An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part III: Analytical performance.

A time-of-flight mass spectrometer (TOFMS) was evaluated as a mass analyzer for inductively coupled plasma mass spectrometry (ICP-MS). The long-term drift of signals was in the range of 7-8% relative standard deviation, whereas the short-term precision was between 5 and 20%, somewhat 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 consequence of its ability to extract all measured ions simultaneously from the inductively coupled plasma. This feature was demonstrated by monitoring the (206)Pb/(208)Pb ratio with boxcar averagers. In this ratioing mode, precision was improved to approximately 0. 5%. Detection limits were measured with two alternative signal processing systems: (1) discriminator-gated integration and (2) integration of digitized spectra. Both 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 poorer for most elements than from the best commercial ICP-MS instruments. 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 times lower than is found in other instruments. Virtually simultaneous multielement and multiisotope analysis is possible for masses from (7)Li to (209)Bi with minimal mass bias and detection limits on the 0. 4-2-ppb level.