Clouds of glowing interstellar gas have become familiar to us from photos taken by the Hubble Space Telescope. These vast clouds, which mostly consist of hydrogen and helium but also heavier elements, originate from exploded stars.
At the cold temperature of space, 2.7 Kelvin (-270.5 °C), we would expect to find simple, unreacted elements as there is insufficient energy to drive the formation of more complex compounds. However, spectroscopic measurements reveal the presence of complex organic compounds previously only thought to be created at the higher temperatures found on a planet.
What could be causing these compounds to form? In dense clouds, elements can join together by nucleating on the surfaces of dust particles but in diffuse clouds this pathway becomes far less probable.
The answer is ion-molecule chemistry. Many atoms in the clouds are ionised by radiation ranging in energy from ultraviolet all the way up to cosmic rays. Ionised atoms have electric fields which attract other species. The fields are tiny and the distances between them are huge, but in the absence of any other forces, they gradually come together and interact.
We have observed organic molecules as complex as CH3CO2H (acetic acid), CH3OCH3 (dimethyl ether) and hydrogen cyanide (HCN) in interstellar clouds. The interactions forming them occur at very low energies and this low energy interaction is exploited in SIFT-MS, where we react ionic species with target organic compounds without fragmenting those compounds. It is this non-destructive, low-energy interaction that makes SIFT-MS such a useful tool for analysis of delicate organic compounds.
Ion chemistry may be somewhat new to us, but the universe has been doing it for billions of years.