Spectra of ions produced by corona discharges

A mass spectrometric study of ions extracted from both positive and negative DC corona discharges, initiated in point-to plane electrode system, has been carried out in ambient air at low air pressure (5 - 30) kPa. The average relative humidity of air was typically 40-50 %. Ions were extracted through a small orifice in the plane electrode into an intermediate gap where the low pressure prevented further ion-molecule reactions. Mass analysis of negative ions formed in the negative corona discharge using ambient air has shown that the yield of individual ions is strongly affected by trace concentrations of ozone, nitrogen oxides, carbon dioxide and water vapour. In dry air the CO3- ion was found to be dominant. In presence of water this is converted very efficiently to cluster ions CO3-.(H2O)(n) containing one and more water molecules. The yield of O-3(-).(H2O)(n)clusters or core ions was found to be considerably lower than in some other studies at atmospheric pressure. The mass spectrum of ions extracted from drift region of a positive corona discharge was simpler being dominantly cluster ions H3O+.(H2O)(n) most probably formed from O-2(+) ions, a two step process being active if water molecules are present in the discharge gap even at relatively low concentration.