Carbon-14 analysis isotopic enrichment

Whereas the structural assignment by one-dimensional 13C NMR spectroscopy is unambiguous in the case of D2-Cj(,2,51 Achiba and co-workers were able to determine the carbon atom connectivity by 2D 13C NMR INADEQUATE (incredible natural abundance double-quantum transfer experiment) analysis performed on an isotopically enriched sample (20% 13C).58 In particular, they found that the observed chemical shifts correlate well with the curvature of the spheroid, the more strongly pyramidalized carbon atoms being shifted toward lower magnetic field. 

Over the next decade, if the stringent requirements for the exclusion of modern carbon contamination in sample preparations can be met, developments in AMS may permit routine extension of the 14C time frame beyond the current 40,000- to 60,000-year range for the typical archaeological sample (8). However, the potential of AMS to extend the 14C time frame into the 70,000- to 100,000-year range for samples from archaeological contexts may be possible only with the development of practical methods of 14C isotopic enrichment that can be routinely used for samples that contain less than 1 g of carbon. The possibility of using a laser-based approach for 14C enrichment before AMS analysis is being studied (9, 10). 

In addition, fewer chemically distinct types of nitrogen moieties, compared to the number of carbon moieties, in polymers should result in simpler spectra, and easier identification of resonances and their quantitative analysis. These properties of15N NMR spectra should also enable the detection of byproducts and minor products which would probably be undetected in the relatively complicated 13C spectra. However, the small gyromagnetic ratio (about 40% of that of 13C) and low natural abundance of the 15N isotope (Table 1) result in about 30 times less sensitivity compared to 13CNMR. Therefore, isotopic enrichment is usually required, particularly for solid state experiments. 

A specific variant of El MS is isotope ratio (IR) MS [46]. It is based on electron impact ionization with maximized ionization probability. IR MS is limited to the analysis of gases of high volatility and low reactivity such as CO2, N2 or SO2. The analytes of interest thus have to be transformed into one of these gases before introduction into the IR MS. Information on the position of C labelings in the analyte can be only obtained, if all carbons are isolated position specific and subsequently combusted. In this context Corso and Brenna [47] showed position specific analysis by IR MS for methylpalmitate through pyrolytic fragmentation. IR MS exhibits an extremely high precision of 0.00001 % for the isotope ratio measurement and is optimal to quantify low label enrichments [48]. This is especially important for in vivo studies with ani-... 

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