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ICP-MS - The High-Resolution Mass Analyzer

This analyzer consists of several components acting in unison.

First, the ion beam passes through a narrow slit, which only allows those ions traveling along the correct axial plane of the mass spectrometer to pass through. This results in a narrow beam of ions all traveling parallel to each other.

The next stage of the mass analyzer is the electro static analyzer (ESA). It consists of two curved plates with applied DC voltage. The inner plate has a negative polarity, which attracts the positively charged ions, and the outer plate has a positive polarity, which repels the ions. The ion beam passes between these two plates and is both focused and curved through an angle of around 40. Because only ions with a narrow range of kinetic energy are able to pass through the ESA, it forms an effective energy filter.

Schematic of the ion optics of the VG Axiom

The next part of the high-resolution mass analyzer is the magnet. When a charged particle passes through a magnetic field it is subjected to a force perpendicular to both its direction of travel and the direction of the magnetic field. This results in a circular trajectory with a radius proportional to the kinetic energy of the particle and its mass/charge ratio (m/e). Because the ion beam has already passed through the ESA, all ions entering the magnetic field have a similar kinetic energy. In a uniform magnetic field, the radius of curvature for particles of the same energy but of different m/e varies according to the square root of their m/e ratio.

The final part of the system is the collector slit. This is another narrow slit, situated at the focal point of the magnet. High resolutions are achieved by making both of the slits very narrow so that the beam reaching the detector has only a very narrow bandwidth of mass at any given time.

The high-resolution mass analyzer is known as a double focusing system because it is able to focus both energy and mass/charge.

ICP Main | ICP Creating Ions | ICP Sampling Ions | ICP Analysis | ICP Quadrupole | ICP High-Resolution
ICP Comparison Quadrupole | ICP Comparison High-Resolution | ICP Analysis: Calibration