Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Patent
1991-08-05
1993-07-13
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
250282, 250287, 250423R, B01D 5944, H01J 4900
Patent
active
052276287
DESCRIPTION:
BRIEF SUMMARY
This invention relates to ion mobility detectors.
Ion mobility detectors are used to detect the presence of materials in an environment, for example contaminants in atmospheric air. A library of known possible contaminants is built up and the measurements known for these are then compared with the results from an unknown species to decide whether a sample contains a contaminant and if so whether it has already been identified. Measurement of concentration or an indication of concentration can be given as well as qualitative identification of the species.
Typical prior art ion mobility detectors have an ionization source, an ion reaction region, an ion drift region, e.g. in the form of a tube, an ion injection shutter or grid interposed between the ion reaction region and the ion drift region, and an ion detector. The systems operate at atmospheric pressure where the mean free path of the contained gas molecules in the drift region is a small fraction of the dimensions of the container. A carrier gas, normally purified atmospheric air (particularly purified to remove water vapor which can interfere with the detection of certain types of charged species) is introduced into the ion mobility detector with a sample gas or vapor of the material whose identity is to be determined by characterization of its ion mobility properties. The carrier gas containing the sample is introduced through an inlet so as to be exposed to the ionization source. This causes portions of both the carrier gas and the sample to be directly ionized at the ionization source. The molecules of the carrier gas are present in far greater numbers than the sample and so more of these are ionized. The gaseous mixture is located within the reaction region at this stage and since the mean free path is many times smaller than the dimensions of the reaction region, multiple collisions between the molecules of the carrier and the sample gas(es) occur, the result of which is that the ion charge tends to be transferred by these collisions from the carrier molecules to the sample molecules thus resulting in a secondary ionization process which ionizes an increased number of the molecules of the sample. The reaction region is normally arranged to be under the influence of a potential gradient which moves the charged mixture towards the ion injection grid which is electrically charged to prevent transfer of ions from the reaction region to the drift region but which can be deenergized so as to let a pulse of ions pass through into the drift region. Accordingly, periodically the grid is de-energized for a short time and a number of ions are introduced into the drift region. This period is called the cycle time and can be varied. The drift region is arranged to be under the influence of an electrostatic drift field or potential gradient which acts to move ions in the drift region down the tube away from the ion injection grid towards a detector electrode which collects the charge from the ions and is located at the end of the drift region. The time of arrival of each ion at the detector grid relative to the time that the ion injection grid was opened is determined by the mobility of the ion in the carrier gas occupying the drift region. Heavier ions move more slowly through the drift region and take longer to travel to the detector than lighter ions. Ions with the same mobility have their velocities modified slightly due to diffusion effects; when they arrive at the detector electrode they are spread in an error function, the peak of which enables one to determine the time taken between the opening of the grid and arrival of the group at the detector. This can be used to characterize the ions.
However, some molecules which it is wished to detect ionize to form positive species and others form negative species and some form both species. If detection of both species could be simultaneously achieved on the same sample this would be attractive. Thus using a single conventional device a delay will be incurred in switching from positive to negative operating co
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patent: 4855595 (1989-08-01), Blanchard
patent: 5021654 (1991-06-01), Campbell et al.
patent: 5070240 (1991-12-01), Lee et al.
Anderson Bruce C.
Graseby Dynamics Limited
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