EP0960431A1

EP0960431A1 - A method for element-selective detection, a micro plasma mass spectrometer for use in the method and a plasma ion source, together with applications thereof

Info

Publication number: EP0960431A1
Application number: EP98904444A
Authority: EP
Inventors: Cato Brede; Stig Pedersen-Bjergaard; Tyge Greibrokk; Elsa Lundanes
Original assignee: PEDERSEN BJERGAARD STIG
Current assignee: PEDERSEN BJERGAARD STIG
Priority date: 1997-02-14
Filing date: 1998-02-12
Publication date: 1999-12-01
Anticipated expiration: 2018-02-12
Also published as: EP0960431B1; NO970707L; NO304861B1; JP2001512617A; DE69804772T2; DE69804772D1; AU719247B2; NO970707D0; CA2278807A1; AU6231498A; WO1998036440A1

Abstract

In a method for element-selective detection of chromatographically or electrophoretically separated compounds there is employed a micro plasma mass spectrometer with a plasma ion source provided in the mass spectrometer's high vacuum chamber. The plasma ion source is equipped with a capillary channel surrounded by a radio-frequency electrode connected to a radio-frequency generator and around or adjacent to the capillary channel there are provided one or more earth electrodes. When plasma gas is introduced into the channel, the plasma is created by discharges between the radio-frequency electrode and one or more of the earth electrodes. The method, the plasma mass spectrometer and the plasma ion source can be installed in existing, commercial mass spectrometers.

Description

A method for element-selective detection, a micro plasma mass spectrometer for use in the method and a plasma ion source, together with applications thereof

The invention concerns a method for element-selective detection of chromatographically or electrophoretically separated compounds, wherein for the detection there is employed a micro plasma mass spectrometer with a radio-frequency generator, a mass analyser and an ion detector. The invention also concerns a micro plasma mass spectrometer, especially for element-selective detection of chromatographically or electrophoretically separated compounds, wherein the micro plasma mass spectrometer comprises a radio-frequency generator, a mass analyser and an ion detector. The invention further concerns a plasma ion source for use in a mass spectrometer, especially a micro plasma mass spectrometer for element- selective detection of chromatographically or electrophoretically separated compounds, the mass spectrometer comprising a radio-frequency generator, a mass analyser and an ion detector, wherein the plasma ion source has an inlet at one end and an outlet at the opposite end, and wherein the plasma ion source is arranged in the mass spectrometer in such a manner that the inlet and the outlet are located in the mass spectrometer's high vacuum chamber. Finally, the invention concerns applications of the method, the micro plasma mass spectrometer and the plasma ion source.

Element-selective detection of chromatographically and electrophoretically separated compounds can be performed with a variety of detectors, but the common feature of most of these is that they only respond to a few elements. The so-called plasma atomic emission detector (AED) can be employed for simultaneous detection of many elements (J.J. Sullivan and B.D. Quimby, Anal. Chem. 62, 1990, pages 1034- 1043; B.D. Quimby and J.J. Sullivan, Anal. Chem. 62, 1990, pages 1027-1034), but due to high costs and limited sensitivity its propagation has been limited. An alternative to AED is to employ plasma mass spectrometry (E.H. Evans, J.J. Giglio, T.M. Castelliano and J.A. Caruso, "Inductively coupled and microwave induced plasma sources for mass spectrometry", The Royal Society of Chemistry, Cambridge, UK, 1995). In detectors based on plasma mass spectrometry the compounds are atomized and ionized in a plasma and the ions are detected by means of mass spectrometry. Inductively coupled plasma mass spectrometry (ICP-MS) permits detection of several elements and has high sensitivity, but the equipment is expensive to purchase and also to operate since it consumes large amounts of noble gases, for example 15 1/min argon or helium. ICP-MS has high detection limits for non-metals and is therefore used preferably for determining metals. Another detector which is based on radio-frequency glow discharge mass spectrometry (RF-GD-MS), can be employed at a gas flow rate of 0.6 1/min., but this detector is not commercially available at present.

In order to reduce the operating costs in the use of microwave-induced plasma (MIP) attempts have been made to employ discharge chambers with smaller dimensions. The introduction of the so-called "Beenaker" cavity has made it possible to generate plasma by microwave induction down to flow rates of 50 ml/min. Mass spectrometers with microwave-induced plasma (MIP-MS) are not commercially available at present.

