Detectors conversion dynode

The most common way to detect the ions is to eject them from the trap and have them hit a detector situated outside the trap, as seen in Figs. 2.16 and 2.17. A standard detector is the conversion dynode together with an electron multiplier. Ions ejected from the trap are accelerated towards the detector and then amplified (see Section 2.3.3). 

Fig. 4.62. Detector configuration with conversion dynode. By courtesy of JEOL, Tokyo.

A further alternative to the Faraday cup - the Daly detector13 - is illustrated in Figure 4.6 a. In the Daly detector a conversion dynode, which is at a high negative potential ( — 40 kV), is applied to convert ions into electrons. The Daly detector was developed from an earlier device using a scintillator (e.g., of phosphorus) for the direct detection of positive ions. 

We should not forget that an appropriate detector, a Faraday cup or a secondary electron multiplier equipped with a conversion dynode, is needed for ion detection. Most commercial instruments are equipped with a secondary electron multiplier, which can be operated in a low amplification mode, the analogue mode, and with a high gain, the counting mode, where each ion is counted. With this dual mode, a linear dynamic range of up to nine orders of magnitude can be achieved, so that major and minor components of the sample can be measured in one run. 

Mass spectrometers work equally well for negative and positive ions by reversing voltages where the ions are formed and detected. To detect negative ions, a conversion dynode with a positive potential is placed before the conventional detector. When bombarded by negative ions, this dynode liberates positive ions that are accelerated into the electron multiplier, which amplifies the signal. 

Linear trap with slots cut in two opposite rods. Sizes are 12 mm for sections A and C and 37 mm for B. Detectors D are placed off-line and ions are attracted by the conversion dynodes. The slots are 30 x 0.25 mm. Drawn according to the data from Schwartz J.C., Senko M.W. and Syka J.E.P., A Two-Dimensional Quadrupole Ion Trap Mass Spectrometer , Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Florida, 2002. 

The electro-optical ion detector (EOID) combines ion and photon detection devices. This type of detector operates by converting ions to electrons and then to photons. The most common electro-optical ion detector is called the Daly detector. As shown in Figure 3.7, this type of detector is made up of two conversion dynodes, a scintillation or phosphorescent screen and a photomultiplier. This device allows the detection of both positive and negative ions. As for the electron multipliers, ions from the analyser strike a dynode. In the positive mode, ions are accelerated towards the dynode that carries a negative potential, whereas in the negative mode, ions are accelerated towards the positive dynode. Secondary electrons... 

Electro-optical ion detector an ion-to-photon detector that combines an ion and photon detection device. This type of detector operates by converting ions to electrons and then to photons. Ions strike a conversion dynode to produce electrons that in turn strike a phosphor and the resulting photons are detected by a photomultiplier. 

Using mass selective instability with resonance ejection, ions are scanned out of the trap through slits in the center of two opposite center section rods and focused onto two separate conversion dynodes. In the case of the QIT, where ions are scanned out of both end cap electrodes, the only place for a detector is behind the end cap opposite the ion entrance, so that only half of the ions scanned out of the trap are detected. Both the QJT and LIT operate at unit mass resolution with similar scan rates and both have the capacity to generate higher resolution spectra at slower scan rates. 

The conversion dynode of a photomultiplier detector generates electrons that impinge on a phosphor, which subsequendy generates photons that are detected... 

An electron multiplier can be thought of as a point detector in that a single conversion dynode and multiplier are configured to detect the ion signal. Ions to be detected must first be maneuvered to a single precise position. An alternative possibility, in which numerous electron multipliers are configured together to provide an array [73] requires miniaturization and juxtaposition of the individual electron multipliers into a continuous detector. 

A variant of the EM is the Daly detector (Daly, 1960) in which ions are accelerated by 30 KV (this is called post-acceleration because it occurs after mass analysis) into a conversion dynode, which generates a significant number ( 10) of secondary electrons. These electrons are accelerated into a scintillator and converted into light, which is detected with a photomultiplier. The Daly detector offers high gain, low noise, and excellent stability. Other variants of the post-acceleration detector exist a simple configuration uses a metal plate to convert the ions into electrons for detection with an EM. Another variant of the EM is the microchannel plate (Coplan et al 1984 Odom et al 1990 Wiza, 1979). MicroChannel plate EM detectors have excellent sensitivity but poor gain stability. When operated in... 

