Detectors multichannel plate

Figure 6.12 Experimental two-color setup featuring an IR beamline, to generate intense shaped IR pump pulses, and a VIS probe beamline, to provide time-delayed probe pulses of a different color. Both beams are focused collinearly into a supersonic beam to interact with isolated K atoms and molecules. Photoelectrons released during the interaction are measured by an energy-calibrated TOE spectrometer. The following abbreviations are used SLM, spatial light modulator DL, delay line ND, continuous neutral density filter L, lens S, stretcher T, telescope DM, dichroic mirror MCP, multichannel plate detector.

Figure 16.22—MS detectors, a) Multiple stage electron multipliers (reproduced by permission of ETP Scientific Inc.) b) channeltron the conical shape of the cathode allows the detection of ions with slightly different trajectories c) electron multiplication within a channeltron d) entrance of a multichannel plate detector (microchanneltron).

MicroChannel plate detectors are particularly useful in time-of-flight mass spectrometry, as they are flat, minimizing time spread and subsequent mass resolution of homologous ion packets. In addition, they have reasonable gain (104—107 per plate) and fast response time (100-psec time resolution). The major limitation of multichannel plate detectors is the recovery time needed for the detector to rechaige. When a channel is discharged, a recovery time on the order of 10 nsec is typical. This becomes problematic if an ion follows another into a particular... 

Two of the three laser ionization methods have already been discussed, namely one-photon PI and multiphoton MPI. The third type is resonance enhanced MPI, or REMPI. In the latter method the laser is tuned so that an intermediate state of the molecule is excited with one, two, or perhaps three photons. The excitation of the intermediate state determines the overall cross section for the process because the absorption of additional photons to reach the ionization continuum is generally rapid. In contrast to PI and MPI, REMPI is state selective if the absorption process is resonant between two bound and reasonably long-lived states of the molecule. It is an extremely sensitive method for product detection because the result of the REMPI process is an ion which can be detected with near 100% efficiency. Not only is the ion collection efficiency of the detector (e.g., by channeltron electron multiplier or a multichannel plate detector) extremely high (ca. 50%), but all ions regardless of their initial velocity vector can be collected by the application of appropriate electric fields. This is a major advantage... 

Schematic diagram of the experimental setup for laser desorption with jet cooling and postionization. Abbreviations sb, sample bar sk, skimmer g, ion source grids m, moveable mirror pd, photodiode mcp, multichannel plate detector LiF, plano-convex lithium fluoride lens c, annular cooler h, annular heater w, fused silica window al, achromatic lens.

Ions arriving from the analyzer release electrons from a conversion dynode (except multichannel plate detector). 

The multichannel plate detector (MCP) is a porous glass plate in which each pore acts as a mini electron multiplier. The MCP is a set of parallel multipliers. 

Figure 3.31. Conceptual diagram of a multichannel plate detector.

Ion detection is provided by Faraday cups, electron multipliers, channel electron multipliers, cryogenic detectors, multichannel plate detectors, and electroop-tical detectors. 

Because only time is measured, the mass range is almost unlimited in linear ionization, which is particularly attractive for MALDI ionization. On the other hand, with conventional multichannel plate detector the response decreases with the increase of m z of the ions. The resolution is strongly dependent on the ability to produce a very focused ion beam and to avoid energy dispersion in... 

While most mass analyzers such as quadmpole, ion trap, or TOF require destructive detectors such as electron multiplers or multichannel plate detectors, in Fourier transform mass spectrometry (FTMS) the detector uses a nondestructive detection mode. Ion cyclotron resonance (ICR) and the orbi-trap uses Fourier transform detection. 

In spite of the major assets of high resolution and the ability to record mass spectra at high frequencies, TOF analyzers now represent a minority on the GC-MS coupling market. Aside from financial and other considerations, TOF analyzers have a reputation of low efficiency for quantification (quantification of analytes is indispensable for most processes in industrial analysis laboratories), in particular because of the use of microchannel plates to detect ions. Researchers often criticize multichannel plate detectors because they do not supply an electric current proportional to the number of ions that collide with them due to the plate regeneration time. Regeneration involves rehomogenization of the electronic distribution in the micro-channels after a massive impact of ions. 

---