We have already studied the various methods of ionization and mass analysis in lectures 10 and 11. This lecture discusses the properties of the mass spectra, their interpretation, and their applications, particularly in biomolecular analysis.

Characteristics of mass spectra
There are three basic characteristics of mass spectrometry: exact mass, isotopic abundances, and fragmentation.

Exact mass: When we talk of molecular weights in general chemistry, we typically refer either to the nominal mass or the average molecular mass. Nominal mass is calculated by adding the atomic masses of the predominant isotopes of all the elements rounded off to the nearest integer. Average molecular mass is the weighted average of the masses of all the isotopes (without rounding off). Stable isotopes of hydrogen, carbon, nitrogen, and oxygen as well as their relative abundances are listed in table 13.1.

The nominal and average molecular masses of, say methane are 16 Da and 16.0428 Da, respectively. A mass spectrometer, however, detects the exact masses of the ions.

Isotopic abundances: Isotopic abundances are reflected in the high resolution mass spectra of the compounds and allow easy identification of small organic compounds. Let us take an example of methane. The different possible isotopologues of methane are listed in table 13.2 along with their natural abundances.