Ion ejection

In TOF-SIMS, the source of primary ions is pulsed at a rate of a few kHz. The pulse width is on the order of 1 ns. Secondary ions ejected from the sample surface are accelerated through a potential V and then drift through a field-free TOF analyzer with different velocities, depending on their masses. The drift velocity of an ion with charge-to-mass ratio zjm can be determined from the expression ... 

The most common way to detect the ions is to eject them from the trap and have them hit a detector situated outside the trap, as seen in Figs. 2.16 and 2.17. A standard detector is the conversion dynode together with an electron multiplier. Ions ejected from the trap are accelerated towards the detector and then amplified (see Section 2.3.3). 

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. 

Franzen, J. The Non-Linear Ion Trap. Part 5. Nature of Non-Linear Resonances and Resonant Ion Ejection. Int. J. Mass Spectrom. Ion Proc. 1994,130, 15-40. 

Aksouh, F. Chaurand, P. Deprun, C. Della-Negra, S. Hoyes, J. Le Beyec, Y. Pinho, R.R. Influence of the Laser Beam Direction on the Molecular Ion Ejection Angle in MALDI. Rapid Commun. Mass Spectrom. 1995,5,515-518. 

Linear ion trap refers in general to 2D ion trap ion ejection is either axial or radial... 

We have seen that ZTRID can be successfully observed and interpreted for cluster ions. It is of interest to look as well at covalent molecular ions for new thermochemical information. The parent ion of tetraethylsilane illustrates these possibilities. The ion is formed in adequate abundance directly in the FTICR cell by electron impact, and the more abundant triethylsilyl ion is readily removed by ion ejection. Temperatures substantially above room temperature are needed to give measurable ZTRID rates. Figure 10 shows the low-pressure dissociation chemistry at 403 K. At this temperature, some water vapor outgasses in the cell and reacts with the tetraethylsilane parent ion to give the EtjSi(H20) ion, but this competing bimolecular reaction is well behaved and easily allowed for in the kinetic fitting. The parent ion undergoes the ZTRID process. 

FT/ICR experiments have conventionally been carried out with pulsed or frequency-sweep excitation. Because the cyclotron experiment connects mass to frequency, one can construct ("tailor") any desired frequency-domain excitation pattern by computing its inverse Fourier transform for use as a time-domain waveform. Even better results are obtained when phase-modulation and time-domain apodization are used. Applications include dynamic range extension via multiple-ion ejection, high-resolution MS/MS, multiple-ion simultaneous monitoring, and flatter excitation power (for isotope-ratio measurements). 

Figure 5. Dynamic range enhancement via SWIFT multiple-ion ejection. Top Normal broad-band heterodyne-mode FT/ICR mass spectrum of perfluorotri-n-butylamine. Middle Same spectrum, in which the vertical scale has been expanded such that the peak at m/z = 503 is full scale. Bottom FT/ICR mass spectrum of the same sample, obtained following prior SWIFT multiple-ion ejection of the (23) peaks whose magnitude-mode peak heights exceeded a threshold of 1.6 % of the height of the biggest peak 1n the original spectrum. Note the higher mass resolution, reduced noise, and flatter baseline in the SWIFT multiple-ion ejected spectrum.

During the trapping time as defined in Fig. 2, ion/molecule reactions can take place where the ions have nearly thermal velocities and the molecules thermal velocities. Unwanted ions can be removed from the cell during this time by application to the transmitter plates of the cell of a series of ion-ejection pulses (see Fig. 2), which are appropriate to increase the radii of the ion orbits so much that the ions strike the walls of the cell, are discharged and pumped away. Other methods of removing unwanted ions from the cell... 

An important development for ion/molecule reaction studies by FA is the extension of the method using so-called selected ion flow tube (SIFT) facilities (Adams and Smith, 1976). In the latter configuration ions are generated in an external ion source, extracted and separated by a quadruple mass filter, after which ionic species of a single mass-to-charge ratio are injected into the flow tube. This set-up permits the ion/molecule reactions of mass selected ions to be studied in the absence of ions of other masses (similar to studies of mass selected ions in FT-ICR after application of so-called ion ejection techniques see above) and neutral precursors, while a wide choice of neutral substrates is possible. 

Ion trap with an RF voltage applied to the ring electrode, providing the fundamental frequency v and its associated variable amplitude V. Instead of injecting ions, electrons may be injected for internal ionization. Variable RF voltage can be applied to the end caps for ion excitation or ion ejection. 

The IT is used to accumulate ions and to perform ion selection and activation in MS/MS experiments before analysis in the TOF analyser. All the ions accumulated in the trap are then ejected in the RTOF analyser. Therefore, the TOF analyser is used for mass analysis instead of the classical ion ejection methods used with ITs, namely mass selective ion ejection at the stability limit or resonant ejection. In comparison with TOF instruments, higher sensitivity is achieved by ion accumulation in the IT. In comparison with IT instruments, the analysis by TOF reduces the time as the TOF analyser allows faster mass analysis, extends the mass range, and gives a better resolution and much better accuracy. 

Moradian, A. and Douglas, D.J. (2005) Axial ion ejection from linear quadrupoles with added octopole field. Proceedings of the 53rd ASMS Conference on Mass Spectrometry and Allied Topics, San Antonio, Texas, June. 

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