Ultrahigh-resolution Fourier transform mass spectrometry of biomolecules above m/z 5000

Ions with a mass-to-charge ratio (m/z) greater than 5 000 have been made by matrix-assisted laser desorption/ionization (MALDI) and detected at high mass resolving power by Fourier transform mass spectrometry (FTMS). The FTMS instrument used for the investigations has a MALDI source mounted outside of the magnetic field in a separate, differentially-pumped chamber. Ions were extracted from the source and transported efficiently to the FTMS analyzer cell by a r.f.-only quadrupole ion guide. To optimize the mass resolution for high mass ions, the operating parameters of the instrument were varied systematically. It was found that the parameters for formation of ions in the MALDI source, such as laser irradiance and amount of sample ablated, do not affect the mass resolution significantly. The biggest effects resulted from changing the ion detection parameters in the FTMS analyzer cell, in particular the trapping voltage, excitation r.f. level, background pressure, and number of ions stored. With optimal tuning, the external ion source FTMS method gave a mass resolving power of M/Delta M(1/2)= 830 000 for human insulin at m/z 5 807 and 81 000 for cytochrome C at m/z 12 360.