Dry Mount Slide Preparation – Materials, Procedure and Uses

Dry mount slide preparation is a simple method in which a dry specimen is placed directly on a clean microscope slide for observation. No liquid mounting medium is added. A coverslip (cover glass) is placed over the specimen when required, and the prepared slide is examined under a compound light microscope.

This preparation is used for specimens that can be observed without water or any other mounting liquid. Hair and textile fibres are common examples. The specimen should be sufficiently thin so that light can pass through it and the structures can be seen clearly. If the specimen is thick, it may be difficult to focus and may not fit properly under the coverslip. Such specimens can also be examined without a coverslip, depending on their size and the microscope used.

Dry mount slides are generally temporary preparations. In a wet mount, the specimen is placed in a drop of liquid, whereas in dry mounting the specimen is placed directly on the slide.

What is Dry Mount Slide Preparation?

Dry mount slide preparation is a simple microscopy technique in which a suitable specimen is placed directly on a microscope slide without using any liquid mounting medium. It is also known as “dry preparation”, and the prepared slide is called a dry mount slide.

The principle of dry mounting is based on observing the specimen in air without adding water, glycerin or any other liquid mounting medium. A small, dry specimen is placed on a clean glass slide. No liquid is added. Specimens such as hair, fibres, and small pieces of paper can be examined by this method, usually with a coverslip placed over the specimen. The dry mount slides are generally temporary, as the specimen is not permanently fixed to the slide.

A dry mount is different from an air-dried smear, where a thin layer of sample is spread on a slide and allowed to dry. The term dry objective is also different. It refers to a microscope objective which uses air between the objective lens and the specimen or coverslip instead of immersion liquid.

Objectives of Dry Mount Slide Preparation

The main objectives of dry mount slide preparation are as follows-

1. To examine suitable specimens under a microscope without using water or any other liquid mounting medium.
2. To observe the physical characteristics of the specimen, such as its shape, size, surface texture and arrangement of parts.
3. To carry out a simple and quick preliminary examination of the specimen without requiring complex preparation or staining.
4. To prevent changes in certain specimens that may occur due to contact with liquid. Water-soluble crystals may dissolve when water is added, which can change their original shape and structure.

Materials Required for Dry Mount Slide Preparation

Equipment and Materials

The following equipment and materials are required for the preparation of a dry mount slide-

1. Clean Glass Microscope Slides- A clean and dry glass slide is used to place the specimen for microscopic examination. The slide should be free from dust, grease and other unwanted particles.
2. Coverslips- A thin glass coverslip is used to cover the specimen and keep it in position. It is not required for all dry mount preparations, especially when the specimen is thick or may be damaged by the coverslip.
3. Forceps or Tweezers- These are optional tools used to pick up and place small specimens on the glass slide without touching them directly with fingers.
4. Dissecting Needle or Toothpick- It is used to separate, spread or adjust the specimen on the slide when required.
5. Compound Light Microscope– The microscope is used to magnify and observe the specimen. It is mainly suitable for thin specimens through which sufficient light can pass.

Suitable Specimens for Dry Mount Slides

The specimens selected for dry mount preparation should be small, sufficiently thin and preferably dry. Some of the suitable specimens are as follows-

1. Hair- Small strands of human or animal hair can be used to observe their shape, thickness and surface characteristics.
2. Textile Fibres- Thin fibres obtained from cotton, wool or synthetic fabrics are suitable for dry mounting. The fibres can be examined for their shape, arrangement and surface structure.
3. Feathers- Small pieces of feathers can be used to examine the arrangement of barbs and barbules.
4. Plant Fragments- Small and thin pieces of dried leaves or other plant materials may be used for dry mount preparation. The specimen should be thin enough to allow light to pass through the parts being examined.
5. Salt or Sugar Crystals- Small crystals can be placed directly on the slide to examine their shape and arrangement. No water is added, as these crystals are soluble in water.

The selection of specimen depends on its size, thickness, dryness and transparency. Thin and translucent specimens are generally suitable for examination under a compound light microscope. Thick or opaque specimens may not allow sufficient light to pass through them. Such specimens are more suitable for observation under a stereomicroscope or by using reflected light.

