MicrobeHunter.com Microscopy Forum

Perhaps this is common knowledge but to me it came as a surprise so I thought I'd share.

I tried to make permanent slides of diatoms I cleaned in bleach as proposed here:
https://www.jstor.org/stable/3226387?se ... b_contents

I used nail polish mounting medium (NPM) because I like using house hold stuff if possible. I put the water containing diatoms (and some dirt, to be honest) on the cover slip, let it dry and dropped it in a drop of NPM. Nice! The frustules looked good and I put the slide aside. A few days later (I think about a week) I took the slide to get my dose of diatoms.

Much to my surprise I could not find a single diatom. Everything I didn't want in the first place was clearly visible but not one diatom!

A number of weeks later I started finding some frustules, but just a few. They're not broken but look ' distorted'. They only show if a bubble forms (must be gas formed by solvents) and don't show the finer features, just the outline and often only vaguely so.

My best guess is that the refractive index of the frustules in relation to the NPM plays a role but I cannot be completely sure.

Can someone shine some light on this and any ideas on ' household' mounting media for permanent slides?

peytr wrote:Perhaps this is common knowledge but to me it came as a surprise so I thought I'd share.

I tried to make permanent slides of diatoms I cleaned in bleach as proposed here:
https://www.jstor.org/stable/3226387?se ... b_contents

I used nail polish mounting medium (NPM) because I like using house hold stuff if possible. I put the water containing diatoms (and some dirt, to be honest) on the cover slip, let it dry and dropped it in a drop of NPM. Nice! The frustules looked good and I put the slide aside. A few days later (I think about a week) I took the slide to get my dose of diatoms.

Much to my surprise I could not find a single diatom. Everything I didn't want in the first place was clearly visible but not one diatom!

A number of weeks later I started finding some frustules, but just a few. They're not broken but look ' distorted'. They only show if a bubble forms (must be gas formed by solvents) and don't show the finer features, just the outline and often only vaguely so.

My best guess is that the refractive index of the frustules in relation to the NPM plays a role but I cannot be completely sure.

Can someone shine some light on this and any ideas on ' household' mounting media for permanent slides?

Hi Petyr,

I think you may be right about the refractive index.. Here is some talk about it.. There are some "special" mountants for diatoms..

http://www.mikrohamburg.de/Tips/TE_Mountingmedia.html

BillT

Hi Peytr,

the high refractive index of these mountants comes from the chemical properties of some of the ingredients. Such stuff is not found in household materials. Its mostly harmful and difficult and dangerous to make.
Whether you really need a high r.i. also depends on the diatoms you have. For fine diatoms you need a high r.i. for good visibility.

Early diatomists used air as mountant and had a good visibility. Here the high r.i. difference comes from the low value of the air. The downside is, that you are limited to objectives of n.A. 1 and below. This might be the easiest start for you.

Regards,

Bob

Hi Peytr
One liquid that is not a "harmful chemical" by definition is Cinnamon bark oil. It has a refractive index of about 1.55 and to my eyes appears to be a good mountant for diatoms. It is available from natural products stores/suppliers. So the price is not prohibitive and it is freely marketed. You can use it "as is" - no objectionable solvents to add.

However it does have some drawbacks:
1. It is a non-drying liquid that contains volatile constituents. So it is not a permanent mountant.
2. Some of its constituents dissolve in plastics and polymers. So, if you mount the diatoms in Cinnamon oil and seal around the coverslip with nail-polish, for example, the oil will gradually evaporate, migrate to the (solid) nail polish ring, dissolve in it and pass through, so the diatoms are left within air bubbles.
3. The oil delivers a strong scent of Cinnamon (naturally...) which may be a nuisance to some.

The plastic material PS - polystyrene (styrofoam) is quite a "household" stuff. Its refractive index is 1.6 - fairly high. In theory, a thick solution of PS in Cinnamon oil might be used to create a thin transparent PS film after most or all of the oil has evaporated. Whether this film will sufficiently adhere to the slide and coverslip so that a permanent sandwitch will be formed remains to be seen... a PS film is brittle - if a plasticizer will be needed to make it flexible, the formulation might be hard to optimize...

I hope to update the readers on the performance of this PS-oil solution in due time.