METHOD AND APPARATUS FOR CONCENTRATING IONISED MOLECULES

Abstract

The invention provides an apparatus for increasing a number concentration of molecules of an analyte of interest in real time from a sample gas flow containing ionised molecules of the analyte, aerosol particles and other molecules, the apparatus comprising: an ion-concentrating chamber having: an inlet for receiving the sample gas flow; an ion outlet; and at least one other outlet; the ion-concentrating chamber being connected or connectable to a gas flow generating and controlling device for establishing a gas flow velocity field within the ion-concentrating chamber to direct the aerosol particles and other molecules of the sample gas flow to the at least one other outlet; and one or more electrodes arranged in an axially spaced apart manner along the ion-concentrating chamber for creating an electric field within the ion-concentrating chamber to direct the ionised analyte molecules in the sample gas flow to the ion outlet, wherein the electrodes are configured such that an absolute value of the strength of the electric field increases progressively along the ion-concentrating chamber.

Claims

1. An apparatus for increasing a number concentration of molecules of an analyte of interest in real time from a sample gas flow containing ionised molecules of the analyte, aerosol particles and other molecules, the apparatus comprising: an ion-concentrating chamber having: an inlet for receiving the sample gas flow; an ion outlet; and at least one other outlet; the ion-concentrating chamber being connected or connectable to a gas flow generating and controlling device for establishing a gas flow velocity field within the ion-concentrating chamber to direct the aerosol particles and other molecules of the sample gas flow to the at least one other outlet; and one or more electrodes arranged in an axially spaced apart manner along the ion-concentrating chamber for creating an electric field within the ion-concentrating chamber to direct the ionised analyte molecules in the sample gas flow to the ion outlet, wherein the electrodes are configured such that an absolute value of the strength of the electric field increases progressively along the ion-concentrating chamber.

2. An apparatus according to claim 1 wherein the progressive increase of the strength of the electric field along the ion-concentrating chamber is created by progressively increasing electric potential differences between the electrodes along the ion-concentrating chamber.

3. An apparatus according to claim 2 wherein the progressively increasing electric potential differences are achieved by increasing electric potential differences between the electrodes along the length of the ion-concentrating chamber in such a way as to progressively increase a voltage gradient therein in accordance with the expression dV/dX=ΔV/ΔX where dV/dX is the average gradient of the electric potential inside the ion concentrating chamber, ΔV is the voltage difference between two adjacent electrodes and ΔX is a gap defined by the presence of an electrical insulator between the electrodes

4. An apparatus according to claim 1, wherein the electrodes are mounted on or in a surface of the ion-concentrating chamber.

5. An apparatus according to claim 1, wherein the electrodes are spaced apart by regions of electrical insulator material.

6. An apparatus according to claim 1, wherein the ion-concentrating chamber is an elongate chamber having a cross-section which is elliptical, oval, or polygonal.

7. An apparatus according to claim 1, wherein the ion-concentrating chamber has a cross-section which is substantially constant along the greater part of the length of the chamber.

8. An apparatus according to claim 1, wherein at least one heater is located at the inlet, the heater being operable to heat the sample gas flow to a temperature sufficient to evaporate analyte molecules adsorbed/absorbed on/in aerosol particles to increase the analyte concentration in the sample.

9. An apparatus according to claim 1 comprising an ionising device located at or near the inlet to the ion-concentrating chamber for ionising molecules of the analyte, wherein the ionising device is selected from: an X-ray source; a corona discharge electrode; a spark discharge electrode; a UV source; a radioactive source; an arc discharge; and combinations thereof.

10. An apparatus according to claim 1 wherein the ion-concentration chamber, or/and the inlet and/or the ion outlet of the ion-concentrating chamber has a rectangular cross-section.

11. An apparatus according to claim 1, configured as a two-dimensional concentrator in which the electrodes are arranged to produce an electric field that urges the ionised analyte molecules progressively closer together as they move through the ion-concentrating chamber predominantly in a single axis that is perpendicular to the direction of flow of the sample gas, so that the ionised analyte molecules form an elongate ion cloud.

12. A combination comprising a plurality of apparatuses of claim 1 connected in series or in parallel.

13. The combination of claim 12, comprising an ion-processing device located in-line between a pair of apparatuses.

14. The combination of claim 13, wherein the ion-processing device is configured to remove unwanted ions while passing ionised analyte molecules to the second apparatus of the pair.

15. An apparatus according to claim 1 wherein the gas flow generating and controlling device comprises one or more fans and/or pumps for moving the sample gas stream into the chamber inlet and through the apparatus; one or more flow meters; and an electronic controller for controlling the operation of the one or more fans and/or pumps in response to flow measurements received from the one or more flow meters.

16. An apparatus according to claim 15 wherein the one or more fans and/or pumps are located downstream of the ion-concentrating chamber and serve to draw the sample gas flow through the inlet into the ion-concentrating chamber; optionally wherein the one or more fans and/or pumps are in fluid communication with the said at least one other outlet.

17. An apparatus according to claim 1 which is connected via the ion outlet to an ion detector; optionally wherein the ion detector is selected from an Ion Mobility Spectrometer (IMS), Mass Spectrometer (MS), Differential Mobility Spectrometer (DMS), Field Asymmetric Ion Mobility Spectrometry (FAIMS), a Variable Electric Field Mobility Analyser (VEFMA) and an ion Differential Mobility Analyser (DMA).

