Phenyl- and cyclopentylimino derivatization for double bond location in unsaturated C₃₇-C₄₀ alkenones by GC-MS

Abstract

A method for the identification of double bond locations in polyunsaturated long chain alkenones adapted to nanogram amounts as currently analyzed by gas chromatography coupled to mass spectrometry (GC-MS) has been developed. The method is based on interpretation of the electron impact mass spectra of the imino derivatives of the carbonyl groups using either cyclopentyl or phenyl substitutents. Other complementary derivatization methods such as elaidization and hydrogenation have also been used for structural characterization of these compounds. This application has led to the identification of a novel homologous series of di-, tri-, and tetraunsaturated ketones with carbon number chain lengths between 37 and 40 in coastal hypersaline sediments. The novel series identified shows a distribution in which the double bond position between different homologs is established by reference to the distance from the carbonyl group whereas the previously known alkenones were constituted by unsaturated homologs with double bonds located at defined distances of the terminal methyl. This difference points out to a dissimilar, but still unknown, biogenic precursor of these novel alkenones. (J Am Soc Mass Spectrom 2004, 15, 1161-1172

Reassessment of the structural composition of the alkenone distributions in natural environments using an improved method for double bond location based on GC-MS analysis of cyclopropylimines

“…However, attempts to locate the positions of the double bonds based on GC-MS studies of vicinal bistrimethylsilyl ethers [8] or dimethanethiols [9] had limited success because they generated derivatives with much longer retention times than the original compounds [10]. Recently, the application of a novel technique based on the mass spectra of phenyl and cyclopentylimines has been used to characterize distributions of alkenones found in hypersaline sediments and coastal tidal ponds [1].However, the study of alkenone structures in samples from old sediments, e.g., those covering the last 250 k years of the history of the Mediterranean SST [6], showed that in some cases the sensitivity of the methods available [1] was not sufficient for the unambiguous determination of the position of the unsaturations because of the low concentrations of these compounds. Keeping in mind the previous experience with the use of C 5 amines, smaller molecular weight homologues with a lower boiling point were tested.…”

“…However, attempts to locate the positions of the double bonds based on GC-MS studies of vicinal bistrimethylsilyl ethers [8] or dimethanethiols [9] had limited success because they generated derivatives with much longer retention times than the original compounds [10]. Recently, the application of a novel technique based on the mass spectra of phenyl and cyclopentylimines has been used to characterize distributions of alkenones found in hypersaline sediments and coastal tidal ponds [1].…”

“…The data presented in this work unify the structures of the known alkenones in the present and the recent past under a common metabolic pathway. (J Am Soc Mass Spectrom 2006, 17, 710 -720) © 2006 American Society for Mass Spectrometry I n a previous manuscript, a method for double-bond location in C 35 -C 41 alkenones was proposed [1]. These compounds currently encompass mixtures of straight chain methyl and ethyl ketones with one to four unsaturations and all trans-configuration.…”

“…However, the study of alkenone structures in samples from old sediments, e.g., those covering the last 250 k years of the history of the Mediterranean SST [6], showed that in some cases the sensitivity of the methods available [1] was not sufficient for the unambiguous determination of the position of the unsaturations because of the low concentrations of these compounds. Keeping in mind the previous experience with the use of C 5 amines, smaller molecular weight homologues with a lower boiling point were tested.…”

“…The usefulness of these three C 3 isomeric amines has been assayed, both on synthetic standards and on environmental samples. Their study has provided a new method involving significant improvements in both preparation ease and sensitivity compared with the amino derivatives previously described [1]. The application of the technique to algal cultures, marine sediments, and particulate matter has allowed characterizing the double-bond positions in all the alkenones known to occur in environmental samples.…”