Probabilities of peak occurrence and information content of the NBS/EPA/MSDC mass-spectral data base

The probabilities of peak occurrence and contents of binary information were calculated for the 43 990 mass spectra in the 1987 NBS/EPA/MSDC data base. The median molecular weight of compounds in the data base was 230. Compounds composed of combinations of C, H, N, and O comprised 64% of the data base. The numbers of base peaks per mass channel are tabulated. A subset of compounds (30480) with low molecular weights was selected as a volatile-compound data base; the median molecular weight of this group was 189. The probabilities and information contents for the whole set of spectra and the volatile hydrocarbons, oxygenated hydrocarbons, chlorocarbons and chlorohydrocarbons and bromohydrocarbons were calculated. The most common peak in the entire data base and in the volatile set occurred at mass 41. All peaks in both of these sets of spectra with probabilities greater than 0.50 occurred below mass 78. The probabilities over the total and volatile-compound data base showed a general decrease with increasing mass channel with a division into odd- and even-mass curves which converged at high masses. Mass channels with 0.90–1.0 bit information content occurred below ca. mass 100. Information contents decreased with increasing mass and the two odd- and even-mass curves were superimposed on the general trend.