Discrete Dynode Detector

Typical scan over isotope ion peaks measured with compact discrete-dynode detectors... 

Before I go on to describe discrete dynode detectors in greater detail, it is worth looking at two of the earlier designs—the channel electron multiplier (Channel-tron ) and the Faraday cup—to get a basic understanding of how the ICP-MS ion detection process works. 

Although most discrete dynode detectors are very similar in the way they work, there are subtle differences in the way the measurement circuitry handles low and high ion count rates. When ICP-MS was first commercialized, it could only handle up to five orders of dynamic range. However, when attempts were made to extend the dynamic range, certain problems were encountered. Before we discuss how modem detectors deal with this issue, let us first look at how it was addressed in earlier instrumentation. 

FIGURE 11.4 Dual stage discrete dynode detector measurement circuitry. (From E. R. Denoyer, R. J. Thomas, and L. Cousins, Spectroscopy, 12[2], 56-61, 1997. Covered by U.S. Patent Number 5,463,219.)... 

Discrete-dynode multipliers consist of an array of separate dynodes with high secondary electron yield surfaces. CEMs consist of a lead-silicate glass tube processed to have a resistive inner surface with a suitably high secondary electron emission to multiply electrons. The following discussion is restricted to discrete-dynode detectors, which are the most common type used in ICP-MS. However most of the principles described can be readily applied to CEMs. 

Figure 3.16 Schematic diagram showing the functional layout of a simultaneous mode discrete-dynode detector.

Figure 3.23 Schematic layout of a discrete dynode detector. The DC power supply should be able to deliver a potential difference of 200-300 V between adjacent dynodes (using the potential divider circuit shown) and so for a 10-dynode detector 2-3 kV needs to be delivered across the entire detector. Care must be taken to apply this voltage only when the detector is under high vacuum, otherwise damage due to electrical breakdown (arcing) can occur.

Discrete dynode and channeltron detectors have many similar characteristics, which is not surprising given the similarity of their operating mechanisms. Discrete dynode detectors generally have longer lifetimes and higher detection sensitivities than channeltrons because they possess a larger active surface area. The discrete dynode detector is also more physically robust than a channeltron but the latter can be obtained in a more compact form and is usually cheaper than a discrete dynode detector. 

A dynode is a metallic plate covered with an alloy (lead or lead oxide most of the time) that is very rich in electrons. Under the effect of a chock corresponding to the arrival of ions, the alloy emits electrons. In the case of a discrete dynode detector (Figure 2.4a), a difference in potential accelerates these electrons to a second dynode where their arrival provokes the emission of other electrons (more numerous) that are then accelerated to a third dynode and so on. This is referred to as an electron avalanche. ... 

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