AN INDUCTIVELY-COUPLED PLASMA TIME-OF-FLIGHT MASS-SPECTROMETER FOR ELEMENTAL ANALYSIS .3. ANALYTICAL PERFORMANCE

A time-of-night mass spectrometer (TOFMS) was evaluated as a mass anal yzer for inductively coupled plasma mass spectrometry (ICP-MS). The lo ng-term drift of signals was in the range of 7-8% relative standard de viation, whereas the short-term precision was between 5 and 20%, somew hat worse than is typically reported for commercial ICP-MS instruments (5%). However, precision can be improved considerably in the TOFMS by ratioing isotopic peaks or through internal standardization, a conseq uence of its ability to extract all measured ions simultaneously from the inductively coupled plasma. This feature was demonstrated by monit oring the Pb-206/Pb-208 ratio with boxcar averagers. In this ratioing mode, precision was improved to approximately 0.5%. Detection limits w ere measured with two alternative signal processing systems: (1) discr iminator-gated integration and (2) integration of digitized spectra. B oth methods improved the signal-to-noise ratio by a factor of from 10 to 100, although detection limits were still 1-2 orders of magnitude p oorer for most elements than from the best commercial ICP-MS instrumen ts. The dynamic range of the discriminator-gated integration system is over 4 orders of magnitude, but can be extended to 10(6) with planned increases in primary ion-beam current, which is currently 10-100 time s lower than is found in other instruments. Virtually simultaneous mul tielement and multiisotope analysis is possible for masses from Li-7 t o Bi-209 With minimal mass bias and detection limits on the 0.4-2-ppb level.