Detection methods based on ICP-MS, RF-GD-MS and MIP-MS respectively employ the same method for transferring the ions from the plasma to the mass analyser which works in a high vacuum. The ions are transferred via a so-called "sampler" and a "skimmer". In this process the ions are transferred from atmospheric pressure or low pressure to high vacuum, but the drawback is that as much as 99% of the ions are lost. In the light of the disadvantages of the above-mentioned detection methods, an object of the invention is therefore to provide a new element-selective detector based on micro plasma ionization and mass spectrometric detection of the ions.

A further object of the invention is that the detector should be able to be used for detection of all elements in the periodic table. Yet another object of the invention is that it should be possible to directly transfer ions from the plasma to the mass analyser under vacuum conditions in the mass spectrometer. Yet a further object of the invention is to be able to use low gas flow rates, typically less than 25 ml/min, preferably less than 10 ml/min and most preferably from 1-4 ml/min, and is able to employ all gases suitable as plasma forming gases, such as helium, neon, argon, hydrogen, nitrogen etc. Finally, it is also an object of the invention to provide a simple plasma probe which can be used in existing commercial mass spectrometers, e.g. as an option in addition to commonly used ion sources. The above-mentioned objects are achieved according to the invention with a method which is characterized by providing a plasma ion source in the mass spectrometer's high vacuum chamber, connecting a radio-frequency electrode in the plasma ion source with the radio-frequency generator, introducing into the plasma ion source plasma gas which carries one or more separated compounds which are to be detected in the mass spectrometer, and creating a radio-frequency potential on the radio-frequency electrode, thus creating plasma in the plasma ion source by discharges between the radio-frequency electrode and an earth connection provided at the plasma ion source, whereby the separated compound(s) which are to be analysed are atomized with the creation of atomic ions which are subsequently expelled from the plasma ion source into the mass spectrometer's high vacuum chamber for separation in the mass analyser and detection in an ion detector provided near the mass analyser; a micro plasma mass spectrometer which is characterized in that it comprises a plasma ion source provided in the mass spectrometer's high vacuum chamber, the plasma ion source being connected to the radio- frequency generator for the creation of atomic ions in a gas introduced into the plasma ion source; and a plasma ion source which is characterized in that between the inlet and the outlet it is equipped with a capillary channel, that around the channel there is provided a radio-frequency electrode which is arranged to be connected to the radio-frequency generator for the creation of a radio-frequency potential on the radio-frequency electrode, and that around or adjacent to the capillary channel there are provided one or more earth electrodes, with the result that when plasma gas which carries one or more of the compounds which have to be detected is introduced at the inlet of the channel, and when the radio-frequency potential is impressed on the radio- frequency electrode, a plasma is capacitively created in the channel by discharges between the radio-frequency electrode and the earth electrode or electrodes, whereby the compound or compounds are atomized into atomic ions.

The method, the micro plasma mass spectrometer and the plasma ion source are employed according to the invention for selective detection of halogens and carbon.

The plasma ion source is employed according to the invention for installation in existing, commercial mass spectrometers. The invention will now be explained in more detail in connection with the accompanying drawing, in which

Fig. 1 illustrates a micro plasma mass spectrometer according to the invention, Fig. 2 illustrates a first embodiment of a plasma ion source according to the invention,

Fig. 3 illustrates a second embodiment of the plasma ion source according to the invention,

Fig. 4 illustrates a third embodiment of the plasma ion source according to the invention,

Fig. 5 illustrates a fourth embodiment of the plasma ion source according to the invention,

Fig. 6 illustrates a first plasma probe which realizes a practical embodiment of the plasma ion source according to the invention for use in a mass spectrometer,

Fig. 7 illustrates a second plasma probe which realizes a practical embodiment of the plasma ion source according to the invention for use in a mass spectrometer, and

Fig. 8 illustrates an apparatus set-up for element-selective detection and use of the micro plasma mass spectrometer according to the invention combined with gas chromatographic separation.

