with continuous flow systems that integrate a... 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

The elements whose isotopes are routinely measured with gas inlet mass spectrometers are carbon (12C and 13C, but not 14C), oxygen (160, 170, l80), hydrogen ( H, 2H, but not 3H), nitrogen (14N and 1SN) and sulphur (32S, 33S, 34). Stable isotopes of H, C, N, O, and S occur naturally throughout atmosphere, hydrosphere, lithosphere, and biosphere. They are atoms of the same elements with a different mass. Each element has a dominant light isotope with the nominal atomic weight (I2C, 160,14N, 32S, and H) and one or two heavy isotopes (l3C, nO, 180, 15N, 33S, 34S, and, 2H) with a natural abundance of a few percent or less Table 1). 

Stable-isotope dilution analysis is an analytical technique in which a known quantity of a stable-labelled isotope is added to a sample prior to extraction, in order to quantitate a particular compound. The ratio of the naturally abundant and the stable-labelled isotope is a measure of the naturally abundant compound and can be determined only by gas chromatography-mass spectrometry since the naturally abundant and the stable-labelled isotope cannot be completely separated gas chromatographically. 

Because the natural variations in stable isotope abundances are usually very small (see above), and since routine measurements are usually made in an isotope ratio mass spectrometer which compares the relative intensities of the mass resolved beams of the sample with those of some standard material (Section 7.2.2), it is standard practice to report abundance ratios using the dimensionless 8-value notation. 

A measure of the amount of a stable isotopic label that exceeds its natural abundance in unlabeled tracee. This is most directly accomplished using an ion ratio mass spectrometer to measure the ratio of ion currents for isotopomers such as C02 at mass 44 and 2 at mass 45. From the difference between the ion current ratio for a sample (Zsampie) and the ion current ratio for a reference gas (Zreference), the atom percent excess (APE) can be estimated. See Tracer/Tracee Ratio Compart-mental Analysis Isotope Exchange Kinetics... 

A quantitative measure of isotope composition relative to the abundance of all isotopic forms found in nature. Values for those stable isotopes most commonly employed in biological tracer experiments are H, 99.985% ... 

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 average relative abundances of isotopes in the Earth s crust, oceans, and atmosphere, commonly expressed as stable isotope ratios, are shown in table 7.5. Small differences in the ratios of a particular element in natural samples can be detected using mass spectrometry, however, it cannot be achieved with high precision and accuracy (Nier, 1947). The solution to this problem, as explained earlier for 14C measurements, is measuring isotope ratios in a sample concurrently with the standard this does allow for adequate precision and accuracy. The equation used to describe this relative difference or del (<5) value is as follows ... 

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