A low-pressure penning ion source for volatile organic compounds

A Penning ion source for volatile organic compounds is described. Design, operating conditions, and properties of the ions generated are reported. The source operates at pressures ranging from 10−5 to 10−3 mbar; a sensitivity of 1.4 × 10−4 A/mbar is achieved. Mass spectra of 4-methyl-2-pentanone, ethyl benzoatc, dimethyl sulfoxide, trichlorofluoromethane and 1,1,2-trichlorotrifluoroethane obtained at low discharge currents are almost identical with their corresponding electron impact spectra at 70 eV. Two different discharge modes, depending on pressure, were found. The average energy of the ions is proportional to the potential of the ion source (〈E〉 = 0.64U, energy given in eV). Multiply charged Ar ions, Arn+, up to n = 4, have been observed.