Isotopic enrichment, measurement

Precision The precision of the absorption value depends upon tfe precision of P, and E measurements This method for isotopic enrichment measurements by mass spectrometry has a precision of 2% as does the measurement of F by atomic absorption This precision is adequate for absorption and bioavailability studies with zinc and copper (Table I) since zinc and copper absorption are in the range of 30-70% Only fairly large changes in iron absorption can be discerned because non-heme iron absorption is typically less than 10% This may not be a serious problem in bioavailability studies since it is doubtful that very small changes in iron absorption from single foods are biologically significant ... 

Jcc and have been measured in isotopically enriched molecules, but in this case again the experimental values are in poor accordance with those calculated using the CNDO/2 approximation (Table 1-40) (113). 

Grzesiek S, Kuboniwa H, Hinck AP, Bax A. Mutiple-quantum line narrowing for measurement of Ha-Hp J couplings in isotopically enriched proteins. J Am Chem Soc 1995 117 5312-5315. 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Linked Systems Isotopic Enrichment. The power of advanced measurement techniques can often be extended by linking them with other techniques. Such is the case for the detection of trace quantities of natural radionuclides and isotope enrichment. We have already found this extremely valuable for gas proportional counting of 37Ar and accelerator atom counting of 14C [8,9]. The first nuclide was enriched by means of thermal diffusion (Ar) the... 

Practicable isotopic enrichment has the following prerequisites adequately short time for the enrichment process, acceptable asymptotic enrichment factor, and adequate accuracy for the estimation of the enrichment factor. (When total activity, rather than specific activity, is limiting, one must also pay attention to losses during enrichment.) For the argon and carbon enrichments referred to above, enrichment factors of about 100 and 500 were obtained within a week and a few hours, respectively and enrichment factors were deduced from direct observations of adjacent, stable isotopes. The 14C enrichment process provided extra dividends for AMS measurement the sample was implanted in an ideal form for the accelerator ion source, and it was spatially localized (depth) which gave added signal-to-noise enhancement. 

Rutherford, W. M., Evans, J., Currie, L. A., The Application of Isotopic Enrichment and Pulse Shape Discrimination to the Measurement of Atmospheric 37Ar, Anal. Chem., 48, 607 (1976). 

Solid-phase peptide synthesis offers a fast and convenient route for many peptides when isotope-enriched compounds are not required. Classical synthesis additionally permits the use of non-natural amino acids and allows site-specific isotope labeling. Although Fmoc protected 15N-labeled amino adds are commercially available, the cost of such synthesis is usually prohibitive, and the peptides from chemical synthesis require perdeuterated detergents and unfortunately exclude investigation of internal dynamics through measurement of 15N relaxation. 

Cross-correlated dipolar relaxation can be measured between a variety of nuclei. The measurement requires two central nuclear spins, each of which is directly attached to a remote nuclear spin (Fig. 16.4). The central spin and its attached remote spin must be connected via a large scalar coupling, and the remote spin must be the primary source of dipolar relaxation for the central spin. The two central spins do not need to be scalar coupled, although the necessity to create multiple quantum coherence between them requires them to be close together in a scalar or dipolar coupled network. In practice, the central spins will be heteroatoms (e.g. 13C or 15N in isotopically enriched biomolecules), and the remote spins will be their directly attached protons. 

Apart from the need for isotopic enrichment and synthesis there are other problems in applying whole molecule mass spectrometry to measure isotope ratios. Assume, for example, that we want to determine isotopic composition of chlorine from the spectrum of chlorobenzene presented in Fig. 7.7. The peaks at 114 and... 

Although the sediments in these systems accumulate Se over time, the small isotopic contrast suggests that dissimilatory reduction is not the dominant accumulation mechanism. If dissimilatory reduction of Se(VI) and/or Se(IV) to Se(0) by bacteria were the dominant mechanism, one would expect the accumulated Se(0) to be enriched in the lighter isotope. In the San Francisco Estuary case, this assumes that the isotopic fractionations measured by Ellis et al. (2003) can be extrapolated to much lower concentrations. Incorporation of Se into algae and macrophyte tissues, followed by decay of some material and conversion of its Se to Se(0), is more consistent with the observed Se isotope data. Notably, the mean Se isotope composition of the Se(0) in the sediments of the Herbel et al. (2002) study was identical to that of the macrophytes. 

Immature soil samples have S Te values that are indistinguishable from lunar rocks, whereas submature and mature soils have 5 Fe values that are greater than those of lunar rocks, and S Te values are positively correlated with Ig/FeO values (Fig. 12). Lunar regolith samples in general tend to have heavy isotopic compositions as compared to lunar rock samples, as demonstrated by isotopic analyses of O, Si, S, Mg, K, Ca, and Cd (Epstein and Taylor 1971 Clayton et al. 1974 Russell et al. 1977 Esat and Taylor 1992 Humayun and Clayton 1995 Sands et al. 2001 Thode 1976). The origin of isotopic compositions that are enriched in the heavy isotopes has been presumed to reflect sputtering by solar wind and vaporization, where preferential loss of the lighter isotope to space occurs. In contrast to previous isotopic studies, the Fe isotope compositions measured in the Lunar Soil Characterization Consortium samples can be related to a specific phase based on the positive correlation in Ig/FeO and 5 Fe values (Fig. 12). 

Example Isotopic enrichment is a standard means to enhance the response of an analyte in nuclear magnetic resonance (NMR). Such measures gain importance if extremely low solubility is combined with a large number of carbons, as is often the case with [60]fullerene compounds. [19] The molecular ion signals, IVT, of Qo with natural isotopic abundance and of C-enriched Cgo are shown below (Fig. 3.11 for EI-MS of [60]fullerenes cf. Refs. [20-22]). From these mass spectra, the enrichment can be determined by use of Eq. 3.1. For Qo of natural isotopic abundance we obtain Mrceo = 60 x 12.0108 u = 720.65 u. Applying Eq. 

The results of the simulations are shown in Figures 1 and 2, superimposed on the experimental results. The agreement between calculated and experimental spectra is very good. Numerous simulations were performed in order to assess the effect of the various parameters. The results indicate that the simulated spectra are very sensitive to the choice of the distribution parameters and to the values of the residual widths AH and AH . Given the limited possibilities of measuring ESR spectra at S-band, we believe that computer simulations are a viable alternative. We also feel that the error margin in the parameters deduced by computer simulation can be decreased if ESR spectra of isotopically enriched Cu are measured and Simulated 4. 

A parameter used as a measure of isotope enrichment in stable isotope exchange and compartmental analysis experiments ... 

Turning now to lithium, we have two nuclides available for NMR measurements Li and Li. Both are quadrupolar nuclei with spin quantum number / of 1 and 3/2, respectively. The natural abundance of Li (92.6%) provides enough NMR sensitivity for direct measurements, but also Li (7.4%) can easily be observed without enrichment. However, isotopic enrichment poses no practical problem and is advantageous if sensitivity is important, as for measurements of spin-spin coupling constants in solution and of quadrupole coupling constants in the sohd state. 

Couplings between N and C are difficult to measure without isotopic enrichment. Values of 7( RN, RC) are generally less than 20 Hz. The sign can be positive or negative. If the value of 7 is close to zero, it can be exceeded by 7 or 7 couplings. Some examples of known "7( RN, RC) couplings are given in Table 16. 

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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