Abstract
The development is described of a new ion source for atomic mass spectrometry which could lead to much wider applications of the technique. The source is based on an atmospheric pressure high temperature inductively coupled plasma. Samples are introduced to the plasma as aqueous solutions by a pneumatic nebulizer. Ion extraction from the plasma at temperatures up to 8000 K is based on sampling gas through an aperture placed in the tail flame. Initially apertures of 0.07 mm diameter were used which were protected from the full plasma temperature by the formation over them of a cooler boundary layer. High sensitivity and good resolution were obtained up to solute concentrations of 10 mug.ml-1, but matrix effects were severe. Conditions were then established in which ions could be sampled direct from the plasma core without passage through a boundary layer. Continuum flow through apertures up to 0.5 mm diameter enables plasma ion populations to be frozen rapidly in the supersonic expansion, before reactions can occur to disturb the composition. Much better matrix performance is obtained in this mode and sample concentrations up to 1000 mug.ml-1 can be introduced. Detection limits below 1 ng.ml-1 are obtained for a wide range of elements. Multi-element analysis is feasible at the rate of one sample per minute and isotope ratio determinations on solutions at about 30 mug.ml-1 are possible with precisions approaching 0.1% for integration times of 5 minutes. The performance of the system is discussed theoretically and reasonable agreement demonstrated with the experimental behaviour. The source already provides a viable analytical technique for samples introduced by any of the methods available for ICP-AES. These include solution nebulization, thermal evaporation, laser ablation and arc aerosol generation. Considerable scope exists for improving ion transport efficiency to provide sensitivity at least as high as any other routine analytical technique.