Vacuum Technology Book, Volume II

6.1 Introduction, operating principle

Mass spectrometry is one of the most popular analysis methods. A mass spectrometer analyzes the composition of chemical substances by means of partial pressure measurement under vacuum.

Total and partial pressure measurement

Figure 6.1: Total and partial pressure measurement

Analyses are typically performed in the field of research & development and in the production of products that are used in daily life:

  • Research & Development
    • Catalysis research
    • Drug development
    • Development of new materials
  • Monitoring production processes
    • in metallurgy
    • in chemical synthesis
    • in semiconductor production
    • in surface technology
  • Trace and environmental analysis
    • Aerosol and pollutant monitoring
    • Doping tests
    • Forensic analysis
    • Isotope analysis to identify the origin
  • Analysis of products in
    • the chemical industry
    • high purity gas production
    • pharmaceuticals
    • the automotive (supply) industry (leak detection)
    • Quality assurance of food products

Mass spectrometers are used to analyze gases. Solid or liquid substances can also be analyzed if they are vaporized in an upstream inlet system. The gas is diluted by pumping it down to a low pressure (molecular flow range) in a vacuum chamber and ionizing it through electron bombardment. The ions thus generated are separated in a mass filter according to their charge-to-mass ratio.

Components of a mass spectrometer

Figure 6.2: Components of a mass spectrometer

Figure 6.2 shows the typical components of a mass spectrometer system.

  • The substances to be analyzed are admitted to a vacuum chamber through the inlet system via a capillary or metering valve, for example, and then are
  • pumped down with the vacuum system to the system’s working pressure.

The actual analyzer is located in the vacuum and consists of the following components:

  • The ion source ionizes neutral gas particles, which are then
  • separated in the mass filter on the basis of their mass-to-charge ratio m/e.
  • The ion current is measured using a Faraday detector or a secondary electron multiplier (SEM) after the ions have left the separating system. The current is a measure of the partial pressure of the respective gas molecules or a measure of fragments that may have been generated in the ion source.
  • A data analysis system processes the ion currents measured with the aid of the detector and presents these currents in various forms. Today, data analysis software programs are capable of supporting the user in interpreting mass spectra.

Mass spectrometers differ as a result of the wide variety of available versions. The main difference consists of the separating systems. The following four types of mass filters are in widespread use today:

  • Sector field devices use the deflection effect of a magnetic field on moving charge carriers.
  • Time-of-flight mass (TOF) spectrometers utilize the differing velocities of molecules of equal energy for the purpose of separation.
  • In ion traps, the trajectories of the ions are influenced by a high-frequency field.
  • Quadrupole mass spectrometers utilize the resonance of moving ions in a high-frequency field (similar to ion traps).

Our discussion will be confined to sector field mass spectrometers and quadrupole mass spectrometers, as these are the mass spectrometers that are most widely used in the field of vacuum technology.

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