NOISE IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY - SOME PRELIMINARY MEASUREMENTS

Fundamental limiting noise sources in a non-commercial inductively cou pled plasma mass spectrometer were investigated, using colour photogra phic evidence and signal processing which is manipulated via computer software. Noise sources were found to be 50 Hz from the mains power su pply, peristaltic pump rotation, plasma audiofrequency peaks and their harmonics, and l/f noise (which is mainly caused by the sample introd uction system). These noise sources adversely affect the precision of inductively coupled plasma mass spectrometry (ICP-MS) for a given meas urement bandwidth. To reduce their contributions in the noise power sp ectra (NPS) of the signal, the use of a bonnet device was investigated . This reduced the 1/f noise and peristaltic pump rotation frequency a nd completely removed the plasma audio frequency peak in the NPS. This latter frequency has been observed by a number of workers in ICP emis sion spectrometry and is associated with instability at the boundary o f the plasma, where it enters the surrounding atmosphere. This audiofr equency peak has been removed in emission spectrometry by using an ext ended torch arrangement. In this present work the position of the plas ma audiofrequency peak in the NPS changed with sampling distance, radi ofrequency power and coolant gas flow rate and the peak was removed by the use of a torch extension bonnet.