18. An apparatus for increasing the number concentration of molecules of an analyte of interest in real time from a sample gas flow containing ionised molecules of the analyte, aerosol particles and other molecules; the apparatus comprising: (a) an ion-concentrating chamber having: (a-i) an inlet for receiving a stream of gas containing an analyte of interest, the inlet having an inlet cross sectional area, and the stream of gas having an inlet flow rate as it enters the inlet; (a-ii) at least one first outlet through which gas can leave the ion-concentrating chamber; and (a-iii) at least one second outlet through which ionised analyte molecules can leave the ion-concentrating chamber; and (b) one or more electrodes for creating an electric field within the ion-concentrating chamber; and (c) optionally an ionising device located at or near the inlet to the ion-concentrating chamber for ionising non-ionised molecules of the analyte of interest; (d) the apparatus being connected or connectable to a gas flow generating and controlling device for establishing a gas flow velocity field within the ion-concentrating chamber; (e) the apparatus being configured so that: (i) the analyte molecules in the sample gas flow are ionised by the ionising device; (ii) the sample gas flow moves along the chamber mainly under the influence of the velocity field; (iii) the electric field acts on ionised analyte molecules in the sample gas flow as they pass along the chamber to concentrate the ionised analyte molecules into a reduced cross sectional area smaller than that of the inlet; (iv) ionised analyte molecules concentrated into the reduced cross sectional area are directed out through the second outlet; (f) and wherein the apparatus is connectable or connected to an ion detector for detecting and/or identifying and/quantifying ions collected from the second outlet.

19. An apparatus according to claim 18 wherein the one or more electrodes are arranged in an axially spaced apart manner along the ion-concentrating chamber for creating an electric field within the ion-concentrating chamber to direct the ionised analyte molecules in the sample gas flow to the ion outlet, wherein the electrodes are configured such that an absolute value of the strength of the electric field increases progressively along the ion-concentrating chamber.

20. An apparatus according to claim 18 wherein the electric field is created by a plurality of electrodes arranged in a spaced apart manner along the ion-concentrating chamber which has a rectangular or polygonal cross-section formed by a number of side surface sections of a rectangular shape or trapezoid shape or a curved shape and wherein the apparatus is configured and set up such that: (a) there is a gradual change in electric potential differences between the electrodes in side surfaces along the length of the ion-concentrating chamber and that the said potential differences may be identical on all side surfaces or different on some or all side surfaces; or (b) there is a gradual increase/change in electric potential differences between the electrodes in side surfaces along the length of the ion-concentrating chamber; or (c) there is a gradual increase in electric potential differences between the electrodes in side surfaces in the entire length of the ion-concentrating chamber or at least in a part of the chamber length; or (d) there is an increase in electric potential differences between the electrodes in side surfaces along the length of the ion-concentrating chamber in such a way as to progressively increase a voltage gradient therein in accordance with the expression dV/dX=ΔV/ΔX where dV/dX is the average gradient of the electric potential inside the ion concentrating chamber, ΔV is the voltage difference between two adjacent electrodes and ΔX is a gap defined by the presence of an electrical insulator between the electrodes; (e) the electric potential differences between the electrodes form a geometric progression where ΔV is proportional to n.sup.m: ΔV˜n.sup.m where n and m (the power) are real or integer numbers that may be different for some side surfaces; (f) the electric potential differences between the electrodes are described by a function of X (where X is the axis along the length of the chamber) ΔV=F(X) wherein the said function is a combination of concave, convex, constant and linear sections; (g) the electric potential differences between the electrodes are described by a function of X (where X is the axis along the length of the chamber) ΔV=F(X,t) wherein t is the time and the said function is a combination of concave, convex, constant and linear sections; (h) are any combination of (a) to (g).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0170] FIG. 1 is a schematic illustration showing ion trajectories focussed together by an electric field in a cylinder.

[0171] FIG. 2 is a schematic illustration of the focussing of ions in a simple metal ring to which a positive voltage has been applied.

[0172] FIG. 3 is schematic side sectional view of an apparatus according to a first embodiment of the invention.

[0173] FIG. 4 is a schematic side sectional view of the embodiment shown in FIG. 3 but with an ionisation device shown at the inlet of the ion-concentrating chamber.

[0174] FIG. 5 is schematic sectional view of a further embodiment of the invention which is similar to the embodiment shown in FIG. 4 but has a grid positioned between the ionisation device and the inlet of the ion-concentrating chamber.

[0175] FIG. 6a is a schematic side sectional Z-X view (vertical cross-section) of an apparatus according to an embodiment of the invention wherein the ion-concentrating chamber has a rectangular cross section.

[0176] FIG. 6b is a schematic sectional top (horizontal cross-section) Y-X view of the apparatus of FIG. 6a.

[0177] FIG. 7 is schematic side sectional view of an embodiment of the invention similar to the embodiment of FIG. 5 but having a flow distribution chamber downstream of an outlet for non-ionised gas molecules.

