Selective mass instability

The mass-selective instability mode of operation permits the selection and trapping of all ions created over a specified period with subsequent ejection to the detector.26 Ions with different m/z values can be confined within the ion trap and scanned singly by application of voltages that destabilize the orbits of the ions and eject them to the detector. Ion trap instruments interface readily with liquid chromatography, ESI,15 and MALDI.27 The motions of the ions and the dampening gas (e.g., helium) concentrate around the middle of the ion trap, thereby diminishing ion loss through collisions with electrodes. 

One method of acquiring a mass spectrum is the mass selective instability scan. As the RF voltage increases, the ions with lowest m/z become unstable and are ejected through small holes in the end cap to hit a detector. As the RF voltage is further increased, heavier ions become successively unstable and are ejected, thus yielding a mass spectrum. 

Fig. 4.44. Timing sequence used for mass-selective instability mode (about 1.5 cycles shown). With an external ion source the ionization time is replaced by the ion injection pulse. Reproduced from Ref. [150] by permission. Elsevier Science, 1984.

At the stability boundary, ion motion is in resonance with this modulation voltage, and thus ion ejection is facilitated. Axial modulation basically improves the mass-selective instability mode of operation. 

Chemical ionization (Cl) mass spectra were first obtained by using the mass-selective instability mode of the QIT. [154,155,170] The reagent gas was admitted into the QIT, ionized and then allowed to react with the analyte. 

Mass analysis is based on mass selective instability. DC and RF voltages applied to the trap electrodes are such that ions over the entire m/e range of interest can be trapped within the field imposed by the electrodes. After this storage period, the parameters U and V0 are changed so that the trapped ions of consecutive values of m/e become... 

Figure 9 Glow discharge mass spectra of NIST SRM 1103 Free Cutting Brass, demonstrating selective ion accumulation by using mass selective instability (a) low-mass cutoff, m/z 15 (b) low-mass cutoff, m/z 45. (From Ref. 36.)...

Dynamic range extension in GD quadrupole/ion-trap MS based on selective ion-accumulation (e.g. by mass-selective instability, single-frequency resonance ejection, combined rf-dc and entrance end-cap dc methods) allows the selective accumulation of the analyte ions and enables the dynamic range to be increased by a factor of 105 [233]. The linearities and relative trapping efficiencies of the previous methods were assessed with respect to the injection time and the methods were used for the GD ion-trap MS determination of major and minor constituents in NIST SRM 1103 Free Cutting Brass. 

Using mass selective instability with resonance ejection, ions are scanned out of the trap through slits in the center of two opposite center section rods and focused onto two separate conversion dynodes. In the case of the QIT, where ions are scanned out of both end cap electrodes, the only place for a detector is behind the end cap opposite the ion entrance, so that only half of the ions scanned out of the trap are detected. Both the QJT and LIT operate at unit mass resolution with similar scan rates and both have the capacity to generate higher resolution spectra at slower scan rates. 

Mass-Selective Instability Scan with Collisions and... 

Individual collision events are another possible source of chemical mass shift during a mass-selective instability scan. Based on results from simulations using ITSIM, Plass et al. [96] reported that elastic collisions causing a change in the ion s oscillation amplitude can reduce the normal ejection delay, particularly when there are scattering events causing a sudden increase in axial displacement. 

Wells proposed an ion isolation method [33], which consisted of two steps for the isolation of precursor ions. The method employed a modified mass-selective instability technology (axial modulation technology) to ejea sequentially ions with m/z less than that of the molecular precursor ion, M+, for example. Combined with an empirical calibration procedure, ions with mass/charge ratio > M Th are ejected resonantly with a broadband waveform. Figure 15.17 shows the scan function of this isolation method. 

Louris, J.N. Schwartz, J.C. Stafford, G.C. Jr. Syka, J.E.P. Taylor, D.M. The Paul ion trap mass selective instability scan trap geometry and resolution. Proc. 40th ASMS Conference on Mass Spectrometry and Allied Topics, Washington DC, 1992, 1003-1004. 

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