Carbon relative isotopic abundance

Carbon has two common isotopes, and with relative isotopic abundances of, respectively, 98.89% and 1.11%. (a) What are the mean and standard deviation for the number of atoms in a molecule of cholesterol (b) What is the probability of finding a molecule of cholesterol (C27H44O) containing no atoms of... 

Mass spectrometers determine atomic and molecular isotope ratios. Table 2 lists the relative isotopic abundance of elements commonly encountered in pharmaceutical analysis [3,4]. The values in Table 2 have been empirically determined and refinements in the values are necessary as atomic mass measurements improve, but for this discussion any inaccuracies in the table are insignificant. For some elements there are only two naturally occurring isotopes. For example, if you were to randomly sample carbon atoms in nature, 99% of the time you would find 12C, and roughly 1% of the time a 13C would turn up. Other elements, such as chlorine and bromine, have elemental isotope ratios that are not as heavily... 

Measurements of relative isotope abundances in atmospheric gases can provide very useful information on chemical reactions in the atmosphere. For instance, the stable isotope content of atmospheric CO2 provides information about the ecosystems carbon-water and biosphere-atmosphere and the carbon exchange between these subsystems. In many cases samples of air are collected and analysed in the lab. This is, however, a more tedious and not momentary analysis. Absorption spectroscopy with a tunable diode laser allows in situ measurements on vibrational transitions of all relevant molecules in the atmosphere and can measure the isotope ratios with high accuracy. 

Ordinarily electrical amplification is used to compensate for differences in isotope abundances in the gas being measured. Thus, for carbon dioxide all three Faraday collectors are used with relative signal amplification at m/z = 44, 45, and 46 of 1 91 500 (since the normal abundance ratios 12C/13C 91, and 160/180 500). The amplified signals from all three detectors are thus comparable in intensity. Because of this feature, however, IRMS should only be used on gases with isotope composition close to natural abundance. Enriched material should not be used without careful recalibration since there is no guarantee of a linear response of electric signal to ion current for widely different isotope ratios. 

Note The calculation of relative molecular mass, Mr, of organic molecules exceeding 2000 u is significantly influenced by the basis it is performed on. Both the atomic weights of the constituent elements and the natural variations in isotopic abundance contribute to the differences between monoisotopic- and relative atomic mass-based values. In addition, they tend to characteristically differ between major classes of biomolecules. This is primarily because of molar carbon content, e.g., the difference between polypeptides and nucleic acids is about 4 u at Mr = 25,000 u. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10-25 ppm in Mr which is significant with respect to mass measurement accuracy in the region up to several 10 u. [41]... 

Applications to Biological Samples. - Methods of distance measurements were compared for four doubly spin-labelled derivatives of human carbonic anhydrase.53 The distances between the spin labels were obtained from continuous wave spectra by analysis of the relative intensity of the half-field transition, Fourier deconvolution of the line-shape broadening, and computer simulation of line-shape changes. For variants with interspin distances greater than 18 A, the DEER method also was used. For each variant, at least two methods were applicable and reasonable agreement between distances obtained by different methods was obtained. The useful distance ranges for the techniques employed at X-band with natural isotope abundance spin labels were estimated to be half-field transition (5-10 A), line-shape simulation (up to 15 A), Fourier deconvolution (8 - 20 A), and four-pulse DEER (> 18 A).53... 

The introductory chapter is brief but provides an ample introduction to mass spectrometry and leaves one comfortable as he/she moves on to the historical and instrumentation chapters that follow. A few of the basic equations are given as part of the review of basic concepts. In these few pages Dr Becker clearly introduces the concepts of atomic mass units relative to carbon, isotopes and isotope abundance. Figures 1.1 and 1.2 go hand in hand in providing the reader with the three major parts of a mass spectrometer (source, ion separation, detection) and show various alternatives for each of these. The subtle use of color in these and subsequent figures adds an attractive benefit for the reader. 

The atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 isotope. The relative atomic mass of an element is the weighted average of the isotopes relative to I/12(of jhe carbon-12 isotope. For example, the atomic mass of neon is 20.17 amu and is calculated from the following data neon-19 (amu of 19.99245, natural abundance of 90.92%), neon-20 (amu of 20.99396, natural abu dan c of 0,260%) and ncon-21 (amu of 21.99139, natural abundance ofc 82%) ... 

Due to the distinctive mass spectral patterns caused by the presence of chlorine and bromine in a molecule, interpretation of a mass spectrum can be much easier if the results of the relative isotopic concentrations are known. The following table provides peak intensities (relative to the molecular ion (M+) at an intensity normalized to 100%) for various combinations of chlorine and bromine atoms, assuming the absence of all other elements except carbon and hydrogen.1 The mass abundance calculations were based on the most recent atomic mass data.1... 

The study of the fiillerenes by C NMR spectroscopy is not straightforward. C has a low natural isotopic abundance (1.11%) and a low relative receptivity. In addition, the carbon atoms in fiillerenes have relatively long relaxation times and spectra for the higher, less symmetric fiillerenes can only be obtained within a reasonable time by using paramagnetic relaxation reagents, typically Cr(acac)3. 

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