Analyseurs de masse : présentation

Common mass analyzers include magnetic sector, double-focusing, quadrupole, time-of-flight, and ion trap analyzers. Each type separates ions according to their mass-to-charge ratios using different physical principles. Magnetic sector analyzers use a magnetic field to deflect ions, while double-focusing analyzers combine electrostatic and magnetic sectors for enhanced separation. These mass analyzers common types serve different analytical needs depending on resolution requirements and sample complexity.

Resolution describes the ability to differentiate ions with similar mass-to-charge ratios. It is expressed as the ratio of a peak's nominal m/z value to the smallest resolvable difference between it and an adjacent peak. Unit resolution indicates the ability to distinguish masses differing by one integer unit, such as m/z 50 from m/z 51. Commercial spectrometers typically achieve resolution ranging from 500 to 500,000.

A simple magnetic sector analyzer requires ions to have approximately the same kinetic energy when entering the magnetic field to ensure reliable deflection to different trajectories. Minor kinetic energy variations caused during fragmentation and acceleration make ion separation less effective. This limitation reduces the analyzer's ability to differentiate ions with similar m/z values, compromising overall resolution performance.

A double-focusing analyzer employs an additional electrostatic sector field to correct kinetic energy differences before ions enter the magnetic sector. The electrostatic field focuses the ionic beam to minimize kinetic energy spread, while the magnetic sector sorts ions based on deflection. This dual-focusing approach can achieve as much as a tenfold increase in resolution compared to simple magnetic sector analyzers.

In a magnetic sector analyzer, a magnetic field is applied to deflect moving charges along a curved path. For a given magnetic field strength, the radius of curvature depends on the m/z value of each ion. Ions with specific m/z values follow particular curved trajectories and pass to the detector. By continuously varying the magnetic field, the instrument sequentially passes ions across a broader m/z range to the detector.

After the vaporized sample is bombarded with a high-energy electron beam in the ionization chamber, generating radical cations and fragments, a series of negatively charged accelerator plates accelerate the cations into the mass analyzer. These accelerated ions then enter the analyzer where they are separated according to their mass-to-charge ratios before reaching the detector.

Unit resolution indicates the ability of a mass spectrometer to differentiate between masses with a difference of one integer unit. For example, an instrument with unit resolution can distinguish m/z 50 from m/z 51, m/z 100 from m/z 101, or m/z 500 from m/z 501. This metric helps evaluate whether an analyzer can resolve closely spaced peaks in a mass spectrum.