The invention is a process for the preparation of at least one nucleic acid duplex by the steps of: (a) synthesizing a first single strand oligonucleotide to produce a crude solution of the first single strand oligonucleotide; (b) purifying the first single strand oligonucleotide from the crude solution by first liquid chromatography to produce a solution of purified first single strand oligonucleotide in a first liquid chromatography eluant; (c) synthesizing a second single strand oligonucleotide to produce a crude solution of the second single strand oligonucleotide; (d) purifying the second single strand oligonucleotide from the crude solution by second liquid chromatography to produce a solution of purified second single strand oligonucleotide in a second liquid chromatography eluant; (e) mixing the solution of purified first single strand oligonucleotide in the first liquid chromatography eluant with the solution of purified second single strand oligonucleotide in the second liquid chromatography eluant so that the first and second single strand oligonucleotides can complex to form a nucleic acid duplex in the mixture of first and second liquid chromatography eluants.