Very high resolution mass spectrometry of HCNpolymers and tholins

Abstract

HCN polymers are complex organic solids resulting from the polymerization of hydrogen cyanide (HCN) molecules. They have been suspected to contribute to the refractory carbonaceous component of comets as well as the distributed CN sources in cometary atmospheres. Titan's tholins are also organic compounds produced in a laboratory setting but result from the complex chemistry between N₂ and CH₄ induced by UV radiation or electric discharges. Some of these compounds have optical properties in the visible range fairly similar to those of Titan's aerosols or those of the reddish surfaces of many icy satellites and small bodies. It has been proposed that HCN polymers are constituents of tholins but this statement has never received any clear demonstration. We report here on the comparative analysis of tholins and HCN polymers in order to definitely establish if the molecules identified in the HCN polymers are present in the tholins as well. First, we present a global comparison of HCN polymers with three kinds of tholins, using elemental analysis measurements, infrared spectroscopy and very high resolution mass spectrometry of their soluble fraction. We show that the chemical composition of the HCN polymers is definitely simpler than that of any of the tholins studied. Second, we focus on six ions representative of the composition of HCN polymers and using mass spectrometry (HRMS and MS/HRMS), we determine that these tholins contain at best a minor fraction of this kind of HCN polymers.