An automatic variable laser attenuator for IRMPD spectroscopy and analysis of power-dependence in fragmentation spectra.

Infrared ion spectroscopy (IRIS) requires reproducibility and standard protocols in order to be fully appreciated as an analytical tool. An important parameter in IRIS measurements is the infrared laser power (energy per pulse), which is often difficult to control precisely on a day-to-day basis. We have developed an automatic variable attenuator to keep the laser power at a well-defined value, independent of IR frequency and time, while maintaining full overlap of the laser beam with the ion cloud in our quadrupole ion trap mass spectrometer. The device has been used to record IR spectra of the metabolite prostaglandin D2 at different laser pulse energies, allowing us to assess the effects on the IRMPD spectra of excessive ion depletion, a threshold pulse energy below which dissociation is not observed, and unobserved fragments due to electron detachment or fragmentation to below the low mass cut-off of the trap. Image 1 • IRMPD intensities are reproducible when an automatic variable laser attenuator is used to control the laser pulse energy. • IRMPD spectra are recorded as a function of laser pulse energy in a quadrupole ion trap mass spectrometer. • Effects on the IR spectra due to an elevated dissociation threshold and excessive ion depletion are discussed. • Effects due to electron detachment and the formation of fragment ions below the low mass cut-off of the MS are discussed. [ABSTRACT FROM AUTHOR]