The principle of the Glen Research gel filtration column, Glen Gel-Pak™ , is based on size exclusion chromatography that separates molecules according to the hydrodynamic volume of the molecule in aqueous solutions. In gel filtration, the mobile phase for size exclusion is an aqueous solution and the stationary phase is a porous resin. The pores of the resin are sized such that they allow small molecules to enter the pores, yet exclude larger molecules from the pores. The small molecules, such as salts and hydrolyzed protecting groups, diffuse into the pores of the resin and move slowly through the column. The larger molecules, such as DNA or proteins, are excluded from the pores and move quickly through the column. The end result is that the larger molecules elute first in the column void volume while the small molecules are still flowing through The resin of the column. Glen Gel-Pak columns are ideal for desalting and reaction clean up. They can be used for removal of the ammonium hydroxide deprotection solution and hydrolyzed protecting groups after deprotection. The columns can also be used for the clean up of NHS-labelling reactions to separate the labelled oligo and unlabelled oligo from the unreacted NHS ester, the hydrolyzed label, and n-hydroxysuccinimide, thereby greatly simplifying the downstream purification steps.