Type 3A molecular sieves should be used to dry dehydration solvents for electron microscopy. Most common solvents (acetone, ethanol, and methanol, etc.) need to be anhydrous for electron microscopy embedding work using epoxy resins, yet they have a tendency to pick up atmospheric water when bottles are opened. Molecular sieves are used to dehydrate the solvents used in the final stages of dehydration and embedding . Molecular sieves are typically zeolite compounds that strongly adsorb water and have carefully controlled pore sizes. While both the solvent and the water will adsorb strongly to the molecular sieve surfaces, the large surface area within the pores is only accessible to the smaller water molecules, so they are effectively removed from the solvent. From the table below it can be seen that water (1.93A) will enter that 3A pore size while acetone (3.08A) will largely be excluded. Water will be able to occupy the large surface area inside the pores and thus be removed. If the solvent could also enter the pores, it would compete with water for the surface area and there would be little or no removal of the water from the bulk solvent. Type 4A molecular sieve is not suitable for drying ethanol, methanol, or acetone since the pore size does not exclude these solvents. The Advanced Specialty Gas Equipment catalog lists Type 3A for drying ethanol and methanol.
Running the solvent slowly through a column of the molecular sieves would be the most effective way to remove water but, ordinarily, dried sieves are just placed into the bulk solvent container (about 5%-10% by volume) to remove most water. The sieve material is ceramic, so be very careful to not stir up any fines when the solvent is withdrawn since it could eventually end up damaging sectioning knives.
Regenerate the molecular sieves at 250 C for 2 hr or more in a shallow layer. Place the container of hot, regenerated sieves on the porcelain plate of a glass desicator and place under vacuum while they cool. Put dry sieves into bottles with polyethelene cap liners to keep dry until needed. Type 3A molecular sieves will reduce water in an air stream to 0.001 mg/liter air. Residual moisture is probably higher for sieves dumped into bulk solvent containers, but functionally it is sufficiently effective in avoiding water contamination problems.
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