Tandem mass spectrometry using continuous-wave infrared multiphoton dissociation in an electrostatic linear ion trap

The electrostatic linear ion trap (ELIT) can be operated as a multi-reflection time-of-flight (MR-TOF) or Fourier transform (FT) mass analyzer. It has been shown to be capable of performing high-resolution mass analysis and high-resolution ion isolations. Although it has been used in charge-detection mass spectrometry (CDMS), it has not been widely used as a conventional mass spectrometer for ensemble measurements of ions, or for tandem mass spectrometer. The advantages of tandem mass spectrometer with high-resolution ion isolations in the ELIT have thus not been fully exploited.

A homebuilt ELIT was modified with BaF₂ viewports to facilitate transmission of a laser beam at the turnaround point of the second ion mirror in the ELIT. Fragmentation that occurs at the turnaround point of these ion mirrors should result in minimal energy partitioning due to the low kinetic energy of ions at these points. The laser was allowed to irradiate ions for a period of many oscillations in the ELIT.

Due to the low energy absorption of gas-phase ions during each oscillation in the ELIT, fragmentation was found to occur over a range of oscillations in the ELIT generating a homogeneous ion beam. A mirror-switching pulse is shown to create time-varying perturbations in this beam that oscillate at the fragment ion characteristic frequencies and generate a time-domain signal. This was found to recover FT signal for protonated pYGGFL and pSGGFL precursor ions.

Fragmentation at the turnaround point of an ELIT by continuous-wave infrared multiphoton dissociation (cw-IRMPD) is demonstrated. In cases where laser power absorption is low and fragmentation occurs over many laps, a mirror-switching pulse may be used to recover varying time-domain signal. The combination of laser activation at the turnaround points and mirror-switching isolation allows for tandem MS in the ELIT.