Tuesday, 7 June 2016

Improving our Gas Chromatograph

As mentioned before, we finished assembling and soldering our gas chromatograph some weeks ago so that it is now externally finished. Currently, we are testing and improving  it as well as adapting the mobile phase to our mission.

Gas chromatography uses the interaction of different chemical substances provoked by their chemical polarity or unpolarity. When a sample is injected, it meets two other substances. On the one hand, there is a inert or unreactive carrier gas, the mobile phase. The samples as well as the carrier gas stream through a column that contains the stationary phase, a microscopic layer of liquid or polymer on an inert solid support. Due to different intensities of the interaction of the individual substances of the sample and the stationary phase, these substances elute the column at a particular time for each compound. This retention time is characteristic of each substance so that  it is possible to investigate the composition of a sample as well as the quantity of the individual components based on this time.

Our own gas chromatograph is based on a self-construction kit made for educational purposes (e.g. chemistry classes at school) that suggests air as an inexpensive and easily accessible carrier gas. Obviously, this cannot work out for our mission of analysing an air sample. Consequently, we have replaced it by helium. Helium as an inert, unreactive and noble gas is usually the most commonly used carrier gas for gas chromatography. It is also very suitable for our particular analyse because air contains helium only as a trace gas with an amount of 0.000425% that is not that interesting for our particular investigation. Helium emerges at every radioactive alpha-decay. The tiny atom rises from the ground and as it is much more lightweight than the other components of the air, it can quickly escape the atmosphere and get into space. These circumstances make the gas quite irrelevant not only for our mission of investigating whether there is life "outside of earth", but also for someone observing the environment in the framework of climate change, air pollution and so on.

Having chosen helium as the carrier gas, we faced the challenge of adapting the gas chromatograph itself to the change of the mobile phase. While the stationary phase that is provided in the kit is unaffected of any modifications, the technical settings of the measuring device had to be adjusted. As the whole system is usually supposed to work with air, this constellation up to now has got the best performance in precision and error susceptibility. In the last time, we have experimented with different settings and are in close contact to the manufacturer of our self-construction kit so that we are now receiving pretty reliable chromatograms (although improvements are still to come). Using different equilibrium reactions involving calcium chloride, we prepared samples of different air humidity. When we measured the humidity with our sensor SHT15 and then compared it to our graphs, we were able to see a very clear tendency of a large surface area of the peak at low humidity and, the other way rund, a small surface area at high humidity. In the last few weeks before launch, we will expand this procedure and create more and broader series of tests in order to improve our experiment's scientific significance.

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