Mass spectrometry (MS) is one of the foremost analytical methods of the modern age. It involves ionizing chemical compounds to generate charged molecules or molecule fragments, which are then separated and detected based on their mass-to-charge ratios (m/z). The resulting mass spectrum provides detailed molecular weight, structure, and composition information. Accuracies of within 1-2 parts per million (ppm) and measurement deviations below 1% make MS indispensable across the entire gamut of science and industry.
Quadrupole mass spectrometry (QMS) is a specific sub-method of MS known for its fast scanning capabilities, reliability, versatility, and ability to provide real-time analysis. This blog post will explore the working principles of a quadrupole mass spectrometer and how it differs from other types.
How Does Mass Spectrometry Work: Step by Step
A general mass spectrometry setup includes an ion source, a mass analyzer, and a detector. The ion source ionizes the sample, converting molecules into charged ions. These ions are then directed into the mass analyzer, where they are separated based on their m/z ratios. This is the basic premise of most forms of MS. Differences typically arise in the type of mass analyzer used (quadrupole, time-of-flight, ion trap, etc.). Finally, the separated ions reach the detector, which records their abundance, generating a mass spectrum.
How Does a Quadrupole Mass Analyser Separate Ions?
Quadrupole mass spectrometry differs from other MS forms by using four rods with RF and DC potentials to filter ions by m/z ratio. It offers high scanning speed, compact design, and broad mass range analysis, unlike time-of-flight or ion trap MS, which utilize different ion separation mechanisms.
As ions pass through the quadrupole, only those with stable trajectories, determined by the applied electric fields, reach the detector. The quadrupole mass spectrometer consists of three main components: the ion source, the mass analyzer, and the ion detector.
Ion Source
The ion source is the first critical component in quadrupole MS. Typically, electron impact ionization is used, where a heated filament emits electrons that collide with sample molecules, resulting in ionization. Chemical ionisation is, likewise, common. Other, more specialised methodologies, include threshold ionisation mass spectrometry (TIMS) and plasma-assisted desorption ionisation (PADI).
Mass Analyzer
The ions produced in the ion source are then accelerated into the quadrupole mass analyzer. This analyzer consists of four parallel rods arranged in a square configuration. Opposite rods are connected electrically and have radio frequency (RF) and direct current (DC) potentials applied to them. The combined RF and DC fields create a dynamic electric field that influences the trajectory of the ions.
Ions of a specific m/z ratio will have stable trajectories and pass through the quadrupole, while others will be filtered out due to unstable trajectories. By varying the RF and DC potentials, the quadrupole can selectively allow ions of different m/z ratios to reach the detector, generating a mass spectrum.
Ion Detection
Ions that successfully traverse the quadrupole are detected using devices such as Faraday cups or electron multipliers. The detection process converts the ion signal into an electrical signal, which is amplified and recorded. The resulting data is processed to produce a mass spectrum that displays the abundance of detected ions as a function of their m/z ratio.
Where is QMS Used?
Quadrupole mass spectrometry excels in high sensitivity and rapid scanning capabilities, making it indispensable for precise molecular analysis across diverse applications.
- Quadrupole MS monitors and analyses the composition of gases in environmental studies, industrial processes, and research applications. It provides high sensitivity and accuracy in detecting trace gases and volatile organic compounds (VOCs).
- QMS is also essential in catalysis research, enabling the study of reaction intermediates and products. It helps understand catalytic mechanisms and optimise reaction conditions for improved efficiency and selectivity.
- In electrochemistry, quadrupole MS analyses products generated during electrochemical reactions. Techniques such as Differential Electrochemical Mass Spectrometry (DEMS) combine electrochemical cells with mass spectrometers to study real-time reaction mechanisms and kinetics.
Want to Learn More About Quadrupole Mass Spectrometry?
The quadrupole mass spectrometer is a powerful tool for chemical analysis, offering high sensitivity, precision, and versatility. Its ability to filter and detect ions based on their m/z ratios makes it invaluable in numerous scientific and industrial applications. If you want to learn more, read our blog on the basics of quadrupole mass spectrometry or explore our research papers to find specific use cases.