Further development of a simple glow discharge source for direct solid analysis by on-axis time of flight mass spectrometry

Different chamber design parameters have been investigated in order to improve the sensitivity of a simple glow discharge (GD) prototype constructed in our laboratory for the direct analysis of solid samples and designed to be easily and quickly exchanged with the inductively coupled plasma (ICP) source of a commercial ICP-time-of-flight mass spectrometer (TOFMS) instrument. Results showed that one of the most important parameters to be considered was the actual design of the channel(s) introducing the plasma gas into the GD. In fact, it was observed that the GD-MS sensitivity can be improved about one order of magnitude by using two opposite gas inlets in the chamber in order to obtain a better sampled plasma, as compared to the use of just one gas inlet. Further improvements on the GD-(TOF) MS sensitivity were achieved by reducing, from 12 mm to 5.5 mm, the distance between the skimmer cone of the (TOF) MS interface and the sample. Also, the use of a skimmer cone with an orifice of 0.7 mm instead of the 1 mm commercial one proved to be necessary to allow GD operation at pressures commonly used in analytical work. Advantages of this prototype include its capability of analysing directly solid samples with different shapes and sizes, the simplicity of the proposed GD design and its versatility because it can be easily "coupled to/removed from'' the (TOF) MS commercial interface.