A wet mount is the most fundamental technique for preparing a specimen for microscopic observation. This simple method involves suspending a sample, which may be a living organism or non-fixed tissue, in a drop of liquid mounting medium on a glass slide. The primary purpose is to observe specimens in an aqueous environment, which closely mimics their natural state. This approach is useful for studying dynamic biological processes, such as the motility of microorganisms like protozoa and algae in pond water. The wet mount is temporary, providing a clear, immediate view of an unfixed specimen without complex staining or drying procedures.
Essential Tools and Materials
Preparing a successful wet mount requires assembling specific items to create a sealed, thin viewing space. The foundation is a standard, clean microscope slide, a rectangular piece of glass providing the stable platform for the specimen. A cover slip is a smaller, very thin square of glass placed over the liquid sample. It serves multiple functions, including flattening the liquid drop to a uniform thickness, protecting the microscope’s objective lens from the liquid, and slowing the rate of evaporation.
The liquid used to suspend the sample is called the mounting medium, typically distilled water or a buffered solution like saline. Using these ensures the specimen remains hydrated and alive during observation. For samples not already in a liquid, such as a thin slice of onion skin or cotton fiber, a pipette or dropper is needed to precisely control the amount of medium added. Tools like fine-tipped tweezers or an inoculating loop are also helpful for carefully transferring the specimen onto the center of the slide before the liquid is applied.
Step-by-Step Guide to Preparation
The preparation of a wet mount begins with ensuring the glass slide is completely clean and free of dust, oils, or fingerprints, as any debris will interfere with image clarity. Once the slide is ready, the first step is to apply the specimen and the mounting medium to the center of the slide. If the sample is solid, such as a piece of leaf, it should be placed first, and then a single, small drop of the liquid medium, like distilled water, is added directly over it.
Controlling the size of this liquid drop is important, as the volume should be slightly smaller than the area of the cover slip. Using a pipette or dropper allows for the precise transfer of only one or two drops, which is typically sufficient. The most crucial part of the process is the application of the cover slip, which must minimize the introduction of trapped air.
To achieve this, the cover slip is held at an approximately 45-degree angle, with one edge touching the slide next to the drop of liquid. The cover slip is then gently and steadily lowered over the liquid, using a needle or toothpick to control the rate of descent. This controlled, angled approach causes the liquid to spread evenly beneath the glass, pushing air out and preventing the formation of air bubbles. Once the cover slip is fully lowered, the sample is sealed and ready for viewing. The final step involves using the corner of an absorbent paper towel to wick away any excess mounting medium that may have seeped out from under the edges of the cover slip.
Avoiding Common Wet Mount Errors
The most frequently encountered problem is the presence of air bubbles, which appear as dark-edged spheres under the microscope. These bubbles result from dropping the cover slip flat onto the liquid instead of using the 45-degree angled technique. If small bubbles persist, a gentle tap on the cover slip can sometimes encourage them to move to the edge and dissipate.
Another common issue relates to the volume of the mounting medium, which must be carefully balanced. If too much liquid is used, the cover slip will float, causing the sample to drift and making focusing difficult. Conversely, using too little medium leads to rapid drying, which can distort or kill live specimens quickly.
Rapid drying can be addressed by creating a makeshift seal. Applying a thin layer of clear nail polish or petroleum jelly around the cover slip perimeter can slow evaporation significantly. If the specimen is delicate, avoid pressing down too hard when wicking away excess fluid, as this can crush larger cells and distort their structure.