Apart from the FT-ICR/MS most spectrometers use a device which converts the ion beam into a usable signal by destruction of the ions. A number of means have been developed to do this but virtually all modem instruments use a conversion dynode and an electron multiplier to accompUsh this. Faster signal processing required by ToF spectrometers, use array detectors. 

This type of detector is made up of two components, a phosphorescent screen and a photomultiplier. After ion impact with the conversion dynode the secondary electrons given off are accelerated towards the screen and are converted into photons. The screen is earthed to prevent a build-up of charge on its surface. The photons enter a photomultiplier behind the screen and cascade to generate a detectable current. The advantage of this detector is its long lifetime compared to electron multipliers but the overall dynamic range is slightly reduced (10 ). 

These detectors consist of conversion dynodes, a scintillator (normally a phosphorescent screen), and a photomultiplier tube (Figure 1.28). Cations and anions are accelerated to the negative and positive conversion dynodes, respectively. The ensuing ion-surface collisions lead to the generation of secondary elecfrons which strike the phosphorescent screen where they are... 

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

The Daly detector uses a photomultiplier rather than an electron multiplier. Ions leaving the analyzer are directed onto a conversion dynode, and the ejected electrons are accelerated onto a plate coated with a fast-acting scintillant. Each electron releases a photon from the scintillant. The photons then enter a photomultiplier tube and impact on a photocathode, producing electrons (photoelectric effect) and initiating an electron cascade (Pigure 2.40). The output from the photomultiplier is further amplified electronically, similarly to the output of dynode type electron multipliers. The level of amplification is similar to that of electron multipliers. Photomultiplier tubes last longer than electron multipliers, but the scintillant-coated plates require replacement every few years. 

In photomultiplier-based detectors, the incoming ion beam is first converted to a photon beam when ions strike a scintillation material. The emitted photons are amplified and detected by a conventional photomultiplier. The construction of a photomultiplier is similar to that of an EM, except that the conversion dynode, called the photocathode, is coated with a photoemissive material that emits electrons when struck by photons. Photomultipliers are usually employed in combination with postacceleration devices, which are discussed next. 

A postacceleration-based detector that can be used for the detection of positive and negative ions is shovyn in Figure 3.30. It contains two conversion dynodes, one for positive ions and one for negative ions, a scintillation material (phosphor), and a photomultiplier. For positive-ion detection, the conversion dynode is maintained at —10 to —20 kV. For the detection of negative ions, the incoming beam is first deflected toward a cylindrical conversion dynode that is held at half the phosphor voltage. The secondary electrons emitted in both cases are accelerated toward the phosphor. The photons hv) released from the phosphor are transmitted to the photomultiplier for further detection. 

It must be remembered that each of the points in Figure 2.13 is defined by the Oz and equations above and, thus, variance in their values can represent variance in m/z, V,f, or (potentially also Q or % but these are usually fked). Therefore, by slowly ramping up V or U (or both), ions can be shifted from stable to unstable trajectories and ejected from the trap. If done correctly, these ions can be made to impact on a conversion dynode (which ejects an electron when hit with a positive ion with sufficient kinetic energy) and a secondary electron multipHer detector. When the election current signal on the detector is plotted versus the or values calculated from the V r U ramp, a mass spectrum is produced. Normally, the conversion dynode/electron multiplier detector is placed on the z-axis, so that the q = 0.908 value as marked in Figure 2.13 is the transition point where an ion becomes unstable in the z-direction (at = 0, or = 0). 

The secondary electron multiplier (SEM) detector is the key to the role of mass spectrometry as an extremely sensitive analytical technique with wide dynamic range and compatibility with on-line coupling to fast chromatographic separations. The SEM was a natural development from the invention of the photomultiplier (Zworkin 1936, 1939), in which photoelectrons produced by photons falling on a conversion dynode with a photo-sensitive surface are amplified in an avalanche fashion by accelerating the original (first strike) photoelectrons on to a... 

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