NanoSIMS 50 sample preparation

Sample preparation is extremely important for getting the best results from your NanoSIMS session.

This page outlines considerations for sample preparation, including specific guidelines for biological samples. If in doubt, contact Ion Probe Facility staff for advice before commencing sample preparation.

General principles

All samples must be solid, flat, vacuum tolerant, and electrically conductive.

The vacuum in the analysis chamber is nominally 10^-10 torr, which is significantly higher than an SEM or EPMA. Due to the close proximity of the primary and secondary lens, which carry a total 16 kV, it is essential that the ultra-high vacuum (UHV) is maintained to avoid electrical arcing between the lenses.

Irregular-shaped samples can be embedded in epoxy resin or Wood's Alloy depending on the nature of the material, and polished down to the surface of interest. In some cases, samples may be pressed into Indium, although with this method it is difficult to ensure sample flatness.

Samples should enter the vacuum system as far in advance of the analytical session as possible — if using a large resin mount, please send it to the Facility at least two weeks in advance.

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Sample size

NanoSIMS holders can accommodate flat polished discs with a nominal diameter of 10mm, 13mm (1/2 inch) and 25mm (1 inch).

The exact tolerances are:

• The ideal disc thickness is 4mm, but this is not critical — glass slides and Si wafers are suitable.
• Discs are held in place by a small lip on the top surface (see right) and a clip applying pressure from below.
• Samples must not be smaller than the diameter of the lip.
• Discs must not have bevelled edges as this makes it difficult to ensure the flatness and may lead to the sample protruding through the hole.

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Sample holder configurations

Sample holders are available in the following configurations:

• Biology holder: 8 x 10mm discs
• Geology holder: 1 x 1 inch, 2 x 1/2 inch, 2 x 10mm discs
• IMS1280 holder: 1 x 1 inch (can be used in NanoSIMS)
• TEM grid holder: 3 x 3mm standard TEM grids

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Resin

Some resins degas over prolonged time scales (days to weeks), so it is critical that the resin is well prepared and properly cured.

Minimise the amount of resin in the mount by using metal rings or reducing the thickness of the disc.

Recommended resin formula

Araldite 502 26g

Dodecenyl Succinic Anhydride (DDSA) 24g

Benzyldimethylamine (BDMA) 1.4g

1. Mix the Araldite and DDSA thoroughly, avoiding creation of bubbles
2. Stand the mixture on the bench for 30 minutes to allow any bubbles to disappear
3. Mix in the BDMA thoroughly, again avoiding bubbles
4. Use as necessary
5. Cure at 60C for at least 24 hours

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Polishing

Polish samples to the highest degree achievable — minimum 1 µm diamond.

Use of colloidal silica is very good for ceramics and metals, but can be detrimental for some softer materials.

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Coating

To insulate samples we typically coat with 5–10 nm gold or 10 nm carbon when gold is an element of interest.

Platinum coating is also possible.

We recommend that sample coating is performed at CMCA immediately prior to insertion in the vacuum system.

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Biological samples

Cells and tissues must be prepared in a manner that preserves and immobilises the elements and isotope(s) of interest.

Typically, samples are fixed, dehydrated and embedded in araldite resin, then microtomed and sections mounted on silicon wafers or metal discs. Techniques for this process depend on the composition of the sample.

Additional sections must be mounted on glass slides and stained to provide navigational images of the sample.

For compounds or isotopes that are neither mobile nor highly soluble

1. Fix conventionally in glutaraldehyde
2. Dehydrate through ethanols
3. Infiltrate and embed in araldite resin
4. Mount sections 500 nm-1 µm thickness on wafers or discs

For ions and compounds that are mobile or soluble in water, ethanol, acetone

1. Cryo-fix by high pressure or plunge freezing
2. Dehydrate by either freeze-substitution in ether + acrolein or freeze drying.
3. Infiltrate and embed in araldite resin under anhydrous conditions
4. Dry cut and mount sections approximately 1µm in thickness

For correlative analysis by high-resolution structural imaging by TEM

Samples can be imaged by TEM and then transferred to the NanoSIMS for correlative analysis.

1. Prepare samples in resin as above
2. Cut sections on water* to 150 nm thickness and mount on carbon-filmed TEM finder grids

* Some loss of ions may occur if highly soluble in water.

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