Isotopomer peak

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. 

The negative entries above result when the first emd second isotope peaks are more intense than the lowest isotopomer peak. The remedning 6855 spectrum-structure pairs are distributed over another 360 mass differences. Knowledge of this mass... 

Fig. 13 Isotopic line splitting of the V3 stretching vibration in single crystalline (see also Fig. 12(a)), after [108, 109], The origin of each absorption band is indicated by an isotopomer present in crystals of natural composition. While the absorption could be fitted by a Lorentzian band profile, the remaining peaks were dominated by the Gaussian contribution in the Voigt band shapes (solid lines below the spectrum). The sum result of fitting the isotopic absorption bands is inserted in the measured spectrum as a solid line...

Perhaps the first clear observation of a reactive resonance in a collision experiment was recently made for the F + HD —> HF + D reaction.65-67 This reaction was one isotopomer of the F + H2 system studied in the landmark molecular beam experiments of Lee and co-workers in 1985.58 Unlike the F + H2 case, no anomalous forward peaking of the product states was reported, and results for F + HD were described as the most classical-like of the isotopes considered. Furthermore, a detailed quantum mechanical study68 of F + HD —> HF + D reaction on the accurate Stark-Werner (SW)-PES69 failed to locate resonance states. Therefore, it was surprising that the unmistakable resonance fingerprints emerged so clearly upon re-examination of this reaction. 

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

The measurements of the labeled metabolites may be performed with GC- or LC-MS, or by NMR. Because it is the most commonly used method, we will only consider GC-MS based approaches here. Obviously and unfortunately, it is not possible to directly measure the isotopomer enrichments by GC-MS, because the apparatus only yields total masses of molecules or fractions thereof, but not directly the position of a label. Each MS peak is produced by all isotopomers with the same molecular weight that is, the same number of labeled carbon positions. Sometimes this concept is also called mass isotopomers [264]. In a so-called retrobiosynthetic approach, it has been shown that the labeling state of many intracellular pools can be determined indirectly by measuring the labels in macromolecular biomass components at steady state for example, the labeling state of alanine from hydrolyzed protein reflects the label of pyruvate [265]. Using this approach, it is possible to quantify fluxes into storage components. 

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. 

Identical or at least almost identical ionization efficiency (Chap. 2.4) for a pair of compounds is only given for isotopomers (Chap. 3.2.9). As these differ from the nonlabeled target compound in mass, they can be added to the mixture at known concentration to result in a special case of internal standardization. The relative intensity of the corresponding peaks in the RICs or SIM traces is then taken as the relative concentration of labeled internal standard and target compound hence the term isotope dilution. [46]... 

The isotope ratio traces of the GC peaks exhibit a typical S shape. The heavier isotopic species of a compound are eluted more rapidly than the light species. Similar effects can be observed for all chromatographic processes, whereas the size of the isotope fractionation and the elution order of the isotopomers de-... 

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. 

Another example of the separation of isotopomers was observed with deuterated benzoic acid, eluted with acidified 30% aqueous methanol. Deuteration, especially at meta and para positions, decreases retention.136 Ratios of retention times are presented in Table 9, where the IEs of monodeuterated benzoic acid or of benzoic-2,6-% acid are omitted because they are too small to produce resolvable peaks although they do lead to detectable peak broadening. Because the IEs are strongest at positions more distant from the carboxyl group, and because the elution conditions were sufficiently acidic that the benzoic acid was not ionized, an IE on the pKa was rejected as responsible. Instead some sort of lipophilic interaction was suggested, and deuteration at the ortho positions is less effective because the carboxyl group hinders those positions. However, it should be noted that the data in Tables 2 and 3 indicate that the IE on acidity does not fall off at meta and para positions.24,32... 

Protonation of the dinitrogen complex I gives the NNH2 complex [WF(NNH2)(depe)2]BF4 (III). As Raman spectra of III were found to be almost featureless, vibrational information on this system has exclusively been obtained from infrared spectroscopy. Figure 4 shows the MIR and FIR spectra of III and its 15N and 2H isotopomers 15N-III and 2H-III, respectively. Seven peaks can be recognized to shift upon... 

Each of these gives rise to an AB pattern due to 13C-13C splitting ( Jcc for the first two and 2JCC for the third species). These additional 13C signals appear as weak satellite peaks (0.55% of the main peak) around the main peaks from the first three species, and because signal-to-noise ratios are typically much less than 200 for 13C spectra, these signals will be buried in the noise. Finally, there is one isotopomer with three 13C nuclei ... 

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