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  • Mass spectroscopy

    Terms used in Mass spectroscopy:

    1. Molecular ion peak:

    When the vaporised organic sample passes into the ionisation chamber of a mass spectrometer, it is bombarded by a stream of electrons. These electrons have a high enough energy to knock an electron off an organic molecule to form a positive ion. This ion is called the molecular ion. and the peak corresponding to molecular ion in mass spectra called Molecular ion peak

    1. Base Peak:

    The peak corresponding to the separated ion beam which has the greatest intensity called as base peak.

    1. Isotopic peak:

    Ιonic fragments of the same chemical formula are usually represented by multiple adjacent peaks attributable to ions of different isotopic compositions called as isotopic peak. (M + 1)+ peaks or (M + 2)+ peaks.

    1. Metastable ion peak:

    Some fragment ions, undergo secondary fragmentations in the analyzer tube of the mass spectrometer, the resulting “signals” or peaks represent neither the m/e of the first ion nor that of the second ion; instead, “metastable ion” peaks are observed.

    F1+ (m/z)1  à F2+(m/z)2

    Metastable ion peak :   m* = (m2)2/m1

    1. Nitrogen Rule:

    Molecular ion having evan molecular weight possess either no nitrogen or even number of nitrogen atoms and molecule with odd molecular weight should contain an odd number of atoms.


    Principle of Mass spectroscopy:

    A sample in form of vapour state is bombarded with high energy electron beam. à result in usually one electron is knocked off from molecule à formation of molecular ion, which has positive charge on it and one free electron.

    e.g.  When  a beam of  electron is bombarded on vapour of ketone à one electron from lone pair of electron on oxygen atom is lost by the ketone à produces molecular ion (M+) on oxygen atom à molecular ions in terms produces fragments (F1+ and F2+)

    Mass spectrometer separate and detect positive ion or negative ion depend upon the polarity of the instrument based on mass to charge (m/z) ratio.

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