Characterization of harmonics and multi-charged peaks obtained by Fourier transform ion cyclotron resonance mass spectrometry

The differences between harmonics and multi-charged peaks in mass spectrometry are often not obvious. This work conducts experiments using both electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources with Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) to investigate the difference between harmonics and multi-charged peaks for the first time. In particular, organometallic compounds, which were characterized by the isotope distribution in ESI, were investigated in detail. A comparison of the peaks of the three charges with the three frequency doubling results at high-resolution mass spectra demonstrated that these peaks may be clearly differentiated from one another. Fullerene (C-60) was characterized using APCI in the negative mode by FTICR MS provided further evidence. In addition, harmonics are unavoidable, but can be relatively weakened. These results are helpful to obtain accurate and comprehensive spectral information from FTICR MS.