Adapting a Fourier Transform Ion Cyclotron Resonance
Mass Spectrometer for Gas-Phase Fluorescence Spectroscopy Measurement
of Trapped Biomolecular Ions
posted on 2021-11-16, 20:11authored byRi Wu, Jonas B. Metternich, Prince Tiwari, Renato Zenobi
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 mass-selected 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).