Polarization dielectric discharge source for IMS instrument

Abstract

An IMS ionizer comprising a wire, a second conductor, and a dielectric, when the first conductor and second conductor are energized to an ionization voltage, discharge ionization occurs. The dielectric is a glass element formed in a tubular shape defining an inner wall. The wire is formed in coils in contact with said inner wall. The second conductor is positioned to define an outer wall of the tube. The tube has an inlet end for receiving the sample, and an outlet end through which the sample exits after ionization.

Claims

1. An ionizing apparatus for use in ionizing molecules in a test sample to be analyzed using an ion-mobility spectrometer, the apparatus comprising: a first conductor, a second conductor, and a dielectric, the first conductor, second conductor, and dielectric being sized, shaped and mutually positioned such that when the first conductor and second conductor are energized to an ionization voltage, discharge ionization occurs at an interface between the first conductor and the dielectric; the dielectric comprising a glass element formed in a tubular shape so as to define an inner wall of a tube, the dielectric being positioned between the first conductor and the second conductor to insulate the first conductor from the second conductor; the first conductor comprising a wire formed in coils that are in contact with said inner wall, and being electrically connectable to an AC voltage pulse generator; the second conductor comprising a conducting layer positioned so as to define an outer wall of the tube, and being electrically connectable to the AC voltage pulse generator; and the tube comprising an inlet end for receiving the sample, and an outlet end through which the sample exits after ionization, wherein the first conductor is coated with a substance having an affinity for organics to enhance trapping of the sample on the first conductor.

2. An apparatus as claimed in claim 1, wherein the wire comprises a generally helically shaped wire defining a longitudinal axis, and wherein said longitudinal axis is itself shaped generally helically so as to form said coils with said wire.

3. An ionizing apparatus as claimed in claim 1, wherein the substance comprises a nanocarbon.

4. An ionizing apparatus as claimed in claim 1, wherein the conductors are connectable to a DC voltage generator to cause heating of the first conductor to release the sample when it is trapped on the first conductor.

5. An ionizing apparatus as claimed in claim 1, wherein the dielectric comprises borosilicate glass.

6. An ionizing apparatus as claimed in claim 1, wherein the second conductor is composed of steel or aluminum.

7. An ionizing apparatus as claimed in claim 2, wherein the conductors are connectable to a DC voltage generator to cause heating of the first conductor to release the sample when it is trapped on the first conductor.

8. An ionizing apparatus as claimed in claim 1, wherein the conductors are connectable to a DC voltage generator to cause heating of the first conductor to release the sample when it is trapped on the first conductor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Reference will now be made, by way of example only, to preferred embodiments of the invention and in which:

(2) FIG. 1 is an end view of an embodiment of the apparatus;

(3) FIG. 2 is a schematic cross-sectional elevation view of an embodiment of the apparatus in fluid communication with an IMS drift tube;

(4) FIG. 3 is a schematic cross-sectional elevation view of an embodiment of the apparatus;

(5) FIG. 4 is a side view of the wire prior to its being arranged in a coil inside the preferred tubular dielectric;

(6) FIG. 5 is an example graph showing ion peaks for an example molecule being detected by an IMS;

(7) FIG. 6 is an example graph showing ion peaks for an example molecule being detected by an IMS; and

(8) FIG. 7 is an example graph showing GC retention time for an example molecule being detected by an IMS in an example system that includes gas chromatography for pre-separation; and

(9) FIG. 8 is a simplified drawing of the preferred wire.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) Referring now to FIGS. 1-3, the apparatus 10 is shown. The apparatus comprises a first conductor or electrode preferably in the form of wire 12 and second conductor or electrode preferably in the form of a steel or aluminum outer layer 14 (the invention comprehends any suitable conductive material from both conductors). Separating the two conductors 12, 14, and insulating them from one another is a dielectric, preferably in the form of glass layer 16. Glass layer 16 is preferably formed in a tubular shape and defines the inner wall 20 of a tube, which tube is preferably generally the shape of apparatus 10. The tube preferably comprises an inlet end 13 by which the sample enters the apparatus, and an outlet end 15 by which it exits the apparatus toward the IMS.

(11) The wire is preferably formed in a generally helical shape, with the helically-formed wire itself being formed in generally helical coils 18 so that the so that the wire has many points of contact with the inner wall 20 of the tube formed by glass layer 16. Stated differently, the wire 12 is formed in a helix which defines a longitudinal axis 22 (see FIG. 4). The wire 12, already including small coils as a result of its generally helical shape, is then further formed into larger coils by forming the wire so that the longitudinal axis itself is formed in a generally helical coil shape (see FIG. 8). Preferably, the coils 18 of the wire 12 are consistently positioned in contact with glass 16 to maximize contact between wire 12 and glass 16.

