The cleavage of the glycosidic bond in purine nucleotides, depurination, can occur due to the strongly acidic conditions needed to remove the DMTr group , which limits maximum oligonucleotide length in standard phosphoramidite chemistry, but not in the photolithographic approach, as the photodeprotection takes place under the mild conditions of the exposure solvent. Photolysis occurs in the triplet state via a photoinduced βelimination pathway promoted by a weak organic base , and for this we use an exposure solvent consisting of 1% (m/v) imidazole in dimethyl sulfoxide (DMSO). DMSO additionally aids in error reduction by matching the index of refraction of quartz, preventing an unwanted reflection as the light exits the flow chamber into the quartz block (Figure ). Like DMTr deblocking, photodeprotection follows first order kinetics and we have determined the irradiance required for 95% removal of NPPOC, Bz-NPPOC and SPh-NPPOC, with the differences based on their respective molar absorptivity and photolysis quantum yield (Table ) .