Ion-mobility spectrometry

An ion mobility spectrometer (English: ion mobility spectrometer, IMS ) is a device for chemical analysis. It was first known under the name plasma chromatograph and is characterized by low detection limits (lower ppb range ), short response times and the detectability of different chemical classes of compounds at ambient pressure.

Operation

The analyte is ionized and the ions are of an electric field is "pulled " through a gas. Due to collisions with the gas molecules, the ions are slowed, said " friction force " for large molecules is stronger than that of small molecules. Therefore, small molecules typically move at a higher velocity in the gas. It is crucial that the ions gain energy in the electric field between collisions and take it to one shock again. Since this is very fast and the ions reach a characteristic for it middle speed for the drift in an electric field, the drift velocity. As this drift velocity is different mainly because of molecular size, but also because of other physical parameters ( polarizability ) for various analyte ions of the molecules, they can be distinguished from each other. Often, it is also possible to separate the isomers, although have equal masses but different geometric structure and functioning of other impact parameters and therefore different drift velocities.

There are two main types, which can be distinguished in the strength of the electric field in the drift tube:

  • Low energy with DC electric fields around 300 V / cm
  • Higher energy with DC and AC fields well above 1000 V / cm

Typical orders of magnitude for the drift velocity of 10 m / s, are ms to 10 for drift distances of 10 cm is required.

If no complete distinction can be achieved by the drift velocity on their own, chromatographic methods are used (chromatography or gas chromatography), for admitting the molecules in the ideal case, one after the other in the ionization chamber. This creates three-dimensional characteristic dependences of the retention time, the drift time and the ion current.

The main difference to the gas sensors is that there also change on the sensor acting chemical reactions electrical conductivity.

In mass spectrometers, operating in contrast to the IMS in a high vacuum, with IMS at ambient pressure. Therefore, mass spectrometers require high vacuum and are significantly larger than in the IMS rule. In physical terms, the probability that an ion, a molecule on the drift path "meets" the mass spectrometer near zero. At the IMS, this probability is very close to 1 Usually occur very many collisions, so after a very short distance, a constant drift velocity is achieved in an electric field.

Many applications of IMS based on the fact that air can be used as a carrier gas directly.

Applications

  • Detection of explosives ( eg airports ), drugs, chemical warfare agents and molds
  • Use in the odorisation of natural gas
  • Use in the diagnosis and treatment of lung diseases, such as lung cancer, chronic obstructive pulmonary disease or sarcoidosis, graft rejection after lung transplantation and bacterial colonization, see respiratory gas analysis

Research

In the field of Ionenmobiltätsspektrometrie will continue intensive research. It is all about the resolution, as well as their sensitivity to increase. It takes place every year the conference ISIMS. In addition, at regular intervals, the IMS User Group meeting will be held in Germany. In addition, there is a scientific journal in the Springer publishing the International Journal for Ion Mobility Spectrometry.

Literature on the method

  • I. J. Baumbach, G. A. Eiceman: Appl. Spectrosc. 1999, 53, 338A - 355A.
  • Gary Eiceman & Zeev Karpas, Ion Mobility Spectrometry, CRC Press, 2005, ISBN 0-8493-2247-2
  • Bengt Nolting, Methods in Modern Biophysics, Springer Verlag, 2005, ISBN 3- 540-27703 -X
  • J. Stach, J. I. Baumbach: Int. J. Ion Mobility Spectrom. 2002, 5, 1-21.
Gas detector Breath gas analysis
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