Gas dynamics of the ICP-MS interface: impact pressure probe measurements of gas flow profiles

A versatile ICP-vacuum interface was constructed to investigate the gas dynamics and gas flow profiles of the ICP-MS interface. This new interface combines-a low interface pressure, greater than or equal to 0.24 Torr, with the ability to accommodate a variety of skimmer designs at sampler to skimmer spacings up to 80 mm. Plasma beams were extracted by placing a 0.9 or 2.0 mm diameter skimming orifice at distances of 6.7-47 mm behind, the sampling orifice. Gas flow profiles of each resulting plasma beam were measured using an impact pressure probe placed at distances of 35-105 mm behind the skimmer. The centreline flux and width of the gas beam were compared with those calculated for an ideally skimmed beam. The results for the 0.9 mm diameter skimmer orifice showed that placing the skimmer at 11.0 and 27.3 mm downstream of the sampler formed the highest intensity and narrowest beams. In contrast, by placing the skimmer closer to the sampler, as in common interface designs, the beam profile is less intense on the centreline and much wider. Using a larger 2.0 mm diameter Skimming Orifice at 16.5-26.8 mm downstream of the sampler produced a more intense beam than any arrangement using the 0.9 mm skimming orifice. However with the 2.0 mm diameter skimmer, centreline intensities were still about half those of an ideally skimmed beam.