Nitrogen rule of mass spectrometry

The nitrogen rule of mass spectrometry says that a compound containing an odd number of nitrogens has an odd-numbered molecular ion. Conversely, a compound containing an even number of nitrogens has an even-numbered M+ peak. Explain. 

The spectra in Fig. 7 can also be used to illustrate another important rule in mass spectrometry, the nitrogen rule. The nitrogen rule states that any common organic molecule or odd-electron ion that contains odd number of nitrogen atoms has an odd (nominal) molecular mass. For example, tributyl amine contains one nitrogen atom thus, the nominal MW must be odd, and so it is (185 Da). However,... 

For most elements in organic compounds, such as C, H, O, S, P, and the halogens, the mass (of the most abundant isotopes) and valence are either both even or both odd, so even molecular weights always result. Nitrogen is a major exception the atomic weight is 14, but the valence is 3. This phenomenon has led to the nitrogeii rule in mass spectrometry, as expressed in this exercise. 

Restricting to the more common elements in organic mass spectrometry (H, B, C, N, O, Si, S, P, F, Cl, Br, I, etc.), a simple rule holds valid With the exception of N, all of the above elements having an odd number of valences also possess an odd mass number and those having an even number of valences have even mass numbers. This adds up to molecular masses fulfilling the nitrogen rule (Tab. 6.7). 

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. 

Whilst there are a number of advantages in using a radioactive label in preliminary experiments to establish correct experimental conditions prior to the use of the more expensive stable isotope label, it is not essential. Indeed, in the case of nitrogen 5 and oxygen-18 studies, the absence of a suitable radioisotope rules out such a procedure. Furthermore, mass spectrometry may also... 

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