High-Resolution MS (HRMS) for Quantitation of Oligonucleotides HRMS in full-scan mode provides selectivity and can improve the detection limit considerably. Therefore, HRMS is especially advantageous for the analysis of oligonucleotides. The method achieved a 5–10 ng/ml lower limit of quantitation (LLOQ) for double stranded oligonucleotides with an 18–22-mer oligonucleotide in each strand using TripleTOF mass spectrometer system. Materials Chemicals such as 1,1,1,3,3,3-hexafluoro-isopropanol (HFIP), diisopropylamine (DIPA), tetrahydrofuran, ammonium acetate and ammonium bicarbonate, were purchased from Sigma-Aldrich Inc. (MO, USA). HPLC grade methanol and acetonitrile were obtained from Fisher Scientific (PA, USA). Clarity OTX lysis buffer and SPE cartridges were obtained from Phenomenex Inc. (CA, USA). Preparation of stock & working solutions Oligonucleotide solution was prepared in deionized water to give a final concentration of 2 mg/ml. IS stock solution concentration wa