Characterization of an Improved dc Plasma Excitation Source

Several modifications of the direct current plasma jet have resulted in significantly enhanced analysis capabilities. Reducing the electrode angle, decreasing the aerosol chimney size, and operating in a diffuse mode with helium have provided improved stability, sensitivity, freedom from interferences, and detection limits. While the actual mechanism of the helium enhancement is not clear, no direct evidence was found which supports participation in the excitation step. Rather, improved desolvation, vaporization and possibly sample introduction result from the thermal conductivity of helium being much greater than that of argon.