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HMDS: an alternative to critical point drying

A further chemical alternative to critical point drying for preparing small (or large) flies

by Brian V. Brown, last updated April 17, 2016. Note that prices for these chemicals have changed substantially since this article was published

Proper citation: Brown, B.V. 1993. A further chemical alternative to critical- point-drying for preparing small (or large) flies. Fly Times. 11: 10.



Previously, I wrote about the use of Peldri II as a chemical alternative to critical point drying (CPD) (Brown, 1990). Using Peldri II for specimen preparation has the advantage of not requiring expensive equipment and huge canisters of carbon dioxide, but the chemical is somewhat expensive and requires some equipment, i.e. a hot plate.

Recently, I came across a material that seems to overcome all the drawbacks of Peldri II. This chemical, called hexamethyldisilazane (HMDS) is readily available and cheap, costing $18.00/250g, versus $58.00/250g for Peldri II. No heating or cooling is needed for using HMDS: one merely dehydrates specimens to 100% alcohol, then do two soaks of one-half hour in pure HMDS (i.e. change the HMDS once). I use small vials for the soaks. After the second soaking, I pour the HMDS and flies into shallow depressions or small dishes, and allow the liquid to evaporate under a fume hood. Specimens come out exactly like CPD- prepared specimens, ready for SEM or for general mounting for the collection. Other users agree, find that HMDS is just as or more effective than CPD for producing perfect specimens of various tissues (Adams et al., 1987; Nation, 1983). Note for larger samples (for instance 50 small flies at once), I recommend an extra change of HMDS.

HMDS is available from Polysciences, Inc., 400 Valley Road, Warrington, PA, 18976-2590, USA, 1-800-523-2575.


Adams, J.L., Battjes, C.J. and D.A. Buthala. 1987. Biological specimen preparation for SEM by a method other than critical point drying. Proceedings of the 45th Annual Meeting of the Electron Microscopy Society of America. (ed. by G.W. Bailey). San Francisco Press, San Francisco.

Brown, B.V. 1990. Using Peldri II as an alternative to critical point drying for small flies. Fly Times. 4: 6.

Nation, J.L. 1983. A new method using hexamethyldisilazane for preparation of soft insect tissues for scanning electron microscopy. Stain Technology. 58: 347-351.