Cleaning of Microscope Optics

Clean microscope optics are a prerequisite for successful microscopy and perfect images. In order to protect our precious equipment, it requires a regular cleaning schedule to ensure that the images remain crisp and artifact free. 

Our goal is to completely remove dust and dirt without leaving any residue of the cleaning agent or damaging the optical surfaces. Which image quality do you expect? Let’s start with some considerations before starting the cleaning procedure.

 1.      Optimize all microscope settings: Aperture, K?hler, correction ring, etc. 

2.      Check your specimen 

Use standard with proven quality

Use cleaned specimen 

3.      Check slide and cover glass

4.      Check immersion medium

Same type as embedding medium of the specimen

Is immersion medium from the same lot?

Only use recommended types

No Anisol

5.      Prepare your Tools

The following equipment is required:

– Long, thin wooden sticks, preferably of bamboo (obtainable from Chinese restaurant suppliers) or a comparable not too flexible material.

– High purity cotton (e.g. that used in ophthalmology supplied by KERMA, Germany) or WHATMAN lens cleaning tissue 105.

– Absorbent polyester swabs for cleaning optical components. ITW Texwipe CleanTipsR swabs (TexWipe) represent a very good alternative to the cotton swabs and can be re-used.

– Soft cosmetic cellulose tissue (e.g. KimWipes soft, KLEENEX).

– Dust blower (laboratory suppliers, pharmacies).

– Distilled or demineralized water.

– Freshly prepared solution of 5 –10 drops of a washing-up liquid (e.g. Fairy Ultra, Fit) in 10 ml distilled water.

– Solvent for the removal of greasy or oily dirt, such as the Optical Cleaning Solution L (recipe from Carl Zeiss), pure petroleum ether (analytically pure, boiling point <44oC) or, exclusively for cleaning coverslips, pure acetone. Not all solutions can be recommended to clean the optics of a microscope. Some clean very effectively, but are either toxic (e.g. chloroform, acetone) or environmentally unfriendly (e.g. Freon, tetrahydrochloride); others will leave surface residue (e.g. xylene, toluene, diethylether). Residue forms particularly with the use of xylene and absolute ethanol, and above all when the dirt contains water-soluble components.

Preparation of cotton swabs

Wash hands (powdered, latex gloves are not suitable).

■ Dip the stick into the cleaning solution (aqueous or organic solvent). The cotton fibers attach better to the stick as a result.

■ Dab the stick onto the wad of cotton and loosen some fibers. Do not compact the cotton otherwise, the fibers will not separate easily.

■ Turn the stick so that an even, elliptical cotton bud forms at the tip.

■ Remove the cotton tip after every wipe and replace it with a fresh cotton bud.

■ The stick can be used for a long period of time. Use separate sticks for water-based solutions and organic solvents.

■ To protect the cotton tip from dirt, the stick should be stored in a polythene bag. It should not be handled as perspiration and grease, from the fingers of the users, will significantly affect its ability to clean.

What to Watch Out for When Cleaning Microscope Optics!

? When starting to clean, don’t forget to use a dust blower except when fluids (such as immersion oil) are to be removed.

? Never wipe lenses with dry swabs or tissue – this causes scratches!

? Do not use abrasive materials e.g. leather wipes, dry linen cloths or polystyrene sticks as recommended by some manufacturers.

? Do not apply any solvents before trying distilled water (a film of distilled water can be generated by breathing on the surface), except when grease is to be removed.

? Do not use ethanol or acetone for the cleaning of very old microscopes.

? Do not use any disposable cotton swabs ( e.g. Q-Tip) instead of the described cotton or ITW Texwipe CleanTips swabs, as the former are not free from contamination.

? Beginners should not use any of the occasionally recommended metal rods instead of the wooden (bamboo) sticks, as the front lenses may be more easily damaged.

? Do not use any of the optical spray cans containing pressurized liquid air. The pressurized air from these sprays leaves a slight, but difficult to remove, residue.

? Never use acids or ammonia to clean objective front lenses.

