Natural abundance isotope analyses

In natural abundance isotope analyses, the emphasis is on the relative difference between samples analyzed under the same conditions, as opposed to the exact values obtained. One of the main advantages of the relative measurement approach is precision. The errors associated with relative measurements are generally significantly smaller as certain errors are cancelled out (e.g., errors related to bias), so the total error can be significantly smaller compared to the total error for absolute measurements [1,4]. 

NATURAL ABUNDANCE ISOTOPE EXCHANGE KINETICS COMPARTMENTAL ANALYSIS ISOTOPE DILUTION Isotope effects,... 

Thermal ionization has been used to determine isotopic abundance of virtually all the elements We have recently extnded our own capability in this direction by adapting the silica gel/phosphoric acid filament coating technique (5) to our system Five 1 of a fine silica gel suspension is placed on a filament Five l of the analyte ion solution is coated, dried then coated with 2 pi of a 0 7N phosphoric acid solution and heated until dry again The analysis is performed in a similar manner as before, except that the signal is more transient and somewhat less intense than the calcium analysis With this approach, however, we have made natural abundance isotope ratio measurements on zinc, copper, and magnesium Table II shows our measurements compared to the accepted values, shown in parenthesis, for these elements The isotope used as reference... 

Incorporation of a stable isotopic tag into proteins/peptides in metabolically active cells was first described to quantify protein abundance in yeast (43). Wild-type and mutant cell populations were grown in media containing the naturally abundant isotopes of nitrogen and enriched in 15N, respectively, followed by trypsin digestion and LC-ESI-MS/MS analysis to identify and quantify relative phosphopeptide levels in both populations (43). 

It is not necessary that there be two isotopes in both the sample and the spike. One isotope in the sample needs to be measured, but the spike can have one isotope of the same element that has been produced artificially. The latter is often a long-lived radioisotope. For example, and are radioactive and all occur naturally. The radioactive isotope does not occur naturally but is made artificially by irradiation of Th with neutrons. Since it is commercially available, this last isotope is often used as a spike for isotope-dilution analysis of natural uranium materials by comparison with the most abundant isotope ( U). 

Atoms of elements are composed of isotopes. The ratio of natural abundance of the isotopes is characteristic of an element and is important in analysis. A mass spectrometer is normally the best general instrument for measuring isotope ratios. 

In recent years, together with enantioselective analysis, the determination of the natural abundance of stable isotopes by means of stable isotope ratio mass spectrometry (TRMS) can be very useful for the assignment of the origin of foods and food ingredients, and of authenticity evaluation (24). 

Barrie, A., Bricout, J. and Koziet, J. (1984) Gas chromatography stable isotope ratio analysis at natural abundance levels. Biomedical Mass Spectrometry 11, 439 447. 

Jim, S., Jones, V., Ambrose, S. H. and Evershed, R. P. (2006) Quantifying dietary macronutrient sources of carbon for bone collagen using natural abundance stable carbon isotope analysis. British Journal of Nutrition 95, 1055 1062. 

For investigations with nuclei with low natural abundance, such as 29Si (4.7%), isotopic enrichment is often applied. This is accompanied with difficulties in the analysis of connectivities between selected pairs, because multiple site interactions become more pronounced. Cadars et al. have suggested incorporation of a z-filter that results in a robust method to select local site connectivities and remove complications from multiple site interactions [122]. 

Sigman et al. [134] have described a bacterial method for measuring the isotopic composition of seawater nitrate at the natural-abundance level. The method is based on the analysis of nitrous oxide gas (N2O) produced quantitatively from nitrate by denitrifying bacteria. The classical denitrification pathway consists of the stepwise reduction of nitrate (NOp to nitrite (N02), nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) ... 

Wiedmeyer (1998). One method of testing for interference is to examine the survey data. In this type of analysis a large number of points are recorded across the mass range, rather than a single measurement taken at a particular mass number for each element. Figure 9.5 shows survey data (dashed line) for masses 203 to 210 on a sample believed to contain lead. The natural lead isotopic abundance is superimposed, shown by filled columns. The proximity of the survey data to the actual abundance of lead indicates that lead is present, and that no interfering elements are present. In this case, a single measurement at mass 208 (which is used for most analyses of lead) is sufficient to quantify lead in the sample. 

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. 

Isotopic analysis of amino acids containing natural abundance levels of 15N was performed by derivatization, GC separation, on-line combustion and direct analysis of the combustion products by isotope-ratio MS. The N2 gas showed RSD better than 0.1%c for samples larger than 400 pmol and better than 0.5%o for samples larger than 25 pmol. After on-column injection of 2 nmol of each amino acid and delivery of 20% of the combustion products to the mass spectrometer, accuracy was 0.04%e and RSD 0.23%o19. 

It remains to consider the isotopically heteronuclear systems to complete the symmetry analysis of this system. Because the experiments are performed under natural abundance conditions, only systems containing a single rare isotope ( O or 0) need be considered. However, because the spatial degeneracy of the electronic state of the ion and the neutral differ, the case where either the neutral or the ion is isotopically heteronuclear must be considered separately. The results in Table 4 show that when the neutral is made isotopically heteronuclear the /-based restriction is removed, while that based on is preserved. Conversely, when... 

Sweeney RE, Kaplan IR (1980) Natural abundance of as a source indicator for near-shore marine sedimentary and dissolved nitrogen. Mar Chem 9 81-94 Sweeney RE, Liu KK, Kaplan IR (1978) Oceanic nitrogen isotopes and their use in determining the source of sedimentary nitrogen. In Robinson BW (ed.) DSIR Bull 220 9-26 Swihart GH (1996) Instrumental techniques for boron isotope analysis. Rev Miner 33 845-862 Swihart GH, Moore PB (1989) A reconnaissance of the boron isotopic composition of tourmaline. Geochim Cosmochim Acta 53 911-916... 

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... 

Furthermore, isotope analysis is relevant for determining the atomic weight (Ar(E)) of elements. The Ar(E) is the average of all masses of all naturally occurring stable isotopes (taking into account the abundances of isotopes) of a chemical element (see Appendix I10). By consideration of the masses of isotopes (/ ,) and the known relative abundances of all stable isotopes (Xi) with i = 1 to n of a selected chemical element, the average atomic weight (Ar(E)) of this element can be calculated ... 

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