Mass Analyzers

Mass analyzers separate ions by their mass-to-charge ratio, and their design sets a mass spectrometer's resolving power, mass range, and speed.

Definition

A mass analyzer is the component of a mass spectrometer that disperses or selects gas-phase ions according to their mass-to-charge ratio so that their abundances can be measured.

Scope

This topic covers the principal mass-analyzer types: magnetic-sector, quadrupole, ion-trap, time-of-flight, Orbitrap, and Fourier-transform ion-cyclotron-resonance analyzers. It treats how each separates ions, the figures of merit—resolution, mass accuracy, mass range, and duty cycle—that distinguish them, and the trade-offs that guide instrument choice.

Core questions

• How does each analyzer type physically separate ions by mass-to-charge ratio?
• What determines resolving power and mass accuracy in a given analyzer?
• How do scanning analyzers differ from trapping and time-of-flight analyzers in duty cycle?
• Which analyzer suits targeted quantitation versus high-resolution accurate-mass work?

Key theories

Quadrupole mass filtering

Oscillating radiofrequency and direct-current voltages on four parallel rods create a field that gives only ions within a narrow mass-to-charge window stable trajectories through the rods; scanning the voltages sweeps this window across the spectrum, making the quadrupole a robust, fast mass filter.

Orbital and cyclotron frequency analysis

In Orbitrap and ion-cyclotron-resonance analyzers, ions oscillate or orbit at frequencies determined by their mass-to-charge ratio; measuring these frequencies and Fourier-transforming the signal yields very high resolution and mass accuracy.

Mechanisms

Each analyzer exploits how ions of different mass-to-charge ratio respond to fields. Magnetic sectors bend ion paths by radius; quadrupoles transmit a selected window through oscillating fields; ion traps store and eject ions selectively; time-of-flight analyzers separate ions by their flight time over a fixed distance; and Orbitrap and ion-cyclotron-resonance analyzers measure characteristic oscillation frequencies. A detector records ion abundance versus mass-to-charge ratio to build the spectrum.

Clinical relevance

Analyzer choice directly shapes analytical capability: triple-quadrupole instruments enable sensitive targeted quantitation in clinical and toxicological assays, while high-resolution time-of-flight and Orbitrap analyzers support accurate-mass identification in proteomics, metabolomics, and screening.

History

Early mass spectrometers used magnetic sectors. Wolfgang Paul's quadrupole and ion trap in the 1950s and 1960s introduced compact electrodynamic analyzers, and time-of-flight analysis, conceived earlier, matured with fast electronics. Alexander Makarov's Orbitrap, reported in 2000, brought very high resolution and mass accuracy in a compact electrostatic trap.

Key figures

• Wolfgang Paul
• Alexander Makarov
• William Stephens

Seminal works

• makarov2000
• gross2017
• skoog2017

Frequently asked questions

What is resolving power in a mass analyzer?

Resolving power is the ability to distinguish ions of nearly equal mass-to-charge ratio; high-resolution analyzers can separate peaks differing by a small fraction of a mass unit, which helps confirm elemental composition.

Why choose a quadrupole over a high-resolution analyzer?

Quadrupoles are robust, fast, and inexpensive and, in triple-quadrupole form, excel at sensitive targeted quantitation, whereas high-resolution analyzers are preferred when accurate mass is needed to identify unknowns.