Innovative methods for determining alkenone unsaturation indices
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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 x GC) and high resolution mass spectrometry (HRMS). U-37(K’) indices, defined as [C-37:21]/ [C-37:2 + C-37:3], 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. U-37(K’) indices measured by GC-GC, GC x GC and HRMS for total lipid extracts were generally observed to have lower U-37(K’) indices than their purified alkenone fractions. (C) 2003 Elsevier B.V. All rights reserved.