Method for ionizing gaseous samples by means of a dielectric barrier discharge and for subsequently analyzing the produced sample ions in an analysis appliance

Abstract

A method for ionizing gaseous samples by dielectric barrier discharge and for subsequently analyzing the produced sample ions in an analysis appliance, in particular a mass spectrometer or an ion mobility spectrometer, produces the sample ions by a plasma caused by a dielectric barrier discharge, wherein the dielectric barrier discharge is produced by virtue of a plasma gas being supplied through a capillary made of a dielectric material, wherein a wire-shaped electrode is arranged within the capillary, the electrode being connected to an AC voltage source, wherein the gaseous sample is supplied to the exit region of the capillary, wherein the wire-shaped electrode is connected to the AC voltage source on the high-voltage side.

Claims

1. A method for ionization of gaseous samples by means of dielectric barrier discharge and for subsequent analysis of the sample ions produced, in an analysis device, wherein the sample ions are produced by means of a plasma that is brought about by means of a dielectric barrier discharge, wherein the dielectric barrier discharge is generated in that a plasma gas is supplied through a flexible, bendable capillary composed of a dielectric material, wherein a wire-shaped electrode is arranged within the capillary, which electrode is connected with an alternating voltage source on the high-voltage side, wherein the gaseous sample is supplied to the exit region of the capillary, and wherein the wire-shaped electrode is inserted only in the back end of the capillary.

2. The method according to claim 1, wherein the ground of the alternating voltage source is connected with an electrode arranged on the outside of the capillary.

3. The method according to claim 1, wherein helium, argon, hydrogen, nitrogen or air is used as the plasma gas.

4. The method according to claim 1, wherein a square-wave voltage is used as the alternating voltage.

5. The method according to claim 4, wherein the frequency of the square-wave voltage amounts to 5 to 60 kHz.

6. The method according to claim 4, wherein a square-wave voltage in the range of 1 to 4 kV is used.

7. The method according to claim 1, wherein a capillary composed of quartz glass with an outer coating of polyimide is used.

8. The method according to claim 1, wherein a capillary with an inside diameter between 50 and 400 μm is used.

9. The method according to claim 1, wherein the gaseous sample is supplied through the capillary.

10. The method according to claim 1, wherein the gaseous sample is supplied to the exit region of the capillary from the outside, without passing through the capillary.

Description

(1) In the following, the invention is explained in greater detail as an example, using the drawing. This drawing shows, in a magnified schematic representation, in each instance, in:

(2) FIG. 1 a basic representation of a capillary with one electrode,

(3) FIG. 2 a basic representation of a capillary with two electrodes,

(4) FIG. 3 the capillary according to FIG. 1 with a plasma P1,

(5) FIG. 4 the capillary according to FIG. 1 with a plasma P1 and a plasma P2,

(6) FIG. 5 the capillary according to FIG. 2 with a plasma P3,

(7) FIG. 6 the capillary according to FIG. 1 with the plasmas P1, P2, and P3, and in

(8) FIG. 7 the capillary according to FIG. 2 with the plasmas P1, P2, and P3.

(9) In FIG. 1, a capillary 1 for carrying out the method according to the invention is shown schematically. This capillary 1 consists of a dielectric material, for example quartz glass, and is preferably configured to be flexible and bendable. For this purpose, the capillary 1 is provided with a coating of polyimide on the outside, for example, not shown. In this regard, the capillary 1 has an inside diameter between 50 and 400 μm. At the back end 1a of the capillary 1, a wire-shaped electrode 2 is inserted into the capillary 1. This electrode is connected, on the high-voltage side, with a high-voltage source, not shown, preferably a square-wave voltage source. This square-wave voltage is indicated with a voltage symbol 3. This square-wave voltage has a magnitude of 1 kV to 4 kV, and a frequency between 5 kHz to 60 kHz. In the embodiment according to FIG. 1, a further electrode is not provided. Bordering on the front end 1b, an inlet into a mass spectrometer is arranged at a slight distance of 1 cm, for example, or the front end 1b opens into an ionization chamber of an ion mobility spectrometer. In this embodiment of the capillary 1, the second electrode is formed by the surroundings or the housing of the analysis device. The surroundings of the capillary 1 then act as a capacitor.

(10) A plasma gas is introduced into the back end 1a of the capillary 1, as indicated by broken-line arrows 4. The sample gas to be ionized is either also introduced through the back end 1a of the capillary 1 and thereby passed through the capillary 1, or it is supplied to the region of the exit from the capillary 1 from the outside, in other words at the front end 1b, without flowing through the capillary 1.

(11) In FIG. 2, the same capillary 1 as in FIG. 1 is shown, with the difference that on the outside, for example in the central length region of the capillary 1, a second electrode 5, preferably in ring shape, is arranged, which is separated from the interior of the capillary 1 by means of the dielectric wall of the capillary 1. The second electrode 5 is connected with the ground of the high-voltage source, as indicated by a ground symbol 6.

(12) In FIGS. 3 to 7, different examples of the different plasma formation are shown:

(13) In FIG. 3, a plasma P1 is shown, which extends from the tip of the electrode 2 all the way to the front end 1b of the capillary 1.

(14) In FIG. 4, a situation is shown, in which not only the plasma P1 forms, but also a plasma P2 at the exit from the capillary 1.

(15) In FIG. 5, the capillary 1 is shown with the first electrode 2 and the second electrode 5, wherein a third plasma P3 forms only in the region of the electrode 2.

(16) FIG. 6 shows a situation with the capillary 1 without a second electrode 5, in which all the plasmas P1, P2, and P3 are formed.

(17) Finally, FIG. 7 shows a situation with the capillary 1 with the second electrode 5, in which also all the plasmas P1, P2, and P3 are formed.

(18) The presence of the different plasmas P1, P2, P3 and the spread of the respective plasma depends on the geometries selected (inside diameter of the capillary 1 and diameter of the wire-shaped electrode 2) and the plasma gas used in each instance. Fundamentally, the situation is that first the plasma P1 at the tip of the electrode 2 ignites, and this plasma P1 then spreads out further and further along the channel within the capillary, as the voltage increases; this spread can be promoted by the additional electrode 5. After the back end 1b of the capillary 1 is reached, the smaller plasma P2 occurs in addition to the plasma P1.

(19) If the voltage is increased further, the further plasma P3 can occur in addition, in particular if helium or argon is used as the plasma gas.

(20) Of course, the invention is not restricted to the exemplary embodiments shown. Further embodiments are possible without departing from the basic idea. Fundamentally, the capillary 1 does not have to be configured to be bendable, as shown; it can also be rigid and run in a straight line; the essential thing is that the wire-shaped electrode 2 arranged in the interior is connected with the high-voltage source on the high-voltage side.

REFERENCE SYMBOL LIST

(21) 1 capillary 1a back end 1b front end 2 electrode 3 voltage symbol 4 arrows 5 electrode 6 ground symbol P1 plasma P2 plasma P3 plasma