Langmuir probe potential measurements for reduced-pressure inductively coupled plasma mass spectrometry
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A floating Langmuir probe was used to measure the apparent de offset potential of a reduced-pressure inductively coupled plasma near a substitute sampling orifice of a mass spectrometer. The experimental results demonstrate that the de offset potential causes the secondary discharge at the sampling orifice. The plasma potential is in the range +3.5 to +20 V and varies with the plasma operating conditions. The manner by which a water-cooled torch is shielded has a substantial effect on the plasma potential, then the secondary discharge. The measured values of the potential give a good explanation for the enhanced capacitive coupling effect in reduced-pressure ICP-MS reported previously.