Tag: LCMS

Source, transport and fate of soil organic matter inferred from microbial biomarker lipids

06/09/2016 UPDATE: The paper has been accepted and is now published. The final version is available from the journal.

Our international team of East Siberian researchers currently has a paper in open review at Biogeosciences. The discussion paper, and its interactive comments, can be downloaded from the  journal website.

The paper studies a group of compounds called “bacteriohopanepolyols” (BHPs for short), which are found in the cell membranes of a range of microbes and are therefore one of the most common organic compounds around. They are found in modern and ancient sediments from all over the world. This study has concentrated on two groups of these. Group 1 is the soil marker compounds. These are only found in soils, and so have been used as tracers for soil material in rivers, lakes and offshore. Here is how they are spread across the East Siberian Artic Shelf:

BHPfig2a (Custom)
Soil marker compounds across the Arctic Shelf

Note how the soil marker concentrations are highest (orange colours) near to the rivers and coastlines. By measuring the concentration next to the river mouths, and in the sediments being washed away by coastal erosion, we show that it is not just rivers that are delivering the soil markers to the Arctic Ocean.

There is no single compound that is a true tracer for carbon produced in the ocean itself, but the compound bacteriohopanetetrol (BHT) is most abundant in marine settings despite being found in soils as well. Therefore if your sample is rich in BHT, and poor in soil markers, it is likely dominated by carbon from the ocean. Here’s a map of BHT across the East Siberian Arctic Shelf:

BHPfig2b (Custom)
BHT, a marine marker, is present across the Arctic Shelf

The BHT results show a fairly constant amount across the ocean floor. If we compare the soil marker concentrations to the BHT concentrations, we can see which areas are rich in soil carbon (more soil markers than BHT) and which are rich in marine carbon (more BHT than soil markers). This comparison is called the R’soil index, and is shown below:

R'soil index on the Arctic Shelf
R’soil index on the Arctic Shelf

The R’soil index shows that all along the East Siberian Arctic coastline, offshore sediments are dominated by carbon from the land. As you go further offshore, especially in eastern parts nearer to the Pacific Ocean, marine carbon is more important. This result shows a similar pattern to that seen using stable carbon isotopes, but is different to the pattern shown by the BIT index. Therefore these two indices, both based on microbial biomarkers, are tracing different parts of the carbon cycle.

What is: Liquid Chromatography (LC)?

Liquid Chromatography is the separation of molecules from a mixture using a liquid carrier medium.

An organic geochemist will often have the problem of having a test tube containing a large amount of different molecules, but only being interested in one or two of them.  Much like a child might place felt tip pen and water onto some filter paper to watch the colours run and separate out (with some very artistic results), with liquid chromatography (LC) we split our mixture of molecules up into its individual components so that they can then be analysed. An LC, sometimes known as HPLC where (HP means High Performance or High Pressure), consists of three main parts: the autosampler, the pump, and the column, labelled in the picture below:

Agilent LCMS
Agilent LCMS

The autosampler collects a syringe full of sample from a vial and injects it at the right time onto the column. You can do this job yourself if you have good timing, are very accurate, and have an exceptionally high boredom threshold; the autosampler allows you to queue up a large amount of samples ready to run while you are away from the machine.

Autosampler
Autosampler

The pumps mix two solvents together and push them through the column. Each chemical in your mixture will behave differently when exposed to each solvent, it will be more soluble in one than the other, and will be differently soluble to the other chemicals in the mixture. Often one of the solvents will be polar (have an electrical charge associated with it, e.g., isopropanol alcohol) and the other will be non-polar (be electrically neutral, e.g., hexane). During the sample run, the amount of each solvent being washed through the column will change, and therefore different chemicals will wash through at different times.

Pumps
Pumps

The column is a metal tube filled with small solid particles. These are slightly ‘sticky’ to the molecules passing by (they adsorb onto the particle surfaces) and so the chemicals in the samples are not washed through the column that quickly. The solvents that are being used in the experiment remove the molecules from the column particles and move them through the system, until they reach the end of the column and can be detected. At the end of a sample run, the flow on the column is reversed and anything that did not make it through to the end is cleaned out (the ‘backflush’)

Column
Column