27 MS Analyzers*

The analyzers in mass spectrometers separate or filter ions based on m/z ratio. Analyzer types include magnetic sector, quadrupole, time-of-flight, and ion traps. Each has advantages and disadvantages in terms of size, cost, speed, and mass resolution.

Types of Analyzers

Sector Analyzer. Sectors were the first analyzers. A magnetic sector produces curved trajectories of different radii for accelerated ions of different m/z. Ions with the correct trajectory are passed through a slit to be detected, with scanning of different m/z values via changes in magnetic field strength. Resolution can be improved by adding an electrostatic analyzer (ESA) to make a dual sector or double focusing MS, where the ESA narrows the distribution of ion kinetic energies. Although historically important in the development of MS, the popularity of these analyzers is much lower in modern MS, in part due to large size, high cost, and slow scan speed.

Quadrupole Mass Filter. A quadrupole is perhaps the most common type of analyzer at present. It has the advantages of being compact, robust, relatively inexpensive, and fast scanning (10 Hz or better). Its main disadvantages are relatively low resolution and a limited mass range. Quadrupole analyzers consist of four parallel rods (typically ~1 cm diameter, ~25 cm length) in a diamond arrangement. One pair of opposing rods is held at direct negative potential; the other pair of opposing rods is held at direct positive potential. Out-of-phase alternating potentials are then applied to each pair of rods. When accelerated down the central axis of the quadrupole, ions take on an oscillating trajectory. Ions with too small or too large of a m/z have unstable trajectories, and only the target m/z ratio passes through the aperture at the end of the quadrupole to be detected. Scanning of m/z is achieved by varying the frequency of the alternating potential or by scanning the magnitudes of the direct and alternating potentials (while keeping their ratio constant).

Time-of-Flight (ToF) Analyzer. Another popular analyzer is a ToF analyzer. Packets of ions are accelerated and raced to the end of a flight tube (~1 m long), with ions of larger m/z taking longer than ions of smaller m/z. The arrival time at the detector enables determination of the m/z value. ToF provides very high scan speeds (up to kHz) with high resolution at reasonable cost. Resolution is further improved by adding an ion mirror, which approximately doubles the flight length and narrows the initial distribution of ion kinetic energies for a given m/z.

Quadrupole Ion Trap. A ring-shaped electrode and two end-cap electrodes trap ions in stable trajectories within a box-like chamber using applied direct and radio-frequency alternating potentials, similar to a linear quadrupole. The applied potentials are adjusted to generate axial instability and eject ions of a specific m/z ratio, with high resolution, through a hole in an end-cap electrode. A linear trap design also exists.

Ion Cyclotron Resonance (ICR). This technology is an ion trap that uses electric and magnetic fields to induce ions to take on a circulating trajectory, with motional frequencies that depend on m/z. Electrical currents are induced in electrode plates within the trap as ions pass close to them. A short radio frequency pulse excites these motions to greater radii that then decay back to smaller radii. The image currents likewise increase and then decay with the ion circulation frequencies superimposed. A Fourier transform of this data extracts all of the m/z values. Although large and expensive, ICR-MS provides extremely high resolution and fast scans. Newer “Orbitrap” technology operates similarly to ICR, but does not require a magnetic field and is thus far smaller and less expensive.

Analyzers for Tandem MS

Quadrupoles have made tandem MS practical. In a triple quadrupole configuration, the first and third quadrupoles select for m/z and the middle quadrupole is operated as a collision cell that produces fragments. Two initial quadrupoles can also be used similarly with a final ToF analyzer (q-ToF). Ion traps are also paired with other analyzers for tandem MS, or are themselves switched between analyzer and collision cell modes of operation.

Connections

• Mass analyzers have a role in mass spectrometry that is analogous to the role of monochromators (Ch. 4) in optical spectroscopies.
• Similar to how monochromators select a desired wavelength of light (Ch. 4), sector analyzers rely on a slit or aperture for selection of a desired m/z ratio.
• Higher resolution monochromators isolate a narrower range of wavelengths of light for measurement (Ch. 4); higher resolution mass analyzers physically (or conceptually) isolate a narrower range of m/z ratios.
• ToF MS analyzers use acceleration by an electric field to separate ions, which has conceptually similarities to electrophoresis (Ch. 22). Because MS occurs in a vacuum (without a frictional force), there is a relationship to mass-to-charge ratio (instead of the charge-to-hydrodynamic radius ratio that applies in solvent).
• ICR and Orbitrap analyzers are the mass spectrometry analogs of FTIR (Ch. 13).

Post-Reading Questions

1. What are two examples of analyzer components that narrow the distribution of ion kinetic energies?
2. Given the operating principles, why is a ToF analyzer used after a quadrupole mass filter in tandem MS, rather than the opposite?

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Quantify mass resolution.
• Relate the parameters used or measured by a mass analyzer to the m/z ratio of an ion.
• Understand how a narrower range of ion kinetic energies improves resolution.
• Draw and label simple diagrams of mass analyzers.
• Understand simple technical considerations in matching ion sources with mass analyzers.
• Recommend (with explanation) a type of mass analyzer for a given analysis.