AMS measurement of samples smaller than 300 μg at center for applied isotope studies, University of Georgia

The center for applied isotope studies has developed a protocol for the measurement of radiocarbon in the range 0.01-0.3 mg of carbon. We analyzed a series of radiocarbon-free and modern standards using the same sample preparation procedure as for unknown samples. We have studied the size dependency of 12C and 13C beam currents and 14C/13C ratios from cathodes containing less than 1 mg of carbon. The iron catalyst used for graphite preparation contains some carbon with activity about 60 pMC and the level of contamination varies between batches from the same supplier. The cathodes in the range smaller than 50 μg showed a very significant dependency between current and the sample size. In this range the current increased by about fifteen-fold; between 50 and 100 μg the current increased two-fold. Samples larger than 300 μg have shown negligible current variation. The 14C/13C ratio was significantly affected only for the samples less than 50 μg. Larger samples have shown very little variation in the 14C/13C ratio. For the measurement of ultra-small unknown samples in the range of 10-50 μg, we have found it to be important to use blanks and modern standards with exactly the same amount of carbon, which makes it possible to apply a mass-balanced correction.