40 years of Fourier transform ion cyclotron resonance mass spectrometry

Abstract

This article reviews the development of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in several respects: (a) a strong static magnetic field serves to convert ion mass-to-charge ratio into cyclotron frequency. Because frequency is the most accurately measurable property, ICR MS inherently offers higher mass resolution and mass accuracy than any other mass analyzer. (b) Coherent excitation followed by induced charge detection yields a time-domain signal whose discrete Fourier transform produces a mass spectrum of ions spanning a wide m/z range simultaneously. By simple analogy to weighing objects with a mechanical balance, that “multiplex” advantage can be shown to be equivalent to “multichannel” detection by an array of individual single-channel detectors. (c) FT-ICR MS performance benefits from near-elimination of magnetic field inhomogeneity by inherent ion cyclotron rotation and ion axial oscillation; inherent nearly quadrupolar electrostatic trapping potential and nearly uniform rf electric field homogeneity near the center of the ICR ion trap (both improved even further by recent ICR cell designs); and theoretically optimal excitation and mass selection produced by stored-waveform inverse Fourier transformation (SWIFT). (d) External ion accumulation allows efficient coupling of atmospheric pressure continuous ionization sources (notably electrospray ionization) with pulsed high-vacuum FT-ICR MS excitation/detection, and injection of externally trapped ions through the “magnetic mirror” into the ICR ion trap has been optimized based on ion trajectory simulations. (e) MS/MS can be performed either inside (e.g., electron capture dissociation, infrared multiphoton dissociation) or outside (e.g., collision-induced dissociation, electron transfer dissociation) the ICR ion trap. (f) Finally, FT-ICR MS instrumentation and experimental event sequences have benefited from striking parallels to prior nuclear magnetic resonance spectroscopy developments. Similarly, non-ICR FT MS development (notably the orbitrap) has benefited from FT-ICR precedents.