Ion ejection technique

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. 

We have recently shown that by varying the strength of the bases (B), it is possible to determine the onset for reaction (13) fairly accurately. " This is conveniently done by means of FT ICR. Full experimental details are given in Refs. 54 and 55. Briefly stated, BH" (g) is formed by chemical ionization following the electron ionization of a mixture of B(g) and R-X(g) (in general, fragment ions from B and/or R-X act as proton sources) and then isolated by ion ejection techniques and allowed to react. The formation/absence of R (g) is monitored for periods ranging from 1 to (sometimes) 60 s. 

If the trapping potential is changed periodically with a definite frequency, the ion ejection technique developed by Beauchamp and Arm-... 

With a time-lag ion-ejection technique the rate constcints for the forward ( f) and the reverse ( r) reaction have been determined ... 

J.L. Beauchamp and J.T. Armstrong, An ion ejection technique for the study of ion-molecule reactions with ion cyclotron resonance spectroscopy. Rev. Sci. Instr. 40, 123-128 (1969). 

When the device is operated as a mass spectrometer, ions produced by an electron-impact or chemical ionization source are admitted through a grid in the upper end cap. The ionization source is pulsed so as to create a burst of ions. The ions over a large mass range of interest are trapped simultaneously. The ion trap can store ions for relatively long times, up to l.o minutes for some stable ions. A technique called mns.s-.te/ct/ri C ejeaion is then used to sequentially eject the trapped ions in order of ma,ss by increasing the radio-frequency... 

Ions prcxluced in the center of the trap (usually by electron impact) are constrained by the magnetic field and applied potentials on the trapping plates to follow orbits within the cell and only slowly diffuse toward the walls. During the time the ions are trapped, they may have maiy collisions with neutral molecules in the trap, and these collisions can lead to thermalization of the ions and may result in chemical reactions. At a neutral pressure of 10 torr, the approximate pseudo first-order rate constant for ion-molecule collisions is 30 s. The ionic products of ionization and subsequent reactions in the cell can be detected at aiy time mass spectrometrically application of a broad range of radio frequencies and detection of resonance at cyclotron frequencies corresponding to m/z values of the ions present. Techniques are available that allow isolation of one or more m/z value in the ion population by ejection of all other ions, and this allows the reactions of a particular ion to be followed directly. Fourier transform methods, intrcxluced by Marshall and... 

The principles of ISS and SIMS are similar, in the method of bombarding the sample, the utilization of high vacuum, and the measurement of ions. In ISS the scattered ion beam is examined and the energies of the ions measured in SIMS the ions ejected from the surface are examined and their mass-to-charge ratio measured. Both techniques are... 

Ion probe microanalysis is a technique in which the sample is bombarded by a well focused beam of primary ions (diameter less than 10 pm) and the secondary ions ejected from the sample are detected in a mass spectrometer. 

Commercial introduction of ion isolation/ejection techniques to reduce background interferences by the use of waveform generators coupled to digital function generators and personal computer systems has greatly contributed to customization of GC/ITMS instruments. These instruments are now providing with an extended simplicity numerous methods for the utihzation of customized sequences for specific analytical applications. 

A better approach is to pulse the reactant ion beam and to eject the product ions by pulsing a repeller electrode within the collision chamber as soon as the reactant ions have left the chamber. Figure 3 shows some data obtained by this pulsed ejection technique for the asymmetric charge-transfer reaction Ar -I- CH4-> Ar -I- CHj" -I- The minor... 

It is clearly difficult to employ this technique at high pressures since reactant and product ions ejected by the ejection pulse may be scattered differently on their passage out of the source. Under such conditions, it is apparently preferable to rely on thermal effusion, provided high enough pressures are employed. 

The longitudinal tandem technique, effective above energies of 1 eV, will surely continue to be used, but it must produce uncertain results to the extent that product ion detection efficiencies are uncertain. The pulsed ejection technique provides a hopeful, but difficult means of circumventing this problem. ... 

MSP Mass-spectrometer ion source—pulse technique. TOP Time-of-flight mass analysis. LP Low-pressure experiment < 10 /xm. MP Medium-pressure experiment < 100 /im. HP High-pressure experiment > 100 /im. ICR Ion cyclotron resonance. CR Constant repeller field—variable reaction time. IMP Impulse technique. PPE Pulsed product ion ejection. MS Mass-spectrometer ion source. 

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