ANALYSIS OF PRODUCTS FROM TITAN SIMULATION EXPERIMENTS
Analysis Methods The identification of products from experiments simulating the chemistry occurring in Titan's atmosphere provides a solid foundation for understanding the data transmitted by the Huygens probe as it analyzes the chemistry of Titan. In these simulation experiments, mixtures composed of approximately 10% methane in nitrogen, representing the Titan atmospheric composition, are subjected to electrical discharges or UV photons. Bonds in these molecules are broken and the reactive pieces recombine to yield a wide variety of new molecules. A number of these, for example hydrogen cyanide, ammonia and cyanoacetylene, undergo further reactions. In a matter of hours or even minutes, a wide variety of organic products are formed, gaseous, liquid and solid, depending on the conditions of the simulation experiment, with a wide range of molecular sizes or weights, compositions, and structures. The solid products are commonly called tholins.
Complete molecular level analysis of these complex mixtures is very challenging, akin to profiling the molecular composition of petroleum or coal. Several techniques are used. Distillation, gas and liquid chromatography allow separation into individual compounds or groups of compounds and these can be analyzed by IR, UV-Vis and NMR spectroscopy and mass spectrometry to provide structural information. Each of these techniques has its strengths and weaknesses. However, for analyzing small amounts of complex mixtures, the powerful instrument combination of Gas Chromatography directly couple to Mass Spectrometry, GC-MS, is often employed and this type of instrument is part of the Huygens probe instrument package.
Unfortunately, GC-MS cannot be applied to materials of high polarity or molecular weight, including polymers, since these materials will not pass through a gas chromatography column even at high temperatures. To derive some structural insight into these materials, they are broken down into smaller molecules by heating them to high temperature (>500¡C) in the absence of oxygen and these molecules are then analyzed by GC-MS, a technique called pyrolysis-GC-MS (Py-GC-MS). This technique is also incorporated into the Huygens probe. It is important to realize that this technique effectively increases the complexity of the analysis because each high molecular weight molecule is thermally degraded and broken down into several lower molecular weight molecules. Also, even if all the pyrolysis products can be identified by the MS, which is rarely possible, it is even more difficult to figure out how all the molecular pieces from pyrolysis were connected together in the parent molecule(s). The process is a lot like trying to figure out what all the original china looked like after a bull has completely trashed a china shop. Never the less, the pieces provide considerable insight into the composition of complex materials, particularly when a well characterized reference material, such as a protein, yields a similar suite products from Py-GC-MS analysis.