  Titan, the biggest moon of Saturn, has a thick atmosphere which presents
similarities with the one thought to be on Earth at its beginning. The study of
Titan s photochemical haze is thus a precious tool in gaining knowledge of the
primitive atmosphere of Earth. The chemistry occurring in Titan s atmosphere
and the exact processes at act in the formation of the hazes remain largely
unknown. The production of analogs samples on Earth has proved to be a useful
tool to improve our knowledge of the aerosols formation on Titan. Such solid
organic analogs samples, named tholins, were produced with the PAMPRE
experiment (French acronym for Aerosols Microgravity Production by Reactive
Plasma). PAMPRE tholins were found to be mostly insoluble, with only one-third
of the bulk sample that can be dissolved in methanol. This partial solubility
limited the previous studies in mass spectrometry, which were done only on the
soluble fraction. The goal of the present study is to compare the two fractions
of PAMPRE s tholins (insoluble and soluble) using a ultra-high resolution
Fourier transform ion cyclotron resonance mass spectrometer (FTICR) equipped
with a laser desorption/ionization source. Using modified Van Krevelen
diagrams, we compare the global distribution of the molecules within the
samples according to their Hydrogen/Carbon ratio and Nitrogen/Carbon ratio.
Major differences are observed in the molecular composition of the soluble and
the insoluble fraction. The soluble fraction of tholins was previously
identified as a set of polymers of average formula (C2H3N)n. In this work we
observe that the insoluble fraction of tholins is comprised of a significantly
different set of polymers with an average composition of (C4H3N2)n.
