NONCRYOGENIC PURIFICATION OF NANOMOLE QUANTITIES OF NITROGEN GAS FOR ISOTOPIC ANALYSIS

The noncryogenic purification of nitrogen, prior to isotopic analysis, by a mixture of CaO granules and Cu has been evaluated for nanomole-sized quantities of N. Duplicate experiments were performed on two batches of CaO granules prepared using identical procedures. For both batches, the rate of absorption of CO2 by CaO was investigated in the temperature range 400-650 degrees C. The interactions between NO2 and CaO in the temperature range 500-900 degrees C and between NO2 and CaO, onto which between 47 and 254 mu mol of CO2 had been absorbed, were also investigated. Although the precise behavior of the CaO granules depends upon several factors, such as the length of time that they have been in use and, more importantly, the thermal characteristics of the reaction vessel used to heat the granules, the general characteristics have been observed. A temperature setting (600-725 degrees C) can be found at which CaO absorbs CO2 rapidly to leave a temperature dependent residual pressure of CO2 of between 1 and 10 mbar. At this temperature setting there is only minimal absorption of NO2 by CaO. When NO2 is exposed to CaO onto which CO2 has been absorbed, the residual CO2 apparently catalyzes the reaction NO2 --> NO + 0.50(2). At higher temperatures (800-900 degrees C), in the absence of CO2, the reaction NO + NO --> N-2 + O-2 is catalyzed. After the addition of 1 g of Cu wire to the CaO granules, the rate of reduction of NO2 was investigated at the aforementioned temperature settings (600-725 degrees C). Reduction was complete in under 500 s. On the basis of these results, a new system was designed and built for the extraction and purification of nanomole-sized quantities of nitrogen, and it has been applied successfully to the determination of the nitrogen content of diamonds. Carbon and nitrogen have been separated from a sample with an initial C/N (atomic) of 78000. Carbon can be recovered quantitatively from the CaO without modification of the initial C-13/C-12. The method offers a cheaper and more efficient means of purifying nitrogen than conventional cryogenic techniques and can probably replace such techniques in high-vacuum lines associated with mass spectrometers for the determination of N-14/N-15. This step would aid greatly in the automation of such systems.