Molecular fragments detection from mass spectra

Often but not necessarily, the peak at highest m/z results from the detection of the intact ionized molecule, the molecular ion, M. The molecular ion peak is usually accompanied by several peaks at lower m/z caused by fragmentation of the molecular ion to yield fragment ions. Consequently, the respective peaks in the mass spectrum may be referred to as fragment ion peaks. 

Also in the mass spectrum of 3-hydrazino-5,6-diphenyl-l,2,4-triazine (82) the elimination of nitrogen from the molecular ion could not be detected. The dominant process in this case is the fragmentation of the molecular ion into a highly delocalized diphenylacetylene radical ion at 178 mass units (7lOMS(5)i085). 

Two aspidofractinine-type compounds, lahadinines A (233) and B (234), obtained from Kopsia pauciflora are remarkable for having a cyano-substituent at C(21) [162]. The mass-spectra of these alkaloids are characterized by a strong molecular ion (usually also the base peak), with the odd mass indicating the presence of a third nitrogen. Fragments attributable to loss of CN and HCN were also detected in the mass-spectrum and the IR spectrum showed a weak band at ca. 2250 cm. The characteristic H(21) signal was absent in the H NMR... 

Such expectations typically involve attempts to infer what spectromet-ric data do not imply. Few conclusions concerning sample purity, for example, follow from a routine mass spectrum. The intensity of a molecular ion peak in the spectrum observed is not necessarily correlated to the concentrations of components in the sample investigated. A volatile but stable minor component can produce an intense molecular ion peak. The most abundant component, however, if it is comparatively involatile or insensitive to the ioiiization method, may yield a relatively weak peak. Alternatively, the molecular ion formed by the major component may fragment into smaller ions faster than it can be detected. In these circumstances, it outrages logic to conclude that a compound is absent from a sample because a mass spectrum failed to show the corresponding molecular ion. Such negative evidence deserves no trust. 

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