The ions that are separated in the rod system on the basis of their mass-to-charge ratio can be electrically detected by means of various types of detectors:
- By means of a Faraday cup for direct measurement of the ion current using an electrometer amplifier
- Using a secondary electron multiplier (SEM) of discrete design with individual dynodes
- By means of a continuous secondary electron multiplier (C-SEM) Detector selection will primarily be based upon requirements that relate to detection sensitivity, detection speed and signal-to-noise ratio. However it will also be governed by other application-specific requirements that relate to stability, thermal and chemical resistance, as well as space requirements.
In the simplest case, the ions strike a Faraday collector (Faraday cup), where they emit their electrical charge.
The resulting current is converted to a voltage that is proportional to the ion current by means of a sensitive current / voltage inverter (electrometer amplifier). Because it is necessary for the input resistance R of the current amplifier to be extremely high, time constants τ = R · C where 0.1 s < τ < 100 s occur together with the capacities C of the measurement lead. Depending upon the time constant, the measurement limit is between 1 · 10-16 and 1 · 10-14 A.
In addition to its simple, robust design, a Faraday detector is characterized by its long-term stability and its ability to withstand high temperatures. To keep the time constants small and to avoid other interfering effects, the electrometer amplifier is connected directly to the analyzer and its output signal is supplied directly to the data analysis system. This is why the Faraday Cup is also present in all Pfeiffer Vacuum mass spectrometers. It is only suitable for detecting positiveions.
If extremely small ion currents are to be measured or if an extremely high measuring speed is required, physical pre-amplifiers, so-called secondary electron multipliers, are used.
Figure 4.16 shows the design of such an amplifier. Cylindrically shaped pieces of sheet metal (dynodes) are coated with a layer that affords a low level of electron work function. Depending upon its kinetic energy, an ion or an electron generates multiple secondary electrons upon striking this layer. Connecting multiple stages in series produces an avalanche of electrons from a single ion. Positive voltages of approximately 100 V are applied between the dynodes to accelerate the electrons. Technical implementation of this arrangement is produced by supplying a high voltage (approximately 3,000 V) to it by means of a resistance chain, with the individual dynodes being connected to the taps of this voltage. The positive high-voltage pole is grounded to keep the escaping electrons at approximately ground potential. These types of arrangements produce current amplification factors of 107.
A secondary electron multiplier offers the following advantages over a Faraday cup:
- It dramatically increases the sensitivity of the instrument, affording sensitivity increases of up to 10 A / mbar
- This means that lower partial pressures can be scanned at shorter intervals of time with the downstream electrometer amplifier
- The signal-to-noise ratio is significantly higher than that of an electrometer amplifier, which means that the detection limit can be lowered by several orders of magnitude. This applies only if a lower dark current (noise portion) is also flowing in the SEM at high amplification. An increase in sensitivity in its own right is of little value
However an SEM also has disadvantages:
- Its amplification can change due to contamination or a chemical change in the active layer
- The number of electrons (conversion factor) that generate a colliding ion (approximately 1 to 5 electrons) will be a factor of the ion energy (mass discrimination)
Amplification changes as a result of this effect. Consequently, it must be calibrated from time to time. Changes in amplification can easily be adjusted by modifying the high voltage. The conversion factor can be kept constant by supplying the first dynode with a separate high voltage that seeks to equal the energy of the various ions.
Extremely fast measurements are possible with the aid of secondary electron multipliers. As can be seen from Table 4.2, the measuring speeds are significantly higher than with a Faraday cup.
In addition to operation as current amplifiers, discretely designed SEMs are also suitable as ion counters. Extremely low count rates of 1 ion per 10 s can be attained with this configuration. High count rates are also possible, producing an extremely broad dynamic range by comparison with operation as a current amplifier.
In the counting mode, the speed of the SEM serves as the upper limit of the dynamic range. With a pulse width of 20 ns, non-linearity begins at a count rate of 106 events per second. Given its pulse width, the SEM must be suitable as a counter.
What all secondary electron multipliers have in common is that they are restricted to operating
at pressures of less than 10-5 mbar. At pressures of more than 10-5 mbar, the
layer of water on the dynodes can lead to pyrolysis in operation, and thus to premature aging.
Due to the high voltages involved, gas discharges that could destroy the SEM can occur at high pressures.
A C-SEM (Figure 4.17) is a continuous secondary electron multiplier, in which ions trigger an electron avalanche through secondary electron emissions. It consists of a glass tube whose interior is coated with a conductive layer that has high resistance and a low work function. High voltage is applied to the layer in order to obtain a uniform voltage gradient throughout the length of the tube. Ions from the quadrupole system are routed to the conversion dynode and generate secondary electrons that trigger an electron avalanche in the tube. Current amplification factors of 106 are attained at an amplification current of 2.5 kV.
Here, too, amplification and dark current govern the signal-to-noise ratio, and the maximum current / dark current ratio of 106 the current amplification factor. Thanks to a C-SEM arrangement that is slightly offset relative to the axis of the quadrupole, both a Faraday cup as well as a C-SEM can be used next to one another in the analyzer, with changeover from one detector to the other even being possible when necessary.
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