Generation and detection of coherent magnetron motion in Fourier transform ion cyclotron resonance mass spectrometry

摘要

One of the most useful recent ion cyclotron resonance (ICR) developments is the conversion of magnetron motion to cyclotron motion by azimuthal quadrupolar excitation in the presence of ionhyphen;neutral collisions. The technique offers a masshyphen;selective means for lsquo;lsquo;shrinkhyphen;wrappingrsquo;rsquo; an ion cloud into a tight packet along the central axis of an ICR ion trap for enhanced signal to noise ratio, mass resolving power, and other advantages. However, the process itself is not directly observable. In this paper, we show that the conversion may be rendered observable by converting coherent magnetron motion (produced by offhyphen;axis ionization during a period short compared to the magnetron frequency) to coherent cyclotron motion, followed by subsequent dipole detection at ohgr;+(reduced cyclotron frequency) or quadrupolar detection at ohgr;c(unperturbed cyclotron frequency) and 2ohgr;+. Detection at ohgr;celiminates the ICR frequency shift due to the electrostatic trapping potential, providing for increased mass accuracy; detection at 2ohgr;+may offer increased mass resolving power. The observed signal behavior as a function of excitation amplitudendash;duration product is predicted theoretically and confirmed experimentally for both types of detection. Unlike the otherwise analogous 180deg; pulse in nuclear magnetic resonance (NMR), the magnetronhyphen;tohyphen;cyclotron interconversion may be phasehyphen;coherent with respect to both initial and final states. Finally, we show how coherent magnetron motion of two ion packets of different magnetron phase can be converted to cyclotron motion of two ion packets of different cyclotron phase, and we discuss the implications of that process. thinsp;