Attomole Measurement of Labeled Compounds

Introduction Accelerator mass spectrometry (AMS) is an ultrasensitive analytical technique for measuring rare nuclides. Single atoms of one isotope can be counted in the presence of 1015 atoms of a neighboring isotope. AMS is used mainly for measuring radionuclides since stable nuclides are usually sufficiently abundant to measure with smaller instruments. The potential for AMS to make possible major advances in several areas of biomedicine has come to the fore in recent years. Several successful applications have been described in review articles (1-7). AMS can now be used routinely for studies that use the radionuclides 10Be, 14C, 26Al, 36Cl, 41Ca, 129I with several others under development. Possible studies include ADME (absorption, distribution, metabolism, and excretion), pharmacokinetics, and long-term nutritional studies. A few projects currently underway are summarized elsewhere in this paper. Stable isotope tracers have the advantage that they do not produce radiation damage and have no disposal problems. However, the detection limit for stable isotopes is limited by their relatively high abundance in all natural materials. For example, 13C is 1% abundant so that 13C-labeled compounds are not detectable when diluted by about five orders of magnitude with natural carbon, since the 13C from the natural carbon would overwhelm the signal from the labeled material. Natural modern 14C is only about 10-10% abundant and, therefore, if measured by AMS, has a far greater dynamic range than 13C. While 14C has a potential dilution range of 12 orders of magnitude, the useful sensitivity is constrained on one side by the dose (limited by cost, radiation damage, and waste disposal) and on the other side by the efficiency of detection. Since AMS counts 14C atoms (as opposed to the number of decay events) it has a detection efficiency 103-105 times greater than conventional decay counting. This superior detection efficiency makes AMS ideal for studies requiring low detection limits. AMS can measure attomole (1 x 10-15 moles) amounts of labeled compounds in milligram-sized samples. The dose of a labeled compound is computed from the product of 1) the fraction that survives or is metabolized into the compound being measured, 2) the fraction that is recovered in the sample collected, 3) the efficiency for extraction from the sample, (for example, by liquid chromatography), and 4) the efficiency of the AMS measurement. For many studies, the dose can be so low that there is negligible radiation damage and no waste disposal problem. In most cases, the dose is only a fraction of the total radiation already present in the body. For example, a 160 pound adult human has 90 nCi of 40K and 50 nCi of 14C already present in the body. Many AMS studies require Attomole Measurement of Labeled Compounds