6.4 Portfolio overview

Pfeiffer Vacuum offers two basic mass spectrometer models:

PrismaPlus

This is a compact device whose entire electronics are attached to the analyzer and can be removed for bake-out. The PrismaPlus offers the following features:



The PrismaPlus is used as a standalone device and can also be integrated into modules and analysis systems.

HiQuad

These devices offer the utmost in accuracy and possess the following features:

Modules

Modules are special process monitoring or gas analysis devices that are equipped with various gas inlet systems and are attached to dry running turbo drag pumping stations for evacuating the analyzer:

Benchtop mass spectrometers

Pfeiffer Vacuum has complete systems for analyzing gases at atmospheric pressure based on the PrismaPlus. The optimized gas inlet systems use closed ion sources to obtain maximum detection sensitivity.



These devices, or elements of them, are installed by OEM customers in complete devices which may have a wide range of additional functions such as upstream treatment of the substances to be analyzed (such as vaporisation of aerosols) or the supply of calibrating gas mixtures for automatic calibration.

6.4.1 Advantages of Pfeiffer Vacuum mass spectrometers

The potential curve in a Pfeiffer Vacuum ion source is shown in Figure 6.20. 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 effective filament to anode potential V₂ = 80 V of 80 V accelerates the electrons into the formation area (100 V), where they ionize incoming neutral gas molecules. The ions are accelerated through an orifice at a potential V₅ = -150 V, and are again decelerated to V₃ = 80 V by the focusing electrode. The injection orifice accelerates the ions once again before they 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 electrically biased ion source as described above and their field axis technology.

Electrically biased ion sources

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 into the mass filter. Given these field conditions, however, electrons can also strike other surfaces in the vacuum, where they trigger EID (electron impact desorption) ions. This results in undesirable background noise and can cause considerable gas desorption when the filament is energized if there are highly-populated surfaces in the chamber.

Pfeiffer Vacuum ion sources have a positive potential (approximately 100 – 150 V). Electrons emitted from them are repelled from all surfaces having a lower potential (e.g. ground) and are thus kept away from these surfaces to avoid triggering interfering EID 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 RF fringing fields, and initially move toward the mass filter at high energy. Ideal conditions for injecting ions into the quadrupole field are attained in this way, even without a pre-filter, unlike other mass spectrometers which require the use of a pre-filter. The mass filter, itself, is appropriately biased to the field axis voltage, which decelerates the ions again to a kinetic energy of approximately 15 eV upon entering the filter. This energy, dubbed field axis voltage, and the ion mass determine the velocity of the ions and thus their time of flight in the mass filter. The favorable injection conditions thus produced result in a high transmission of ions through the mass filter over a broad mass range, thereby resulting in the high sensitivity of the entire system.

Right-angled arrangement of the secondary electron multiplier

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 (90 degrees off-axis SEM, Figure 6.21).

If the SEM 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. To prevent this, the ions exiting 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 SEM, which significantly improves the signal-to-noise ratio.

Apart from a very few special models, all HiQuad analyzers are equipped with this technology.

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 are separated from the undesired particles.

Mass discrimination

The number of secondary electrons that are created for each of the ions striking the conversion dynode depends on the ion mass and energy and the type of ion (atomic or molecular ion). The conversion rate is a function of the mass. This effect is called mass discrimination, and is less pronounced with an SEM of discrete design than with a C-SEM. Mass discrimination can be reduced by accelerating the ions to a high energy before they strike the conversion dynode.

Summary

Both a stable RF 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.

6.4.2 Dara analysis system

A quadrupole mass spectrometer provides a large amount of information within a short time which is ideal for displaying and storing on a PC. This is the reason why the electrical controls on Pfeiffer Vacuum mass spectrometers only have rudimentary control and display elements. QUADERA® software is used both for controlling purposes as well as to display, analyze and save data on a PC.

Pfeiffer Vacuum’s QUADERA® mass spectrometer software is a modular system for use with PrismaPlus and HiQuad devices. The PC can communicate with the mass spectrometers via Ethernet which means that the length of the cable between the spectrometer and computer is immaterial.

To perform certain measurement tasks, the PC transfers parameter records to the mass spectrometer in order to set the device. The data read out during or after the measurement is transferred to the computer, where it can be analyzed, displayed or stored.

Typical display formats are:



Typical measuring tasks, such as residual gas analysis or leak detection, are pre-programmed and can be launched with a mouse click.

If quantitative analysis is to be performed, the mass spectrometer must be calibrated beforehand. If this involves recurring processes, such as calibration with subsequent quantitative analysis, these processes can be programmed with VSTA (Visual Studio Tools for Applications). Programming skills are not required, as pre-engineered modules are available for this purpose.

To solve complicated measurement tasks, a library containing fragment ion distributions for several frequently occurring gases and compounds is available in the QUADERA® software. However these and other distributions obtained from spectra libraries can only be viewed as guideline values, as they are influenced by various parameters, such as ionization energy, temperature or the transmission characteristics of the mass analyzer.

In analyzing mixtures containing multiple gas components, the problem of overlapping ion currents of differing origin on the same mass numbers is one that frequently occurs. There are mass numbers whose intensity is produced exclusively by a single gas component (e. g. argon on mass number 40, oxygen on mass number 32, carbon dioxide on mass number 44 and water on mass number 18).

In the case of other mass numbers, the overall intensity of the detected ion current is governed by the overlapping of various concentrations of fragment ions from different gas components. Depending upon the composition and concentration ratios in the gas mixture to be analyzed, suitable algorithms and calibration procedures must also be formulated for the measurement task in question. Before carrying out quantitative gas analyses by applying suitable calibration gas mixtures that each have non-overlapping components, the calibration factors for each single gas component must be determined for all overlapping mass numbers. The concentration and/or partial pressure for these gases can then be determined by a matrix calculation. The QUADERA® mass spectrometer software performs the matrix calculation and provides the necessary gas-specific calibration routines.