Peak widths molecular ions

It follows, that the peak width of a solute could give an indication of its molecular weight and, although the data may not be precise, approximate values could be extremely valuable when dealing with very large molecular weight substances such as polypeptides and proteins. In particular, the technique would be very useful for those substances that are extremely difficult, or impossible, to vaporize in the ion source of a mass spectrometer to provide mass data. 

Example The influence of relative slit width on peak shape and resolution is demonstrated on the second isotopic peak of toluene molecular ion, m/z 94 (Fig. 4.25). With the entrance slit at 50 pm and the exit slit at 500 pm the peak is flat-topped (left), because a narrow beam from the entrance sweeps over the wide open detector slit keeping the intensity constant as the scan proceeds until the beam passes over the other edge of the slit. Closing the exit slit to 100 pm increases resolution to 2000 without affecting the peak height (middle), but reduces the peak area by a factor of 4 in accordance with an increase in resolution by the same factor. Further reduction of the exit slit width to 30 pm improves... 

We summarize here the features of the UPS and UVA data which lead to the molecular ion concept for these aromatic pendant group polymers. First, the spectra of the polymers PVP and PS are essentially identical to those of condensed model molecular moieties 2-vinyl pyridine and ethyl benzene, respectively. Second, the solid-state spectra are related to the gas-phase spectra of these model moieties by an essentially constant shift to higher energy (lower binding energy) of all the ionization peaks by = 1.5 0.1 eV. Third, the width in energy of the solid-state ionization peak is Air = 1.0 0.1 for both polymers... 

This is particularly helpful when analyzing a mixture of tryptic peptides covering a wide mass range. Table 13.1 shows the molecular ion peak widths for a mixture of tryptic peptides from BSA in the normal delayed extraction mode, and in the MCA mode. In both cases, the instrument was tuned optimally for the mass of insulin (m/z 5733) to around 4 ns. In the normal delayed extraction mode, peak widths range from 8.6 ns at m/z 2832 to 11.4 ns at m/z 928. In the MCA mode, peak widths are in the range of 2.8 to 4.5 ns. 

In the linear mode of a TOP instmment, however, the fragments from molecules fhat decompose after the acceleration zone have nearly the same velocity as the intact molecular ions. Thus these metastable ions are detected at the same flight time, but with a slight increase in peak width. In the reflection mode of a TOP instrument, ions that undergo fragmentation cannot be detected at their correct molar mass. ... 

FIGURE 4.10 Peak widths of the singly, doubly, and triply charged molecular ions of (a) lactal-bumin and (b) lysozyme, as a function of pH. (Reprinted with permission from reference 26). 

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