  Two sorts of solid organic samples can be produced in laboratory experiments
simulating Titan atmospheric reactivity: grains in the volume and thin films on
the reactor walls. We expect that grains are more representative of Titan
atmospheric aerosols, but films are used to provide optical indices for
radiative models of Titan atmosphere. The aim of the present study is to
address if these two sorts of analogues are chemically equivalent or not, when
produced in the same N2-CH4 plasma discharge. The chemical compositions of both
these materials are measured by using elemental analysis, XPS analysis and
Secondary Ion Mass Spectrometry. The main parameter probed is the CH4 N2 ratio
to explore various possible chemical regimes. We find that films are
homogeneous but significantly less rich in nitrogen and hydrogen than grains
produced in the same experimental conditions. This surprising difference in
their chemical compositions could be explained by the efficient etching
occurring on the films, which stay in the discharge during the whole plasma
duration, whereas the grains are ejected after a few minutes. The higher
nitrogen content in the grains possibly involves a higher optical absorption
than the one measured on the films, with a possible impact on Titan radiative
models.
