Increased Fragmentation Efficiency of Ions in a Low Pressure Linear Ion Trap with an Added dc Octopole Field

In-trap fragmentation of ions in a hybrid linear ion trap triple quadrupole mass spectrometer occurs at pressures about 5e-5 torr. At these low pressures, efficient fragmentation of heavy ions (such as the singly charged homogenously substituted triazatriphosphorine of mass 2721.89 Da) can take a long time because of the relatively low collision frequency with the background gas and the high internal energy content required to produce fragmentation. Increasing the amplitude used for dipolar excitation leads to loss of the ion upon the quadrupole rods. In the work presented here, the addition of a dc octopolar field to a linear ion trap is described. The dc octopolar field was created by the addition of four auxiliary electrodes situated between the quadrupole rods at a distance of 10 mm from the axis. The inclusion of the dc octopolar field was shown to cause the ions’ frequency of motion to shift out of phase with the excitation signal at high radial amplitudes. This resulted in beat-like trajectories with periods of excitation and de-excitation as the ions’ frequency of motion shifted in and out of phase with the excitation signal. This led to a reduction in the loss of ions on the quadrupole rods during the excitation process. The result is an increased fragmentation efficiency relative to the fragmentation efficiency obtained when using an LIT constructed of round rods only. The inclusion of the dc octopolar field allowed the ion to be fragmented more efficiently in a relatively short excitation period

Resonant excitation in a low-pressure linear ion trap

AbstractIt has been shown that through the process of resonant excitation the fragmentation of ions confined in a low-pressure (<0.05 mTorr) linear ion trap (LIT) can be accomplished while maintaining both high fragmentation efficiency and high resolution of excitation. The ion reserpine, 609.23 Da, has been fragmented with efficiencies greater than 90% while a higher mass ion, a homogeneously substituted triazatriphosphorine of mass 2721.89 Da, has been fragmented with 48% efficiency. This was accomplished by extended resonant excitation by low-amplitude auxiliary RF signals. Computer modelling of ion trajectories and analysis of the trapping potentials have demonstrated that a reduction in neutralization of ions on the rods (or losses on the rods) and increased fragmentation is a consequence of higher order terms in the potential introduced by the round-rod geometry of the LIT