The present invention relates to ion mobility spectrometry (IMS)
IMS equipment is increasingly used for the detection and location
of gases and vapours of interest in ambient atmosphere. The principles of operation
of such equipment are well known in the art and are described, for example, in
"Plasma Chromatography" ed. T W Carr, Plenum Press (1984).
One disadvantage of the IMS technique is that water vapour and other
contaminants present in the equipment can interfere with the detection of many
vapours of interest. This has led to the practice of providing a flow of dry clean
air within the equipment into which samples are introduced, the dry clean air flow
being provided either from an external source, or, in the case of portable equipment,
such as the applicants' CAM (RTM) chemical agent monitor, by means of an internal
circulatory system including filters and driers. Such a system is described in
US Patent No. 4 317 995, which corresponds with EP-A-0,021,518.
The need to provide a continuous flow of dry clean air has constrained
miniaturisation of IMS equipment as continual operation of an electric fan or pump
to maintain airflow in the circulatory loop calls for a power source far larger
than would otherwise be necessary and considerably complicates the internal design
and the manufacture of the equipment.
It is an object of the invention to at least partly obviate these
drawbacks and thereby enable IMS equipment to be further miniaturised.
According to the present invention there is provided IMS equipment
as set out in claim 1. The invention is characterised in that the IMS cell and
the body of absorbent material are disposed within a diffusion compartment, the
water vapour or other interfering species diffusing within the diffusion compartment
to the body of absorbent material.
Preferably, the sample means comprises pressure-pulse means arranged
to create a negative pressure pulse within the compartment, thereby drawing in
the sample via the inlet. The pressure-pulse means may be a loudspeaker or, alternatively,
The invention also extends to IMS equipment comprising a hermetically
sealed compartment containing an IMS cell and a body of absorbent material, into
which compartment samples may be introduced to enable the detection or identification
of gases or vapours of interest present in such samples, water vapour or other
interfering species introduced into or otherwise present in the compartment diffusing
within the compartment and being absorbed by the body of absorbent material, whereby
a dry clean atmosphere may be maintained within the compartment.
IMS equipment in accordance with the invention, in which there is
no requirement for a continuous flow of dry clean air, may be made smaller, simpler,
less bulky and more rapidly operable than hitherto.
The invention may be carried into practice in a number of ways, and
one specific embodiment will now be described with reference to the accompanying
drawings, in which:
- Figure 1 is a diagrammatic representation of an IMS instrument in accordance
with an embodiment of the present invention;
- Figure 2 is a cross-sectional view illustrating more specifically the construction
and layout of part of a preferred embodiment of an IMS instrument;
- Figure 3 is a plan view of part of the embodiment of Figure 2; and
- Figure 4 shows an end view of the embodiment of Figure 2, looking in the direction
of the arrow IV of Figure 2, with the end closure removed.
Referring to Figure 1, IMS equipment (shown here schematically) comprises
a sealed case 10, within which is mounted an IMS cell assembly 12, a gauze-faced
box 14, containing a body of molecular sieve material, and a dopant permeation
The IMS cell assembly 12 comprises an ionizing source 18, internally
coated with a radioactive source material, typically Nickel-63, to ionise incoming
vapour molecules; an electrode structure comprising a gate electrode 19 and a
series of electrodes 20, to establish an electrostatic field along the length of
the cell 12; and a collector electrode 22, connected to instrument signal processing
and control circuitry 24, which in turn is connected, inter alia, to a display
or alarm unit 26.
A power supply 28 provides appropriate voltages and currents for
the IMS cell 12, and for the processing and control circuitry 24.
The case 10 contains a pin-hole aperture 30 in a wall 32 thereof,
directly forward of the ionizing source 18 of the IMS cell 12, with means, here
shown as a cap 34, for sealing the aperture 30 when the equipment is not in use.
An end wall 36 of the case 10 has an aperture 38 and carries externally
a small moving coil loudspeaker 40 mounted directly over the aperture 38, such
that the inner face of a cone 42 is in pneumatic contact with the interior of
the case 10; but the aperture is otherwise sealed.
The power supply 28, signal processing and control circuit 24 and
display or alarm unit 26 may conveniently be contained in a housing 42, attached
to or formed as an extension of the casing 10. The house 42 also serves to protect
the loudspeaker 40 and may carry low voltage primary or secondary cells 44 for
powering the power supply 28 if the equipment is to be totally self-contained.
