How to prepare a slide for a microscope

Almost everything you want to observe under a microscope first needs to be ‘mounted’.

To get the best view – and to prevent your microscope getting dirty or contaminated – you’ll need to prepare your samples beforehand using glass slides and coverslips.

Not to worry, mounting your samples is quick and easy. 

To get you started, here’s a quick look at 3 of the most common mounting techniques.

Dry mount

Dry mounts are the most basic technique. You simply place the sample on a glass slide and top it off with a coverslip.

This approach is best used for dry specimens, such as powders pollen, particles, hair or parts of plants and insects.

Samples that are too thick or particularly opaque might have to be ‘thinned down’ first. To do this, you make a very fine slice of your sample that will be thin enough to fit under the coverslip and allow enough light to get through.

Dry mounted slides will last a long time, so you can hang onto them for future viewing.

Wet mount

Wet mounts are a little more involved.

Instead of just resting on the slide, your samples are suspended in a liquid medium such as water or glycerine. This approach lets you view living specimens in their natural habitat without drying them out, but it involves a few extra steps.

• If your sample is already a liquid, place a small drop in the middle of a clean, dry slide. If it’s a solid, put your sample on the slide and add one drop of water to it.

• Take a coverslip and lower one edge onto the slide at the outer edge of the drop of liquid.

• Slowly lower the rest of the coverslip on top of the rest of the liquid.

• If the coverslip is floating around, you have too much liquid. Take the edge of a paper towel and touch it to the liquid at the edge of the coverslip. The absorbent paper towel will soak up the excess.

By lowering the coverslip at an angle instead of just dropping it on top, you avoid trapping any air bubbles.

Common wet mount slide samples might include pond water, yoghurt culture, and blood cells.

While this technique is versatile and useful, wet mount slides often have a short lifespan. The liquid inside will evaporate over time and living specimens will eventually die.

Smear

Smears are a variation mount used when a liquid is too thick or deeply coloured for a normal wet mount. This technique is commonly used for blood samples.

A droplet of the sample is placed on a slide then carefully smeared across it so the sample is thinned out and easier to see.

While making a smear isn’t complicated, it takes a bit of practice to get a nice, even spread.

• Place a small drop of your sample about one-third of the way down a glass slide.

• Take a second glass slide and hold it at an angle (30 to 45 degrees).

• While holding the bottom slide in place, touch the end of your angled slide to the droplet and wait until the liquid flows along its width.

• Push your angled slide out along the length of the bottom slide in a smooth spreading motion, creating a smear. Try to maintain an even contact and keep the slide at the same angle. A good smear will have a ‘feathered edge’.

• Leave the slide to dry so that it can be stained or place a coverslip over the smear, depending on what your sample needs.

By adjusting the angle of the slide, you can make the smear thicker or thinner – bigger angles will give you a thicker smear.

Different samples can require different smearing techniques. For example, blood smears usually don’t place the angled slide directly into the droplet. Instead, they place it on the bottom slide itself before pulling it back into the droplet.

Stains

Staining is an important part of slide preparation with specimens that are transparent or hard to see.

Stains allow you get a better look at their details and structure. You can even use stains to differentiate between different microbial species.

Staining methods can range from the very simple to some fairly complex multistage preparations. But to get you started, here’s how to perform a simple stain on a wet mount.

• Take your already mounted sample slide.

• Add a drop of stain at one edge of the coverslip.

• Place a piece of paper towel at the opposite edge of the coverslip.

• Watch as the stain is pulled through the sample by capillary action.

Stains are handy but also usually lethal to a living sample. So, if you’re trying to study live microbial activity or movement, you’ll need to find an alternative technique to staining.