Gas chromatography of all C 15C 18 linear alkenes on capillary columns with very high resolution power

Abstract

Conditions were studied for the separation of mixtures of all the 60 isomers of C 15C 18 n-alkenes in the presence of the corresponding n-alkanes in highly efficient glass capillary columns ( N ≈ 0.5·10 6 effective plates for n-alkenes with k ≈ 3), coated with C 87 hydrocarbon, Apiezon L or Carbowax 20M as stationary phase. The separation of cis- and trans-isomers of n-alkenes is made possible at a sufficiently high column efficiency by the different dependences of their retentions on temperature. The separation of positional isomers of n-alkenes having even carbon numbers for the pairs with the most centrally situated double bonds can be achieved only by use of highly efficient columns with polar or non-polar stationary phases ( n req ≈ 10 6 plates). Correlations between the structure and increments H, δ I, d I/d T and Δ I, derived from retention indices of n-alkenes, were used for identification as they reflect the position of the double bond, the geometrical arrangement and the length of the carbon chain in various isomers. The so-called propyl effect was confirmed for trans-4-alkenes, disturbing the regularity of all correlations between structure and retention. The influence of adsorption of n-alkanes and n-alkenes at the polar phase-carrier gas interface on the retention characteristics is discussed. Reproducibility of retention indices of n-alkene isomers is a function of their capacity ratios (thickness of the stationary phase film in the column), decreases with polarity of the stationary phase and the isomer under analysis and with the difference between a reference standard and the n-alken ein question. These effects are so significant for the columns with Carbowax 20M that a different retention sequence was found for some n-alkene isomers in columns with relatively small differences in the thickness of the stationary phase.