Procedure for Dry Mount Slide Preparation

The following are the steps involved in the preparation of a dry mount slide-

1. First, select a small and dry specimen which is sufficiently thin to allow the light to pass through it during microscopic examination.
2. Take a clean and dry glass slide. The slide should be free from dust, grease and other unwanted particles. Hold the slide by its edges and do not use cracked or broken glass.
3. With the help of forceps or a dissecting needle, place the specimen near the centre of the glass slide. If the parts are overlapping, spread them out gently. No water or other liquid mounting medium is added, as the specimen is examined in its dry condition.
4. A clean coverslip is placed gently over the specimen if it is thin enough. Lower the coverslip carefully without applying pressure. In the case of thick or hard specimens, the coverslip can be left out, as forcing it down may break the glass or damage the specimen.
5. Rotate the nosepiece of the microscope to bring the lowest-power objective (usually 4×) into position. Lower the microscope stage and place the prepared slide on it. The specimen is positioned over the stage aperture and the slide is held in position with stage clips.
6. Switch on the microscope light and adjust the illumination. The specimen is first examined under low magnification. Use the coarse adjustment knob carefully to bring the specimen into focus, keeping sufficient distance between the objective lens and slide. The fine adjustment knob is used to obtain a clear image. Adjust the diaphragm if required.
7. Move the slide to bring the required part of the specimen into the centre of the field of view. The specimen is then observed under suitable higher magnification to examine its shape, size, arrangement and surface characteristics. At higher magnification, only the fine adjustment knob is used and the objective lens should not touch the slide.
8. Record the observations by preparing a labelled drawing or taking a microscopic image. The magnification used is also noted down. After examination, rotate the nosepiece back to the lowest-power objective, lower the stage and carefully remove the glass slide.

Observations and Results of Dry Mount Slide Preparation

The observations of a dry mount slide vary according to the specimen used. The following are some of the expected observations-

• The shape, size, colour and thickness of the specimen can be observed under the microscope. At low magnification, the general appearance of the specimen may be seen. Some finer details and the arrangement of its parts can be observed at higher magnification.
• In the case of hair, the specimen may appear as a long thread-like structure. The hair shaft and its thickness can be examined. Fine surface details may not be clearly visible at the magnification used.
• Textile fibres may be seen as thin strands, which can be separate or overlapping with one another. The fibres may have different thicknesses and arrangements.
• Salt or sugar crystals may show angular shapes with visible edges. Some crystals may be irregular or broken. The appearance of the crystals is not always the same, as it depends on the material and the individual particles being examined.
• The name of the specimen and the actual magnification used are noted down in the laboratory notebook. A labelled drawing or microscopic image of the specimen is prepared. Only the structures which are clearly visible under the microscope should be recorded.
• The actual results are recorded according to what is observed during the examination. The expected features may not be seen in every dry mount preparation. If any structure is not clearly visible or cannot be identified, it should be noted in the observation rather than recording an assumed result.

Precautions for Dry Mount Slide Preparation

The following precautions should be taken during dry mount slide preparation-

• The specimen selected should be small, dry and sufficiently thin for microscopic examination. Thick or opaque specimens may not be suitable for examination under a compound light microscope.
• Use a clean and dry glass slide. The slide and coverslip should be free from dust, grease and other unwanted materials.
• Handle the glass slide by its edges. Avoid touching the surface with fingers. Cracked or broken slides should not be used.
• The specimen should be placed near the centre of the slide using forceps or a dissecting needle if required. Avoid overlapping or overcrowding of the specimen.
• No water or other liquid mounting medium should be added to the specimen. Some specimens, especially water-soluble crystals, may dissolve when they come in contact with water.
• The coverslip should be lowered gently over the specimen without applying pressure. Do not force the coverslip onto thick or hard specimens, as it may break the glass or damage the specimen. In such cases, the coverslip can be left out.
• The prepared slide should be properly placed on the microscope stage and secured with the stage clips before examination.
• Always begin the observation with the lowest-power objective. At higher magnification, only the fine adjustment knob should be used for focusing. The objective lens should not touch the slide or coverslip.
• After examination, return the microscope to the lowest-power objective and lower the stage. Remove the slide carefully. Do not leave the slide on the microscope after use.

