Preparing geological samples for SEM analysis
Preparing a Vesuvian lava for SEM analysis
Ensuring your sample is prepared correctly is key to making sure you collect the best quality data possible during SEM analysis. For techniques such as
Energy Dispersive Spectroscopy
,
Automated Mineralogy
and
Electron Backscatter Diffraction
, samples need to be flat and polished to a fine standard to avoid shadowing of electrons from the detector that could cause gaps in the desired dataset. At PEMC, we have a series of sample preparation equipment that allows us to prepare a range of samples in house and this is available commercially to our customers if required. Here's an example of how a typical geological sample would be prepared, from collection in the field to analysis using our
JEOL IT800 FE SEM
.
Step one – what's the sample?
The first thing to work out when preparing samples is what the sample is made of as this may effect what you use to prepare your samples. The way you prepare a sample can depend largely on what the sample consists of, for example, if you're preparing water sensitive samples you need to ensure the suspensions you use during the polishing process are free of water (in some cases ethanol or oil can be used instead).
The sample in this case study is volcanic in origin, collected on the way up to Mt Vesuvius, Italy and so can be prepared with the standard processes carried out for geological sample preparation.
Step two – cutting to size
To make resin blocks, the samples are required to fit into either 25 mm, 30 mm or 40 mm resin moulds. As a result if the samples are larger than this, they need to be cut down in order to fit into the moulds.
Typically we do this using our IsoMet 1000 precision saw where the sample is secured into a sample holder before being cut with a diamond saw blade. The samples are cut to size aiming for the area of interest to be on the flat surface as this will be placed on the base of the mould to be polished once the resin is cured.
Once cut, the samples are ready to be mounted.
Step three – making the resin block
The next stage of the process is to make the resin block. First, we apply a releasing agent on the the mould to make sample removal after the resin is cured a bit easier; the sample is placed into the mould with the desired analysis side face down. A mixture of resin and resin hardener is then mixed up, we use EpoFlo Epoxy Resin and Resin Hardener at a ratio of 3.3:1, after a gentle stir the mixture is poured over the sample into the resin mould.
The filled mould is then placed into a vacuum oven to draw air out of the resin before leaving it to cure on a hot plate for ~24–48 hours. This is also the perfect opportunity to insert a label into the resin so that the sample can be identified once it is prepared. See an example of the cured resin block on the right.
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Step four – polishing the sample
Once the block has been cured, it's time to grind and polish the block. Depending on the nature of the sample this process can vary, for example, the use of different polishing cloths, speeds of polishing or polishing lubricant if the sample is water soluble. To prepare this Vesuvian lava, we started grinding the block using silicon carbide papers at a coarse grit size (usually starting ~600 grit) to expose the sample.
Once exposed, the sample was polished using 9 µm Al2O3 powder and tap water before working through polishing stages using diamond polishing suspension at 6 µm, 3 µm and 1 µm size. This is important to reduce the sizes of the scratches until they are small enough to minimise the effect of sample preparation on the data collected during analysis. This sample is now ready for
Energy Dispersive Spectroscopy
. If we wanted to carry out
Electron Backscatter Diffraction
, however, we would polish this sample further, down to 0.02 µm.
Step five – analysis!
Once the sample preparation is complete, it's time to carbon coat it to ensure the sample surface is conductive. Once this stage is done, analysis can begin! Here are some examples of the data that can be collected on this sample once it's prepared to a 1 µm polish.
For geological samples, the initial investigation using backscattered electrons is a great way to visualise the distribution of lighter and heavier elements (and subsequently lighter and heavier mineral phases) across the sample. On the right is an image acquired using backscattered electrons, where the brighter phases are accessory minerals consisting of atomically heavy elements (likely iron and titanium) whist the darker phases are rock forming minerals that are atomically lighter (likely silicon rich).
Techniques such as
Energy Dispersive Spectroscopy
allow us to take this analysis a step further and produce X-ray element maps of any site across the sample. Each element in a X-ray element map is assigned a colour, and each map can be combined to produce a layered image (left) allowing the identification of mineral phases in a sample based on their elemental composition. EDS also allows us to quanitfy the concentration of each element in a mineral, providing more detail about the formation of the sample.
Below is an example of a backscatter image (left) where you can see a bright grain of an atomically heavy mineral phase with a slightly darker, bladed mineral phase occurring within. Using X-ray element maps we could work out that the bright grain was iron rich (the green X-ray element map on the right), and the darker mineral phase occurring within was Ti-rich.
