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Drying specimens from alcohol

Several methods have been proposed for drying specimens from alcohol. The three most commonly used methods are described below, but several others have been proposed, eg freeze-drying, Peldri II, acetone vapour, xylene and hexane (see Heraty & Hawkes, 1999).

(1) Air drying
This method is not highly recommended as it is the least satisfactory method of drying specimens from alcohol. Specimens dried in this way, especially small thinly sclerotised species, will probably collapse or shrivel when drying. However, it is the cheapest method and can be used when the other methods of drying described below are not available.

Place specimens individually in a small drop of 70% alcohol onto a fairly absorbent piece of card. Good quality, unglazed library record cards are best for this, although smaller specimens may benefit from being dried on good quality Bristol board since this prevents matting of the fringes of the wings. Once the specimen is on the card, lay it flat on its back and position the wings so that they are held flat against the card. It may be necessary to hold the specimen whilst the alcohol evaporates to ensure that the wings stay flat against the card. When the specimen is just dry remove it gently and mount as described below. Specimens are safe to mount for up to about 5 min after drying, after this period they will become rather brittle. It this is the case repeat the operation.


(2) Critical-point drying
This is an excellent method of drying specimens from alcohol using a Critical Point Drier (see Gordh & Hall, 1979). To prevent the risk of specimens exploding during the critical point drying process, all water must be removed. This involves removing all the water from the specimens by transferring them through an alcohol series until they have been completely dehydrated in 95-100% alcohol. Sometimes acetone or Freon is used for this process. After dehydration, the specimens are transferred to a high pressure chamber which is sealed. The alcohol, acetone or Freon is completely replaced with liquid CO2 under pressure. Once all the alcohol has been replaced the chamber is warmed until it passes the "critical point" for CO2 (1100 lbs/sq in [c78kg/cm2]and 31.5°C). At this point the liquid CO2 passes into a gaseous phase. The CO2 gas is then drawn off slowly leaving the specimens in the chamber. If they have been preserved under ideal conditions then drying using this method will produce material that is a delight to study. Specimens will not collapse and will remain slightly flexible so that appendages can be arranged to their most advantageous position for taxonomic study. The only disadvantage of this method is that Critical Point Drying machines are expensive and generally available only to those who work in research establishments such as Universities. Another disadvantage is that many of the older machines have relatively small drying chambers that allow only a limited number of samples or specimens to be dried at any given time. However, many of the more modern machines have larger chambers which can allow at least 20 samples and several thousand specimens to be dried at one time.

(3) Using HMDS
This has been termed "the poor man's critical point drier". It is a relatively cheap method that can be used to dry specimens from alcohol that achieves similar results to using a critical point drier. HMDS (Hexamethyldisilazene) is a relatively low cost chemical solvent that has a low volatility. It can be purchased from most laboratory suppliers. The alcohol from the specimens is replaced by the HMDS which is then allowed to evaporate to leave the specimens dry and in perfect condition. Specimens in 95-100% alcohol are transferred to HMDS in a glass dish for about half an hour. These are then transferred to a second glass dish of HMDS for another half an hour and then the HMDS is allowed to evaporate in a fume cupboard (or out of doors). Material treated in this way are in nearly as good condition as those dried using a critical point drier (see Heraty & Hawkes, 1998), although they do not remain quite as flexible.

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Last updated 19-Aug-2003 Dr B R Pitkin