Development and comparison of chromatographic methods for the analysis of long chain diols and alkenones in biological materials and sediment

Abstract

We have compared and assessed the suitability of several chromatographic methods for the analysis of long chain alkenones and long chain diols and the associated paleotemperature proxies (UK’37 and LDI). We evaluated the traditional methods for the analysis of the UK’37 and the LDI, gas chromatography (GC) − flame ionization detection (FID) and GC mass spectrometry (MS) using selected ion monitoring (SIM), respectively, and developed a new method using GC–MS/MS in multiple reaction monitoring mode (MRM) for the analysis of long chain diols as well as a method for automatic silylation of diols using a robot autosampler. Finally, we evaluated liquid chromatography (LC) methods to simultaneously measure the UK’37 and the LDI, using ultra high performance LC (UHPLC) with low (nominal mass) resolution MS in SIM mode, and UHPLC with high resolution MS (HRMS). Detection and quantification limits and reproducibility were assessed by means of serial dilutions of culture extracts.Automated silylation by a robot autosampler showed similar reproducibility as off-line silylation while substantially decreasing sample preparation time. The novel MRM method had a slightly lower limit of quantification (LOQ; i.e. 0.3pgC28 1,13-diol injected on-column) than the traditional method (0.5pg) and improved reproducibility while allowing more unambiguous identification of LCDs in complex matrices. For diols, UHPLC–MS using SIM had the highest LOQ (i.e. 15pg) and a comparable reproducibility as GC–MS. UHPLC–HRMS had a LOQ of ca. 1.5pg, and an improved reproducibility for diol analysis. For alkenone analysis, both UHPLC–HRMS and UHPLC–MS using SIM were 2–3 orders of magnitude more sensitive (LOQ ca. 20 and 2pgC37:2 alkenone injected on-column, respectively) than GC-FID (LOD ca. 3ng), with a similar reproducibility of the UK’37 index. Hence, UHPLC–HRMS allows simultaneous analysis of the UK’37 and LDI at an increased sensitivity. In addition, it allows simultaneous measurement of TEX86, a temperature proxy based on the isoprenoid glycerol dialkyl glycerol tetraethers. This reduces the preparation time by excluding the need of derivatization and separation of the ketone (containing the long chain alkenones) and polar fractions (containing the long chain diols and GDGTs). However, synthetic standards are required to fully assess the accuracy of the new methods for determination of the LDI and UK’37.