NOISE SOURCES IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY - AN INVESTIGATION OF THEIR IMPORTANCE TO THE PRECISION OF ISOTOPE RATIO MEASUREMENTS

It has long been accepted that the precision of isotope ratio measurements by inductively coupled plasma mass spectrometry (ICP-MS) is poorer than consideration of counting statistics would suggest is possible. This has been attributed to the coupling of a scanning mass analyser to a noisy ICP ion source, the sample introduction system and the ion extraction method. Thus, the ion output from the source varies as the analyser scans between successive masses so that their ratios vary. A variety of noise sources have been identified, mainly from noise power frequency spectra, but the sensitivity of these spectra as diagnostic tools is limited by the statistics of the signals fed to the analysing software, and it becomes necessary to extend the investigation by using isotopic ratio precision as the ultimate criterion. Even when the effect of (previously identified) major sources are greatly reduced, poor precision is still obtained. In this study the effect of poor ion distribution in the plasma and ICP-MS interface is identified as the principal cause and by avoiding this, precision values are found which agree with statistically expected values.