LANGMUIR PROBE MEASUREMENTS OF THE ION EXTRACTION PROCESS IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY .2. MEASUREMENTS OF FLOATING VOLTAGE AND RADIOFREQUENCY VOLTAGE

A single Langmuir probe is thrust axially through the skimmer into. th e supersonic jet of a typical interface for inductively coupled plasma -mass spectrometry (ICP-MS). Floating voltage V-f is measured from cur rent-voltage curves at various axial positions and is generally in the range +2 to +7 V in the supersonic jet. A disturbance at the skimmer, when present, induces a sharp increase in V,in the vicinity of the sk immer tip. Floating voltage generally decreases as the probe is retrac ted farther behind the skimmer. The measured d.c. voltages are attribu ted primarily to plasma rectification and to the calorelectric effect between two metal surfaces at different temperatures in a plasma. An r .f. voltage of up to 25 V (peak-to-peak) at the frequency of the plasm a generator can also be measured behind the sampler.