Gas chromatography for in situ analysis of a cometary nucleus: characterization and optimization of diphenyl/dimethylpolysiloxane stationary phases

The development of a gas chromatograph for the cometary sampling and composition (COSAC) experiment is described in the context of the preparation for the European Space Agency (ESA) Mission Rosetta for investigation of a cometary nucleus. COSAC is one out of ten experiments on the Rosetta Lander. Its scientific goal is to analyze in situ the chemical composition of the volatile constituents of the nucleus of the target comet P/Wirtanen. Constituted of several (up to eight) capillary wall-coated and porous-layer open tubular columns operating in parallel, the GC system is designed to separate and identify both organic and inorganic compounds which evolve from the comet naturally or are obtained from cometary samples through stepwise heating in a miniaturized pyrolizer. In this first part of our study, dimethylpolysiloxane (DMPS) stationary phases with increasing percentages of diphenyl substituted group (DP) have been investigated. A coupled experimental and theoretical approach has been taken in order to predict chromatographic data. By the use of a four-point experimental calibration (0 to 65% diphenyl group) in conjunction with Pro ezGC modeling software, results in prediction of multicomponent chromatograms with a mean error less than 5% for each compound retention time were obtained, irrespective of the stationary phase's diphenyl content and column physical parameters. The possibility to associate such phases is illustrated by the evolution of coelutions obtained on a non-polar (100% DMPS) and a medium polar (65% DP-DMPS) stationary phase, respectively. This study showed that with a small number of well tuned DP-DMPS columns, the separation and identification of most of the targeted compounds can be achieved with a minimum amount of coelutions and within the experiment requirements.