Alkenones are long-chain methyl or ethyl ketones with 2 to 4 double bonds and 35 to 42 carbon atoms. They are produced by Isochrysidales, an order of haptophyte algae. Because of the strong linear response between the unsaturation degrees of alkenones and environmental temperature, alkenones have been widely used for paleo-sea surface temperature (SST) reconstructions in the past 40 years. Traditionally, alkenones were analyzed utilizing gas chromatography (GC)-based approaches. However, such analysis is challenging when alkenones have low abundances and/or co-elute with other compounds, which requires extensive purification and concentration of sediment samples. Here, we show the exceptional separation of alkenones in culture and sediment samples using high-performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS) under reversed-phase mode. The strong molecular ion peaks of alkenones under EIS-MS allow detection under single ion monitoring, increasing the detection limit and resolving alkenones from other co-elution compounds. Compared with GC methods, analysis of alkenones on HPLC-ESI-MS greatly shortens the time needed for sample preparation that mainly depends on the purification and concentration processes. The relative response factor of tri-unsaturated alkenones over di-unsaturated alkenones was determined to be  2.1. The higher response of tri-unsaturated alkenones permits accurate quantification of alkenones from sediments in the Pacific Warm Pool where extremely low abundances of tri-unsaturated alkenones are observed. This allows temperature reconstructions in these regions. Specially, we demonstrate the high efficiency of HPLC-ESI-MS in analyzing alkenones from culture samples of <i>G. oceanica</i> strain RCC6484 (one alkenone-producer isolated from the Pacific Warm Pool) that were cultured from 24 to 33 °C. The first HPLC-based temperature calibrations of alkenone proxies were thus provided.

Analysis of long-chain alkenones with exceptional sensitivity and selectivity using HPLC-ESI-MS

ACS Fall 2021

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