Electrospray Ionization (ESI) and Mass Spectrometry (MS)
ESI and nano-ESI are ‘soft’ ionization methods, capable of generating intact gas- phase macromolecular ions from solution. If the ESI process is gentle enough, ions are not completely desolvated and clusters containing analyte complexed with multiple solvent molecules can be formed.
We use two trapping mass spectrometers in our lab: an ion cyclotron resonance mass spectrometer (ICR-MS, APEX-Qe, 7 Tesla magnet, Bruker Daltonics) and a quadrupole ion trap (QIT, Esquire 3000+, Bruker Daltonics). Both are equipped with nano-ESI sources. In these instruments, gas-phase ions are trapped in a relatively small volume where they can be stored from minutes (QIT) to days (ICR). Isolation waveforms eject unwanted ions from the storage cell. The mass-selected ions of interest are then probed using a variety of techniques. We employ traditional probes, such as collion-induced dissociation (CID), electron capture dissociation (ECD), and ion/molecule reactions (e.g. gas-phase hydrogen/ deuterium exchange). We have also developed a different probe of ion structure: fluorescence spectroscopy, photodissociation, and any other laser-induced photophysical phenomena (see Research).
We have implemented an optical spectroscopy interface for our QIT mass spectrometer. With this setup, any light source can be sent inside the trapping region of the QIT to interact with the ions. We use one of two light sources to excite the trapped ions: a small CW laser (World Star Tech), which puts out up to 30 mW at 532nm, or a tunable femtosecond Ti:Sapphire Laser system (TSUNAMI from Spectra Physics).
Our optical spectroscopy set-up for the QIT also includes two optical detectors for detecting fluorescence from trapped ions: we use an electron-multiplying CCD camera coupled to a spectrograph (Newton and Shamrock from ANDOR Technologies) to measure steady-state fluorescence spectra, and a single-photon avalanche photo-diode (SPAD) to measure time-resolved fluorescence decays by time-correlated single-photon counting techniques (TCSPC).