[0178] FIG. 8 is a schematic side view of an embodiment of the invention similar to that shown in FIG. 7 but having an array of four metal electrodes on the inner surface of the ion-concentrating chamber.

[0179] FIG. 9 is a schematic side view of an embodiment similar to that shown in FIG. 8 but with a different electrode layout. In this embodiment, there are four cylindrical electrodes positioned along the length of the ion-concentrating chamber, three mounted on the inner surface of the chamber and one on the outer surface of the chamber, and a further circular electrode surrounding the ionised gas outlet.

[0180] FIG. 10 is a schematic side view of the embodiment of FIG. 9 but showing the ion concentrator connected to an IMS drift tube.

[0181] FIG. 11 is a schematic side view of the embodiment of FIG. 9 but showing the ion concentrator connected to an ion DMA.

[0182] FIG. 12 is a schematic side view of the embodiment of FIG. 9 but showing a tandem arrangement in which the ion concentrator has a second ion-concentrating chamber connected in sequence at a downstream end thereof.

[0183] FIG. 13 is a schematic side view of an embodiment showing a tandem arrangement of ion-concentrating chambers similar to that shown in FIG. 12 but with an ion-selecting device positioned between the two ion concentration chambers.

[0184] FIG. 14 shows ion mobility spectra (concentrations of ions measured with an ion selecting DMA vs. the voltage difference between ion separating electrodes of the DMA) for the reagent ions which were obtained (i) with a voltage applied to the ion concentration chamber; and (ii) without a voltage applied to the ion concentration chamber. The ion concentration apparatus used was similar to the apparatus shown in FIG. 11. The DMA used was as described in U.S. Ser. No. 10/458,946.

[0185] FIG. 15 is a magnified version of the ion mobility spectra shown in FIG. 14, where the scale has been expanded along the y-axis so that the spectrum obtained when there no applied voltage in the ion concentration chamber can be seen more clearly.

DETAILED DESCRIPTION OF THE INVENTION

[0186] FIG. 1 illustrates schematically the effect of an electric field on a stream of ionised analyte molecules moving through a cylindrical chamber. As shown in in FIG. 1, the cylindrical chamber (1) has an electric potential on its internal surface that generates a radial electric force directed towards the axial symmetry line. The radial component of the electric force affects the trajectories of ions (2) so that they are “squeezed together” in a smaller space (3). If, at the inlet of the cylindrical chamber, the radius of the ion cloud is R.sub.in, then at the outlet of the cylinder the ion trajectories (2) are squeezed together so that the ion cloud has a smaller radius R.sub.out. In the cylindrical chamber the average flows in the inlet and outlet are equal to one another. If, for simplicity, the velocity profile along the radius of the cylindrical chamber is discounted, then the concentration of ions in the central area of the outlet marked as a dashed circle R.sub.out is greater than the concentration of ions in the inlet (where radius is R.sub.in) by the ratio of (R.sub.in/R.sub.out).sup.2. The local concentration of ions is defined by the degree of compression of the ion cloud while ions are constantly transported along the axial symmetry line of the cylindrical chamber. This local concentration increase forms a non-uniformity in the ion concentration profile. However, the global concentration averaged across the entire cross-section of the outlet is the same as the ion concentration in the vicinity of the inlet. Therefore, the cylindrical chamber shown in FIG. 1 does not act as an ion-concentrating device.

[0187] The concept of ion focussing is well known and widely used in electron microscopy. A simple focussing device is schematically shown in FIG. 2 where a positively charged ring (4) shown as a cross-section of two circles marked with the symbol “+”. The charged ring (4) generates an electric field that has two components: first—an electric field along the axial symmetry line of the ring Ex (5) and second—an electric field along the radius of the ring Er (5). The component Er of the electric field squeezes the ion trajectory bundles (6) together reducing the distance between the ions (6). This example is shown for positive ions. In the case of negative ions, the ring (4) should be charged negatively. The arrangement shown in FIG. 2 works well in a vacuum, but a majority of the devices used for airport security and border control applications operate at atmospheric pressure, rather than under vacuum, and therefore a different approach than simple ion focussing is needed for such applications.

[0188] FIG. 3 illustrates schematically an apparatus according to a first embodiment of the invention. FIG. 3 shows a Z-X cross-section of an ion-concentrating chamber (7) having a cylindrical symmetry. The chamber (7) is provided with means (not shown) for generating an electric field inside the chamber that convolutes or radially constricts ion trajectories into a smaller zone. The chamber has an inlet (8) through which a sample of air or other gas laden with analyte can be introduced into the chamber (7). At the opposite end of the chamber (7), there are provided a pair of first outlets (9) through which air (or other gas), aerosol particles and neutral analyte molecules can leave the chamber, and a centrally disposed second outlet (10) through which air (or other gas) containing concentrated ions can leave the chamber for onward passage to an ion detector.

[0189] Connected to the second outlets (9) is a gas flow generator (40) for controlling the flow of sample gas through the apparatus. The gas flow generator (40) comprises at least one fan or pump (41) for moving the sample gas stream into the chamber inlet and through the apparatus; one or more flow meters (42); and an electronic controller (43) in electronic communication with the fan/pumps and flow meter(s) for controlling the operation of the one or more fans and/or pumps in response to flow measurements received from the one or more flow meters.

