Mass-selected instability scan

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. 

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. 

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. 

Like most successful clever ideas, the mass selective instability scan mode seems rather simple once someone had conceived it. However, it turned out (Stafford 1984) that full experimental reahzation of the potential of the method required another innovation, the exploitation of the collisional coohng/focusing effect of a low pressure of helium, discussed above. It was found (Stafford 1984) that the presence of helimn at a relatively high pressure of approximately 10 torr significantly enhanced the mass resolution, sensitivity and detection limit of a Paul ion trap operated as a mass spectrometer using the mass selective instability scan to obtain the mass spectrum. 

One result of the collisional focusing caused by the low pressure of helium is to initially restrict ions to orbits in the trap that have maximum displacements from the centre of about 0.1 rg ( 1 mm) or less (i.e. well away from the electrodes) this effect directly reduces ion losses arising from initial positions and velocities of the ions, and thus also the detection limit. A less direct but still significant effect of the coUisional focusing arises because, during the mass selective instability scan, ions of a given mJz spend most of their lifetime in the center of the trap where the theoretical field is zero (see discussion of Equation [6.28]) so that field imperfections, caused by mechanical errors in the fabrication of the electrode structure, are at a minimum. 

Figure 6.21 Partial spectra illustrating the dramatic effect of use of helium as a damping gas in a Paul trap on its paformance as a mass spectrometer using the mass selective instability scan mode. Peaks on the left are for mtz 69, those on the right for m/z 502. The top spectra were obtained without damping gas, and the bottom spectra by using 10 torr of helium. Reproduced from Stafford, Int. J. Mass Spectrom. Ion Proc. 60, 85 (1984), copyright (1984), with permission from Elsevier.

The mass selective instability scan represented an enormous advance in the exploitation of the Paul trap as a mass spectrometer but was limited with respect to the available 650 (see above) for a trap of reason-... 

Figure 6.22 Scan function used to obtain an El mass spectrum (one microscan) using a Paul trap with mass selective instability scanning with concurrent axial modulation. The scan function shows the ionization period A (this could be replaced by a period of ion injection from an external source), followed immediately by activation of the axial modulation waveform and the analytical ramp of Vg. Note that the pre-scan for the automatic gain control function is not shown. Reproduced from March, J. Mass Spectrom. 32, 351 (1997), with permission of John Wiley Sons, Ltd.

The quadmpole mass filter has seen continued performance improvements, but it is still operated fundamentally as it was when it was first introduced. The QMF is in demand today in the form of single-quadmpole, triple-quadmpole, or hybrid mas s spectrometers. The birth of the QIT as a scanning mass spectrometer in 1983 by Finnigan Corporation occurred later because of the need for the often overlooked computer control which was essential for implementation of the scan function and the mass selective instability scan. ... 

Major ion trap developments have occurred over the last 15 years. In 1995, Bier and Syka patented the use of the mass selective instability scan from high-charge-capacity ion trap geometries such as the linear quadmpole ion trap (LQIT), toroidal trap (TQIT), curved or banana traps (CQIT), and elliptical traps (EQIT). The LQIT with radial ejection was eventually commercialized in 2002, and it is used as a stand-alone mass analyzer and has been combined with QMFs, Fourier transform ion cyclotron resonance (FT ICR), and the orbitrap to form hybrid instmments. The LQIT analyzer has eclipsed the conventional ... 

A dipole signal (Vres cos(2nf. esi)) is applied to the end caps as shown in Figure 9.3a, for resonance excitation and/or resonance ejection. For either of these processes to occur efficiently, however, the frequency used must equal the fundamental resonant frequency of the ion, fz-Ks- For example, during the mass selective instability scan supplemented with resonance ejection (see Figure 9.3b), ions are ejected from the trap axially as they are scanned into resonance at a frequency that corresponds to a 2 eject = 0.83, Pz-eject = 0.736. Resolution, peak height, and sensitivity are greatly improved when using the mass selective instability scan mode supplemented with resonance ejection. 

Figure 2.28 Ion trap scan function. The sequence of events used to generate a mass spectrum in a typical Paul trap is shown. This sequence is typically repeated many times with the individual spectra summed or averaged to produce the final spectrum. The generic parts of the scan function shown include (1) ion injection and trapping, (2) ion relaxation/cooling, (3) auxiliary excitation for selective ejection/storage of desired ions and (4) mass-selective instability scan. The timing of the resonant excitation function is also indicated two sine waves shown indicate a first pulse for selective excitation and a second pulse for enhancing the mass selective instability scan...

---