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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Lake Sediment Analysis  /  Use of Freeze Drier

Use of Freeze Drier

For some analyses, e.g. lipid analysis, it is essential that sediment is freeze dried rather than oven dried. Freeze-drying is preferable to oven drying because it removes the water content without greatly altering the physical structure of the sediment. As a result, sediments containing clay are much easier to handle after freeze drying (they remain friable) and diatom breakage resulting from contraction of the sediment during drying, is also reduced.

Operating the MODULO 4k Freeze drier.

Preparing samples. All samples must be frozen in the lab freezers before they can be freeze-dried. The samples should be placed in containers which allow as much of the sample to be exposed to the vacuum as possible. Petri dishes are ideal for sediment.

Preparing the freeze-drier. Switch on the freezer unit (main power socket and on switch located on the front of the freezer unit) and leave for around 10 min. allowing the temperature to stabilise at between -55 and -60oC. Also, at the same time, switch on the vacuum pump (main power socket and switch on the left hand side on the pump unit) ensuring that the drain valve is closed and the flask unit is sealed. This action heats the oil to working temperature and makes freeze drying more efficient. After the temperature has stabilised and the oil heated, switch off the pump (leave freezer on) and release the vacuum by opening the drain valve. Before use, check the level and quality of the oil through the window on the front of the pump. The oil level should be between the upper and lower marks and should be a clear brown colour.

Loading. After the vacuum has been released, remove the top of the perspex flask. Take pre-frozen samples from the freezer and transfer them to the flask as quickly as possible to prevent melting and subsequent water boiling. In the case of petri dishes, samples are placed on plastic trays with the lids removed or slightly open. As each tray becomes filled, levels are built up using small glass phials as props between trays. The trays can be lifted into place using needle-nosed pliers available in the lab. Using petri dishes with the lids slightly open, 7 samples per tray can be dried, making a total of 56 samples on 8 trays.

Switching on. After loading, the unit should be sealed by replacing the perspex flask lid and ensuring that the drain valve is closed. The drain valve requires only slight tightening - over-tightening will damage the valve. Switch on the pump as above.

Freeze-drying. The pressure in the flask should decrease with time indicating that freeze-drying is taking place and that water is being removed from the samples. When the pressure drops to 0.08 - 0.1 mbar, as indicated on the pressure gauge, the sample should be dried (visual inspection can also be used to determine the stage of drying). Drying time varies with the type of material and generally increases with 1) increasing water content, 2) increasing thickness of the sample and 3) decreasing surface area in proportion to sediment size. As a rough guide, for clay-rich sediments in petri dishes, samples can take up to 48 hours to dry and for sandy sediments, up to 36 hours.

Switching off. Switch off both the vacuum pump and the freezer unit. Then release the pressure in the flask before removing samples. To release pressure, the drain valve should be opened to allow air to enter the chamber. It is advisable to place your finger over the drain inlet/outlet whilst opening the valve to assess how much air is entering. It is important to allow air to enter very slowly to prevent a rush of air dispersing dried sediment around the chamber causing contamination. When the flask has reached atmospheric pressure, the lid and the samples can be removed. Make sure that there is nothing attached to the drain inlet/outlet before releasing the vacuum.

Clearing out the ice. After the samples have been removed, open the drain valve fully and attach a rubber pipe to the valve inlet/outlet putting the other end into a container. Ice formed during freeze-drying in the condensation chamber will melt and the water will pass out through the drain valve. Although it is not strictly necessary to do this after every run, it is advised as it improves the efficiency of future runs. The ice can be left to melt overnight or to speed the process up, warm water can be poured over the ice in the chamber. This should be done with care - always ensuring that the angled pipe in the condensation chamber is in place.

Cleaning the freeze drier. After each run, clean the trays, flask and chamber thoroughly using damp paper towel and dry. Never use solvents to clean components of the freeze drier.

Safety Notes

  • Care should be taken whilst loading/unloading the freeze-drier as this involves working from a stool
  • Users should handle the perspex flask carefully and check regularly for any any abrasions or fractures which might impare the strength of the chamber. Remember, the during freeze drying the flask is under a high vacuum any weaknesses could cause the chamber to implode.
  • Do not touch the inside of the refrigeration chamber whilst the fridge is on or immediately afterwards as inside temperatures can be as low as -60oC