? Never try to clean the internal optical surfaces, cameras or adaptor optics

? Never use ammonium containing glass cleaner like Sidolin, Sparkle, etc.

? All tools should be absolutely clean

 

Where to clean and where not

·        Where to clean

Front lens of objective

Cover glass of camera sensor

Coverslip and specimen slide

C-mount

Condenser lens

Misc. glass surfaces

·        Where not to clean

Filter Cubes

All inside optics of a microscope

How to locate & Recognize dirt

If the image sharpness or contrast is not optimal, then there is a high probability that the microscope optics are not clean.

In order to determine the location of the dirt, please proceed as follows:

·        Carefully rotate objectives a small amount within their thread.

·        Check the slide and coverslip by moving the specimen while focusing initially on the upper and then the lower surfaces.

·        Check the condenser while moving it up and down and if applicable, by swiveling the front lens slightly.

The affected optical surface is identified when a suspected optical component is moved and the dirt follows this movement.

·        Carefully rotate cameras a small amount within their thread.

Dirt located within the camera will not move when the camera is moved!

·        Control the relevant elements macroscopically

A macroscopic check for larger dust particles and scratches on optical surfaces can be carried out using either a magnifying glass (with a magnification of 3 –6x) or an eyepiece held the wrong way round.

N.b. A soiled objective front lens is easily determined by looking at an evenly lit surface through the wrong end of the objective.

 The final check should always involve an assessment of the achieved improvement in image quality.

Different kinds of dirt

·        Loosely or not permanently attached dirt

Glass (broken slides, coverslips, etc.)

Skin, dander

Pollen, etc.

·        Attached dirt

Water-soluble e.g. coffee, Coke, etc.

Solvent soluble e.g. immersion oils, fingerprints

Unhardened paints, cements

In practice often a mixture

How to clean

1.      Blow all loose dust particles away with a dust blower.

2.      Remove all water-soluble dirt with distilled water. If this is unsuccessful repeat using a solution of diluted washing-up liquid. Remove any remaining residue with a dry cotton swab, but breathe on the surface first to generate a film of moisture. In so doing, be careful not spray droplets of saliva.

3.      To remove oily dirt, use a solution of dilute washing-up liquid initially. If this does not produce a satisfactory result, repeat the cleaning using a solvent (Optical Cleaning Solution L, petroleum ether).

4.      Greasy dirt must always be removed using a solvent.

6.      After cleaning check the surface (see the section “How to locate & recognize dirt”).

Place the objectives, eyepieces, and cameras on a dust-free surface (e.g. fresh aluminum foil). All other optical components to be cleaned should be as accessible as possible.

Dip the cotton swab or the ITW Texwipe CleanTips swab into the cleaning solution and shake off excess liquid.

The solvent should remove as much dirt as possible. In order to increase the retention time of volatile organic solvents in the cotton bud, some users chill the solvent (–10oC to –20oC). A more suitable and recommended way to improve the retention time of a solvent is to add isopropanol, for example.

Cleaning is achieved using a spiral motion from the center to the rim. Never wipe using zig-zag movements as this will only spread the dirt.

With larger optical surfaces (e.g. tube lenses) the spiral motion starts initially at the rim before moving to the middle and is only then followed by a center to rim cleaning motion.

Normally several spiral wipes are recommended.

I recommend pure, volatile petroleum ether or the Optical Cleaning Solution L from Carl Zeiss.

Acetone can be recommended when oil and grease are to be removed from coverslips. Acetone attacks most types of plastic as well as rubber, and as a consequence, its use to clean eyepieces can be problematic, for example. It cannot be excluded that acetone may attack cemented optical components (e.g. objectives, TV-adapters, eyepieces) when used frequently.

Acetone can also dissolve specialized organic coatings.

References:

• The Clean Microscope Dr. Michael Z?lffel Carl Zeiss MicroImaging GmbH · Carl-Zeiss-Promenade 10 · 07745 Jena · Germany
• Cleaning of Microscope Optics flyer from Leica
• From A to Z - Introduction to Your Microscope (MicroscopeMaster Series Book 1) H M Anderson