ELECTRICAL EFFECTS DURING THE MASS-SPECTROMETRIC SAMPLING OF IONS FROM HOT FLAME GASES AT ATMOSPHERIC-PRESSURE

Two atmospheric-pressure, flat, flames of H2 + O2 + N2, with different final temperatures, have been burnt on an insulated metallic burner. Small amounts of caesium were added to the burner supplies, generating Cs+ ions and free electrons in the flame. The burnt gases were sampled continuously through a small (80-200-mu-m diameter) orifice in the tip of a hollow, conical, nickel nozzle. The gases subsequently expand, adiabatically and supersonically, into a vacuum chamber. A pointed needle was located immediately behind the orifice, on the axis of the hollow sampling nozzle, and collected either the positively or negatively charged species which had passed through the orifice into the vacuum chamber. The currents collected by this probe were studied as a function of the bias voltage on the needle, the potential difference applied between the burner and the sampling nozzle, the electron concentration in the flame, and the diameter of the sampling orifice. The observations can be explained in terms of the formation of a sheath, either of positive ions or of electrons and negative ions, adjacent to the surface of the nozzle. A full consideration of sheath formation at a sampling probe in a flowing plasma is given. The thickness of a sheath varies inversely with the electron density in the flame, so that under certain conditions the sampling orifice is covered by a sheath of either positive ions or electrons. It is concluded that at low electron densities (almost-equal-to 10(7) ml-1), very few electrons were able to pass through the inlet hole, but instead are lost to the sampling nozzle. By contrast, at higher electron concentrations (almost-equal-to 10(10) ml-1) and with a suitably large orifice, some electrons do enter the vacuum chamber. The effects of increasing the negative ion concentration, and reducing the electron density, were studied by the addition of CO2 to the flame. The positive and negative ions were also studied by a mass spectrometer in the vacuum system, after removing the collecting probe. It was found that the optimal sampling of negative ions required the application of a voltage, such that the nozzle was at a positive potential with respect to the burner. The losses of positive or negative ions during sampling can be negligible, compared with the significant loss of electrons. The role of the sheath in determining the magnitude of the losses of any species, under a wide range of experimental conditions, is considered.