On-line Gas Phase Chromatography with Chlorides of Niobium and Hahnium (Element 105)

Abstract

The retention behavior of volatile chlorides and oxychlorides of short-lived isotopes of group 5 elements Nb and 105 (Ha = hahnium) in quartz columns was studied using on-line isothermal gas chromatography. The 15-s ^99gNb was produced from a ²³⁵U-fission target at a reactor neutron beam line and 34-s ²⁶²Ha in fusion reactions of ¹⁸O + ²⁴⁹Bk. The reaction products were continuously and rapidly transported to the chromatography apparatus with a carbon aerosol gas-jet system using He as carrier gas. Volatile chloride molecules were formed in a 900°C reaction oven by adding HCl as reactive gas. Depending on trace amounts of O₂ in the system, either the pentachlorides or the oxytrichlorides, or a mixture thereof, were formed. The isotopes ^99gNb and ²⁶²Ha were unambiguously identified after gas chromatographic separation by measuring the characteristic γ-lines of ^99gNb and by registering ²⁶²Ha-²⁵⁸Lr mother-daughter α-α correlations as well as spontaneous fission decays, respectively. The adsorption enthalpies of the investigated species on quartz surfaces were determined by analyzing the measured retention curves with a Monte Carlo model. Using an empirical correlation, the adsorption enthalpies were converted to sublimation enthalpies. The sublimation enthalpies of 95±16 kJ · mol^-1 and 124±16 kJ ·^-1 determined for NbCl₅ and NbOCl₃, respectively, were in good agreement with literature data. In experiments with Ha-chlorides a yield curve with two components was observed. Sublimation enthalpies of ≤ 120 kJ · mol^-1 and 152±18 kJ · mol^-1 were estimated for HaCl₅ and HaOCl₃, respectively. The estimated sublimation enthalpies were compared with theoretical predictions from relativistic calculations and with empirical extrapolations of chemical properties. In agreement with empirical extrapolations, a lower volatility was found for HaOCl₃ than for NbOCl₃.