Keywords: high resolution mass spectrometry Substances: 5-Fluoro Cytosine (XRECTZIEBJDKEO-UHFFFAOYSA-N) 3-Phenoxybenzoeacid (NXTDJHZGHOFSQG-UHFFFAOYSA-N) 1-Methyl-Benzotriazole (HXQHRUJXQJEGER-UHFFFAOYSA-N) 4-Hydroxy-Benzotriazole (NPZTUJOABDZTLV-UHFFFAOYSA-N)