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-s ized quantities of N. Duplicate experiments were performed on two batc hes of CaO granules prepared using identical procedures. For both batc hes, the rate of absorption of CO2 by CaO was investigated in the temp erature range 400-650 degrees C. The interactions between NO2 and CaO in the temperature range 500-900 degrees C and between NO2 and CaO, on to which between 47 and 254 mu mol of CO2 had been absorbed, were also investigated. Although the precise behavior of the CaO granules depen ds upon several factors, such as the length of time that they have bee n in use and, more importantly, the thermal characteristics of the rea ction vessel used to heat the granules, the general characteristics ha ve been observed. A temperature setting (600-725 degrees C) can be fou nd at which CaO absorbs CO2 rapidly to leave a temperature dependent r esidual 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 ex posed to CaO onto which CO2 has been absorbed, the residual CO2 appare ntly catalyzes the reaction NO2 --> NO + 0.50(2). At higher temperatur es (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 afo rementioned temperature settings (600-725 degrees C). Reduction was co mplete in under 500 s. On the basis of these results, a new system was designed and built for the extraction and purification of nanomole-si zed quantities of nitrogen, and it has been applied successfully to th e determination of the nitrogen content of diamonds. Carbon and nitrog en have been separated from a sample with an initial C/N (atomic) of 7 8000. Carbon can be recovered quantitatively from the CaO without modi fication of the initial C-13/C-12. The method offers a cheaper and mor e efficient means of purifying nitrogen than conventional cryogenic te chniques 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.