Selected-ion accumulation from an external electrospray ionization source with a fourier-transform ion cyclotron resonance mass spectrometer

The coupling of an electrospray ionization (ESI) source to a Fourier‐transform ion cyclotron resonance (FTICR) mass spectrometer provides a facility for the high resolution, accurate mass analysis of large biopolymers. Typically the m/z range of the electrosprayed ions injected and trapped in the FTICR cell is between m/z 500 and 2500 (but can vary considerably with solution conditions). Recent reports on quadrupole excitation have demonstrated the ability to cool the magnetron motion of ions trapped in the FTICR mass spectrometer cell by converting this motion to cyclotron motion which, under appropriate pressure conditions, damps readily to the center of the cell. The use of a broadband waveform (swept frequency or stored waveform inverse Fourier‐transform) for the quadrupole cooling pulse was shown to provide cooling of a wide range of m/z ions, while implementation of a single frequency demonstrated a much narrower m/z response. This report demonstrates the successful combination of single‐frequency quadrupole cooling with external injection of electrosprayed ions into an FTICR mass spectrometer for m/z selected‐ion accumulation. This capability is particularly significant with electrospray ionization since it allows ions of only one charge state to be accumulated in the cell, and greatly increases the potential dynamic range of ESI‐FTICR.