A methanol/acetic acid (100:0.1, v:v) solution infused at 5L/min was electrosprayed as the extraction solvent to generate the primary ions. The sample plume was directed to intersect the ESI plume for extractive ionization of the analytes. The analyte ions were then guided through the heated capillary of the LTQ mass spectrometer for mass analysis. The temperature of the heated capillary maintained at 300◦C. A positive ion detection mode was used for most experiments, with an ESI voltage of +4.0 kV. The pressure of nitrogen gas was set at 2.0 MPa. The default values of voltages for the heated capillary, ion optics, and the detectors were used without further optimization. For full MS scan, a mass range between 50 and 600m/z was scanned for each measurement. The mass spectra were recorded with an average time of 30 s and with background subtracted. Collision-induced dissociation (CID) experiments were preformed by applying 10–35% (arbitrary units defined by the LTQ instrument) of the collision energy to the precursor ions isolated
with a window width of 1.6 mass/charge (m/z) units.