Fig. 1 illustrates a micro plasma mass spectrometer according to the invention. The mass spectrometer comprises a high vacuum chamber 1 , a mass analyser 3 and an ion detector 4. All of this is well known to those skilled in the art. In the high vacuum chamber 1 there is provided a cavity 5 in which there is inserted a plasma ion source 10 according to the invention. The cavity 5 may be formed in a separate block in the high vacuum chamber 1 of the mass spectrometer, and the block can include an electrostatic repeller 6 together with electrostatic lenses 7 for focusing the ion beam, as known from the use of standard ion sources for electron ionization or chemical ionization. The supply of plasma gas and the sample, i.e. the compound or compounds which are to be analysed is performed via a supply line 17. A transition piece 8 leads from the atmospheric pressure outside the mass spectrometer into the mass spectrometer's high vacuum chamber 1. The plasma ion source 10 is attached to the supply line 17 inside the high- vacuum chamber 1, with the result that the plasma ion source projects into the high vacuum chamber. The supply line 17 passes through the transition piece 8. A radio-frequency generator 2 which generates a radio-frequency potential is connected to the plasma ion source 10 via the transition piece 8. A not shown lead 21 from the plasma ion source 10 to earth is also passed through the transition piece 8. The plasma ion source 10 is located as mentioned inside the mass spectrometer's high vacuum chamber. In the plasma ion source 10 the atomic ions which are created flow out of the plasma ion source's outlet 15, are deflected electrostatically by the repeller 6 and focused by the electrostatic lenses 7, whereupon the ions are separated in the mass analyser 3 and detected in the ion detector 4. Via the supply line 17 a suitable plasma gas, for example helium or argon, mixed with the sample which is to be analysed, is introduced into the plasma ion source 10. The supply line 17 is passed into the plasma ion source 10 via the transition piece 8 which forms a seal between the atmospheric pressure and the vacuum in the mass spectrometer's high vacuum chamber 1. The plasma ion source 10 comprises a radio- frequency electrode 11 and one or more earth electrodes 12, as will be discussed later. The radio-frequency generator 2 is connected to the radio- frequency electrode 1 1 , and plasma is generated by the radio-frequency generator impressing on the radio-frequency electrode a radio-frequency electrical potential. The frequency may, for example, be between 100 kHz and 100 MHz. In a preferred embodiment 350 kHz is used. By means of discharges between the radio-frequency electrode 11 and one or more earth electrodes 12 the plasma gas is now mainly capacitively converted to a plasma, and in this plasma the inserted sample is atomized, with creation of atomic ions, which as mentioned flow out of the plasma ion source's 10 outlet 15 and are analysed in the mass spectrometer for determination of the elements.

A first embodiment of the plasma ion source 10 according to the invention is illustrated in more detail in Fig. 2. The plasma ion source 10 has an inlet 14 which is connected to the supply line 17 in Fig. 1 and an outlet 15 which projects into the mass spectrometer's high vacuum chamber 1. Between the inlet 14 and the outlet 15 respectively the plasma ion source 10 is designed as a capillary column or channel 13, for example in the form of a capillary tube whose internal diameter is preferably at most only 2 mm. Furthermore, the tube 13 may preferably be a quartz capillary tube. The inlet 14 of the plasma ion source 10 is connected to an earthed metal tube 16 which contains the supply line 17 for the plasma gas and the sample inserted in the plasma gas. The supply line 17 can also be designed as a quartz capillary tube and may be formed in one piece with the plasma ion source 10, with the result that the capillary channel 13 also acts simultaneously as a supply pipe. Around the capillary channel 13 or the capillary tube there is provided a radio-frequency electrode 11 which is connected to the radio-frequency generator. In addition, around or adjacent to the channel or the tube 13 there are provided one or more earth electrodes 12. In Fig. 2 only one earth electrode 12 is shown, but there is no reason why more earth electrodes cannot be provided. The earthed metal tube 16 can thus itself constitute a second earth electrode. When the radio-frequency generator 2 impresses a radio-frequency potential on the radio-frequency electrode 11, discharges are obtained between the radio-frequency electrode and the earth electrode or earth electrodes 12, the plasma gas introduced into the channel 13 is converted into a plasma and the accompanying sample is atomized with subsequent creation of atomic ions, as discussed above in connection with the description of the micro plasma mass spectrometer in Fig. 1.

The capillary column or channel 13 preferably has an internal cross section which does not exceed 2 mm. The distance between the electrodes 11, 12 may be up to a few cm. If a capillary tube is employed with an internal diameter of, for example, 320 μm, the volume of the discharge space becomes very small and the gas consumption is correspondingly reduced, for example to less than 25 ml/min.

