Elution patterns from capillary GC for methyl-branched alkanes

A common and confusing problem in analyses of insect hydrocarbons is in making sense of complicated gas chromatograms and interpreting mass spectra since branched chain compounds differing by one or two carbons in backbone or chain length may elute from the column at nearly the same time. To address this confusing situation, relative gas chromatography (GC) retention times are presented for typical mono-, di-, tri-, and tetramethylalkanes comprising most of the commonly appearing series of homologous methyl-branched alkanes up to 53 carbons that are found in insect cuticular hydrocarbons. Typical insect-derived methylalkanes with backbones of 33 carbons were characterized by Kovats indices (KI); monomethyl alkanes elute between KI 3328 and 3374, dimethylalkanes elute between KI 3340 and 3410, trimethylalkanes elute between KI 3378 and 3437, and tetramethylalkanes elute between KI 3409 and 3459, depending upon the positions of substituents. A protocol is described for identification of methyl-branched hydrocarbons eluted from nonpolar polysiloxane DB-I capillary GC columns. In this protocol, retention indices (KI values) are assigned to peaks, then the patterns in GC peaks that probably contain homologs are marked to assist subsequent GC-mass spectrometric (GC-MS) interpretation. Use of the KI allows assignment of likely structures and the elimination of others, with demonstrative consistency, as there are no known exceptions. Interpretation of electron ionization mass spectra, can then proceed within narrowed structural possibilities without the necessity of chemical ionization GC-MS analysis. Also included are specific examples of insect hydrocarbons that were assembled from 30 years of the literature, and these are intended to help with confirmation of confusing or contradictory structures.