Study on Mass Spectrometry for Nonmetallic Elements with Reduced-Pressure ICP

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In order to generate an inductively coupled plasma (ICP) at reduced pressure (RP), an electrostatically shielded water-cooled (ESWC) torch with a lengthened top was fabricated. A modified interface between the torch and mass spectrometer was also made. An additional rotary pump was used for the further evacuation of the torch.The RP-ICP could be generated easily under very wide operating conditions. The lower the torch pressure was, however, the higher the ion intensity of background species that originated from the impurities in Ar gas or vacuum leakage became. An increase in the ion intensity of Cu that is the material of a sampler was also observed at lower pressure. This fact indicates an enhancement of the secondary discharge. Molecular ions originated from Ar and the secondary discharge could be suppressed by inserting a resistor into the shielding circuit of ESWC torch. The torch pressure rat her than gas flow rate affected the intensities of analyte and polyatomic ions. The ion intensity of I measured at about 130 Torr was more than 10 times higher than that at atmospheric pressure (AP), which suggests that RP-ICP provides a sensitive ion source for nonmetallic elements. Microliter aqueous samples were introduced into the RP-ICP with electrothermal vaporization method using a tungsten filament. The detection limits of Br and C1 obtained at optimal reduced pressure were ca. one order of magnitude lower than those obtained with AP-ICP.Although a matrix effect was investigated with Na as a matrix element, the significant decrease in the analyte ion intensity was not observed. It was found from above results that RP-ICP-MS is very useful for analysis of nonmetallic elements such as halogens.