Unit-Mass Molecular Ion and Isotope Peaks

So far, we have discussed the mass spectrum in terms of unit resolutions The unit mass of the molecular ion of CyHyNO (Fig. 2.1) is m/z 121—that is, the sum of the unit masses of the most abundant isotopes  

In addition, molecular species exist that contain the less abundant isotopes, and these give use to the iso- 

Elements Isotope Relative Abundance Isotope Relative Abundance Isotope Relative Abundance 

If sulfur or silicon, is present, the M + 2 will be more intense. In the case of a single sulfur atom, 34S contributes approximately 4.40% to the M + 2 peak for a single silicon in the molecule, 30Si contributes about 3.35% to the M + 2 peak (see Section 2.10.15). The effect of several bromine and chlorine atoms is described in Section 2.10.16. Note the appearance of additional isotope peaks in the case of multiple bromine and chlorine atoms. Obviously the mass spectrum should be routinely scanned for the relative intensities of the M + 2, M + 4, and higher isotope peaks, and the relative intensities should be carefully measured. Note that F and I are monoisotopic. 

For most of the Problems in this text, the unit-resolution molecular ion, used in conjunction with IR and NMR, will suffice for determining the molecular formula by browsing in Appendix A. For several more difficult Problems, the high-resolution formula masses— for use with Appendix A (see Section 2.4.2)—have been supplied. 

If these isotope peaks are intense enough to be measured accurately, the above calculations may be useful in determining the molecular formula.  

---

For example, the presence of bromine can be determined easily, because bromine causes a pattern of molecular ion peaks and isotope peaks that is easily identified. If we identify the mass of the molecular ion peak as M and the mass of the isotope peak that is two mass units heavier than the molecular ion as M -t- 2, then the ratio of the intensities of the M and M+2 peaks will be approximately one to one when bromine is present (see Chapter 8, Section 8.5, for more details). When chlorine is present, the ratio of the intensities of the M and M + 2 peaks will be approximately three to one. These ratios reflect the natural abundances of the common isotopes of these elements. Thus, isotope ratio studies in mass spectrometry can be used to determine the molecular formula of a substance. 

The example of ethane can illustrate the determination of a molecular formula from a comparison of the intensities of mass spectral peaks of the molecular ion and the ions bearing heavier isotopes. Ethane, C2H6, has a molecular weight of 30 when it contains the most common isotopes of carbon and hydrogen. Its molecular ion peak should appear at a position in the spectrum corresponding to a mass of 30. Occasionally, however, a sample of ethane yields a molecule in which one of the carbon atoms is a heavy isotope of carbon, This molecule would appear in the mass spectrum at a mass of 31. The relative abundance of in nature is 1.08% of the atoms. In the tremendous number of molecules in a sample of ethane gas, either of the carbon atoms of ethane will turn out to be a atom 1.08% of the time. Since there are two carbon atoms in ethane, a molecule of mass 31 will turn up (2 x 1.08) or 2.16% of the time. Thus, we would expect to observe a peak of mass 31 with an intensity of 2.16% of the molecular ion peak intensity. This mass 31 peak is called the M+ peak, since its mass is one unit higher than that of the molecular ion. 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds... 

This example can be used in reverse to show the usefulness of looking for such isotopes. Suppose there were an unknown sample that had two molecular ion peaks in the ratio of 3 1 that were two mass units apart then it could reasonably be deduced that it was highly likely the unknown contained chlorine. In this case, the isotope ratio has been used to identify a chlorine-containing compound. This use of mass spectrometry is widespread in general analysis of materials, and it... 

The table 5 below summarizes high resolution data from m/z 39 to the prominent molecular ion at 454.1922, which corresponds to C24H32O5FCI. Each fragment ion containing chlorine appears as a doublet peak because chlorine is a mixture of two isotopes (35C1 and 37C1). These peaks are two mass units apart with an intensity ratio of 3 to 1. 

Whether or not a high-resolution mass spectrometer is available, molecular ion peaks often provide information about the molecular formula. Most elements do not consist of a single isotope, but contain heavier isotopes in varying amounts. These heavier isotopes give rise to small peaks at higher mass numbers than the major M+ molecular ion peak. A peak that is one mass unit heavier than the M+ peak is called the M+l peak two units heavier, the M+2 peak and so on. Table 12-4 gives the isotopic compositions of some common elements, showing how they contribute to M+l and M+2 peaks. 

Although the molecular ion of pentane have m/z value of 72, its mass spectrum shows a very small peak at m/z = 73. This is called an M+1 peak because the ion responsible for this peak is one unit heavier than the molecular ion. The M+1 peak occurs because there are two naturally occuring isotopes of carbon 98.89% of natural carbon C and 1.11% is C. The M+1 fragment results from molecular ions that contain one C instead of a C. ... 

Isotope peaks observed for proteins are mainly produced by the 12C and 13C mass difference of 1 mass unit. Once one molecular ion charge state is determined, the other charge states are readily assigned by spectral observation. ESI, MALDI, and FAB are usually used in the positive-ion mode, especially for protein MW determination therefore the proton mass must be used to calculate the ion mass, as shown in Table 15.7. 

---