Common Problems and Troubleshooting in Dry Mount Slide Preparation

Some common problems encountered during dry mount slide preparation and their possible solutions are as follows-

• The specimen may not be visible under the microscope if it is not properly placed over the stage aperture. Start with the low-power objective and move the slide slowly to bring the specimen into the field of view.
• Sometimes the image appears blurred even after adjusting the focus. This may occur due to improper focusing or overlapping of the specimen. Adjust the coarse adjustment knob at low power, followed by fine adjustment. If the specimen is overlapping, spread out its parts before examining again.
• The image may appear too bright or too dark due to improper illumination. Adjust the light intensity and diaphragm to obtain a clear image. Excessive light may reduce the contrast of the specimen.
• Dust, grease or fingerprints on the glass slide and coverslip may appear as unwanted spots during microscopic examination. The glass should be cleaned and the slide prepared again. If the spots remain when the slide is moved, check the microscope lenses and clean them with lens paper.
• In some preparations, the specimen may move from its original position while handling the slide. A coverslip can be used to hold the specimen in place if it is sufficiently thin. The slide should also be secured properly with the stage clips.
• Thick or opaque specimens may not allow sufficient light to pass through them, making their internal details difficult to observe. Use a smaller or thinner part of the specimen. Opaque materials may require a stereomicroscope or reflected-light observation.
• The coverslip may break when it is pressed over a thick or hard specimen. Do not apply pressure. Such specimens can be examined without a coverslip where appropriate.
• When the microscope is changed to higher magnification, the specimen may become blurred or disappear from the field of view. The specimen should first be centred and focused at low magnification. At higher magnification, use only the fine adjustment knob and avoid bringing the objective lens in contact with the slide.
• Some dry specimens may change their original appearance when exposed to moisture. For example, salt or sugar crystals may dissolve if water is added. A fresh dry specimen should be used, without adding any liquid mounting medium.

Uses of Dry Mount Slide

Some of the important uses of dry mount slides are as follows-

• Used to examine small, dry specimens under a microscope without adding any liquid mounting medium.
• Used to observe the shape, thickness and appearance of hair.
• The arrangement and surface characteristics of textile fibres can be studied.
• Used to examine the shape of salt and sugar crystals without dissolving them in water.
• Used to observe the fine structures of feathers and thin plant fragments.
• Dry mount slides are used for simple preliminary examination of specimens in microscopy practicals.

Advantages of Dry Mount Slide

• It is a simple and quick method of slide preparation.
• Only a few materials are required, making the preparation inexpensive.
• No staining or fixation is required for examining suitable dry specimens.
• The specimen can be examined in its natural dry condition without adding water or other liquid mounting medium.
• It prevents the dissolution of water-soluble materials such as salt and sugar crystals during examination.
• The specimen can be easily removed or repositioned, as no permanent mounting medium is used.

Limitations of Dry Mount Slide

• It is not suitable for examining specimens that need to remain in water or other liquid medium.
• Thick or opaque specimens cannot be properly examined under transmitted light. The internal details may not be visible.
• Some dry specimens have very low contrast, making the fine structures difficult to observe.
• The specimen is not permanently fixed to the glass slide. It may move during examination. The preparation is generally temporary and not suitable for long-term preservation.
• The coverslip may damage thick specimens or break when pressure is applied.
• Even small amounts of dust or other unwanted particles on the slide may interfere with microscopic observation.

Dry Mount vs Wet Mount Slide Preparation

Dry mount and wet mount are two methods of preparing specimens for microscopic examination. The main difference is the use of liquid mounting medium.

The differences between dry mount and wet mount slide preparation are given below-

Note- The presence of a coverslip does not determine whether a slide is a dry mount or wet mount. When the specimen needs to remain in liquid, especially living aquatic organisms, wet mount preparation is preferred instead of dry mounting.

References

1. Cappuccino, J. G., & Welsh, C. T. (2022). Microbiology: A laboratory manual (12th ed.). Pearson. https://www.pearson.com/en-us/subject-catalog/p/microbiology-a-laboratorymanual/P200000006788/9780137546527
2. Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. https://openstax.org/books/biology-2e/pages/4-1-studying-cells
3. Parker, N., Schneegurt, M., Tu, A.-H. T., Lister, P., & Forster, B. M. (2016). Microbiology. OpenStax. https://openstax.org/books/microbiology/pages/2-4-staining-microscopic-specimens