15N/14N ratios

Heaton, T. H. E. (1987) The 15N/14N ratio of plants in South Africa and Namibia relationship to climate and coastal/saline environments. Oecologia 74, 236 246. 

Now, GC-IRMS can be used to measure the nitrogen isotopic composition of individual compounds [657]. Measurement of nitrogen isotope ratios was described by Merritt and Hayes [639], who modified a GC-C-IRMS system by including a reduction reactor (Cu wire) between the combustion furnace and the IRMS, for reduction of nitrogen oxides and removal of oxygen. Preston and Slater [658] have described a less complex approach which provides useful data at lower precision. Similar approaches have been described by Brand et al. [657] and Metges et al. [659]. More recently Macko et al. [660] have described a procedure, which permits GC-IRMS determination of 15N/14N ratios in nanomole quantities of amino acid enantiomers with precision of 0.3-0.4%o. A key step was optimization of the acylation step with minimal nitrogen isotope fractionation [660]. 

Isotope-dependent chemistry in these cold clouds can fractionate the 15N/14N ratio. It appears that 15N-richness in N2 molecules and in solid ammonia will... 

Altabet, M.A., Fran9ois, R., Murray, D.W., and Prell W.L.,1995. Climate related variations in denitrification in the Arabian Sea from sediment 15N/14N ratios. Nature, 373, 506-509. 

Schoeninger, M.J. (1985). Trophic level effects on 15N/14N and 13G/12C ratios in bone collagen and strontium levels in bone mineral. Journal of Human Evolution, Vol.l4, pp. 515-525. 

Metges, C. C. and Petzke, K. J. (1997) Measurement of 15N/14N isotopic composition in individual plasma free amino acids of human adults at natural abundance by gas chromatography combustion isotope ratio mass spectrometry. Analytical Biochemistry 247, 158 164. 

Carbonaceous material (Fig. 12.8b) is intimately mixed with silicates and is very abundant (carbon abundance averages 13% and varies up to 50%) in CP IDPs. Some carbon is elemental (graphite), but C-H stretching resonances in infrared spectra show that aliphatic hydrocarbons are also present. Polycyclic aromatic hydrocarbons (PAHs) also occur. Nanodiamonds have been identified in cluster IDPs, but not in smaller CP IDPs. Enormous D/H and 15N/14N anomalies have been measured in bulk IDPs, and the hydrogen isotopic anomalies are correlated with organic-rich domains. Ratios of D/H as high as 25 times the solar ratio suggest the presence of molecular cloud materials. 

Activation with 14 MeV neutrons has been used to determine the oxygen content of various metals such as beryllium 20>, Cl, F, O, Na, Si, and various rare earths in complex molten salt electrolytes 45>, the protein content of food products by means of the nitrogen content 46>, i60/180 and 14N/15N isotopic ratios in stable isotope tracer experiments 47,48), and in a wide variety of other applications. One application we... 

This secondary production by the CN cycle from 12C is not the major source of 15N in the universe most of it cannot arise from the intermediate-mass AGB stars, because their production ratio 15N/14N is too small to make most of nature s 15N even though it does account for most 14N. Most of the 15N production occurs in... 

In general, molecular spectroscopy of the N-bearing molecules in interstellar matter determines the 15N/14N abundance ratio (see 14N). 

Similarly, because the ratios of 15N/14N and 87Sr/86Sr in elephant tusks also vary depending on the region of Africa the elephant inhabited, they can be used to trace the elephant s origin. In fact, using these techniques, scientists have reported... 

Figure 3.2. Stable isotope labeling for quantifying differential protein expression. Cell populations are grown in either 14N or 15N containing medium. Protein lysates are fractionated and separated by 2D gel electrophoresis. Protein spots are excised, digested with trypsin and the mass of the resulting peptides is determined by mass spectrometry. The presence of 15N results in a shift and creates two peaks for each peptide. The ratio of intensities of the peaks is indicative of the relative expression levels of the proteins. Spot A contains a protein that is expressed at similar levels in both cell pools. Spot B contains a protein that is expressed at higher levels in cell pool 2. Figure adapted from Oda et al. (1999).

A choice between mechanisms (b) and (c) was achieved by photolyzing a mixture of a 14N-nitrite in the pregnane series and the 15N-nitrite (29) to half-completion. Rearrangement of the nitroso dimers thus obtained to a mixture of oximes followed by oxidation of the pure oxime from (29) to the ketonitrile and mass spectrometric analysis indicated the N15/N14 ratio to be 1.15 1.00. Analysis of pure unreacted (29) gave a ratio of 1.00 0.00. These results indicate that mechanism (c) is operative in nitrite photolyses. 

Fig. 3.43. Variation of CNO isotopic ratios with Galactocentric distance, deduced from mm wave measurements of molecules in molecular clouds, (a) 12C/13C from CO (triangles) and formaldehyde CH2O (circles). (b)14N/15N from HCN (circles) and NH3 (triangles). (c)160/180 from formaldehyde. Solar-System values are indicated by the O sign and Galactic centre values (or a lower limit in the case of 14N/15N) by a filled circle. Adapted from Wilson and Rood (1994).

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