A LOW-PRESSURE PENNING ION-SOURCE FOR VOLATILE ORGANIC-COMPOUNDS

A Penning ion source for volatiie organic compounds is described. Desi gn, operating conditions, and properties of the ions generated are rep orted. The source operates at pressures ranging from 10(-5) to 10(-3) mbar; a sensitivity of 1.4 X 10(-4) A/mbar is achieved. Mass spectra o f 4-methyl-2-pentanone, ethyl benzoate, dimethyl sulfoxide, trichlorof luoromethane and 1,1,2-trichlorotrifluoroethane obtained at low discha rge currents are almost identical with their corresponding electron im pact spectra at 70 eV. Two different discharge modes, depending on pre ssure, were found. The average energy of the ions is proportional to t he potential of the ion source ([E] = 0.64U, energy given in eV). Mult iply charged Ar ions, Ar(n+), up to n = 4, have been observed.