Biomedical subjects

F A Londry

Publications and source records attributed to F A Londry.

3 recordsLinked to original sources

PubMed2003

Resonant excitation in a low-pressure linear ion trap.

It 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.

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PubMed2003

Mass selective axial ion ejection from a linear quadrupole ion trap.

The electric fields responsible for mass-selective axial ejection (MSAE) of ions trapped in a linear quadrupole ion trap have been studied using a combination of analytic theory and computer modeling. Axial ejection occurs as a consequence of the trapped ions' radial motion, which is characterized by extrema that are phase-synchronous with the local RF potential. As a result, the net axial electric field experienced by ions in the fringe region, over one RF cycle, is positive. This axial field depends strongly on both the axial and radial ion coordinates. The superposition of a repulsive potential applied to an exit lens with the diminishing quadrupole potential in the fringing region near the end of a quadrupole rod array can give rise to an approximately conical surface on which the net axial force experienced by an ion, averaged over one RF cycle, is zero. This conical surface has been named the cone of reflection because it divides the regions of ion reflection and ion ejection. Once an ion penetrates this surface, it feels a strong net positive axial force and is accelerated toward the exit lens. As a consequence of the strong dependence of the axial field on radial displacement, trapped thermalized ions can be ejected axially from a linear ion trap in a mass-selective way when their radial amplitude is increased through a resonant response to an auxiliary signal.

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PubMed2002

Resonance shifts in the excitation of the n = 0, K = 1 to 6 quadrupolar resonances for ions confined in a linear ion trap.

Experiments examining the excitation of the quadrupolar n = 0, K = 1 to 6 resonances for the ion reserpine in a linear ion trap have been shown to produce resonance shifts that were dependent upon either or both of the excitation amplitude and trap pressure (Collings, B. A.; Douglas, D. J. J. Am,. Soc. Mass Spectrom., 2000, 11, 1016-1022). The extent of this dependency was determined by examining the effects of each parameter using an ion trajectory simulator. The simulations indicated that it is the change in excitation amplitude that is mostly responsible for the resonance shifts with a minor dependency upon the trapping pressure. It was found that the higher excitation amplitudes required to observe the higher order resonances resulted in greater shifts relative to the theoretical resonances predicted for an excitation amplitude of zero volts. The nature of these shifts can be understood by examining the equations of motion for an ion trapped in a quadrupolar potential during the excitation process. Rearrangement of the equations of motion lead to a Mathieu stability diagram in which the coordinate and ordinate variables are dependent upon the excitation frequency and amplitude. In such a diagram the resonances occur in the regions of instability. The calculated resonance shifts showed good correlation with the experimental and simulation results.

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