Fig. 3 illustrates a second preferred embodiment of the plasma ion source 10 according to the invention. It differs from the embodiment in Fig. 2 in that the radio-frequency electrode 11 is mounted around the plasma ion source's outlet 15, i.e. the outlet of the capillary channel or the capillary tube 13. In this case the earthed metal tube 16 constitutes the earth electrode 12. Otherwise the mode of operation of the plasma ion source 10 is as mentioned above in connection with Fig. 2. Fig. 4 illustrates a third preferred embodiment of the plasma ion source 10 according to the invention. It corresponds mainly to the embodiment in Fig. 2, but in this case the capillary channel or capillary tube 15 at the outlet 15 is provided with a narrowing 18. The narrowing 18 at the outlet 13 causes the pressure in the plasma gas or the plasma in the channel or the tube 13 to become higher, thus influencing the properties of the plasma ion source, for example by increasing the energy density in the plasma. Otherwise the mode of operation of the plasma ion source 10 is as mentioned above in connection with the embodiment in Fig. 2. Fig. 5 illustrates a fourth embodiment of a plasma ion source 10 according to the invention. In this case too the outlet 15 of the capillary channel or the capillary tube 13 is provided with a narrowing 18. The radio-frequency electrode 1 1 is arranged around the narrowing 18 and in this case too the earthed metal tube 16 acts as an earth electrode 12, with the result that apart from the narrowing the embodiment corresponds to the embodiment illustrated in Fig. 3. Otherwise the mode of operation of the embodiment in Fig. 5 is as mentioned above in connection with the embodiment in Fig. 2, apart from the fact that in this case the narrowing 18 at the outlet 15 also influences the properties of the plasma ion source, as mentioned in connection with Fig. 4.

The plasma ion source 10 according to the invention can be advantageously implemented as a plasma probe, in which form it can be installed in existing, commercial mass spectrometers. A plasma probe of this kind is illustrated in Fig. 6. The plasma ion source 10, whose mode of operation is as mentioned above in connection with one of the figures 2-5, can be implemented by one of the embodiments illustrated in these figures. In Fig. 6 the plasma probe is provided with a radio-frequency electrode 1 1 arranged in the middle of the capillary channel or capillary tube 13 and has an earth electrode 12 at the outlet 15. The radio-frequency electrode 11 is connected via a radio- frequency conductor 19 to the radio-frequency generator 2 provided outside the mass spectrometer, while the earth electrode 12 is connected to earth via an earth conductor 21. As mentioned above the plasma ion source 10 is connected to an earthed metal tube 16 which contains the supply line 17 for the plasma gas and the sample included therein. At the end of the supply line 17 there is a device 24 which enables the length of the supply line to be adjusted in relation to the electrode system 1 1 , 12 in the plasma ion source 10. The supply line 17 is naturally connected as illustrated in Fig. 1 to a feed line 9 for the plasma gas and the sample which has to be analysed. Around the capillary channel 13 there are provided insulating pipes 20 for attaching the electrodes 1 1, 12, while the electrode leads, i.e. the radio-frequency conductor 19 and the earth conductor 21 , may be flexibly connected by means of screw devices 22. The transition piece 8 between the external atmospheric pressure and the mass spectrometer's high vacuum chamber 1 is formed as a cover and the plasma probe is mounted in this cover. The cover 8 is made of an insulating material, such as transparent acrylic plastic. The supply line 17, the radio- frequency conductor 19 and the earth conductor 21 are passed through the cover 8. The metal tube 16 or the supply line 17 together with the radio-frequency conductor 19 are attached to the cover 8 with plugs 23 which seal and insulate them, the plugs possibly being made of Teflon. Since the cover 8 is transparent and electrically insulating, the plasma in the plasma ion source 10 can be observed. The electrically insulating material prevents short-circuiting between the radio-frequency conductor 19 and the earth conductor 21. The plasma probe as illustrated in Fig. 6 is easily attached to the mass spectrometer and is secured by means of vacuum. Between the plasma probe and the opening in the mass spectrometer there is provided a not shown rubber O-ring to ensure a good seal. The illustrated plasma probe is very simple to install and dismantle, thus making it easy to equip existing commercial mass spectrometers, so that they can be employed as a micro plasma mass spectrometer according to the invention. In a preferred embodiment the supply line 17 for plasma gas and the sample included therein is a capillary quartz tube with an internal diameter of 0.32 mm and external diameter of 0.45 mm. The channel or tube 13 in the plasma ion source 10 may also be a capillary quartz tube, formed in one piece with the supply line 17. The capillary tube of quartz is then passed through the radio- frequency electrode 11 and the earth electrode 12 at the outlet. These electrodes 11, 12 may, for example, be in the form of metal tubes with an internal diameter of 0.5 mm and external diameter of 1.6 mm.

