Innovative methods for determining alkenone unsaturation indices

Standards, total lipid extracts and crudely fractionated samples as well as extensively purified alkenone references were analyzed by conventional single column gas chromatography (GC), selective two-dimensional GC (GC–GC), comprehensive two-dimensional GC (GC×GC) and high resolution mass spectrometry (HRMS). U37K′ indices, defined as [C37:2]/[C37:2+C37:3], for independently analyzed standards showed GC–GC average accuracy of 0.014 and precision ±0.008 and HRMS average accuracy of 0.001 and precision ±0.007. The HRMS limit of precision is approximately ±0.008 for “real world samples” applied to the MS probe at greater than a few nanograms of the lesser alkenone component. Both of the two-dimensional GC techniques and HRMS indicate the presence of coeluting compounds in typical alkenone fractions that are not resolved by single dimensional GC. U37K′ indices measured by GC–GC, GC×GC and HRMS for total lipid extracts were generally observed to have lower U37K′ indices than their purified alkenone fractions.