[0190] The electric field inside the chamber is created by an array of one or more (usually more than one) electrodes that can be located in or on an internal surface of the ion-concentrating chamber and/or on an outer surface of the ion-concentrating chamber.

[0191] In order to bring about radial constriction of the ion cloud as it moves along the chamber, the strength of the electric field varies with position along the chamber Typically, the voltage settings for the electrodes (e.g. electrodes in or on an internal surface of the chamber)) gradually change along the length of the ion-concentrating chamber (X-axis) in such a way that the gradient of the electric potential inside the chamber dV/dx=E.sub.x(X,Y,Z) is substantially a non-linear function (concave or concave shape) and increases gradually over at least a part if not all of the entire length of the chamber.

[0192] The electric field can be non-linear along the whole of the length of the chamber, or one or more (e.g. a plurality) of such non-linear potential sections can be combined with (e.g. interspersed with) linear or constant electric field strength sections.

[0193] In FIG. 3, the chamber (7) is shown as having two “first” outlets (9) for air (or other gases). However, it should be understood that the number of “first outlets” can be from 1 to any practical number, e.g. 100, that is possible to accommodate for a given diameter of the outlets and the circumference of the cylindrical chamber (7). In one embodiment, the first outlet can take the form of a circular slot extending around the circumference of the chamber (7).

[0194] The difference between ion focussing in a vacuum and real-time ion-concentrating at atmospheric pressure can be seen in FIGS. 1, 2 and 3. The ion-concentrating ratio for the embodiment shown in FIG. 3 is defined as the ratio of the air flow rate at the inlet (8)—Qin to the flow rate in the second outlet (10)—Qout. For example, if Qin=10 l/min and Qout=0.1 l/min, then the ion-concentrating ratio is 100. Thus, it is the removal of unwanted air mass along with aerosol particles and non-ionised analyte molecules through the first outlet(s) that enables the ion-concentrating of ionised molecules to be achieved.

[0195] The mode of action of the apparatus shown in FIG. 3 is based on the combined effects of a non-linear electric field and a velocity field. A flow of sample gas (e.g. air) containing molecules of analyte enters the inlet (8) of the ion-concentrator body (7). Ions are then formed in zone (11) (schematically depicted with a dashed ellipse) by exposing the analyte to an ionisation device (not shown). Along the internal surface of the ion pre-concentrator chamber an electric potential is applied to generate a radial electric field Er that squeezes ion trajectories (12) together thereby reducing the Y-Z cross-section of the ion cloud. This creates an ion cloud of smaller radius than the radius of the ion cloud near the inlet (8). The flow of air containing concentrated ions is directed to an ion detecting device (not shown) through “second outlet” (10). Neutral (non-ionised) molecules of air and analytes as well as particulate matter leave the chamber through the “first” outlet(s) (9); their trajectories are shown schematically by means of the arrow-headed dashed lines.

[0196] It is important to note that the diameter of the second outlet (10) does not influence the increase in concentration of ions. The concentration of ions is equal to the number of ions divided by the volume of the air (or other gas) in which the ions are dispersed. If it is assumed for simplicity that all ions generated in zone (11) reach the second outlet (10), then the increase in concentration is equal to the ratio of Qin/Qout, where Qin is the flow rate of the sample entering the real-time ion-concentrator via inlet (8) and Qout is the flow rate of the air sample coming out of the second outlet (10). According to the conservation law, Qin=Qout+Qone, where Qone is the flow rate through the first outlet (9). Thus, the ion-concentrating ratio Qin/Qout=1/(1−Qone/Qin) and the real-time ion-concentrating increases when Qone is getting close to Qin.

[0197] It should be understood that the above expression for the ion-concentrating ratio is an approximation for the case when the ion velocity is mainly controlled by the flow in the outlet (10) and a contribution from the electric field can be neglected. It becomes clear if one considers the case where Qout is equal to zero, in which case the ion-concentrating ratio becomes infinitely large; which is obviously impossible.

[0198] In FIG. 4, an embodiment of the present invention is shown with an X-Ray source (15) mounted at the inlet of the ion-concentrating chamber (7). The source (15) generates ions in the zone (11) shown schematically with a dashed ellipse.

[0199] It can be advantageous to the performance of the apparatus to position a grid (16) between the X-Ray source (15) and the ion-concentrating chamber (7) as shown in FIG. 5. Note that in FIG. 5, the trajectories of the neutral (non-ionised) molecules are not shown. In practice, the grid (16) is located at the entrance to the chamber (7) and a particular electric potential is applied to the grid. The grid adds an additional electric field Ex in the chamber (7) that increase velocity of ions and decreases the residence time of ions in the said chamber (7).

[0200] It should be noted that the ionisation zone (11) is the zone where ionisation of analyte molecules mainly takes place. Reactive ions such as N.sub.2.sup.+ and O.sub.2.sup.− are also formed by ionisation of the component gases of air. Such ions are typically formed close to the X-Ray source and some of them subsequently transfer their charge to the analyte.

[0201] In the apparatuses shown in FIGS. 3, 4 and 5, the ion-concentrating chambers (7) are of a circular cylindrical shape. However, other shapes are possible. The ion-concentrating chambers can, for example, have rectangular (e.g. square or oblong) or elliptical crosssections as well as a combinations of these shapes.

