A reduced-pressure argon inductively coupled plasma (ICP) is interfaced to a mass spectrometer to evaluate its possibility of increasing the sensitivity of nonmetallic elements. An electrostatically shielded water-cooled torch is used for the investigation of the secondary discharge at the sampling orifice. Iodine vapor is continuously introduced into the torch as an analyte by using a peristaltic pump. The effects of plasma operating parameters such as gas flow rate, pressure, and power on the intensities of background and iodine ions are studied. It is shown that when the pressure is less than about 30 Torr, an intensive secondary discharge occurs at the sampling orifice if the torch shield is not grounded. The background ion intensity and secondary discharge effect decrease with increasing pressure. The pressure in the torch has an important effect on both polyatomic and analyte intensities. At about 130 Torr of torch pressure, the iodine signal is more than one order of magnitude higher than that obtained at atmospheric pressure, which suggests that low-pressure ICP provides a sensitive ion source for the elements with high ionization potential.
Xiaomei Yan, Tomokazu Tanaka, and Hiroshi Kawaguchi, "Reduced-Pressure Inductively Coupled Plasma Mass Spectrometry for Nonmetallic Elements," Appl. Spectrosc. 50, 182-187 (1996)