In order to make your microprobe session as productive and efficient as possible, there are a few basic things you should know.
SAMPLES
Preparation
In order to get accurate compositional data samples in most cases must be polished. Types of polished samples include polished thick and thin sections, polished blocks (we have holders for 25 mm and 1.25 inch samples, we may be able to accommodate other sizes but please ask beforehand)
In order to avoid sample charging non-conductive samples (most rocks and minerals) need to be coated with a thin coat of carbon. In order to have a good adhesion between the carbon coat and the sample, the samples should be completely free of grease and other residue. If they have been previously coated but the coating is more than a few months old it should also be removed. (This typically means a light repolishing of the sample.)
In order to save time on the day of your appointment it is best to bring the samples to the lab a few days earlier for carbon-coating
Navigation
The minimum magnification of the microprobe’s electron microscope is 40x, this means that locating selected areas can be difficult without previous planning. Depending on your requirements the following can be helpful.
· A high resolution scan of the sample or section using a flatbed scanner is a very useful aid in finding larger grains ore regions. For smaller grains photomicroscope images are useful, especially reflected light images, which correspond more closely to SEM images.
· An alternative (or complementary) navigation aid is to draw small circles directly on the polished sample and connect the circles with lines. The best markers for this purpose are water proof overhead projector markers. Remember to avoid touching the cleaned surfaces.
Analytical considerations
You should always provide lab staff with a detailed list of minerals and elements you want to analyse. Setup for most common silicates will be straight forward, but some setups will require additional work. Elements light than F can be analysed, but setup is not routine and will require extra time. Rare earth elements also require special setup to avoid problems with elemental peak overlaps. High accuracy trace element determination (elements in concentrations less than 1%) will also require extra setup time.
If you are not sure what minerals and elements you need to analyse, you can discuss with your supervisor or schedule some survey time on the SEM.
The JEOL 6610LV scanning electron microscope is an extremely versatile instrument capable of imaging and chemical analysis of a huge range of materials. As well as conventional high-vacuum, high-voltage SEM modes, the 6610LV can be used in low-voltage and low-vacuum modes that allow imaging of delicate and precious samples which cannot be coated. Low-vacuum mode allows chemical analysis of uncoated samples.
As such almost all samples are suitable for electron microscopy but there are some exceptions. The system has no cryostage attachment; therefore, hydrated samples such as muds or suspensions cannot be used. Biological material can be imaged, but will rapidly dessicate, freeze drying is recommended for this type of sample.
Quantitative chemical analysis is best performed on polished, carbon-coated samples. Unpolished material can be analysed if coated or in low-vacuum mode, but due to indeterminate beam incident angles and possible absorption between the site of X-Ray generation and detection, such results should only be considered semi-quantitative. Samples which cannot be coated may be analysed using low-vacuum modes, but the analytical area is in the area of 20-30 µm as opposed for to 1 µm in high vacuum mode
Low voltage mode can bu used for imaging of uncoated samples at high vacuum. This is useful for mineralogical material such as pieces of rock or rare mineral samples that cannot be coated. The low operating voltage precludes x-ray analysis . One advantage of low voltage mode is that the visibility of fine surface features are greatly enhanced.
If you haven't previously used the SEM, you are strongly advised to read SEM A-Z, a quick review of scanning electron microscopy published by JEOL.
For help deciding how to prepare you samples and which imaging mode to use please contact lab staff.