Molecular sieves consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules, such as nitrogen and water can diffuse. A molecular sieve is capable of separating molecules on the basis of size. There are many types of molecular sieves according their applications. Like 3a, 4a, pentasils, mordenite, zeolite etc. The type 3a molecular sieve is used to dry dehydration solvents for electron microscopy. Most common solvents include acetone, ethanol, and methanol, etc.

The type 3a molecular sieve is 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. Type 4a molecular sieve is not suitable for drying ethanol, methanol, or acetone since the pore size does not exclude these solvents.

Type 5a of this kind is used for separating normal straight chain hydrocarbons from branched hydrocarbons. Type 13 is another molecular sieve of highest theoretical capacity adsorbent and very good mass transfer rates. It can remove impurities in large to fit into a type a crystal and is commonly used to separate nitrogen from oxygen. Type y molecular sieve can adsorb hydrophobic molecules in the presence of water and has mild acid resistance. Pentasils is a high silica/alumina ratio crystal that can remove organics from gas and liquid streams containing water. Mordenite is a highly acid resistance absorbent.