In operation, the cap 34 is removed from the aperture 30 and power
is applied to the IMS cell 12 and to the signal processing circuit 24 from the
power supply 28 driven from the low-voltage cells 44.
Discrete samples of ambient atmosphere possibly containing vapours
of interest are drawn into the sealed case 10 through the pin-hole aperture 30
by periodic energisation of the loudspeaker 40, by a suitable drive signal in
the manner described in the present applicants' co-pending PCT Patent Application
If the discrete samples are drawn into the case 10 as single repeated
pulses, these slowly diffuse out of the region of the ionizing source (after analysis
which happens relatively rapidly). The molecular sieve drying agent, held in the
gauze-faced container 14, is able to absorb water-vapour drawn in from the ambient
atmosphere sufficiently to maintain a dry internal atmosphere within casing 10
and allow continued IMS detection of water-sensitive species in the samples. Hence,
the internal atmosphere will be sufficiently dry, by the time the next sample
is taken, to avoid contamination from water vapour that would otherwise still be
present in the case from the previous sample.
The brief negative pressure pulse within the casing, provided by
the loudspeaker 40, causes the sample to be drawn in in the form of a laminar jet
which terminates in an almost stationary vortex ring. The position of the vortex
ring depends upon the amplitude and duration of the pressure pulse and on the nature
of the hole. It has been found that by suitable adjustment of the length and/or
amplitude of the pulse the sample can accurately be deposited at a chosen and controllable
distance from the pin-hole aperture 30. Hence, the sample can be arranged to be
deposited exactly as required, within the ionizing source 18.
The removal of sample and water vapour from the region of the ionizing
source 18 is assisted by an "electric wind" effect in the region of the aperture
30 due to the presence of a body of grounded metal - part of the casing 10 - in
close proximity to the ionizing source 18, which is in operation held at a potential
of about 1kV.
In practice, after the user of the device removes the plug 34, he
simply presses a button (not shown) which actuates the unit. The unit then automatically
takes one negative reference reading, one positive reading, then actuates the
loudspeaker 40 to inject a sample of ambient gas to be analysed, and subsequently
takes a further positive reading and a further negative reading. The unit then
automatically subtracts the positive sample reading from the positive background
reading, to provide a corrected positive signature for the sample, and similarly
subtracts the negative background reading from the negative sample reading to provide
a corrected negative sample signature. The negative and positive corrected signatures
are stored in computer memory (not shown) and may either be viewed by the user
on the display (26) or alternatively down-loaded to an external computer via a
serial port (not shown).
More specifically, by the action of pushing the button the user powers
up the entire unit, and causes the IMS cell 12 to be powered so as to detect negative
ions. The first negative background reading is then automatically taken, following
which the polarity of the cell 12 is reversed, to enable it to detect positive
ions, and the procedure is repeated. On the subsequent actuation of the loudspeaker
40, a sample is drawn in through the pin-hole aperture 30 and, with the cell still
powered to detect positive ions, a positive sample reading is taken. The polarity
of the cell is then switched again, to enable it to detect negative ions, and a
negative sample reading taken. Once the appropriate data have been stored in the
memory, for future reference and analysis, the unit is automatically powered down.
Since the entire procedure, between pressing the button and the unit powering
itself down after having taken the appropriate readings, takes only about half
a second, very little battery power is used. Accordingly the battery life is extremely
long and that means that relatively small and light batteries may be used. This
makes the unit more desirable for use as a portable gas detector.
In an alternative mode, it is possible for the user to instruct the
device to take repeated measurements, say every ten minutes or so, without further
user intervention. The resultant data may automatically be transferred via the
serial port (not shown) to a remote recording and/or analysing computer. Used in
this way, the present embodiment provides a very compact remote sensing device.
The information provided by the unit may be relatively simple (for
example merely a determination that a particular sample to be detected is or is
not present), but may also be considerably more complex. The output of the IMS
cell 12, on each positive or negative measurement, may comprise a detailed spectrum.
