Mass spectrometry is not a true spectroscopic technique because absorption of electromagnetic energy is not involved in any way.
Mass spectrometry is use to characterized organic molecule in two principal ways:
I. To measure exact molecular weight, and by this, exact molecular formula can be determined.
II. To indicate the point at which molecule prefers to fragment; from this, the presence of certain structural units in the compound can be recognized.
A Mass spectrum is a presentation of the masses of the positively charged fragments versus their relative concentrations. The most intense peak in the spectrum, called the ‘Base peak’ is assigned value of 100%, and the intensity of other peaks, are reported as percentage of the base peak.
The commonly used technique in mass spectrometry is ‘Electron Impact’. In this mode spectrometer bombards molecules in the vapor phase with a high energy electron beam and record the result of electron impact as a spectrum of positive ions separated on the bases of mass/charge (m/z). On electron impact, the molecules are energized sufficiently to eject an electron, thus produces molecular ion M+.which further fragments to more daughter ions.
The molecular ion though it may be of low abundance gives highly useful information about the identity of an organic compound. Fragmentation pattern that is the break-up process of the molecular ion into smaller ions gives further information about the structure of compound.
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