Isotopomer, mass spectrum

Example Peptides often contain sulfur from cysteine. Provided there are at least two cysteines in the peptide molecule, the sulfur can be incorporated as thiol group (SH, reduced) or sulfur bridge (S-S, oxidized). Often, both forms are contained in the same sample. At ultrahigh-resolution, the contributions of these compositions to the same nominal m/z can be distinguished. The ultrahigh-resolution matrix-assisted laser desorption/ionization (MALDI) FT-ICR mass spectrum of native and reduced [D-Pen jenkephalin gives an example of such a separation (Fig. 3.25). [39] The left expanded view shows fully resolved peaks due to and C2 isotopomers of the native and the all- C peak of the reduced compound at m/z 648. The right expansion reveals the peak of the native plus the... 

The mass spectrum of cyclohexanone has been examined by deuterium-labeling to reveal the mechanism effective for propyl loss, [M-43], m/z 55, from the molecular ion, = 98. [38,39] The corresponding signal represents the base peak of the spectrum (Fig. 6.11). Obviously, one deuterium atom is incorporated in the fragment ion that is shifted to m/z 56 in case of the [2,2,6,6-D4]isotopomer. These findings are consistent with a three-step mechanism for propyl loss, i.e., with a double a-cleavage and an intermediate 1,5-H shift. 

Fig. 15. Neutral product mass spectrum seen for the DR of various H30+ isotopomers, with a 70% transmission grid intersecting the path of the neutral products prior to detection [199]. Analysis of this mass distribution subsequently yields the reaction s product distribution...

Both chlorine ( Cl 76% and Cl 24%) and bromine Br 51% and Br 49%) exist as two isotopes. Consider the differences in mass numbers for the isotopes—any compound containing either Ci or Br will have molecular ions 2 u apart. The lightest isotopomer of ClBrj is Ci Br3 at 272 u and the heaviest is Cl Brj at 280 u. Three other molecular masses are possible, giving rise to a total of five peaks in the mass spectrum shown in Figure 8.38. The differences in the relative intensities of these peaks are a consequence of the differences in the percent abundance for each isotope. 

Most elements occur naturally as a mixture of isotopes, differing from one another by the number of neutrons present in the nucleus. Natural carbon comprises a mixture of mainly and (98.9 and 1.1% respectively) with a trace of the radioactive isotope Chlorine has isotopes Cl (75.77%) and Cl (24.23%). Thus any mass spectrum will demonstrate a number of molecular ions due to the isotopomers present. Most data systems have programs which allow the input of a molecular formula which generates a theoretical isotopic distribution. This can then be compared with the actual spectrum obtained (Fig. 5.16). It may be necessary to add or subtract a proton from the inputted formula, hydrogen contains 0.015% deuterium. 

FIGURE 15.18 Top, a pseudo-product ion mass spectrum of m/z 264 of PFTBA but without collision-induced dissociation (CID) the mass spectrum was acquired with default isolation parameters and isolation window of 1 Th. Bottom, a full scan mass spectrum of PFTBA obtained with the same duration of ionization and showing both the m/z 264 peak and the C-isotopomer peak at m/z 265. By comparison, loss of the m/z 264 ion during the isolation process is essentially zero for this chemically-stable ion. For chemically-unstable ions, loss of precursor ion during mass-selective isolation can be minimized by using an isolation window of 3-5 Th. 

Fig. 1.40 Excitation spectra of Lis clusters detected by photoionization of the excited states (a) no mass selection (b) spectrum of the Lis = Li Li Li isotopomer (c) spectrum of Lis = Li Li Li, recorded with doubled sensitivity [95]...

Here, part of a multiphoton ionization mass spectrum in the region around the benzene radical cation is represented. While the linear TOF spectrum only reveals the molecular ion and its Cj-isotopomer, the reflectron TOF spectrum additionally shows the fragment ions CgH5 and CgH4 + although the ion source conditions were identical. (The difference in mass resolution will not be discussed here, see the Further reading section and other articles in this Encyclopedia.)... 

Figure 1 Section of high resolution Ceo mass spectrum obtained by multistep electron impact ionization. The black peaks are calculated isotopomers ofthe septuply charged Cg ion normalized to the first isotopomer of the experimental run. After Scheier P and Mark TD (1994) Physical Review Letters 73 54.

Figure 9.68 shows the El mass spectrum of a monochlorinated chlorobenzoqui-none molecule. The relative proportions of certain ion couples allow rapid identification of the chlorinated ions m/z 142/144, 114/116, 88/90, and 60/62. On the same principle, we note that ion m/z 79 is not chlorinated due to the absence of a peak at m/z 81. The case of the m/z 86 ion is more complex. Careful observation of the isotope pattern m/z 88/90 shows that the abundance of ion m/z 88 is proportionally more than it should be in theory for a monochlorinated ion. It is therefore likely that some of the m/z 88 ions constitute the contribution of Cl isotopomers of isotope pattern m/z 86/88. 

The greater the number of isotopomers, the more peaks are included in an isotope pattern and the more the ionic signal is diluted. Beyond a certain number of isotopomers, some ions become too few for detection. As an example, consider the isotopic pattern corresponding to the molecular ion of hexachlorobutadiene in Figure 9.72. With six chlorine atoms, the isotope pattern should present seven main peaks (m/z 258, 260, 262, 264, 266, 268, and 270) in addition to the peaks of C. In reality, the m/z 270 ion does not appear in the mass spectrum. The reason is that the relative abundance of the isotopomers containing six Cl atoms among the isotopomers constituting the molecular ion is 0.24 or 4.6 x 10 . This corresponds to a number of ions in the source below the limit of detection of the mass spectrometer. 

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

In calculating the exact masses of the three isotopomers of the peak M+1 (molecular formulae given in Part One), a = 209.09217 b = 209.095054 and c = 209.09321 amu. The differences between these values are very much smaller than the value of AM calculated from the spectrum. Under the conditions of recording the spectrum these three types of molecules would appear superimposed. 

Figure 1.14 Simultaneously recorded REMPI-TOF spectra of several isotopomers of MoC. Spectra (b) - (f) correspond, respectively, to 98MoC (24.1% natural abundance), 97MoC (9.6%), 96MoC (16.7%), 9SMoC (15.9%), and 94MoC (9.3%). Spectrum (a) is the sum of spectra (b) - (f). It is not the spectrum that would have been obtained without TOF mass-selectivity because the summation does not include the spectra of 92MoC (14.8%) and 100MoC (9.6%) (from D. Brugh, Ph.D. thesis, 1997, courtesy of M. Morse see also Brugh, et at., 1998).

This problem Is a general one. The C nmr spectrum of a multiply labeled metabolite or biosynthesized product is a superposition of subspectra corresponding to isotopomers consistent with the biosynthetic pathways Involved. If the product contains n carbon atoms, there will be 2 isotopomers possible, with 2 -l contributing to the C nmr spectrum. As illustrated above, a detailed analysis of the flux of C through the system cannot always be done using the spectrum alone but frequently requires nmr and mass spectrometric data as well. In the analysis of such data, one of several approaches can be taken. 

The combination of laser- and mass-spectrometry has brought a wealth of new information on the structure and dynamics of molecules [494]. If, for instance a mixture of different isotopomers of a molecular species (these are molecules with different atomic isotopes) is present, the absorption spectra of the different isotopomers might overlap which impedes the analysis of the spectrum. Therefore the separation of these isotopomers by a mass spectrometer will facilitate the unambiguous analysis. The isotope shift of spectral lines gives additional information on the molecular structure. 

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