In the latter case, subtraction of the negative and positive background spectra
from the negative and positive sample spectra provides the user with very detailed
information by way of corrected negative and positive sample spectra, these corrected
spectra being essentially independent of any contaminants that were inside the
case 10 when the negative and positive background spectra were being taken. In
this way, the unit automatically corrects for small traces of contaminants that
might have been left over from the previous sample.
It will be appreciated that IMS equipment constructed in accordance
with the invention requires no external dry air source nor a pumped internal closed-loop
dry air circulatory system.
Dispensing with the need for an internal pump immediately reduces
the power needed to be drawn from the instrument's internal low voltage cells,
enabling smaller and lighter cells to be employed to power the instrument. Removing
the need for a circulatory system simplifies the internal design of the equipment.
The equipment may thus be considerably reduced in volume and weight
to the point where body-worn IMS equipment becomes feasible, even using present
generation IMS cells and processing and control circuitry.
Further advantages arise from the use of IMS equipment the subject
of the present invention. For example the presence of the molecular sieve material
14 inside the case 10 ensures that not only water-vapour but many potentially-contaminating
organic vapours are absorbed whilst the equipment is out of use, for example in
storage, or whilst in operational use but when not deployed, permitting rapid
start-up without the need for any preliminary scouring or cleansing as is required
in prior art equipment.
Rapid start-up and operation in turn means that power supplies can
be further conserved or reduced in capacity as the equipment does not need to be
kept continuously running between operations.
The simpler construction permitted by use of the present invention
obviating the need for external dry air supplies or internal air circulation and
drying systems also means that equipment in accordance with the invention is considerably
cheaper to manufacture than prior art IMS equipment.
Referring to Figures 2, 3 and 4 which show various aspects of a practical
realisation of an IMS instrument in accordance with a preferred embodiment of the
invention, a main body case 50, machined from a solid block of aluminium alloy,
contains longitudinal bores 52 and 54 in which are mounted respectively an IMS
cell 56 and a cylindrical gauze-walled container 58 packed with molecular-sieve
The casing 50 is sealed at one end by a closure plate 60 mounted
upon the end face of the casing 50 by means of screws which pass through holes
62 and engage in tapped bores 64 in the end face 66 of the casing 50. the closure
plate 60 carries a shutter assembly 68 comprising a circular plate 70 mounted by
means of a screw 72 to engage and rotate in an annular counterbore 74 upon the
closure plate 60, either to align a pin-hole aperture 76 in the plate 70 with
a port 78 or to close the port 78. An 'O' ring seal 80 seated in the port 78 maintains
a seal between the prot 78 and the rear face of the shutter plate 70. Blind bores
82 and 84 in the closure plate 60 contain dummy 'O' rings seals to provide a three-point
support for rotating plate 70.
At its other end, the casing 50 is closed by a closure plate 90,
provided with an aperture 96. The aperture 96 which gives access to the interior
of the casing 50 communicates with the inner face of a cone 98 of a miniature
loudspeaker 100, mounted in sealed relationship upon the outer face of the closure
A connector stem 104 of a collector electrode 106 of the IMS cell
56 protrudes through a hermetically-sealed and electrically insulated aperture
108 in the closure plate 90.
A housing, not shown, which may be a rearward extension of the casing
50, but which does not need to be sealed, contains a power supply unit for the
instrument, primary or secondary cells for powering the power supply, instrument
control and signal-processing circuits and a display module for indicating the
presence and/or concentration of gases or vapours of interest in incoming samples.
To operate the instrument, the circular plate 70 is rotated by hand
to align the pinhole aperture 76 with the port 78, so providing communication between
the ambient atmosphere and the interior of the casing 50.
With power applied to the power supply unit from the primary or secondary
cells, the IMS cell 56 and control and processing circuits are energised and the
loudspeaker 100 may be operated under the control of the user and at a pre-determined
rate to draw pulses of ambient atmosphere in through the pinhole aperture 76 and
port the 78 into the entry port of IMS cell 56, where components of the sample
are ionised and the resulting ions passed into the drift region of the cell 56
for separation and collection in a known manner.
Using the physical construction described in relation to Figures
2, 3 and 4, a self-contained IMS personal vapour detector with dimensions approximately
150mm long, 60mm wide and 35mm deep has been realised and demonstrated. It will
be apparent to those skilled in the art that various modifications and variations
can be made to the IMS equipment described herein without departing from the scope
of the invention.