Relationship between mass and electron parity

consider a molecule that contains no nitrogen and is ionized through electron ionization. The ionization process consists of expelling one electron in order to produce a radical cation  

At the beginning the molecule M has an even mass because, hypothetically, it contains no nitrogen, and has an even number of electrons. After ionization, the mass has not changed, but the number of electrons has decreased by one unit, and has become odd. We have obtained a radical cation, which is represented by a dot combined with a plus sign M,+. 

If this molecule M is ionized through chemical ionization, we obtain a protonated molecule, (M + H)+. The protonated molecule has an odd mass the mass, which is even, increased by one because of the new proton. The number of the ion electrons is the same as that of the neutral molecule, that is even. It is a normal cation, not a radical cation. The same deductions apply to negative ions. 

An odd number of nitrogen atoms brings about an odd molecular mass in daltons such as is defined in mass spectrometry NH3 17, CH3NH2 31, and so on. Thus, in the case of an odd number of nitrogens, the earlier rule must be inverted for the ion, the mass parity is the same as the electron parity. 

From an analytical point of view, we see that an odd molecular mass, based on the predominant isotopes, indicates the presence of an odd number of nitrogen atoms. 

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