High-speed video images of helium inductively coupled plasmas for mass spectrometry

High-speed video images of helium inductively coupled plasmas (He ICPs) wer e obtained at a rate of 1000 frames/s under conditions suitable for mass sp ectrometry (MS). These experiments established, for the first time, that th e He ICP is a rotating discharge at the forward powers (500-1000 W) used fo r 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 n ot coupled to the mass spectrometer, the rotational frequency of the He ICP decreased slightly as the forward power was increased. Coupling of the pla sma to the mass spectrometer resulted in enhanced plasma oscillations. The implications of these variations on the sensitivity and precision of analyt ical measurements are discussed. Results are contrasted with data obtained for the Ar ICP. (C) 1999 Elsevier Science B.V. All rights reserved.