Ion mobility spectrometer

An ion mobility spectrometer (IMS) is a spectrometer capable of detecting and identifying very low concentrations of chemicals based upon the differential migration of gas phase ions through a homogeneous electric field. IMS devices come in a wide range of sizes (often tailored for a specific application) and are capable of operating under a broad range of conditions. Systems operated at higher pressure (i.e. atmospheric conditions, 1 atm or 760 Torr) are also accompanied by elevated temperature (above 100° C), while lower pressure systems (1-20 Torr) do not require heating. Elevated temperature assists in removing ion clusters that may distort experimental measurements.

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Operation

In its simplest form an IMS system measures how fast a given ion moves in a uniform electric field through a given atmosphere. The molecules of the sample need to be ionized, usually by corona discharge, atmospheric pressure photoionization (APPI), electrospray ionization (ESI), or a radioactive source, eg. a small piece of ⁶³Ni or ²⁴¹Am, similar to the one used in ionization smoke detectors.

In specified intervals, a sample of the ions is let into the drift chamber; the gating mechanism is based on a charged electrode working in a similar way as the control grid in triodes works for electrons. For precise control of the ion pulse width admitted to the drift tube, more complex gating systems such as a Bradbury-Nielsen design are employed. Once in the drift tube, ions are subjected to a homogeneous electric field ranging from a few volts per centimeter up to many hundreds of volts per centimeter. This electric field then drives the ions through the drift tube where they interact with the neutral drift molecules contained within the system. Separation of chemical species is achieved based upon the ion mobility (a parameter that is dependent of ion mass, size, and shape) where they arrive at the detector for measurement. Ions are recorded at the detector in order from the fastest to the slowest, generating a response signal characteristic for the chemical composition of the measured sample. Often the detector is a simple Faraday plate, however, more advanced ion mobility instruments are coupled with mass spectrometers where both size and mass information may be obtained simutaneously.

Perhaps ion mobility spectrometry's greatest strength is the speed at which separations occur--typically on the order of 10's of milliseconds. This feature combined with its ease of use, relatively high sensitivity, and highly compact design have allowed IMS as a commercial product to be used as a routine tool for the field detection of explosives, drugs, and chemical weapons. In the pharmaceutical industry IMS is used in cleaning validations, demonstrating that reaction vessels are sufficiently clean to proceed with the next batch of pharmaceutical product. As a research tool ion mobility has also shown great strides towards the analysis of biological materials, specifically, proteomics and metabolomics.

References

• Bengt Nolting, Methods in Modern Biophysics, Springer Verlag, 2005, ISBN 3-540-27703-X
• Gary Eiceman & Zeev Karpas, Ion Mobility Spectrometry, CRC Press, 2005, ISBN 0-8493-2247-2

See also

• Electrical mobility
• Viehland-Mason Theory
• Explosive detection