Provided is a method comprising introducing a sample comprising a plurality of oligonucleotides into an ion pair high performance liquid chromatography column having a buffered mobile phase and allowing at least a portion of the oligonucleotides to separate; allowing the oligonucleotides to elute from the column; and introducing the oligonucleotides into a mass spectrometer and quantifying at least a portion of the oligonucleotides by mass spectrometry. In the method, at least a portion of the oligonucleotides are co-eluting oligonucleotides that differ in mass by no more than 20%; and the buffered mobile phase causes at least 50 mole percent of co-eluting oligonucleotides to have the same charge when they enter the mass spectrometer.