Provided are improved processes and apparatuses for solid phase oligonucleotide synthesis, the improvements comprising carrying out detritylation of the nascent oligonucleotide using a composition of an organic solvent, a protic solvent and a selected concentration, or selected concentration range, of water. In some embodiments, an oligonucleotide synthesis apparatus includes means for adding water to a detritylation reagent. In some embodiments, an apparatus can include a water detector for analyzing the water concentration of a detritylation reagent to be reacted with a nascent oligonucleotide. An apparatus can comprise a feedback loop to control the concentration of water at the point of the detritylation reaction and/or to control the detritylation reaction time. The apparatuses and methods reduce batch-to-batch variations in the manufacture of oligonucleotides immobilized on the surface of various substrates.