After purification, the DMT-nucleosides are phosphitylated at the 3′-position using 2-cyanoethyl diisopropylaminophosphorochloridite in the presence of the non-nucleophilic base diisopropylethylamine (DIPEA; Figure 21), to give phosphoramidite monomers. After silica gel column chromatography, precipitation into hexane, and filtration through a microfilter, the phosphoramidite monomers are ready to use in oligonucleotide synthesis.