For many types of microscopes, especially light or bright field microscopes, some form of specimen sample preparation must be done before the specimen can be viewed under the microscope. Take for example a compound light microscope. It makes use of microscope slides to mount the specimen on, before loading it onto the microscope for viewing.
How the specimen slide must be prepared largely depends on its properties, such as whether it’s organic or inorganic, live or fixed, wet or dry, thick or thin, large or small, and so on. In fact, these things determine not only how the specimen preparation will go, but also the type of microscope that is most suited for viewing the specimen.
Below is a step by step guide on three of the most common types of microscope slide preparation, as well as an overview on the different types of microscope slides you can use for every kind of specimen.
How to make a dry mount slide
Perhaps the most common and the easiest type of microscope slide preparation is making a dry mount slide, since it essentially just involves placing the specimen on the slide. This is a technique used for most inorganic specimens and dead matter.
These include insects, body parts of tiny animals such as legs and antennae, air particulates like dust and pollen, and other small specimens such as hair and feathers. No preservation is required when working with these types of specimens, but it may be beneficial to use microscope slides that are resistant against scratches and dust.
Here are the steps on how to make a dry mount slide:
- Prepare the specimen by thinly slicing the portion you wish to view. Darker and more opaque specimens need to be sliced as finely as possible.
- Get a clean piece of a microscope slide, and hold it carefully on its edges. Then, using tweezers, place the specimen on the center of the slide.
- Grab a cover slip, making sure to hold it on the edges, and place it on top of the specimen on the slide.
- Lower the prepared slide onto the microscope stage, positioning it in such a way that the sample is clearly visible through the eyepiece.
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How to make a wet mount slide
A wet mount slide is another common kind of specimen slide preparation when it comes to many forms of microscopy. This method is used for viewing living organisms, liquids, and other specimens that need to remain moist.
Caution must be used when using wet mounts, since many liquids may eventually dehydrate and living specimens may die. If you need to, there are several techniques that you can research to delay this process and preserve the specimen for a longer period of time.
Here are the steps on how to make a wet mount slide:
- Get a piece of clean flat or concave glass slide, and lay it carefully on an even surface, or hold it gingerly at the outer edges.
- If you are working with a liquid specimen, take a few drops of it using a medicine dropper, and place a single drop of the specimen onto the center of the slide.
- Otherwise, place a single drop of fluid (water, brine, oil, or glycerin) on the center of the slide, and place the specimen on top by using a pair of tweezers.
- Set aside the dropper and tweezers, and pick up a clean strip of cover slip, holding it carefully by the edges.
- Gently set the cover slip right on top of the specimen slide, making sure to avoid creating air bubbles. Using a paper towel, wipe off any excess liquid.
- Carefully lower the prepared slide onto the microscope stage, and slowly adjust the position of the slide until the specimen is right beneath the aperture.
How to make a smear slide
For liquid specimens such as blood, a smear slide is often the most suitable option. This slide preparation technique makes use of two sets of flat microscope slides and cover slips, and takes a certain finesse to perfect.
Here are the steps on how to make a smear slide:
- Use a pipette to collect a small amount of the liquid specimen. Take a piece of microscope slide, and place a drop of the specimen onto the slide.
- Using a second piece of a glass slide, smear the specimen around the slide by dragging the smearing slide across the first.
- You can make a longer smear by smearing at a low angle, and a thicker smear by backing the smearing slide or using two drops of the specimen.
- Cover the smeared sample with a cover slip, ideally a rectangular one if a large portion of the slide is smeared with the sample.
- Remove any air bubbles and excess liquid, and place the slide on the microscope stage.
Other types of slide preparation
Aside from dry mounts, wet mounts, and smear slides, many other types of slide preparation techniques exist, such as squashing the specimen using a lens tissue, or staining the specimen with a staining solution.
These methods are designed to make the specimen as clearly visible and high contrast as possible, and allow us to see important details that may otherwise not be visible with just a simple specimen preparation method.
Therefore, knowing the right type of microscope slide preparation is important in studying a specimen correctly. But more than that, it’s also important to know which type of microscope slide you need depending on the specimen.
Bonus: Types of microscope slides
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When we think of microscope slides, we usually visualize a flat piece of glass. But there are actually several more types of microscope slides that are specially designed for specific purposes.
These slides vary both in appearance and composition. As a matter of fact, some of them are treated with reagents to make them more sterile and resistant to contamination, chemical reaction, and even plain water.
The most basic type of microscope slides are tiny rectangular pieces of clear glass or plastic made of soda lime or borosilicate, which measure at around 1 by 3 inches, and are roughly 1 millimeter thick. Flat slides need to be covered with a cover slip when using.
These slides typically have at least one side that is frosted as a means of orientation and sample identification, as well as to enable you to label and categorize prepared samples. These may also come in color, with rounded or clipped corners, and with beveled edges.
When working with flat slides, it’s a must to use a cover slip to protect the specimen from moving, leaking, getting contaminated, and coming into contact with the objective lens of the microscope. It’s a small piece typically made of clear square borosilicate or silicate glass.
There are two types of cover slips available depending on the level of magnification and resolution you need. Number 2 cover slips are for general use, while Number 1 slips are thinner and made for higher magnification microscopes.
On the other hand, concave slides are a special type of rectangular slide that has at least one circular depression to accommodate liquid solutions and larger specimens. These slides may be used with or without a cover slip, since the specimen is already safely secured within the concave part of the slide.
These are also typically more expensive than regular flat slides, and are slightly thicker, but roughly the same length and width as flat slides most of the time. Some concave slides feature dual concavity for performing a side by side analysis of two different samples without the risk of cross contamination.
Aside from these two general versions of microscope slides, there are many more slight but important variations available, all of which are designed to make the specimen analysis and microscopy process as efficient as possible.
Electrostatic charged slides
There are also special types of microscope slides such as electrostatic charged slides, which are used for certain specimens that are not able to adhere on a regular glass slide. These include several kinds of cells, tissues, organ samples, and even a variety of disease-causing elements such as cancer cells.
Etched grid/ graticule slides
Some flat slides may feature a grid-like etching system on one surface. The presence of these grids is helpful in allowing the viewer to better observe and monitor the specimen per section. The grids also make it easier to measure the size and scale of certain elements on the specimen, as well as make plotting and sketching more convenient.
Transparent mica slides
Another variation of the regular glass slide is the transparent mica, which is mainly used for small, dry, and rough particulates that need to be analyzed under a high magnification level. This is because these slides are designed to be glare, scratch, and dust resistant
When preparing a microscope slide, keep these things in mind:
- Always use the correct type of slide and slide preparation technique
- Make sure that the slide is clean and sterilized
- Perform any necessary specimen preparations beforehand
- Work slowly and carefully to avoid incidents
- Remember to clean, store, or dispose everything properly afterward