Characterization and Manipulation of Ion Clouds Inside an FT-ICR Instrument: A Laser Tomography Perspective

The efficiency of many tandem mass spectrometry (MS/MS) techniques depends on the position of the trapped ion cloud and its overlap with a beam of photons or electrons. Here, we report photoactivated tomography profiles measured in a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer to characterize the position and radial extent of the ion cloud. Tomography profiles generated by measuring single-photon processes, namely, fluorescence and ultraviolet photodissociation (UVPD), produced estimates of ion cloud full width at half-maximum (FWHM) of at least 1.7 mm under the experimental conditions employed. The effects of vertical and horizontal beam steering ion transfer optics were characterized using visible light photodissociation (VisPD) tomography to monitor ion cloud centroids, FWHM of the fitted tomography profiles, and maximum photodissociation levels. A robust alignment procedure is described to ensure proper overlap of the laser beam with the ion cloud. The ion cloud behavior characterized in this laser tomography study has made it possible for users to better control the ion cloud prior to optical excitation, thereby increasing the efficiency of in-cell MS/MS processes such as photodissociation and fluorescence. This method for rapid determination of the position of the ion cloud can be implemented on other types of mass spectrometers, which will be beneficial for photoactivation experiments.