Journal of Chromatography A, 1002 (2003) 179–192 www.elsevier.com / locate / chroma Decoding of complex isothermal chromatograms recovered from space missions Identification of molecular structure a, a a a * Maria Chiara Pietrogrande , Ilaria Tellini , Luisa Pasti , Francesco Dondi , b c d Cyril Szopa , Robert Sternberg , Claire Vidal-Madjar a Department of Chemistry, University of Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy b ` SA, UMR CNRS 7620, 91371 Verrieres-le-Buisson, France c ´ LISA, UMR CNRS 7583, 94010 Creteil Cedex, France d LRP , CNRS, 94320 Thiais, France Received 28 February 2003; received in revised form 22 April 2003; accepted 23 April 2003 Abstract A chemometric approach, based on the study of the autocovariance function, is described to study isothermal GC chromatograms of multicomponent mixtures: isothermal GC analysis is the method of choice in space missions since it is, to date, the only method compatible with flight constraints. Isothermal GC chromatograms look inhomogeneous and disordered with peak density decreasing at higher retention times: a time axis transformation is proposed to make retention an homogeneous process so that CH addition in terms of an homologous series yields a constant retention increment. The time 2 axis is transformed into a new scale based on the retention times of n-alkanes, as they are the basis of the universal Kovats indices procedure. The order introduced into the chromatogram by retention time linearization can be simply singled out by the experimental autocorrelation function (EACF) plot: if constant inter-distances are repeated in different regions of the chromatogram, well-shaped peaks are evident in the EACF plot. By comparison, with a standard mixture it is possible to identify peaks diagnostic of specific molecular structures: study of the EACF plot provides information on sample chemical composition. The procedure was 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 (Cassini–Huygens mission) and cometary’s nucleus (Rosetta mission). The employed experimental conditions simulated those applied to GC instruments installed on space probes and landers in space missions. The method was applied to two specific investigations related to space research, i.e., a comparison of retention selectivity of different GC columns and identification of the chemical composition of an unknown mixture.  2003 Elsevier Science B.V. All rights reserved. Keywords: Chemometrics; Autocovariance function; Volatile organic compounds 1. Introduction *Corresponding author. Tel.: 139-0532-291-152; fax: 139- In previous papers it has been demonstrated that a 0532-240-709. E-mail address: mpc@dns.unife.it (M.C. Pietrogrande). chemometric approach based on Fourier analysis 0021-9673 / 03 / $ – see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0021-9673(03)00727-1