4.1 Introduction, operating principle
Mass spectrometry is one of the most popular analysis methods today. A mass spectrometer analyzes the composition of chemical substances by means of 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:
- Analysis of products from the chemical industry
- Drug development
- Doping tests
- Quality assurance of food products
- Monitoring semiconductor production processes
- Isotope analysis
Gaseous or liquid substances that vaporize under vacuum are admitted to a mass spectrometer. The gas is diluted by being partially pumped down to a low pressure (molecular flow range) in a vacuum chamber and ionized through electron bombardment. The ions thus generated are introduced to a mass filter and separated on the basis of their charge-to-mass ratio.
Figure 4.2 shows the typical structure of a mass spectrometer system:
- The substances to be analyzed are admitted into a vacuum chamber through the inlet system via a capillary or metering valve, for example, and then
- Partially pumped down 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
- Sorted 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 measured current is a parameter of the partial pressure of the respective gas molecules or a parameter of fractals that may possibly 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 separation
- 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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