Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
1998-07-30
2001-05-01
Anderson, Bruce C. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S288000
Reexamination Certificate
active
06225623
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to a corona discharge ion source for use in analytical instruments, and in particular for use in ion mobility spectrometers.
2. Background of the Invention
Hitherto, corona discharge sources have been used in ion mobility spectrometers in order to produce the primary ions required for the operation of the instrument. Device is shown in co-pending published PCT application No. WO/9311554.
The principle reason for employing such corona discharge ion sources has been to replace the most frequently used nickel
63
radioactive ionisation source. The corona discharge ion source is significantly cheaper than the nickel
63
source. It is also not subject to the health and safety requirements of a radioactive source and may therefore be more readily transported across borders and so forth.
In substituting a corona ion source for the nickel
63
source, the emphasis has been to replicate as far as possible the ion-molecule chemistry produced by the nickel
63
source, in order to ensure that an ion mobility spectrometer fitted with a corona discharge ionisation source detects the same range of compounds as when fitted with a nickel
63
source.
FIG. 4
shows a typical output of an ion mobility spectrometer in the absence of an introduced compound or impurity to be detected. This peak corresponds to stable molecular ion species which have resulted from a complex series of ion-molecule reactions and is referred to generally as the Reactant Ion Peak (RIP). When a sample to be detected, such as in this example RDX (a major constituent of Semtex), is introduced into the ion mobility spectrometer, a further peak (or peaks) is detected as well as the reactant ion. The problem with this procedure is that, in practice, the sample entering the ion mobility spectrometer contains a significant number of other compounds. If these have a similar mobility to the RDX ions, the signature peak of the RDX is reduced in amplitude and may in certain cases be suppressed by the contaminants to such an extent that the RDX peak is no longer clearly visible. A schematic plot of the output of an ion mobility spectrometer under these conditions is shown in FIG.
11
.
A technique known as chemical doping has been developed to address this problem, and is frequently used in ion mobility spectrometry and chemical ionisation mass spectrometry. Chemical doping may be used irrespective of the ionisation source used to generate the primary ions (i.e. either a corona discharge ion source or a nickel
63
ion source) to change the way in which sample vapour introduced to the device becomes ionised.
The use of such chemical dopants is described in
Analytical Chemistry,
56(11):1794-1797 by Procter and Todd.
In outline, a chemical dopant, typically in the form of a vapour or gas is introduced into the ionisation region of the instrument such that the dopant chemical becomes the dominant reactant ion species in the ionisation region of the instrument and, if an incoming sample vapour molecule is to be ionised, it must undergo an ion-molecule reaction with the dopant reactant ion.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an analytical instrument including a corona discharge ionisation source arranged to generate corona dopant ions.
When the energy density around the point of a corona discharge is above a certain level, new reaction compounds may be generated. Typically, when the discharge is conducted in air, these compounds will include ozone, oxides of nitrogen and excited neutral states of nitrogen. Since these reaction compounds influence the ion-molecule chemistry in an analytical instrument, previous work has been directed towards minimising the concentration of the compounds, so that their effect on the functioning of the instrument is negligible.
It has been found, however, that the products of the corona discharge ionisation source may be employed as dopant ions, whereby the ion-molecule chemistry of the instrument may be altered under external control.
Preferably, the analytical instrument is an ion mobility spectrometer, the ionisation region of which is doped by the corona dopant ions generated by the corona discharge ionisation source.
By means of, for example, electronic control of the corona discharge, the doping regime of an ion mobility spectrometer may be rapidly and easily altered to provide changing ionisation chemistry conditions, thereby to modify instrument sensitivity and/or selectivity readily.
Preferably, the corona discharge ionisation source is arranged to generate the corona dopant ions substantially continuously. Under such conditions, only a few types of sample vapours, such as explosive compound vapours, are capable of efficient ionisation and hence detection. Thus, the selectivity of the analytical instrument to these compounds is better than that of a system undoped by discharge compounds.
Alternatively, the analytical instrument may have switching means for switching the corona discharge ionisation source such that the dopant ions are generated selectively. Thus, the analytical instrument fitted with a corona discharge ionisation source can, at times, operate under conditions such as to produce the corona dopant ions to dope an ionisation region thereof, and can, at other times, operate under conditions such as not to produce those corona dopant ions, the instrument then operating as an undoped system.
The change between a doped system and an undoped system can be accomplished by electronic switching means, for example, within a very short time, typically within a fraction of a second. Thus a sample vapour administered to the ion mobility spectrometer, for example, could be quickly analysed under two different doping regimes.
Analysis of the sample vapour under the two different regimes provides additional identification information. For instance, the sample vapour may be ionised when the system is undoped, but not when it is doped by the discharge compounds, and this may help to indicate that the sample vapour is not that from an explosive compound. In another case, the mobility of the detected ion may be different under the two doping regimes, and indicate that the ion species formed from the sample vapour is different in the two regimes. This has also provided further discriminatory information, provided that all sample vapours ionised in both regimes did not change their mobility by the same amount.
Preferably, the analytical instrument further comprises chemical dopant means arranged to generate different, chemical dopant ions. Preferably the chemical dopant ions are produced when the corona dopant ions are not being produced by the corona discharge ionisation source. The chemical dopant means may be a gas permeable source fitted within a circulating gas flow of the analytical instrument.
This mode of operation can be of value, for example, if the electron or proton affinity of the corona dopant ions produced by the corona discharge ionisation source are greater than those of the chemical dopant ions available from the permeation source.
If the analytical instrument is an ion mobility spectrometer, then the primary ions necessary for ionisation of a sample to be analysed may either be generated by the corona discharge ionisation source which also generates the corona dopant ions, or alternatively may be generated by a radioactive source, such as nickel
63
. In the latter case, the corona discharge ionisation source may not be fitted in the ionisation region of the instrument but instead external to that region, and possibly outside the body of the ion mobility spectrometer, for instance in a gas flow into the instrument. For example, the corona discharge ionisation source may be located in series with an incoming gas flow associated with the cell.
Corona dopant ions generated by the corona discharge would be carried into the cell and act as dopants in the manner described above, without the corona discharge source acting as the means of ionisation of the incoming sample. This method ma
Arnold Paul Douglas
Clark Alastair
Taylor Stephen John
Turner Robert Brian
Anderson Bruce C.
Graseby Dynamics Limited
Wallenstein & Wagner Ltd.
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