The RF Discharge at Atmospheric Pressure and Its Use as an Excitation Source in Analytical Spectroscopy

Abstract

Radio-frequency fields can be used to produce excitation of the spectral emission of various chemical species at low pressure, or at atmospheric pressure. The use of RF excitation under low pressure has a long history dating back to the works of J. J. Thompson in 1891. Successful results have been obtained since then in the excitation and measurements of samples in gaseous and solid form contained in electrodeless, evacuated glass vessels. The use in analytical spectroscopy of RF discharges of flamelike appearance occurring at atmospheric pressure is of more recent origin. This work is concerned with this type of discharge and presents a general review of the field, together with the results obtained with two RF discharges, one of 30 Mc and 250 W and the other of 2450 Mc and 2 kW. This last unit has been specially designed and constructed to be used on an optical bench in a manner similar to the usual spark and arc sources. From the several carrier gases studied, air, N₂, O₂, CO₂, He, and H₂, the last two were chosen for their relatively reduced background and for their ability to permit the excitation of numerous chemical species. Practically all compounds present in a dissociated state can be excited by the 2450-Mc discharge in helium as can be seen from the emission spectra of some 70 elements recorded thus far, producing atomic and molecular radiations. The RF discharge at atmospheric pressure possesses a high electron energy and a comparatively low thermal energy. Various procedures developed for the introduction of solid, liquid, and gaseous samples are discussed and the use of these procedures for analytical purposes are presented. The following chemical species have been investigated more closely with regard to quantitative measurement possibilities: Sn, Pb, Hg, B, Au, Cd, As, Sb.