The Pulsed Ion Source

It is apparent from the above discussion that it would be desirable to perform reaction rate studies by varying the retention time without having simultaneously to vary the relative collision velocity. This problem 

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A collinear scheme for ion- and laser beams enlarges the interaction time between the laser and the negative ions. The synchronization of the laser pulses with the pulsed ion source ensures an optimum time overlap between the ion pulses and the laser pulses. The electrons are focussed by a weak magnetic field onto the electron detector. The remaining negative ions are separated from the neutrals by a perpendicular electric field. 

The ion-transfer optics include at least one quadrupole (or hexa- or octopole) at elevated pressure of about 1 mbar for thermahzation of the ions, to stabiUze them for the longer analysis time. The transfer optics also essentially decouples the TOF analyzer from the ion source for high laser pulse repetition rates the transfer optics actually converts the pulsed ion source into a continuous source. Ions are either continuously transferred into the pulser region of the TOF, or accumulated and extracted discontinuously. The fast pusher then injects them into the TOF section of the instrument The maximum rate of ion injection is Umited by the transit time of the largest ion through the TOF analyzer to typically a few kHz. Unfortunately, oTOF instruments have their Umitations ... 

The hybrid has other advantages of sensitivity, low signal-to-noise ratio, fast switching between MS and MS/MS modes, use with continuous or pulsed ion sources, and use with high- or low-energy collision-induced ion decomposition. 

Very high sensitivity is obtained because almost all the ions formed in the ion source are detected, and the mass range is almost limitless. TOF systems work best when pulsed ion sources are used, and the flight time of the ions is then given by... 

Pulsed ion sources for axial injection such as MALDI and SIMS, and earlier plasma desorption are suitable for TOF analyzers. For continuous ion sources such as ESI the oa configuration is suitable. 

Quadmpole ion traps were originally coupled to continuous ion sources but also work well with pulsed ion sources. The cylindrical QIT is a compact device with a diameter and length of 5 cm and the linear QIT is the size of a quadmpole mass filter, that is, 20 cm long. As stand-alone instruments they are nowadays of benchtop size and, together with quadmpole mass filters, are considered to be standard low-cost devices, which are commonly coupled to LC systems. 

MALDI generated a great demand for a mass analyzer ideally suited to be used in conjunction with a pulsed ion source and capable of transmitting ions of extremely high mass up to several 10 u. [24] Since then, the performance of TOF instruments has tremendously increased. [25,26] TOF analyzers were adapted for use with other ionization methods and are now even strong competitors to the well-established magnetic sector instruments in many applications. [25,27]... 

ToF mass spectrometers as dynamic instruments gained popularity with the introduction of matrix assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) as effective pulsed ion sources for the soft ionization of large biomolecules (up to 10s dalton) due to their high ion transmission.38 ToF mass spectrometers, quadrupole analyzers and/or magnetic sector fields can be combined in tandem mass spectrometers (MS/MS) for the analysis of organic compounds. 

The final system described here is that of Jones et al. (1993), which was developed for positron-hydrogen ionization studies and is illustrated in Figure 5.10. Similar apparatus has been used by Ashley, Moxom and Laricchia (1996) (see subsection 5.4.5 below), Kara et al. (1997a, b) and Kara (1999). Several of the basic features, including the pulsed ion extraction and ion transport systems, are similar to those developed by Knudsen et al. (1990). E x B plates were introduced by Jones et al. (1993) to remove the slow positrons from the fast / + particles, secondary electrons and gamma-ray flux emanating from the source. 

Figure 6. Block diagram of the experimental. RB system, vhich consists of three basic components a laser system capable of producing tunable ultraviolet radiation, a magnetic sector mass spectrometer with a suitably modified thermal atomization source, and a detection and measurement circuit capable of quantifying the pulsed ion currents produced in the experiment.

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