Electrostatically Shielded Water-Cooled Torch for Inductively Coupled Plasma Mass Spectrometry

Abstract

An electrostatically shielded water-cooled torch was constructed in order to minimize the "secondary discharge" at the sampling orifice, which has been a serious problem in the application of a water-cooled torch to inductively coupled plasma mass spectrometry. The outer gas flow rate could be reduced down to 5l/min with an appreciable increase of sensitivity. The intensities of copper ions originating from the sampling orifice decreased to the same level as that of the conventional torch, and doubly charged analyte ions considerably decreased, even compared to those with the conventional torch. A decrease in the plasma potential was also observed with a probe. These facts suggest that the secondary discharge at the sampling orifice was almost completely suppressed by the electrostatic shielding. An increase, however, in the ion signals of analyte oxide was observed. The intensities of ArO⁺ and Ar₂⁺ decreased, but that of ArN⁺ increased. The enhancement of ArN⁺ may have resulted from an entrainment of air into the plasma with the decreasing outer gas flow rate.