Reduction of Space Charge Effects Using a Three-Aperture Gas Dynamic Vacuum Interface for Inductively Coupled Plasma-Mass Spectrometry

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Abstract
A three-aperture gas dynamic vacuum interface for ICP-MS has been developed which reduces the ion current within the ion optics. The effect on performance limitations previously associated with space charge were determined. The gas and ion dynamics within the first stage of the vacuum interface are similar to those of current ICP-MS instruments and allow delineation of the relative effects of space charge in the interface region and within the ion optics. The sensitivity of the instrument to trace elements is similar to that of current commercial instruments. The instrument shows an improvement in the discrimination against low mass ion transmission. It also shows a dramatic improvement in both the magnitude and analyte-ion mass dependence of the signal suppression induced by a high concentration of a heavy concomitant element (Tl). The new design results in more uniform ion kinetic energies as a function of ion mass-to-charge ratio. The performance improvements are ascribed to the reduction of space charge effects resulting from both the reduction of the ion current within the ion optics and the reduction of the mass dependence of the ion kinetic energies. It is concluded that the discrimination against low-mass ions and the magnitude and analyte mass dependence of the signal suppression induced by concomitant heavy elements observed with current ICP-MS instruments derive from space charge effects within the ion optics. These effects are less significant within the first stage of the vacuum interface or within the mass analyzer.