Thursday, 7 August 2014

Analysing lipid residues in archaeological soil and faeces at the NERC LSMSF

It's been a while since I've been in the lab, but last week I got to spend the entire week working in the NERC LSMSF (Life Sciences Mass Spectrometry Facility) in the Organic Geochemistry Unit at the University of Bristol. I have worked with Bristol for many years now as they have arguably the best set up in the world for archaeological and environmental geochemistry. For this visit I was working mostly on samples from the Ecology of Crusading project, with a few extra coprolite samples thrown in from Catalhoyuk and Durrington Walls (more on those at a later date!). The EoC samples are part of a larger programme of geochemical analysis, designed to look at human impacts on the landscape associated with colonisation in the medieval period. We are looking for evidence of increased faecal inputs (lovely!) associated with clearance of land for pasture, and maybe even human 'sewage' inputs from intensification of activity.

Although I have developed the facilities for pottery residue analysis at my lab in Edinburgh, extracting lipids from coprolites and sediments is a bit more complicated. With pottery, we pretty much are just grinding up the pot and shaking it about a lot in solvent, then looking at everything that comes out. Coprolites and soil hold on to their organic molecules a bit better and they are less soluble, so instead of just shaking them with solvents, we need to do something a bit more extreme. This used to mean leaving samples overnight in a special bit of kit called a Soxhlet extractor, that circulates solvent through the sample continuously. Now Bristol have a new exciting microwave extraction kit that does it all in 40 mins! After extracting the residue, we then need to conduct a serious of steps to break up and chemically modify the lipids, firstly so that we can remove any unwanted molecules (the mixtures are much more complex than we see in pottery and there is a lot of stuff that is not very useful for archaeological purposes), and secondly so that the molecules can be easily detected using the GC/MS. So that's what I spent my week doing - rinsing lipid molecules from very old soil and poop, then doing lots of fiddly chemistry to make them suitable for analysis.

Left: sample in tube (white object is magnetic stirrer) Right: adding solvent to the sample and loading into microwave sleeve
Left: Samples loaded in microwave Right: after microwave solvent is emptied into flask, evaporated, and transferred again into a small vial
Left:small vials are placed in heating block with a stream of nitrogen to dry them gently Right: after they are completely dry, the lipid residue looks like this, if you're lucky!

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