Mass spectra were acquired in a mass range from m/z 50 to 2'000 at ca. 20'000 resolution (m/z 622) and at 1.0 Hz rate. The mass analyzer was calibrated between m/z 118 and 2'721 using an Agilent ESI-L low-concentration tuning mix solution (Agilent, USA) at a resolution of 20,000, giving a mass accuracy below 2 ppm. All solvents used were purchased in the best LC-MS quality.