2.2 Mass Spectrometry
The high-resolution ESI-MS and subsequent high-resolution ESI-MS/MS experiments were performed on a Q Exactive HF Orbitrap mass spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) equipped with a standard ESI interface. Nitrogen was used as the sheath gas, auxiliary gas and collision gas. The basic ESI conditions were: spray voltage, 3500 V; capillary temperature, 275 °C; probe heater temperature, 300 °C; sheath gas flow rate, 35 arbitrary units; auxiliary gas flow rate, 10 arbitrary units; while the selected collision energy depended on the dissociation capability of the precursor ions. Data acquisition and analysis were carried out with the Xcalibur software package (version 4.2.47, Thermo Scientific).