Glow Discharge Mass Spectrometry
The glow discharge is a simple, yet effective device which has found application in atomic absorption, atomic emission, and elemental mass spectrometry. It takes the form of a low-pressure gas discharge wherein rare gas ions are attracted to a sample cathode and efficiently sputter atoms from the surface. Figure 1 shows the essential components of the source. The discharge has been powered by dc, pulsed dc, and rf supplies, usually operating in a low power regime, with optimum currents and voltages determined by such factors as source configuration, discharge gas, and pressure. Nearly all the discharge voltage is dropped across the cathode dark space, serving to accelerate the argon ions onto the cathode, which is formed directly from the sample or compacted from a sample/matrix mixture. The glow discharge approach is simple, inexpensive, and shows relatively few matrix effects. Sensitivity in the ppb range is seen for most elements. A disadvantage of the technique is that the gas discharge produces certain background species which can create interferences for some elements, if alternative isotopes are not available.
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