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.

Fig. 1.33 Cu rved channel electron multiplier with conversion dynode. The conversion dynode acts as a post acceleration device of the ions before they hit the surface of the channel electron multiplier.

A further widely used multiplier is the photon multiplier. In this case the ions (positive or negative) elicit secondary ions formed by a conversion dynode, which are further accelerated towards a phosphorescent screen where they undergo conversion into photons detected by a photomultiplier (Fig. 1.34). 

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. 

Extraction Lens Vacuum Housing Conversion Dynode (—15kV)... 

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. 

Schematic diagram of electron multiplier. The first dynode is a conversion dynode to convert ions into electrons.

The amplifying power is the product of the conversion factor (number of secondary particles emitted by the conversion dynode for one incoming ion) and the multiplying factor of the continuous dynode electron multiplier. It can reach 107 with a wide linear dynamic range (104-106). Their lifetime is limited to 1 or 2 years because of surface... 

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. 

All mass spectrometric analyses were carried out on a triple quadrupole mass spectrometer (TSQ-700) from Finnigan-MAT (San Jose, CA) equipp with an electrospray ion source (ESI) operating at atmospheric pressure. Mass spectra were recorded in the positive ion mode. The electrospray needle was operated at a differential of 3-4 kV, the conversion dynode was set to -15 V. The drying gas was nitrogen and the temperature was set to about 200 °C. A sheath flow of 2-methoxyethanol was delivered with 2 pJ/min and the nitrogen sheath gas was set at 50 psi. Samples were directly introduced into the source with a flow rate of 2 p.l/min. Scans were continuously taken every three seconds. 

SOURCE SLIT (FIXED) CONVERSION DYNODE FOCUS (BEAM CENTRE)... 

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. 

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