4.2.7 Detectors
Once the analytes have been ionized, accelerated, and separated in the mass filter, they must be detected. This is most commonly performed with an electron multiplier (EM), much like the photoelectron multipliers 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). The EMs in ICP-MS systems are usually discrete dynodes EMs since these can be easily modified to extend their dynamic range.
A continuous EMs is typically horn shaped and are 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 4-7 illustrates the response of a continuous electron multiplier as ions, separated in a mass filter, strike its surface.
Animation 4.7. Illustration of a Continuous-Dynode Electron Multiplier.
A standard discrete electron multiplier is shown in Animation 4.8 is actually a connected series of phototubes. In a discrete system each dynode is held at a +90 V potential as compared to immediately adjacent dynodes. As a cation hits the first cathode, one or more electrons are ejected and pulled toward the next cathode. These electrons eject more and more electrons as they go forward producing tens to hundreds of thousands of electrons and amplifying the signal by a factor of 106 to 108. This allows for extremely low detection limits in the parts per billion (ppb) to parts per trillion ranges (ppt). Such an EM is shown in Animation 4.8.
Animation 4.8 Illustration of a Standard Discrete-Dynode Electron Multiplier.
In order to extend the dynamic range of an EM to cover relatively high analyte concentration (in the ppm range), some manufactures have incorporated two EM detector in one by including a switch that allows high signals to be counted in a digital manner in order to prevent the overload of signal, and use analogue counting to analyze low concentration samples. Such an EM detector is illustrated in Animation 4.9.
Animation 4.9 Illustration of a Dual (Gated ) 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 electro-optical 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. The positioning of the dispersed beam of cations is easier to visualize for a magnetic sector MS, but the EOIDs have applications in most mass filter systems. Unfortunately, EOIDs have not been adapted as rapidly as expected by instrument manufacturers.
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