In gas chromatographic isothermal separations of multicomponent mixtures, i.e., mixtures where the number of components is high and unknown, the efficiency is low and not enough to allow a complete separation with a consequent severe peak overlapping in the chromatogram. The consequence is a significant loss in analytical information content. Therefore it is practically mandatory to use a chemometric procedure to decode the complex chromatogram, i. e. to deconvolve the overlapping signal to extract from it information on the mixture and the separation system. Isothermal separations are not very popular, due to their low efficiency, but they are very common in in situ analyses during space missions, as a consequence of flight constraints on instrumentation complexity and power (energy saving). In this work a chemometric approach based on Fourier analysis is applied to chromatograms obtained under isothermal or low temperature programming conditions. These conditions simulate those employed in space missions. The procedure has been applied to standard mixtures containing compounds representative of the planetary atmospheres that will be investigated in the near future: in particular, those related to Titan's atmosphere (CassiniHuygens mission) and a cometary nucleus (Rosetta mission), i. e., hydrocarbons and oxygenated compounds with carbon atom numbers ranging from 2 to 8. The original approach, developed for constant peak width, is extended to variable peak width, in particular to the case of peak width linearly increasing with retention time, representing isothermal separations. The proposed approach is able to characterise complex isothermal chromatograms in terms of number of components present in the mixture and of separation efficiency: such results are useful in interpreting data recovered from space missions and for optimising analysis conditions compatible with flight constraints.
Two Wall Coated Open Tubular capillary columns, coated with poly(cyanopropylphenyl-dimethyl)siloxane and poly(diphenyl-dimethyl)siloxane stationary phases, have been selected for use in the COmetary SAmpling and Composition space experiment for the separation and identification of the wide range of volatile organic compounds which could be present in cometary nuclei. This article presents the main characteristics of the tandem column system for the analysis of solutes of cometary interest within the constraints of space instrumental operating conditions. The high efficiency of the columns is demonstrated and the influence of the operating conditions on their separation properties are investigated. The studied columns exhibit complementary retention pattern: their use in a dual column system makes it possible to achieve the separation and the identification of the compounds of interest. Finally, the good analytical behavior of the columns when analyzing samples which include large amounts of water, the main presumed volatile in comets, is demonstrated. The presented results thus show the suitability of the selected tandem columns system for the desired analyses, and their performance on adaptation to in-situ cometary chemical investigation.
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