A microwave-induced plasma (MIP)/inductively coupled plasma (ICP) combination has been evaluated as an "atomization-excitation" tandem source (TS). The results indicate that some of the problems reported for similar sources in the past can be effectively reduced or eliminated. Desolvation of the analyte aerosol before its introduction into the tandem source improved detection limits only slightly. However, power modulation of the microwave plasma served to improve tandem-source performance in two ways. First, noise contributed by the ICP and the MIP was isolated one from the other. Since the modulation frequency was chosen to be far removed from the major noise frequencies observed from either of the two sources, more stable background signals were realized. Second, the intense background signal from the ICP could be discriminated against through use of a lock-in-amplifier tuned to the modulated MIP signal. Relative standard deviations of both the signal and the background were usually below 1% when the MIP power was modulated. Detection limits ranged from 0.53 ppb to 40 ppb for the elements examined, which represents an improvement of 5- to 40-fold over previously reported data for a similar tandem-source configuration. Disappointingly, these improved detection limits were still much worse than detection limits obtained by other investigators utilizing the ICP alone. Addition of a mixed gas did little to improve the performance of the tandem source; for barium and calcium ion lines, an increase of approximately three in signal-to-background ratios was observed upon the addition of hydrogen or helium.
Brian W. Pack and Gary M. Hieftje, "Inductively Coupled Plasma/Microwave-Induced Plasma Tandem Source for Atomic Emission Spectroscopy," Appl. Spectrosc. 54, 80-88 (2000)