An externally air-cooled low-flow torch for inductively coupled plasma mass spectrometry

A low-flow torch specifically designed for inductively coupled plasma mass spectrometry (ICP-MS) was developed and evaluated. The outside of the torch is cooled externally by pressurized air flowing at ∼70 l min−1 through a fourth tube sealed onto the usual ‘outer’ tube of a standard minitorch. This external air flow merely cools the outer tube of the torch and does not enter the plasma. Although plasmas can be sustained at operating power as low as 400 W with a 2 l min−1 outer Ar flow, somewhat higher power and flows are advisable. A stable and analytically useful plasma can be obtained at 850 W with an outer Ar flow rate of only 4 l min−1. Under optimum operating conditions, the externally air-cooled plasma produces comparable sensitivities, M2+/M+ signal ratios, matrix effects and other analytical figures of merit as those produced by a conventional torch while using much less argon. MO+/M+ signal ratios are slightly higher with the externally cooled torch.