Isotopic peaks theoretic intensity

The theoretic intensity of isotopic peaks due to the presence of several 13C isotopes may be easily calculated for any molecule. The intensity of the A + 2 peak (as a percentage of the A peak) for a compound with n carbon atoms may be established by the following formula ... 

Fig. 3.24. Listing of the theoretical isotopic distribution of Ci6H2oOSi at infinite resolution. The contribution of is not considered, and isotopic peaks above m/z 260 are omitted due to their minor intensities.

Fig. 5.10. Illustration of the signal-to-noise ratio. The first isotopic peak of toluene molecular ion, m/z 93, (70 eV El, R = 4000) shows about S/N = 250, whereas the second isotopic peak at m/z 94 has S/N = 10. The theoretical ratio of intensities of m/z 92 93 94 = 100 7.7 0.3, i.e., the ratio of intensities also reflects the S/N ratio.

There is another advantage [70] of the high resolution so obtained for the multiply charged ions produced by ESI, the 12C/13C isotopic peaks must exhibit unit mass spacing so that the number of them in a unit m/z ratio must represent the number of charges. A theoretical intensity distribution may be compared with the experimental distribution. Figure 2.64 displays a spectrum of ubiquitin from McLafferty s laboratory [71]. 

The composition of most FeD3(BR)2 complexes was confirmed by their electron impact (El), plasma desorption (PD), and fast atom bombardment (FAB) mass spectra. The molecular ion peaks were the most intense and corresponded to the theoretically calculated isotope peak series. 

As introduced in Section 8.4, a match value that shows how well the theoretical isotope distribution matches the measured intensities of the isotope peaks in the MS can be... 

Because of the occurrence of 13c, and 1 0, and because mass spectrometry sorts ions according to m/e values, the peak intensities at m/e 252-256 do not represent the true proportion of iron isotopes. Table II is an example of how the abundances of the diligand ions deviate from the abxmdeuices of iron isotopes. Theoretically, the abundances of the diligand species can be calculated from the abundances of iron isotopes (IJ ). Calculated values may be used as a reference for the accuracy of the experimental values. 

Samples spiked with known amounts of stable isotopes Fe> Cu, Zn, Zn were prepared and the measured ion intensities in the mass spectra were compared to theoretical values The response of the mass spectrometer was linear over a wide range of enrichment levels ( ) A computer program included in the mass spectrometer software package was used to calculate the theoretical values for peak intensities in the mass spectra Corrections for isotopic composition of the ligand ( C, H) were made as described previously (2) ... 

It is interesting how the width of the isotopic pattern increases as X+2, X+3, X+4. .. become detectable. In principle, the isotopic pattern of C expands up to X+w, because even the composition is possible. As a result, the isotopic pattern of w atoms of a di-isotopic element consists at least theoretically of w+l peaks. However, the probability of extreme combinations is negligible and even somewhat more probable combinations are of no importance as long they are below about 0.1%. In practice, the interpretation of the carbon isotopic pattern is limited by experimental errors in relative intensities rather than by detection limits for peaks of low intensity. Such experimental errors can be due to poor signal-to-noise ratios (Chap. 1.4.3), autoprotonation (Chap.7), or interference with other peaks. 

---