Adapting a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer for Gas-Phase Fluorescence Spectroscopy Measurement of Trapped Biomolecular Ions

Abstract

Gas-phase fluorescence spectroscopy is still in its infancy, which demands further instrumental developments. In this study, a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS), equipped with a lab-developed data acquisition system, was coupled to a tunable femtosecond laser and a state-of-the-art optical system for fluorescence studies of massselected ions. For excitation, a laser beam was focused (beam size < 1.0 mm) into the cylindrical ICR cell. A wire mesh replaced the back trapping plate, allowing ∼10% of the fluorescence emitted from trapped ions to be collected by a lens installed beside the wire mesh. The collected fluorescence light was then transmitted outside of the mass spectrometer via fiber optics. A novel accumulation during detection (ADD) scheme was developed to increase the duty cycle of gas-phase fluorescence spectroscopy experiments. With ADD, >90% duty cycle for mass spectrometry and fluorescence experiments could be achieved. This instrument was able to perform fluorescence experiments on various ions, from simple rhodamine dyes to large biomolecules (i.e., peptides and proteins) labeled with dyes of various optical properties. A fluorescence lifetime measurement of trapped rhodamine 6G cations was also performed, yielding a value of 5.97 ± 0.23 ns. This setup has a broad mass range and decent fluorescence spectroscopy performance (i.e., the emission spectrum of rhodamine 6G can be acquired with good S/N in a minute). Finally, this setup also allows more challenging gas-phase fluorescence spectroscopy experiments, for example, of low quantum yield fluorophores and large biomolecules in their native state that appear at high m/z, which may not be doable with quadrupole ion traps (QIT).

Robust Fluorescence Collection Module for Wide-Bore Ion Cyclotron Resonance Mass Spectrometers

“…However, these effects cannot account for the discrepancies in the literature. We were surprised at the unaddressed discrepancies between the maxima recently reported from the ETH Zurich ICR fluorescence setup and spectra reported previously by both our lab ,, and their own . The most recently reported spectral maxima were substantially red-shifted from literature values for rhodamine 110 (8 nm) and rhodamine R6G , (13 or 17 nm) and, most surprisingly, blue-shifted by 6 nm for rhodamine B .…”

“…Later, Zenobi and co-workers at ETH Zurich reported the dispersed emission spectrum of rhodamine 19 trapped in an ICR cell. 41 Very recently, the Zurich lab unveiled a setup with dramatically higher levels of fluorescence signal, 33 which allows a direct comparison of two slightly different functioning setups for fluorescence collection for ICR instruments.…”

“…The main difference is that our optical fiber goes inside the vacuum chamber, while theirs was exclusively outside. These two setups are different from the one reported by the Zenobi group who chose a radial excitation of the ions, combined with an axial collection of the fluorescence. , …”

“…These two setups are different from the one reported by the Zenobi group who chose a radial excitation of the ions, combined with an axial collection of the fluorescence. 32,33 Analytes and Experimental Sequence. Several cationic dyes and dye-labeled proteins were transferred to the gas phase via nanoelectrospray ionization (nanoESI) to test the fluorescence capabilities of this new experimental setup.…”

Cryogenic Fluorescence Spectroscopy of Ionic Fluorones in Gaseous and Condensed Phases: New Light on Their Intrinsic Photophysics

“…Efforts to develop gas-phase fluorescence experiments coupled with ESI or MALDI sources over the last two decades have been summarized in ref . These instruments are based on 3D Paul, − Penning, , or LQT (linear quadrupole trap) , ion traps that accumulate and localize the ions into an irradiation volume. While each type of trap has advantages, 3D Paul traps are simple and approximate a point-like trapping volume, making them easily adapted for fluorescence experiments.…”