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Ion Detectors

To obtain a mass spectrum, ions need to be converted into a usable signal by a detector. The simplest form of ion detection is a photographic plate or a Faraday cup for the direct measurement of the charge. In a Faraday cup the induced current is generated by an ion which hits the surface of a dynode and emits

Ions

Signal

Fig. 1.31 Discrete-dynode electron multiplier. When the ions hit the surface of the detector electrons are emitted to form an avalanche of electrons which generates the signal.

electrons. This type of detector is generally insensitive and mounted in isotopic ratio mass spectrometers. The first electron multipliers mounted in mass spectrometers were discrete-dynode multipliers fabricated from beryllium copper alloy. When a positively or a negatively charged ion reaches the detector electrons are produced (Fig. 1.31).

In this type of detector the electrons are accelerated down the channel producing additional electrons to the output signal. The created cascade of electrons results in a measurable current at the end of the detector [77].

Channel electron multipliers (CEM) are fabricated from lead-silica glass (Fig. 1.32) and can have curved or straight forms. In a channel electron multiplier, when the charged particles (positive or negative) hit the surface of the electrode, electrons are produced from the surface which then generate the current.

Channel electron multipliers can be operated either in analog or pulse counting mode. The difference between the two modes of operation is that pulse counting produces output pulses with a certain amplitude while analog detectors produce a wide distribution of output pulses. Therefore, the pulse counting mode is more suitable for high sensitivity mode while analog mode is best suited for intense signals. In modern mass spectrometers, autotune procedures optimize the analog multipliers based on signal-to-noise. The tuning of pulse counting detectors is somewhat different because they operate in a different mode. The sensitivity of a detector decreases almost exponentially with the mass of the ions. One way to improve the signal in the channel electron multiplier detector sensitivity at higher mass is to use a conversion dynode (Fig. 1.33). A conversion dynode is a metal surface which is held at high potential (>3 kV). The role of the dynode

Fig. 1.32 Straight channel electron multipliers (CEM) are typically used in quadrupole-type mass spectrometers.

Fig. 1.32 Straight channel electron multipliers (CEM) are typically used in quadrupole-type mass spectrometers.

Fig. 1.33 Curved 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.

potential is to accelerate ions to a point where good conversion in secondary ions or electrons occurs.

The lifetime of channel electron multipliers is ca. 1-2 years. Neutrals or photons hitting the detector also increase the noise of the detection.

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).

The advantage of the photomultiplier compared to the electron multiplier is the longer lifetime (several years). Channel electron multiplier and photomultiplier are mostly used in quadrupole instruments or ion traps.

Array detectors, such as the multichannel plate (MCP) detector are best suited for mass analyzers where ions are spatially dispersed like in time of flight instruments. Array detectors are detectors [78] which allow simultaneous multichannel detection. The advantages of such detectors are high sensitivity and the possibility to eliminate the accompanying noise. Array detectors are largely used with TOF mass analyzers. Generally, the array consists generally of 106 microscopic glass channels, ca. 5-50 mM in diameter, bound together and electrically connected with each other. Each channel operates as a continuous dynode electron multiplier (Fig. 1.35).

Fig. 1.34 In the photon multiplier detector ions are transformed into photons which are detected by a photomultiplier.

Fig. 1.35 Multi-channel plate multiplier. Each hole corresponds to a single channel detector.

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