High-speed video images of helium inductively coupled plasmas (He ICPs) were obtained at a rate of 1000 frames/s under conditions suitable for mass spectrometry (MS). These experiments established, for the first time, that the He ICP is a rotating discharge at the forward powers (500–1000 W) used for He ICPMS. The rotational frequency of the He ICP ranged from 80 to 250 Hz, depending on the forward power and whether analyte was introduced. When not coupled to the mass spectrometer, the rotational frequency of the He ICP decreased slightly as the forward power was increased. Coupling of the plasma to the mass spectrometer resulted in enhanced plasma oscillations. The implications of these variations on the sensitivity and precision of analytical measurements are discussed. Results are contrasted with data obtained for the Ar ICP.
Iacone, L. A., Minnich, M. G., Masamba, W. R. L., Nam, S-H., Montaser, A., & Michel, R. G. (1999). High-speed video images of helium inductively coupled plasmas for mass spectrometry. Spectrochimica Acta, Part B: Atomic Spectroscopy, 54(13), 1919-1928. https://doi.org/10.1016/S0584-8547