[0202] FIG. 6a shows an X-Z cross-section of a three-dimensional (3D) ion-concentrating apparatus having a rectangular cross-sectioned chamber (7). The apparatus shown in FIG. 6a can be considered as a 3D rectangular device or as a 2D device. The 2D device version is a simplified approximation of the 3D geometry when the width of the rectangular ion-concentrating chamber (7) in, e.g. the Y-axis (FIG. 6b), is substantially greater than the height of the chamber (7) along the Z-axis. In this approximation the side effects of the X-Z boundaries at smallest and largest co-ordinates along the Y-axis (Y=Ymin and Y=Ymax) are disregarded. The mode of action in the 2D case is similar to the mode of action in the 3D case but the shape of the second outlet (10) is different because in the 2D case the ion cloud trajectories (12) at the outlet are predominantly squeezed together only in one direction—along the Z-axis. This is shown schematically by the differences in dimensions of the outlets (10) in FIG. 6a and FIG. 6b.

[0203] It will be noted that, conceptually, for a 2D version of the apparatus, all the vertical crosssections of ion trajectories (12) are identical and they are not influenced by an electric field component along the Y-axis. In a 3D version of ion-concentrating apparatus ion cloud trajectories are deformed along both the Y-axis and the Z-axis. This is a conceptual difference between two rectangular geometry versions of the apparatus.

[0204] In the embodiment shown in FIGS. 6a and 6b, the chamber (7) has two first outlets (9) through which the non-ionised analytes and the mass of neutral gas molecules can leave the chamber. These outlets are located in the top and the bottom walls (13) and they are formed with substantially rectangular shapes, FIGS. 6a and 6b.

[0205] In order to reduce the influence of side effects on the performance of the ion-concentrator having the rectangular cross-section, the Y-dimension of the outlet (10) should be slightly narrower than the internal Y-dimension of the chamber (7) as shown in FIG. 6b. This stops some ions (12) that are near the side walls (13s) from coming out of the outlet (10) but still allows ions (14) to be directed through the outlet (10) to the ion measuring device. It may slightly reduce the number of ions in the outlet (10), but it allows ions with the same residence time to come out of the outlet (10). The residence time near the side walls would be greater due to the boundary conditions on the internal boundaries (13s): v=0. In some cases, an increase of the residence time is not desirable due to ion chemistry that may modify or deplete analyte ions. The above combination of features gives an apparatus which is close to an ideal 2D version of an ion-concentrating device.

[0206] The apparatuses in FIGS. 4 to 6 may also be provided with a gas flow generator (40) as shown in FIG. 3.

[0207] FIG. 7 shows an apparatus similar to the embodiment of FIG. 5 except that the apparatus of FIG. 7 is provided with a flow distributer (17) comprising an annular chamber that encircles the downstream end of the ion-concentrating chamber (7) and communicates with the first outlet(s) (9). In this embodiment, the ion-concentrating chamber can have a plurality of outlets (9) spaced (e.g. equidistantly) around the circumference or it can have a single outlet (9) in the form of an annular slot extending around substantially the entire circumference of the chamber. Where there is a plurality of outlets (9) spaced around the circumference, the circumferential distances between adjacent outlets (9) can advantageously be less than the circumferential sizes of the outlets. This arrangement of the outlets (particularly where the outlet (9) is an annular slot) enables the formation of a uniform (that is not influenced by an angle in the Y-Z plane) axially symmetrical flow of air or other gas out of the opening (9) into the flow distributer (17) and finally out of the flow distributor (17) through outlet (18) to a gas flow generator (40) comprising a fan or pump (41); one or more flow meters (42); and an electronic controller (43) in electronic communication with the fan/pumps and flow meter(s). Streamlines for this flow are shown schematically by means of dashed curved arrows. The flow distributer (17) enables the gas flow through opening (9) to be more uniform or homogeneous. This enhances the performance of the real-time ion concentrating device.

[0208] It should be noted that the presence of the flow distributer or flow homogeniser can be advantageous for any given geometry of the chamber (7) including but not limited to a rectangular chamber, elliptical chamber or polygonal Y-Z cross-section chamber.

[0209] The electrical field within the ion-concentrating chamber can be provided by a plurality of conductive electrodes (19) mounted on an inner surface of the chamber (7), for example as shown in FIG. 8. In one embodiment, electrodes (19) are formed from cylindrical metal rings and secured to the internal surface of the chamber (7), which can be made from a non-conductive material such as glass or a plastics material.

[0210] Where the ion-concentrating chamber (7) is of rectangular cross section (e.g. as shown in FIGS. 6a and 6b), various electrode shapes and configurations are possible depending on the size of the chamber and the sizes of the electrodes. Thus, electrodes can be present on all four sides of the rectangle or on just the top and the bottom surfaces in the X-Y plane. When electrodes are present on all four sides of the rectangle, rectangular electrodes which extend continuously around the inner surface of the chamber in the Y-Z plane can be used. Alternatively, a single continuous rectangular electrode can be replaced by a discontinuous array of individual electrode elements extending around the inner surface of the chamber in the Y-Z plane. Typically a plurality of rectangular electrodes or a plurality of discontinuous arrays of electrode elements are provided at spaced apart locations along the x-axis of the ion-concentrating chamber.