(12) Second conductor 14 is preferably a metal layer bonded to glass 12 and position so as to define an outer wall 21 of the tube.

(13) In operation, preferably, the wire 12 acts as an electrode in contact with glass 12, which glass is polarized upon application of a high voltage pulse. It will be appreciated that the invention comprehends other possible dielectrics, including but not limited to ceramic dielectrics.

(14) Preferably, the wire 12 is coated with nanocarbon, which has affinity for organics, to facilitate trapping and pre-concentration/enrichment of the sample on wire 12.

(15) Preferably, the sample is then desorbed by means of application of a DC voltage to the wire 12. The DC voltage is preferably provided by DC voltage generator 24, to which the conductors are electrically connectable.

(16) The conductors 12, 14 are preferably energized at the same voltage, and polarization of the glass 12 occurs followed by charging of the inner wall 20. The amount of charges formed at the wall 20 by polarization/charging is proportional to the change of the voltage. Preferably, an AC or square wave pulse generator 26 (to which the conductors are electrically connectable) creates AC pulses (ionization voltage) to create the plasma for ionization of the desorbed sample.

(17) Preferably, the AC pulse is timed to occur after the DC voltage desorbs the sample, so that the sample molecules have moved off of wire 12 and are in the tube and available to be ionized. Thus, the apparatus 10 preferably functions to perform desorption of the concentrated sample by applying a DC voltage to the wire 12 and AC pulse to induce soft ionization of the gas phase sample at the dielectric coil interface.

(18) The polarized dielectric discharges are generated at the interface between glass 16 and wire 12i.e. by multiple points of contact between the dielectric material and the wire 12, separating the ionization area (in the tube, away from the glass 12) and the discharge area (at glass 12, at the points of contact with the first conductor).

(19) There are possible benefits to the shape and configuration of wire 12 as described above. One is that it can be formed with substantial length, which increases the total resistance of the coil, thus facilitating the generation of more heat in response to DC through the coil, which in turn facilitates faster evaporation and release of the sample into the drift tube. Another is the provision of increased surface area for trapping the incoming sample.

(20) In the prior art, the plasma is separated from sample introduction for the reason because, inside the plasma, the electron density is too high; therefore organic molecules might be fragmented to smaller ions or non-ionized species. In an embodiment of this invention, the plasma is not separated from the place where the sample is introduced (i.e. the ionization area). The present apparatus, in this embodiment, has not shown extensive fragmentation in either the positive or negative ions' ion mobility spectra.

(21) An example of the present invention in operation is described. Referring now to FIGS. 5-7, separation of PETN plastic explosive by gas chromatography is shown followed by PDBD ionization and detection by ion mobility spectrometer operating in fast switching mode.

(22) PETN retention time at 58 seconds produced two PETN negatively ionized molecules, identified as PETN cluster with chloride dopant to form (PETN+Cl).sup. ion at mass 351 amu and PETN molecular ion PETN.sup. at mass 315 amu at their respective reduced mobility constants of 1.203 cm.sup.2/V.Math.sec and 1.167 cm.sup.2/V.Math.sec.

(23) PETN also is detected in the positive ion mode at the same GC retention time producing a fragment peak at reduced mobility constant of 1.406 cm.sup.2/V.Math.sec corresponding to tentative mass of 206 amu or tentatively loss of CH.sub.2N.sub.2O.sub.4 group.

(24) This example embodiment was operated at a source temperature of 170-200 C. 170 C. is believed to be the preferred temperature.

(25) Ionization of explosives in the gas phase proceeds after the vaporization of the explosive molecules and the microcontroller initiation of the pulsed waveform at optimized voltage. In some embodiments, the pulse have amplitudes between 1000 to 5000V, pulse period of 3-6 sec of in the range of 100-150 kHz.

(26) Energetic electrons produced on the polarized dielectric induce ionization of ambient gases, which in turn through ion-molecule reactions produced relatively high density reagent ions. The introduced gas phase sample is then ionized by these secondary ion-molecule reactions.

(27) Once the sample molecules have undergone ionization for analysis, they travel out through outlet 15 to the IMS.

(28) It will be appreciated that the invention comprehends other forms besides the preferred forms described herein, and may take any form comprehended by the full breadth of this description.