Pfeiffer Vacuum A Passion for Perfection

4.1.4.1 Advantages of Pfeiffer Vacuum mass spectrometers

(Figure 4.20)
Figure 4.20: Potential curve in an electrically biased ion source

The potential curve in a Pfeiffer Vacuum ion source is shown in Figure 4.20. The heated, electron-emitting cathode has a potential of approximately 20 V. The Wehnelt electrode is typically connected to the positive pole of the cathode and prevents electrons from being scattered in the vicinity of the ion source. An anode voltage V2 of 80 V accelerates the electrons into the formation area (100 V), where they ionize penetrating neutral gas molecules. The ions are accelerated through an orifice at a potential V5 of -150 V, and are again decelerated to V3 = 80 V by the focusing electrode. The shot orifice accelerates the ions once more before they then enter the mass filter and are decelerated by the field axis potential V4 = 85 V at an energy of approximately 15 eV (difference between formation area and field axis).

The Pfeiffer Vacuum PrismaPlus and HiQuad mass spectrometers are characterized by their above-described electrically biased ion source and their field axis technology.

Electrically biased ion source

In many quadrupole mass spectrometers, the cathode is grounded or even has a negative potential. The cathode (filament) accelerates the emitted electrons to the formation area (anode), where they ionize neutral gas particles, which are then extracted in the mass filter. Given these field conditions, however, electrons can also strike other surfaces in the vacuum, where they trigger electron stimulated desorption (ESD) ions. This results in undesirable background noise and can cause considerable gas eruptions when the filament is energized if there are highly-populated surfaces in the recipient.

Pfeiffer Vacuum ion sources have a positive potential (approximately 10 – 100 V). Electrons emitted from them are repelled from all surfaces having a negative potential and are thus kept away from these surfaces to avoid triggering interfering ESD ions.

Field axis technology

The ions formed in the ion source are accelerated toward the mass filter at high kinetic energy. As a result, the ions cannot be influenced by the peripheral or interference fields, and initially move toward the mass filter at high energy. This enables optimal shot conditions to be achieved in the quadrupole field, even without the pre-filters that are required with other mass spectrometers. The mass filter, itself, is appropriately biased to the field axis voltage, which decelerates the ions to a kinetic energy of approximately 15 eV again upon entering the filter. This energy – which the industry terms the field axis voltage – together with the mass of the ions determines the velocity of the ions, and thus their time of flight in the mass filter. The favorable shot conditions thus produced result in a high transmission of ions through the mass filter over a broad mass range, thus producing the high sensitivity of the entire system.

SEM: 90 degrees off axis

An additional advantage of Pfeiffer Vacuum mass spectrometers is the arrangement of the secondary electron multiplier (SEM), which is offset by 90° relative to the filter axis (“SEM: 90 degrees off axis”).

If the SEM (4.1.2.3) is arranged in the axial direction behind the mass filter, all colliding particles (neutral particles, ions, electrons, photons) will generate secondary electrons and thus contribute to the background signal

This is why the ions escaping from the filter are deflected by 90 degrees and then accelerated to the first dynode of the SEM. Neutral particles and photons are not deflected at all by the electrical deflection unit, and electrons are deflected to a much greater extent than ions. This means that almost all of the ions that are allowed through the filter will strike the amplifier, which significantly improves the signal-to-noise ratio. Except for a few special versions, HiQuad analyzers are equipped with “90 degrees off-axis SEMs”.

In the PrismaPlus, an axial C-SEM is offered as a current amplifier. In this case, too, the ions exiting the mass filter are deflected slightly toward the C-SEM, and in a weakened state are thus separated from the undesired particles.

(Figure 4.21)
Figure 4.21: Design of the detectors in a QMA 400 HiQuad™ analyzer with Faraday cup and SEM

Mass discrimination

In the PrismaPlus, an axial C-SEM is offered as a current amplifier. In this case, too, the ions exiting the mass filter are deflected slightly toward the C-SEM, and in a weakened state are thus separated from the undesired particles.

Summary

Both a stable HF supply as well as a mechanically precise filter are necessary in order to achieve maximum possible transmission over a broad mass range with a pre-selected mass resolution. A biased ion source with suitably selected field axis technology, as well as the “90 degrees off-axis” arrangement of the SEM considerably improve the signal-to-noise ratio.

Mass discrimination in an SEM or a C-SEM can be reduced with the aid of a conversion dynode to which a high voltage is applied.

Quadrupole mass spectrometers differ from other designs through the following attributes:

With these advantages, the quadrupole mass spectrometer has become the most widely used mass spectrometer.

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