Ion-mobility spectrometer
An ion mobility spectrometer (IMS) is a device for chemical analysis . It was first known under the name of plasma chromatograph and is characterized by low detection limits (lower ppb range), short response times and the ability to detect different chemical substance classes at ambient pressure.
functionality
The analyte is ionized and the ions are “pulled” through a gas by an electric field . The ions are slowed down by collisions with the gas molecules, whereby this “frictional force” is stronger with large molecules than with small molecules. Therefore, small molecules usually move at a higher speed in the gas. It is crucial that the ions absorb energy in the electrical field between two collisions and release it again when there is a collision. Since this happens very quickly, the ions reach a mean speed that is characteristic of them. The drift in the electric field is called the drift speed . Since this drift speed is different for different ions of the analyte molecules, primarily because of the molecule size, but also because of other physical parameters ( polarizability ), they can be distinguished from one another. Often it is also possible to separate isomers that have the same mass, but have different geometrical structures and therefore have different impact parameters and therefore different drift speeds.
There are two main designs that can be distinguished in terms of the strength of the electric field in the drift tube:
- low-energy tables with direct electric fields around 300 V / cm
- higher energetic ones with direct and alternating fields well over 1000 V / cm
Characteristic orders of magnitude for the drift speed are 10 m / s, for drift distances of 10 cm around 10 ms are required.
If a complete differentiation cannot be achieved by the drift speed alone, chromatographic methods are used ( chromatography or gas chromatography ) in order, ideally, to allow the molecules to enter the ionization space one after the other. This creates characteristic three-dimensional dependencies on the retention time, the drift time and the ion current.
The main difference to electrochemical gas sensors is that there chemical reactions acting on the sensor change the electrical conductivity .
In mass spectrometers is in high vacuum worked, in contrast, in IMS under ambient pressure. Therefore, mass spectrometers require high vacuum pumps and are usually significantly larger than IMS. From a physical point of view, the probability that an ion “hits” a molecule on the drift path is close to zero in a mass spectrometer. In the case of the IMS, this probability is very close to 1. As a rule, very many shocks occur, so that a constant drift speed is reached in the electric field after a very short distance.
Many applications of IMS are based on the fact that air can be used directly as a carrier gas.
Applications
- Detection of explosives (e.g. at airports), drugs , chemical warfare agents and mold
- Used in the odorization of natural gas
- Use in the diagnosis and therapy of lung diseases, e.g. B. bronchial carcinoma , chronic obstructive pulmonary disease or sarcoid , rejection reactions after lung transplants and colonization with bacteria , see respiratory gas analysis
research
Intensive research continues in the field of ion mobility spectrometry. The main aim here is to further increase the resolution and its sensitivity.
The ISIMS conference takes place annually. In addition, the IMS user meeting is held in Germany at regular intervals. There is also the International Journal for Ion Mobility Spectrometry, a scientific journal published by Springer-Verlag .
Method literature
- Jörg Ingo Baumbach, Gary A. Eiceman: Ion Mobility Spectrometry: Arriving On Site and Moving Beyond a Low Profile. In: Applied Spectroscopy. Vol. 53, No. 9, 1999, ISSN 0003-7028 , pp. 338A-355A, doi: 10.1366 / 0003702991947847 .
- Gary A. Eiceman, Zeev Karpas: Ion Mobility Spectrometry. 2nd edition. Taylor & Francis, Boca Raton FL et al. 2005, ISBN 0-8493-2247-2 .
- Bengt Nölting: Methods in Modern Biophysics. 3rd edition. Springer, Berlin et al. 2009, ISBN 978-3-642-03021-5 .
- Joachim Stach, Jörg I. Baumbach: Ion Mobility Spectrometry - Basic Elements and Applications. In: International Journal for Ion Mobility Spectrometry. Vol. 5, No. 1, 2002, ISSN 1435-6163 , pp. 1-21.