In order to make ion microprobe analysis possible and facilitate good results, there are strict requirements to sample preparation. The following text presents some guidelines and advice about sample preparation.
Physical dimensions. The NanoSIMS can in principle accept all samples that can be safely mounted in the sample holders (illustrated below). However, we strongly recommend that the shape of the sample be close to a perfectly cylinder with a diameter of either 10, 12.5 or 25 mm and no more than 4 mm in thickness. In order to be introduced into the NanoSIMS, the samples are mounted in holders as illustrated below:
- The 'Biology' sample-holder can carry 8 individual samples mounted in or on top of 10 mm diameter cylindrical sample mounts. This type of sample mounts is therefore the most efficient way to introduce samples into the NanoSIMS and we encourage users to prepare their samples in this manner, if possible.
- The 'Geology' sample-holder can carry one 25 mm, two 12.5 mm and two 10 mm sample mounts. Many geological objects, including meteorite thin sections, are mounted on 25 mm glass-slides. Such samples can only be introduced into the NanoSIMS on the 'Geology' holder. Therefore, only one 25 mm sample can reside in the analysis chamber at any given time. This is impractical and might cause analytical problems if samples have to be shuttled in and out of the analysis chamber frequently. Certainly, time is lost with each such transfer. We therefore encourage users to mount their samples in the 10 mm sample mounts, when possible.
- Many samples are conveniently mounted in epoxy (see below) followed by polishing of the sample surface. Epoxy-mounted samples normally have external diameters of either 10 or 25 mm and are often mounted in custom-made steel cylinders with these dimensions, available from our facility. Steel cylinders for sample mounting can be requested along with the Application Form.
- Many types of samples cannot be mounted in epoxy. If samples are either very small (i.e. less than 200 micrometer in longest dimension) or very thin (e.g. a TEM section) it is often preferable to mount them on top of polished steel cylinders or glass slides - always either 10 or 25 mm in diameter. This can be achieved either by using epoxy to glue these objects to the steel cylinder or simply by using double-sticking C-tape. Samples prepared for TEM analysis and mounted on a standard TEM-grid are successfully mounted with double sticking C-tape. Samples substantially larger than 200 micrometer in size may also be mounted without the use of epoxy, but this requires special precautions in consultation with the facility.
- Very small samples, i.e. particles smaller than 20-10 micrometers in linear dimension require special precaution. Often such samples are prepared for the NanoSIMS by depositing them on, for example, a gold foil using a special solution that prevents the particles from clumping together. Alternatively, such particles can be pressed into a gold foil, which typically is 0.025 mm in thickness. The NanoSIMS laboratory does not supply gold foil, which needs to be pure. Typically we use 99,985 percent pure gold. Such pure gold foil can be purchased from companies specialized in research materials.
Epoxy. Because of the ultra-high vacuum under which the NanoSIMS operates (about 10-10 Torr) not all types of epoxy can be used. We strongly recommend using KÖRAPOX® 439, which can be obtained from, for example:
4, rue des Frères Lumière - BP 75
67172 Brumeth Cedex
Phone: +33 3 88 59 43 70
Email: contact @ ast-colles-mastics.fr (contact @ ast-colles-mastics.fr)
Other types of epoxy might be suitable as well. However, the important consideration is for the vacuum of the NanoSIMS. Samples that cannot be pumped down to the appropriate vacuum on a reasonable timescale (hours to days) very often have to be rejected.
Sample surface. The NanoSIMS is particularly sensitive to the quality of sample preparation for two primary reasons.
1) The NanoSIMS operates at a vacuum around 10-10 Torr. Thus, samples containing a lot of volatiles, either as surface contamination or as a component inherent to the sample, cannot be introduced into the instrument.
2) Due to the high spatial resolution of the NanoSIMS, good results depend on the flatness of the sample surface. Sample surface roughness or scratches can cause imaging artifacts and have to be avoided. In general, samples introduced into the NanoSIMS are polished to a 0.25-micrometer finish, or better. Such a finish can be achieved with diamond paste or Al-oxide. Facilities for sample polishing are available at the LMCM. Users with previous experience are encouraged to polish their own samples.
Coating. Sample surfaces must be coated with a layer of conductive material is a requirement for all surfaces that are not strongly conductive. Conductive materials embedded in non-conductive materials, for example epoxy, also require coating. We offer to coat sample surfaces with either C or Au. Both types of coating are used routinely in ion microprobe analysis. Users with previous experience and access to a sample coater might coat their own samples.
Please direct questions regarding sample preparation to mathieu.roskosz @ mnhn.fr (mathieu.roskosz @ mnhn.fr)
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