Analysing our first water samples

Part of the research that we are conducting is to analyse the silicon concentration and isotopc composition of lake waters. As we are interested in productivity, it is very important to understand silicon cycling through the year (e.g. winter versus summer conditions). This allows us to constrain down core interpretations. To find out more on why silicon is so important and what organisms use it in Lake Baikal, click here.

P1050563Having collected and filtered all water samples in the field, we are now ready to analyse the silicon concentration of the samples. This is conducted at the Brtish Geological Survey, UK. Concentration of trace metals are also given at the same time which is of great interest when looking at pollution stressors on the lake.

The next step is to measure the isotopc composition of the lake waters (δ30SDSi). In order to do this there are a number of purification steps that must be conducted, with samples being passed through a pre-cleaned resin to remove all cations (e.g. Na+, Ca2+) from the waters. This process takes rather a long time as the liquid must pass through the resin at a slow rate to ensure that there is full Si recovery of the sample.

The acid cleaning stage of the resin.

The acid cleaning stage of the resin.

However, this can only be done once the resin has been acid cleaned. Results from the ICP-MS are used at this stage in order to load onto the resin an exact volume of sample to ensure that the concentrations of all samples are high enough to precisely measure their isotopc composition.

Cationic resin preparation of samples and standards for MC-ICP_MS analyses

Cationic resin cleaning step of samples and standards for MC-ICP_MS analyses

Once samples are fully prepared they are measured once more on the ICP-MS to ensure that there is fully silicon recovery and to know the final concentration of the samples. This is important so that we know the volume of sample we need to analyse on the Multi-Collector Intercoupled Mass Spectrometer (MC-ICP-MS) and match concentrations of Si in bracketing standards.

P1050613

A lot of the action happens under the bonnet of the Multi Collector! Samples are injected into a plasma which ionises it, as the sample passes through the magnet the different masses of silicon (28, 29, 30)are separated and collected in three detectors. Based on the ratio between 30/28 we are able to understand the degree of biological uptake by diatoms that has taken place.

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