Analysis of trace amounts of alkenones in complex environmental samples by way of NaBH 4/NaBD 4 reduction and silylation

Abstract

The U 37 K ′ index is widely used to infer paleo-sea surface temperatures from the relative abundances of sedimentary 37:3 and 37:2 methyl alkenones. However, the measurements can be difficult and error prone when the alkenones are present in only trace amount, when one is much more abundant than the other, or when co-eluting compounds are present. Silylated alkenols can be analyzed by way of gas chromatography–electron ionization mass spectrometry (GC–EIMS) with much greater sensitivity and specificity than the parent alkenones and near-baseline resolution can be obtained without the use of very long chromatographic columns and/or slow temperature programs. We show, using particulate matter from the Black Sea and the DYFAMED site in the Ligurian Sea, and surface sediments from the Black Sea and SE Alaska, that conversion of C 37 alkenones to alkenols using NaBH 4 reduction and subsequent derivatization to TMSi ethers allows U 37 K ′ values to be reliably calculated without time consuming purification steps, using the relative abundances of the diagnostic m/ z 117 ion. The response factor for m/ z 117 varies for 37:3 and 37:2 alkenols, so a specific correction is required. The reduction method is easy to use, highly sensitive and widely applicable. Ethyl alkenols give a characteristic ion at m/ z 131 and, if NaBD 4 is used, the ions at m/ z 117 and 131 shift to m/ z 118 and 132, providing further confirmation of the structures.