AMS has a linear dynamic range over 4 orders of magnitude for determination of a 14C:C isotope ratio, the direct result of measurement by AMS. This range is restricted on the high end by excessive deadtime of the microsecond-scale detector electronics or by memory effects in the ion source; and restricted at the low end by either the resolvability above natural 14C levels or the need for reasonable throughput of isotope diluted samples. Liquid scintillation counting (LSC) complements the AMS measurements, extending the upper range of biomedical 14C concentrations by employing the radioactivity inherent in 14C, and overlapping the AMS compatible range. Multiple laboratory intercomparison exercises show the AMS versus LSC equivalence in carbon dating determinations at low concentrations of naturally occurring 14C, but higher levels of 14C in tracer experiments may require use of both technologies in their separately optimized regimes: measurement equivalence at the high end of the AMS range and low end of the LSC range. In a typical kinetic experiment to observe the concentration over time, the initial 14C content is measurable by conventional LSC, declining over time to levels measurable by AMS but not LSC. To show equivalence near the transition point, each sample in a set of human urine samples was measured by both techniques for comparison. A need to modify AMS operation and data acquisition techniques from their carbon dating heritage was recognized, as well as the need for specialized procedures for low level determinations by LSC. A set of 89 samples from 90 to 500 Modern in 14C concentrations had an AMS/LSC ratio of 0.97±0.13. The agreement can be extended by alternative AMS detectors and selective reduction in ion source output.
See more of AMS in Low Dose Biosciences Posters
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)