Accelerator mass spectroscopy (AMS) has been developed over the past twenty-five years as an effective method to count 14C atoms. An alternative 14C measurement technique, suitable for routine laboratory use, combining the optogalvanic effect with laser assisted isotope ratio analysis is now being extended from its current use for precision analysis of 13CO2 to similar analysis of 14CO2. The analyzer uses a unique resonance between a specific molecular transition in the molecule 14CO2 and the narrow band output of a laser based on the same molecular transition. Isobaric background is not an issue as in AMS, and the sample is not consumed in analysis so that long averaging times with small samples is possible.
Proof of principle has been demonstrated using a small sealed 2 W output power laser tube filled to operate only on the 14CO2 transition at 11.8 microns. This laser irradiated a cell with normal carbon dioxide followed by analysis of 13CO2 content using a 13CO2 laser. The signal to noise ratio for 13CO2 was maximized. As expected, the background 14CO2 (10-12 normal enrichment) could not be detected with this system, however, the null result was significant indicating that possible “accidental” laser coincidences with possible impurity molecules was not present. Further, a null result with a nitrogen sample indicated that any non-resonant background was also negligible. Samples of CO2 enriched with 14CO2, prepared by Lawrence Livermore National Labs, were then analyzed. The analyte constituted approximately 5% of the pressure of the discharge cell, with the remaining about 95% N2, yielding a total discharge cell pressure of 6.9 torr. The measured OGE response with 2.5 Watt laser illumination of a cell containing only 7nC 14CO2 was unambiguous and first reported in 2003. The signal to noise of the measurement (10 minute average) was greater than 120. The analysis yielded a system sensitivity exceeding 3 picomoles of 14CO2. A 20 W 14CO2 laser is now being built with an intra-cavity optogalvanic detection cell. Based on the demonstrated sensitivity of order 10-9, with less than 2 W laser power the circulating power of 400 W will yield the desired sensitivity- of order 10-12. The 14CO2 laser assisted ratio analyzer (LARA) device will be considerably smaller, less complex and much lower in cost than an AMS with comparable capability.
See more of AMS in Low Dose Bioscience Workshop
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)