Question

Why do slides need to be mounted and which mounting solution is right for my slides?

Answer

The mounting solution is responsible for holding the sample in position during imaging and preserving it for long periods of time. It also reduces the amount of optical aberrations by making the refractive index of the environment surrounding the sample uniform with that of the objective used. Additionally, it contains anti-fade compounds that reduce the photobleaching rate of the fluorophores used.

The choice of mounting medium largely depends on the type of sample being examined. Numerous mounting solutions exist, which are generally based on organic solvents, such as toluene or xylene, or aqueous solutions. These are available ready-to-use or can be prepared in the laboratory. Aqueous solvents are the most commonly used for sample preparation in optical microscopy. They are divided into two classes depending on whether they solidify or not. The advantage of polymerizing mounting media is that they can be stored for extended periods, even at -20°C, without losing their ability to preserve existing stains. However, they solidify within a few days, which leads to sample crushing. Therefore, they are not the best choice for imaging purposes such as 3D reconstruction of the sample structure or protein colocalisation analysis. In these cases, samples should be mounted using a liquid solution, such as a glycerol-based one. While the 3D structure remains unaffected, the stains are preserved for much shorter periods, only few days. The edge of the coverslip must also be fixed to the slide using VALAP or nail polish.