TIMS analyses require dissolving the sample and extracting the elements of interest, commonly by use of ion-exchange chromatography. Precise concentrations of the elements of interest are obtained using isotope dilution. The sample is "spiked" with a small amount of the element of interest that is abnormally enriched in one or more isotopes of that element. These analytical procedures are often applied to very small samples such that a clean laboratory with low blank capability is required.
For very small samples TIMS analyses typically have high-precision and accuracy, but ionization efficiency is a limiting factor. Isobaric interferences (caused by other isotopes or ion complexes with the same mass) are either eliminated by chemically purifying samples or are capable of being corrected during the analyses. Therefore, the high resolution required by the ionprobe to correct for these interferences is not necessary for a TIMS instrument to perform with high-precision and accuracy.
Mass fractionation (the lighter isotope ionizing at a faster rate than the heavier) is smaller for the TIMS than for the ICP-MS and is a significant advantage for high-precision analyses. For measuring very small beams a TIMS instrument equipped with an analogue Daly can measure a beam of as little as 0.1 millivolt (6000 ions/second) and up to 45 millivolts (270,000 ions/second). This allows for a better sample-to-background ratio and therefore more accurate analyses, particularly for large isotopic ratios such as 206Pb/204Pb in zircon or 238U/234U. When the signal is switched from the Daly detector to the standard Faraday cup collector, signals as large as 10 volts (6x108 ions/second) can be measured. This means that a wide variety of sample sizes and types can be analyzed on a single instrument.
Higher throughput was achieved when TIMS instruments were designed with capability to load multiple samples at one time, but the throughput of the ICP-MS instruments is still significantly higher. TIMS requires a longer analytical time per analysis than does the ICP-MS, but has very high precision and accuracy.
Radiogenic Isotope Laboratory Introduction
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/isotope_geochron_lab/TIMS.html
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Page Last Modified: Tue 14-Feb-2006 13:06:23 MST
