Mass Analyzers: Common Types

A quadrupole mass analyzer uses four parallel cylindrical rods with applied DC and radio-frequency AC voltages. The field strength causes only ions with specific m/z values to resonate with the field and strike the detector. Though cost-effective with high transmission rates, quadrupoles offer low resolution, typically around 1 Da, limiting exact elemental composition determination.

Time-of-flight analyzers measure ion flight time through a linear field-free drift tube to the detector. Since ions enter with similar kinetic energy, lighter ions travel faster and reach the detector sooner than heavier ions. This velocity difference converts flight time to m/z values, though kinetic energy differences between same-mass ions can impair resolution.

Reflectrons, or ion mirrors, slow down energetic ions traveling toward the detector, correcting kinetic energy spread among same-mass ions. More energetic ions experience greater slowdown, improving resolution overall. This increased travel distance and energy correction enables better separation and higher sensitivity in time-of-flight mass spectrometry.

Three-dimensional quadrupole ion traps consist of a ring electrode and two endcap electrodes that trap ions via radio-frequency voltage. Unlike other mass analyzers allowing only small ion fractions to reach the detector, ion traps enable approximately half of ions to reach it, providing superior sensitivity. However, space-charge effects from storing many ions can decrease resolution.

Increasing radio-frequency voltage in a three-dimensional quadrupole ion trap stabilizes progressively heavier ions that oscillate in the ring electrode. As voltage increases, ions of different masses become selectively stable and are ejected through endcap apertures to the detector at different times, enabling mass separation and detection.

In time-of-flight analyzers, all ions enter the drift tube with approximately equal kinetic energy. Since kinetic energy equals half mass times velocity squared, lighter ions must travel at higher velocities to maintain the same energy. Consequently, lighter ions reach the detector faster than heavier ions, producing shorter flight times.

Quadrupole mass analyzers typically achieve a resolution of 1 Da (or 1 u), meaning they can distinguish between ions differing by one atomic mass unit. This low resolution prevents precise determination of exact elemental composition but makes quadrupoles cost-effective instruments with high transmission rates for routine analytical applications.