[0211] When, as exemplified above, electrodes are present on all four sides of the rectangle, the ion-concentrating apparatus can carry out ion concentration in a three dimensional (3D) mode with constriction of the ion cloud taking place along both the Y and Z axes.

[0212] The ion-concentrating chambers can also be configured to operate in a two dimensional (2D) manner where constriction of the ion cloud takes place either along the Y axis or along the Z axis, but not (to any significant extent) along both Y and Z axes at the same time. In this case, the electrodes could be present on just two opposing walls (e.g. the top and bottom surfaces) of the chamber. Thus, with reference to FIG. 8, in an embodiment where the ion-concentrating chamber is of rectangular cross section in the Y-Z plane, in a 2D mode there would be electrodes on two opposing sides of the chamber as shown in FIG. 8, but there would be no electrodes on the other two opposing side walls of the chamber. By contrast, when configured for operation in a 3D mode, there would be electrodes on all four side walls of the chamber.

[0213] The number of electrodes (normally—between 1 and 100) and their length to be used in any particular case can be determined by trial and error. In the embodiment illustrated in FIG. 8, when four annular electrodes (19) were used in a cylindrical chamber (7), and the ion-concentrating chamber was connected to an instrument for selecting and quantifying ions, an increase in sensitivity (i.e. concentration of ions) of more than 100 times was achieved, compared to the same instrument but without the ion-concentrating chamber.

[0214] It should be understood that the voltages on the electrodes (19) should follow the pattern described above, i.e. the voltage difference along axis X at the internal electrodes should gradually increase along the length of the ion concentrating chamber (X-axis) in such a way that the gradient of the electric potential inside the chamber dV/dx=E.sub.x(X,Y,Z) is substantially a non-linear function of the X co-ordinate. As indicated above, the electric potential should increase gradually along at least part of the length of the chamber, if not the entire length of the chamber.

[0215] It is important to notice that the sign of the electric potential in the apparatus of this invention is influenced by the choice of analyte ions (positive or negative ions). Therefore, in the foregoing and following description of the present invention, references to a gradual increase in the electric potential should also be understood as referring to a gradual decrease in the electric potential where the apparatus is set up to concentrate ions of the opposite ion polarity.

[0216] In the case of an axial symmetry it is advantageous to apply voltages to the electrodes that generate a radial electric field Er that directs ions of interest to the centre of the chamber (R=0). To achieve this, electric potentials applied to electrodes (19) should form a certain pattern that can be represented as a series of voltages Vi, where i is a number of an electrode, e.g. from left to right 1<i<Nmax (Nmax—the number of electrodes).

[0217] In one embodiment of the present invention the series of voltages forms a non-linear set of rational or integer numbers, for example V1, V2=2*V1, V3=4*V1, VNmax=2.sup.Nmax*V1. In this case the difference between Vi+1 and Vi is a non-linear function of a number of an electrode and, therefore the position of the electrode along the axis X. Voltage applied to the grid (16) may or may not be equal to V1.

[0218] In another embodiment of the ion-concentrator of the invention, the voltages Vi form a gradually decreasing pattern when the difference between Vi+1 and Vi (for 1<i<Nmax) is gradually increasing with number i. The optimal difference between voltages is influenced by the geometry of the concentrator and the flow rates. In practice these voltages can be optimised by trial and error for each different geometry using methods familiar to the person skilled in the art of handling ions in the air.

[0219] In further embodiment of the ion concentrator, the voltages Vi form a gradually increasing pattern when the difference between Vi+1 and Vi (for 1<i<Nmax) is gradually decreasing with number i.

[0220] In the each of the embodiments of the ion concentrating device of the invention, a combination of different patterns of voltages applied to electrodes (19) (essentially a non-linear pattern, linear pattern, gradually decreasing or gradually increasing patterns) can be used to achieve better real-time ion concentrating efficiency.

[0221] In another embodiment of the ion concentrating device shown in FIG. 9 some electrodes (20) are positioned outside the non-conductive body of the ion-concentrator (7) chamber.

[0222] Electrically conductive electrodes can also be positioned inside the non-conductive (electrically) material.

[0223] FIG. 9 also shows how some electrodes, e.g. (21) can be positioned on the internal or external (electrically non-conductive) surfaces that are perpendicular to the axis X of the concentrating device (7).

[0224] It should be also understood that electrodes (19), (20) and (21) may or may not be axially symmetrical in case of a circular chamber (7). It is especially important if the ion detecting device inlet (10) is not circular but a rectangular or elongated ellipsoidal shape as for example for an ion DMA (U.S. Pat. No. 7,855,360 B2, the disclosure in which is incorporated herein by reference).

[0225] The ion concentrating apparatus of the invention can be linked to various ion detecting systems (such as IMS, MS, DMS, FAIMS, VEFMA and ion DMA (e.g. US20070278398, US 20060054804, U.S. Pat. Nos. 7,572,319, 6,787,763, the disclosures in each of which are incorporated herein by reference)) where it will act as a real time ion-concentrator and will be of benefit by increasing the sensitivity of detection of various volatile and semi-volatile organic compounds as well as inorganic compounds. Examples of combinations of the ion concentrator apparatus of the invention with ion detecting devices, where the apparatus of the invention functions as a real time ion-concentrator are shown in FIGS. 10, 11 and 13.

