Intensity of molecular ion peak in electron ionization mass spectra

Compounds from NIST'08 and Wiley 8th databases were considered as a representative subset of the general population of organic compounds which can be analyzed using mass spectrometry with electron ionization. The percentage of organic compounds as a function of intensity of molecular ion (M+aEuro cent) peak normalized to the base peak was determined for the first time. It was shown that only 26% compounds have high-intensity M+aEuro cent peaks (greater than 50% of base peak). Intensity of M+aEuro cent peak is less than or equal to 1 or 5% of base peak for 24 or 37% compounds respectively. It means that in case of trace-level analysis M+aEuro cent peak may not be registered for quarter (or even more) of organic compounds. It is well-known that the absence of M+aEuro cent peaks in electron ionization mass spectra reduces reliability of unknown compound identification based on library search. Therefore determination of molecular mass by independent technique (e.g., mass spectrometry with chemical ionization) is necessary for increasing the identification reliability.