LOW-ENERGY, LOW-TEMPERATURE MASS-SPECTRA .15. ELECTRON IONIZATION MASS-SPECTRA OF SOME MONOTERPENE HYDROCARBONS

The electron ionization mass spectra of 19 monoterpenes are reported at 70 eV and ca. 500 K and at 12 eV and ca. 350 K. The former agree well with those published previously, whereas the latter show simplification, especially at low m/z values. Temperature has little effect on the limonene spectrum at electron energies of 12 or 70 eV. Variation of the electron energy at 350 K and 500 K has a marked effect. The effect of electron energy on the spectrum of myrcene is fairly small, apart from the decrease in the relative intensity (RI) of the molecular ion. In the cases of limonene and DELTA-2-carene it is fairly large. A correlation is found between the 12 eV mass spectra and published linked scan spectra at constant B/E for some of the monoterpene hydrocarbons. The compounds are divided into three classes showing, respectively, low, medium and high fragmentation in their low-energy mass spectra. The percentage fragmentation along with the ratios RI(m/z 121)/(m/z 93) serve in most instances to distinguish the terpene hydrocarbon.