[0226] FIG. 10 shows how the apparatus of the invention can act as real-time ion concentrator when interfaced with an Ion Mobility Spectrometer (IMS) device (22). Inside the IMS a linear electric field is created to move ions from the BN-gate (Eiceman, 2002) shown as a dotted line (23) to the Faraday plate detector (34). It is important that the electric field between the outlet (10) of the ion-concentrator (7) and the BN-gate (23) is strong enough to pull ions from the ion-concentrator to the IMS (22).

[0227] FIG. 11 schematically shows the apparatus of the invention can function as a real-time ion concentrating device when connected to an ion DMA (U.S. Pat. No. 6,787,763). An ion DMA (24) is an ion selecting and ion detecting device and should be interfaced with the apparatus (7) of the invention by choosing a voltage in the inlet of the DMA that generates an electric field that is sufficiently strong to pull ions from the outlet (10) of the concentrator to the inlet of the DMA (24).

[0228] FIG. 12 shows an embodiment of the present invention wherein two ion-concentrators of the invention are connected in series in a tandem system to increase the ion-concentrating ratio. The second ion-concentrator (25) is connected to the outlet (10) of the first ion-concentrator (7). The second ion-concentrator does not require ionisation means, but an additional ionisation facility can be used if required. The second ion-concentrator contains electrodes (not shown) to provide a required electric field, and has first outlet(s) (26) and a second outlet (27) that provide a high concentration flow of ions to be used by any ion measuring device connected downstream of the ion-concentrators of the invention

[0229] The mode of action of the tandem ion concentrator is similar to a single ion-concentrator. Ions formed and concentrated in the first ion-concentrator (7) enter the second ion concentrator (25) via the outlet (10) of the first-concentrator (7). The non-linear electric field generated in the second ion-concentrator (25) along with the velocity field concentrate ions (12) further to a narrower stream (28). The neutral analyte and air molecules are directed to the first outlet(s) (26). Depending on the geometry of the chamber (25) the shape of the first outlet may be a circular slot (for axial symmetry geometry), rectangular slots (for 2D or 3D geometries) or any suitable shape that as desired. The outlet(s) (26) are connected to a flow distributer/homogeniser (29) where the outlet (30) is connected to a pump with a flow control system (not shown) to maintain the flow rate of the gas through the outlet. The narrow stream of concentrated ions (28) passes through the second outlet (27) of the second ion-concentrator (25) and may be connected to any ion quantifying or collecting instrument.

[0230] This system enables the concentration of analytes to be increased further to the level that enables remote detection of explosives and contraband substances when the ion-concentrator of the invention is used in conjunction with any currently used or any known device.

[0231] It should be understood that a plurality of ion-concentrating devices can be used connected to each other either sequentially or in parallel to achieve a higher concentration ratio and simultaneous detection of different types of ions, for example positive and negative ions, heavy and light ions, etc.

[0232] FIG. 13 shows an embodiment of the ion-concentrator having tandem chambers (7) and (25) with an ion selecting device (31) positioned between them to eliminate/reduce volume charges by filtering out some ions from the ion cloud with analytes, for example reagent ions. In operation, analyte molecules are first ionised in the first ion-concentrator (7) as described above. Ions concentrated in the first chamber (7) are directed to the ion selecting device, e.g. a DMA (31). In the DMA reagent ions (such as N.sub.2.sup.+ and O.sub.2.sup.+) shown as two dashed lines (33) are deflected from the outlet (35) of the DMA (31). The analyte ions (32) are then directed through the DMA outlet (35) to the inlet of the second ion-concentrator (25) where the ion bundle (28) becomes further concentrated and directed to the outlet (27) of the second pre-concentrator (25).

[0233] The ion separating device (31) enables removal of reagent ions (33) from the ion cloud and therefore reduces the volume charge effect caused by the repulsion forces. Thus, the volume-charge-limit is removed, and the ion-concentrating ratio can be increased further.

[0234] It is noted that an ion separating device (31) placed between two ion-concentrating devices (7) and (25) may or may not be a low-resolution device. If an ion separating device (31) is a high-resolution device, then it may increase the resolving power of the final ion characterisation instrument connected to the outlet (27).

[0235] The apparatuses in FIGS. 8 to 13 may also be provided with a gas flow controller (40) as shown in FIG. 7. Alternatively, where an ion detector is present (such as in the apparatus shown in FIG. 10), the gas flow controller (40) can be connected downstream of the ion detector (22), rather than to the outlets (9) or (18).

[0236] The ion-concentrators of the invention can be operated at ambient temperature or either the whole system (containing ion-concentrators and an ion separating device), or parts of the system, can be operated at elevated temperatures to reduce adsorption of analytes on the internal walls and increase sensitivity and resolving power.

