Making a wet mount for microscopy

The sample to be viewed floats in a layer of water which is between the slide and the cover glass. The water performs an important optical function. Without it, the resolution is lower.

The general procedure of making a wet mount

  1. Place a drop of water on the center of the slide. It is also possible to first place the specimen on the slide, but small specimens usually separate more easily from the tweezers or needle if dipped into the drop of water.
  2. Place the specimen into the drop of water and if the specimen floats, add another drop of water on top of it. This reduces the possibilities of air bubbles forming.
  3. Carefully lower the cover glass so that it touches with one side the drop of water. The cover slip should form an angle of about 45 degrees with the slide. Touch the cover glass on the sides only to prevent finger prints. Alternatively, use tweezers to hold the cover glass.
  4. Then lower the cover slip completely. Placing the cover slip at an angle prevents the formation of air-bubbles.
  5. Remove excess water with a filter paper or tissue paper

Possible problems of making a wet mount

  • The cover glass floats and moves: This is due to too much water. Remove water with the help of a tissue paper. Under no circumstances should there be water droplets on top of the cover glass. This water may get into contact with the objectives.
  • The liquid streams and does not settle: This could be due to evaporation. Add more water between coverslip and slide.
  • Air bubbles start to become visible: If bubbles were not present before and start to form, then this could be an indication of oxygen production due to photosynthesis. This depends on the oxygen saturation of the water and the amount of photosynthetic algae present.
  • Air bubbles are present: Often the cover glass was not lowered from the side at an angle, but placed horizontally on the water drop. It may also be that the the specimen is hydrophobic (fatty) and /or fluffy. In this case, the the water may have problems reaching all of the areas of the speciemen and there is much air caught by the fine structures. Wet the specimen briefly in alcohol and then transfer directly from the alcohol to water. Alternatively you can try to break the surface tension of the water by adding a small amount of surfactant, such as soap or shampoo. Be aware that alcohol or soap may have adverse effects on living organisms.