As published in the Journal of the American Society for Mass Spectrometry, researchers in the Glish Group used a focused laser to make infrared multiphoton photodissociation (IRMPD) more efficient in a quadrupole ion trap mass spectrometer. Efficient (up to 100%) dissociation at the standard operating pressure of 1 × 10−3 Torr can be achieved without any supplemental ion activation and with shorter irradiation times. The axial amplitudes of trapped ion clouds are measured using laser tomography.
Laser flux on the ion cloud is increased six times by focusing the laser so that the beam waist approximates the ion cloud size. Unmodified peptide ions from 200 Da to 3 kDa are completely dissociated in 2.5–10 ms at a bath gas pressure of 3.3 × 10−4 Torr and in 3–25 ms at 1.0 × 10−3 Torr. Sequential dissociation of product ions is increased by focusing the laser and by operating at an increased bath gas pressure to minimize the size of the ion cloud.
Lasers have been used as a tool for the photodissociation of ions in mass spectrometers for at least 35 years. Trapping instruments such as quadrupole ion trap mass spectrometers (QITMS) in particular are advantageous for photodissociation because they serve to hold the ions in the laser path, allowing multiphoton processes such as infrared multiphoton dissociation (IRMPD). As published in the Journal of Physical Chemistry, researchers in the Glish Group used IRMPD combined with ion trajectory simulations to obtain probability maps of ion position as a function of different operating parameters in a quadrupole ion trap mass spectrometer. The factors that contribute to the depth of the pseudopotential trapping well are analyzed, and their effects on the efficiency of IRMPD are demonstrated. Ion trajectory simulations are used to substantiate experimental results and demonstrate in greater detail the dynamic nature of the ion population's positional distribution.
In particular, it is shown that the so-called "qz value" used during photodissociation can be of great consequence, as can the frequency of ac trapping voltage applied to the ring electrode. The results reveal that parameters which increase the pseudopotential well have the effect of decreasing the size of the ion cloud and maximizing overlap between the irradiating laser and the ions. Thus, while the common understanding of IRMPD dictates otherwise, IRMPD fragmentation efficiencies really depend on many ion trap operating parameters, much as collision-induced dissociation does.