The effects of the introduction of capillary supercritical fluid chromatography mobile phases, CO<sub>2</sub> and N<sub>2</sub>O, into a helium microwave-induced plasma (He-MIP) are investigated. The plasma and mobile-phase flow rates are maintained at conditions comparable to those typically used for these systems. Spectral characteristics are monitored in both the ultraviolet-visible and near-infrared regions. Molecular bands from these "foreign" gases are assigned and their relative intensities are noted as a function of the doping gas flow rate. Possible molecular band interferences are noted for nonmetal atomic emission determinations with He-MIPs. Nonmetal atomic emission reduction by these gases is studied with the use of chlorine atom and ion lines as test signals. Decreases in chlorine atom and ion emission are seen as CO<sub>2</sub> and N<sub>2</sub>O flow rates are increased. It is found that ion line emission intensity is reduced more significantly than atom line emission. Reduction of both atom and ion line emission is more pronounced with CO<sub>2</sub> than with N<sub>2</sub>O. The electron density of the plasma does not change as the CO<sub>2</sub> flow rate is increased from 0 to 0.25 mL (STP)/min.
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