[0237] It will be appreciated that a plurality of both ion-concentrators and ion selecting or separating devices can be connected in parallel and in series. Also, the ion-concentrating device and a system that includes one or several ion-concentration devices can be used with various ion measuring instruments such as IMS, DMS, DMA, FAIMS, Variable Electric Field Mobility Analyser (VEFMA) (U.S. Pat. No. 8,378,297, the disclosure in which is incorporated herein by reference) and MS.

[0238] It will also be appreciated that air is not the only medium where the real-time ion-concentrating of ions can be arranged. The ion-concentrating device can work in any gas medium, e.g. hydrocarbon based natural gas, clean gases used in microelectronics, other (even corrosive) gases and gas mixtures.

[0239] The real-time ion concentrator can also operate at a reduced atmospheric pressure (rarefied gas) or in a vacuum when the vacuum medium can be considered as a fluid medium, for example so called “high-pressure” Mass Spectrometers.

[0240] It will be appreciated that the methods and apparatuses according to any of the above embodiments can be used without ionisation means when analyte ions of interest are already present in a gas sample. Thus, an apparatus without an ionisation means, or where an ionisation means is switched off, can be used in applications when trace quantities of ions in the air and any other gases have to be detected or identified, for example in atmospheric research or for quantification of extremely low ionising levels of radiation in nuclear physics or geophysics.

[0241] It should be noted that the prior art pre-concentrator apparatuses and methods referred to in the introductory section above focus on deposition and evaporation of molecules, but not ions. The concentrating of ions using these prior art pre-concentrators is practically impossible.

EXAMPLES

Example 1

[0242] An ion-concentrator apparatus having cylindrical geometry (shown schematically in FIG. 8 and FIG. 11) was manufactured from Perspex and aluminium with the internal dimensions of 5 cm ID and 8 cm length with 3 aluminium electrodes of graduating size separated with PTFE insulators. A soft X-Ray source of 4.9 kV was used to ionise molecules in the air flow. The flow rates were from 0.3 l/min to 2.0 l/min. The voltages applied to the grid and the first electrode were from 200 V to 2,000 V.

[0243] The ion-concentrator apparatus was connected to an ion-selecting device (U.S. Pat. No. 10,458,946) where ions were selected in a DMA and then ions of the selected mobility were directed to an individual ion counter built according to U.S. Pat. No. 7,372,020 (the disclosure in which is incorporated herein by reference). In operation, an air sample flow was introduced into the inlet (8) FIG. 11 into the ion-concentrating device (7) ibid where ion concentration occurs and a concentrated ion cloud (12) ibid is moved into the DMA (24) ibid. The DMA was interfaced with an individual ion counter, not shown. In the DMA variation of the electric potential difference between the DMA electrodes enables selection of ions of different mobility and the recording of mobility spectra.

[0244] FIG. 14 shows two groups of mobility spectra of reagent ions formed in the real-time ion-concentrating device at the flow rate 0.3 l/min that were recorded with and without voltage applied to the real-time ion-concentrating device. An individual ion-counting device was used as an ion detector to count the number of ions coming out of the DMA. A group of large peaks at 240 V was obtained when a voltage was applied to the ion-concentrating device. A very small peak at the DMA separation voltage was recorded with the ion-concentrating device voltage switched off. The average magnitude of counts for spectra with the voltage on was ˜25,000 counts per second. The average magnitude of the signal without the voltage was ˜260 counts per second, see FIG. 15. Therefore, the ion count rates at the maximum of the peaks recorded with the ion-concentrating device in operation was almost two order of magnitude (i.e. almost two hundred times) greater than the concentration when ion-concentrating device was switched off.

[0245] The individual ion counter used in the apparatus described above was designed to operate at a very low ion count rate, normally below 2,000 counts per second. The measured magnitude of the peak (about 25,000 counts per second) is higher than the upper limit of the individual ion counting device. In this case, an individual ion counting device operating at higher count rates may be saturated, resulting in broader spectra widened. A more precise evaluation of the ion concentration denoted by of a saturated peak is given by the total number of ions obtained by integration of the ion spectra. For the apparatus described above, the ratio of the integral ion concentrations measured with and without the voltage applied to the real-time ion concentrator is circa 300 times. Therefore, the real-time ion concentrator of the invention enables concentrations of ions to be increased by more than two orders of magnitude. In practical terms, the invention makes possible the remote detection of explosives, contraband goods and other threats in real time with greatly improved sensitivity.

REFERENCES

[0246] UK patent Application GB 2560565 A by B. Gorbunov [0247] U.S. Pat. No. 7,572,319 B2 by A. Tipler, G. Campbell, M. Collins [0248] U.S. Pat. No. 7,855,360 B2 by J. Fernandez de la Mora, A. Casado [0249] G. A. Eiceman. Ion-mobility spectrometry as a fast monitor of chemical composition. trends in analytical chemistry, vol. 21, no. 4, 2002. (IMS for explosives) [0250] U.S. Pat. No. 7,199,362B2 by A. L. Rockwood, E. D. Lee, N. Agbonkonkon, M. L. Lee [0251] Cheng et al., Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Sensitive Detection of Explosives, Anal. Chem. 2013, 85, 1, 319-326. [0252] Gaik et al., Anal. Bioanal. Chem. (2017) 409:3223-3231, DOI 10.1007/s00216-017-0265-2

[0253] The disclosures in each of the above references is incorporated herein by reference.