Research Interests
Dstl, Salisbury
The combination a wide variety of equipment is available. This includes Ion Mobility Spectrometers(IMS), a Selected Ion Flow Tube (SIFT)
with electron impact (EI), chemical ionization (CI) and flowing afterglow (FA) ion sources, and an Ion Trap Mass Spectrometer (ITMS,
with atmospheric pressure chemical ionization and ESI sources) allow a wide ranging study of ion-molecule chemistry to be studied over nine orders of magnitude in pressure.
The results of the study will not only yield valuable fundamental information about the complex area of atmospheric pressure ion-molecule chemistry but will also play an
integral part in the design of future ion-chemistry based atmospheric pollutant detectors.
IMS is a technique used primarily for the monitoring of atmospheric pollutants.
IMS utilizes ion-molecule reactions that occur at atmospheric pressure following ionization (usually by radiolysis).
Product ions that are formed by reaction of the sample gas with ions produced in air travel along a potential gradient through
a region of counter-flowing pure air (drift tube) towards a Faraday plate detector. As the drift tube is at atmospheric pressure the ions
undergo many collisions and interactions with the air and through these different ions separate out in time.
The current at the Faraday plate displayed as a function of time gives the mobility spectrum. Mobilities (which are inversely proportional to the drift time
of an ion, and are characteristic of an ion) are dependent upon a number of factors including mass, shape, size and polarizability of the drift tube gas.
Different samples may respond in either the negative ion or positive ion mode (or both) and changing the potential gradient of the drift tube allows both
ion polarities to be observed. The technique can detect atmospheric pollutants at the ppb level and below. In the laboratory we have a number of IMS
systems coupled to quadrupole mass spectrometers via a small orifice in the Faraday plate. The drift tube is at atmospheric pressure whilst the mass spectrometer
chamber is at about 8x10-5 Torr. This combination enables us to determine the nature of the ions in the mobility spectrum and to study the ion molecule chemistry
of the system at atmospheric pressure. We have three tandem ion mobility spectrometer/mass spectrometer (IMS/MS) systems which can all be configured in different ways
(e.g. different pressure regimes, inlet systems, drift tubes, temperatures, etc.) allowing for detailed investigations into IMS processes.
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