5.6 Ion Detectors
Once the analytes have been ionized, accelerated, and separated in the mass filter, they must be detected. This is most commonly completed with an electron multiplier (EM), much like the ones used in optical spectroscopy. In MS systems, the electron multiplier is insensitive to ion charge, ion mass, or chemical nature of the ion (as a photomultiplier is relatively insensitive to the wavelength of a photon). EMs for MS systems can be a series of discrete dynodes as in the photomultiplier or they can be continuous in design. Most commonly, continuous EMs are used. Continuous EMs are horn shaped and are typically made of glass that is heavily doped with lead oxide. When a potential is placed along the length of the horn, electrons are ejected as ions strike the surface. Ions usually strike at the entrance of the horn and the resulting electrons are directed inward (by the shape of the horn), colliding sequentially with the walls and generating more and more electrons with each collision. Electrical potentials across the horn can range from high hundreds of volts to 3000 V. Signal amplifications are in the 10 000 fold range with nanosecond response times. Animation 1.10 illustrates the response of a continuous electron multiplier as ions, separated in a mass filter, strike its surface.
Animation 5.10. Illustration of a Continuous-Dynode Electron Multiplier.
Animation 5.11. Illustration of a Discrete-Dynode Electron Multiplier.
Another form of MS detector is the Faraday Cup that counts each ion entering the detector zone. These detectors are less expensive but provide no amplification of the signal and are not used in typical instruments due to their poor detection limits.
One of the latest detectors to reach the market is a microchannel plate, a form of an array transducer also called an electrooptical ion detector (EOID). The EOID is a circular disk that contains numerous continuous electron multipliers (channels). Each channel has a potential applied across it and each cation reaching the detector will generate typically up to 1000 electrons. The electrons produce light as they impinge on a phosphorescent screen behind the disk containing the channels. An optical array detector using fiber optic technology records the flashes of light and produces a two dimensional resolution of the ions. The advantage of an EOID is their ability to greatly increase the speed of mass determinations by detecting a limited range of masses simultaneously, thus reducing the number of discrete magnetic field adjustments required over a large range of masses. EOIDs have not been readily incorporated into instruments as initially anticipated.
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