(57) A plasma (24) is generated within an induction coil (12) and the plasma is sampled
through an orifice (30) into a vacuum chamber (28) for mass analysis_of trace ions
in the plasma. Arcing at the orifice is prevented by grounding the induction coil
at or near its centre, thus eliminating ultraviolet noise and reducing average ion
energies and ion energy spread, as well as preventing destruction of the orifice.
The elimination of arcing at the orifice allows the use of a sharp edge orifice structure
(92) to prevent formation of a cool boundary layer over the orifice and also permits
direct sampling of the plasma. The direct sampling and the lack of cooling prevent
recombination and reaction of the ions with oxygen and improve the response to elements
of high ionization potential, increasing the desired ion signal and greatly reducing
the presence of oxides which would otherwise complicate the spectrum.
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