Plasma potential measurements for inductively coupled plasma mass spectrometry with a centre-tapped load coil

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Abstract
A floating Langmuir probe is used to measure d.c. potentials in an inductively coupled plasma (ICP) while the latter is being sampled for mass spectrometry (MS). With the centre-tapped load coil configuration the plasma potentials are found to be low (+0.5 to –3.5 V) and only moderately dependent on plasma operating parameters. The plasma potential measurements are in basic agreement with previous measurements of ion kinetic energies and mass spectral characteristics for this type of ICP-MS device. Both the numerical values of the plasma potentials and their variation with plasma parameters differ considerably from previous measurements with other load coil configurations that yield a secondary discharge from the plasma to the sampling orifice. Thus, the fundamental processes occurring during ion extraction differ somewhat for the two basic types of ICP-MS device.

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