Sample Requirements

We have prepared a list of sample requirements to help you prepare your samples and determine if your sample is suitable for NanoSIMS analysis.

For more specific information about planning your experiment and sample preparation please contact the NanoSIMS group.

General sample restrictions:

  • The sample must be stable under vacuum and not degas (the analysis chamber is at 10-10 torr)
  • The sample surface must be flat as topography will artificially enhance signals at edges. Highly polished samples (down to 1 µm diamond or colloidal silica) or microtomed samples are ideal. Polished samples should not be bevelled, sloping or rounded at the edges.
  • If the sample is non-conducting it will be necessary to coat the sample with a thin layer of platinum (this is best done in the department just prior to sample loading)

Other experimental considerations:

  • A maximum of 8 samples can be loaded into one holder for analysis in one session therefore minimising sample exchange time. Having more than one replicate on a stub will help fit more samples onto the holder.
  • If the analysis involves the detection of trace or uncommon elements, standards of high concentration must be provided. If many standards are required they should all be mounted together so that they only take up one position on the sample holder (sample size restrictions still apply). The NanoSIMS group has some standards already prepared which may be applicable.
  • The field of view on the NanoSIMS is limited so it is absolutely necessary to have an optical or SEM map of the whole sample at reasonable magnification. The areas of interest should be marked. The field of view of the optical camera is 400 µm by 300 µm.
  • Ideally samples should be loaded into the vacuum system a few days before the start of the session to allow them to de-gas and be pumped down to high vacuum.

Sample Size:

The following size restrictions must be adhered to exactly otherwise samples will not fit into the holders. 

  • The maximum thickness of sample that can fit into the sample holder is 5 mm (4 mm is the ideal thickness). Thinner samples can be loaded, e.g. Si wafers.
  • If the sample is cylindrical it must have a diameter between 9.5 and 10.2 mm.
  • If the sample is rectangular or square then the diagonals must be between 9.5 and 10.2 mm.
  • Irregularly shaped samples should be embedded in resin (fully cured) or wood’s metal, and the mounting medium must be cut or turned down to the correct size as above. It will probably be necessary to polish back down to the surface of interest.
  • Samples are mounted with respect to the front of the sample holder so cannot be simply stuck to a metal cylinder as they will protrude from the front and scratch the lens.
 

Sample preparation for biological samples:

Sample preparation of biological materials for NanoSIMS analysis is much more complex than for say a metallographic sample. 

The NanoSIMS operates under ultra-high vacuum and this means that hydrated cells cannot be directly analysed. Therefore the water must be removed.

Prior to the removal of water it is necessary to fix the samples to prevent redistribution of the elements of interest. There are two possible routes to fix the samples, the choice depends on the nature of the sample/experiment.

If the element/isotope you are interested in is not mobile or highly soluble

Use chemical fixation (same procedure used for TEM sample preparation):
  • Fix in glutaraldehyde
  • Dehydrate in a series of increasing ethanol solutions
  • Infiltrate and impregnate in resin (such as LR white, TAAB low viscosity resin or Spurr’s)
  • Microtome 1 µm sections onto a suitable substrate*

If the element/isotope is mobile or soluble in water, ethanol or acetone

Use cryo-fixation:
  • Freeze using plunge or high pressure freezing
  • Freeze substitute with acetone adding in a fixative such as osmium tetroxide or glutaraldehyde
  • Infiltrate and impregnate in resin (such as LR white, TAAB low viscosity resin or Spurr’s)
  • Microtome 1 µm sections onto a suitable substrate*

*The NanoSIMS group has suitable substrates cut to the right size which microtome sections can be mounted onto.