Micrometer scale imaging of sedimentary climate archives - Sample preparation for combined elemental and lipid biomarker analysis

Mass spectrometry imaging (MSI) can be used for the mu m scale mapping of target molecules on intact sample surfaces. Recently we demonstrated that MSI of non-disturbed sediments can be used for paleoenvironmental studies; using matrix-assisted laser desorption/ionization coupled to Fourier transform-ion cyclotron resonance-mass spectrometry we visualized the spatial distributions of archaeal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids. There is a pressing need for implementing sample preparation procedures that allow exploiting the full potential of sediment MSI. Here we present a suite of sample preparation steps, optimized for the analysis of GDGTs in marine sediments. It considers the crucial requirements for successful MSI and optional combination with elemental imaging via micro X-Ray Fluorescence Spectroscopy (mu-XRF). Preservation of the sediment's spatial distribution is achieved with freeze-drying and subsequent embedding in a mixture of gelatin and carboxymethyl cellulose. This enables sectioning the sample into sequential slices from 20 to 500 mu m in thickness. Thinner sections showed enhanced signal intensity in MSI, but elemental mapping by mu-XRF is more accurate for thicker sections; 100 mu m thick slices provide satisfactory results for both analyses and are recommended for congruent elemental and biomarker imaging. When applied to the uppermost similar to 5 cm of marine sediment from a Santa Barbara Basin box core, the optimized sample preparation yields reproducible ultra-high-resolution GDGT records from sequential slices, thus demonstrating the robustness of the method. Congruent mu-XRF results aid the establishment of a contextual framework regarding supply of terrigenous and marine detritus as well as the assignment of molecular data to annual layers. (C) 2018 The Author(s). Published by Elsevier Ltd.