(3) Cold Fracture Stage

The cold fracture stage: this unit has two separate functions:

1. To allow cryo-fracturing of the sample at a precisely controlled temperature
2. To allow the user to load and store up to 2 samples in a cold 'parking area'

The fracture stage unit is LN2 cooled, and can be maintained at -180°C. The whole unit moves in and out to allow samples to be parked in the 'parking area', picked up for placement in the fracture unit, or transferred (following fracture) into the sample analysis chamber.

ToF-SIMS researchers have found empirically that water-containing samples should be prepared by fracturing at between -105°C to -110°C. If the temperature is too cold then subsequent surface spectra are dominated by water molecules. Too high a temperature appears to allow excessive mobility of surface species leading to smearing of surface features. Thus, accurate control of the sample temperature during fracture is essential.

During fracturing a large pressure burst is observed. Condensation of this water back onto the sample will tend to obscure the freshly fractured surface. To minimise this, both the 'knife' and the surrounding material of the fracture stage are held at -180°C during fracture. Also, the inside of the preparation chamber is lined with a cryo shield.

The fracture tool itself is specifically for the preparation of fresh surfaces on biological materials. It too is actively cooled with LN2. The sample usually comprises a layer of biological/soft sample material sandwiched between two flat plates (Si wafers, copper plates, etc), and it is this 'sandwich' that is cleaved apart by the fracture tool. The fracture tool is not strictly a knife. The tool is really a 'finger' that can be hooked under the top plate in order to pull it free, thereby causing fracture.

Once the sample has been fractured, the sample stub is picked up again on the transfer rod and transferred through the analytical chamber gate valve and onto the cold stage.