Quadrupole Ion Trap MS

Glossary

space-charge effects: too many ions in the trap distort the electric fields, leading to significantly impaired performance.

fundamental rf: a 0.76 MHz potential applied to the ring electrode. The amplitude of this potential determines the mass range of the instrument and is ramped to eject ions.

q_z: a dimensionless parameter that determines stability of ion trajectories depending upon their mass-to-charge ratio, the size of the ion trap, and the amplitude and frequency of the fundamental rf.

q_z = 4eV/mr²w². For the case of the LCQ^TM traps, r = 1 cm, w/2PI = 0.76 MHz, and V ranges from 0 to 8,500 V_0-p.

secular frequency: the frequency, dependent upon the q_z value, with which an ion oscillates in the trap.

resonance: an ac voltage is applied to the endcap and the q_z value of an ion of interest is changed until the secular frequency of the ion matches the frequency of the applied ac voltage. When resonance occurs, the amplitudes of ion trajectories linearly increase with time. A high-amplitude ac voltage will cause resonance ejection, while a low -amplitude ac voltage will cause resonance excitation

ac voltage: also called supplementary or auxiliary potential, is a voltage placed on the endcap electrodes. It can be used to induce resonance excitation and resonance ejection and to isolate ions.

tickle voltage: an ac voltage applied to the endcap electrodes during an excitation period. The amplitude of the voltage is generally small so as to enable fragmentation of the ions by collisions with the helium damping gas rather than ejection.

bath gas, damping gas, target gas: helium gas in the trapping volume at a pressure of about 1 mtorr serves to cool ion kinetic energies and focus ion trajectories into a tight packet at the center of the trap. Helium is also used as a collision partner for fragmenting peptide ions via collision- induced dissociation.

high resolution: an experiment where peaks corresponding to carbon isotopes may be resolved. The mass scan speed of the instrument is reduced, resulting in an increase in the density of data points per unit m/z, thus increasing mass resolution.