Mass Spectrometers

A mass spectrometer serves to examine the mass of particles. With a mass spectrometer, the substance is first converted into the gaseous phase, then ionised and finally accelerated in an electric field. This particle beam, generated by the mass spectrometer, is split in the analyser according to the mass-to-charge ratio (m/q). A mass spectrometer is characterised by two parameters: mass resolution and mass accuracy. The least possible measurable distance of two particles is the mass resolution. The mass accuracy indicates how accurately the mass of a particle can be determined.

History of the mass spectrometer

In the early 19th century, the British chemist William Prout published his hypothesis that each atom has a distinct property, being its mass or atomic weight. Atoms were claimed to behave with regard to their mass as an integer multiple of the hydrogen atom. However, Berzelius and Turner later developed hypotheses that disproved Prout's hypothesis, for chlorine has 35.45 times the mass of hydrogen. In the mid-19th century, the influence of magnetic fields on the glow of glow-discharge lamps was established by Julius Plücker. In 1886 and 1898, the anode rays and their deflection by fields were published by Eugen Goldstein and Wilhelm Wien. In 1897, J. J. Thomson published several experiments concerning the deflection of cathode rays at various elements in a vacuum tube with electromagnetic fields. In 1919, the first operable mass spectrometer was built by Francis William Aston.

Architecture and functionality of a mass spectrometer

A mass spectrometer consists of an ion source, an analyser and a detector. The ionisation of the substance occurs in the ion source. Different ion sources can be used according to the nature of the analyte. The ions are pulled by an electrical field from the ion source and carried into the analyser. Inside the analyser, the ions are separated according to their mass. In mass spectrometry, the detector often is an electron multiplier, a photomultiplier or a Faraday cup. In the case of a quadrupole mass spectrometer, the generated ions are accelerated by a static electric field and fly centrally through four parallel lying rod electrodes whose intersections with a plain that is oriented vertically on the cylinder axis create a square, the so-called quadrupole. In the alternating field between the quadrupoles, a selection by m/q takes place. This means that only particles with a defined mass can pass the field. If the samples are very complicated, mass spectrometers are often coupled with chromatographs. This coupling is frequent with GC/MS and LC/MS.

Literature

• http://de.wikipedia.org/w/index.php?title=Massenspektrometrie&oldid=87004140 (called: 30.03.11).

• http://www.chemgapedia.de/vsengine/popup/vsc/de/glossar/m/ma/massenspektrometer.glos.html (called: 30.03.11).