Fig. 7 illustrates a second embodiment of the plasma ion source 10 implemented as a plasma probe. This embodiment is similar to the one given in Fig. 6, except that the part of the capillary tube 13 which protrudes out of the earthed metal tube 16, is encapsuled by an outer fused silica tube 25. The fused silica tube 25 is provided with a strong narrowing 18 at the outlet 15 and is tightly attached to the earthed metal tube 16 by a Teflon™ tubing 26. Thus the encapsulation of the capillary tube 13 is almost tight and a satisfactory pressure of the plasma gas can be maintained for very small gas consumptions. The gas consumption may be as low as 1 ml/min, but it is preferred to employ a consumption of 2.25 ml/min, which is the output of the gas chromato graph. The radio-frequency electrode 11 and the outer earth eletrode 12 are made of steel wire which is twisted around the fused silica tube 25.

Fig. 8 illustrates an apparatus set-up for element-selective detection in micro plasma mass spectrometry and the use of gas chromatographic separation. The actual micro plasma mass spectrometer is designed as illustrated in Fig. 1, and reference is therefore made to the above discussion of this figure. A gas chromatograph has an open tubular column 37 which ends in a T- connection 38 in order to mix the sample with oxygen-doped helium which is used as plasma gas and supplied from a helium gas supply 30 with pressure regulator and gauge, while the oxygen, which in this case is used as scavenger gas, comes from the oxygen gas supply 31 which is similarly equipped with pressure regulator and gauge. A T-connection 32 divides the helium gas flow into a carrier gas flow and an external helium flow. An external helium flow gauge 33 and an external helium flow regulator 34 are provided between the T-connection 32 and the T-connection 35, the T- connection 35 being used to introduce oxygen to the external helium flow through, for example, a 20 μm microcapillary column of fused silica. The helium carrier gas line is conveyed to a "split-splitless" injector 36. The plasma gas doped with oxygen is transported from the T-connection 38 at the end of the tubular column 37 and the separated sample is added through a heated feed line 9 with a temperature control unit 39 to the supply line 17 and the inlet 14 of the plasma ion source 10. It should not be necessary to provide a detailed description of this apparatus set-up, since the technique will be well known to those skilled in the art.

For the implementation of the method according to the invention, helium was employed as plasma gas in the plasma ion source. Helium has a high ionization potential, providing a plasma with high energy, thus enabling the method and the micro plasma mass spectrometer according to the invention to be successfully employed for the detection of elements with a high ionization potential. It is assumed that the flow rate of helium influences the plasma energy, the plasma pressure and the extension of plasma from the foremost electrode in the plasma ion source's channel. Since collisions in the plasma create the atomic ions, the amount of colliding species and their energy will, for example, be important factors.

In the implementation of the method according to the invention, a scavenger gas was added to the plasma gas in order to remove carbon deposits which were formed on the quartz wall in the capillary tube 13. As mentioned in connection with Fig. 8, oxygen was employed as scavenger gas, since oxygen is considered to be effective with respect to chlorine-selective detection. With the use of the method and the micro plasma mass spectrometer according to the invention for detection of chlorine, a detection limit of 3.3 gs"l was achieved. With the use of hydrogen instead of oxygen as scavenger gas a somewhat higher detection limit for chlorine was achieved. Gas flow rates of less than 25 ml/s were employed, but higher flow rates were also possible. A radio-frequency potential of 350 kHz was used, but the frequency can be higher or lower, for example in the range 100 kHz to 100 MHz. An internal diameter of the tube or channel of only 320 μm gave a narrow ion beam from the outlet of the plasma ion source. The small volume of the channel resulted in a power output of only 2.0 watt being employed for the discharge.

Thus it will be seen that the plasma ion source as specified above and employed in a mass spectrometer effectively realizes a micro plasma mass spectrometer according to the invention.

Experiments, however, also showed that the method and the micro plasma mass spectrometer according to the invention, together with the plasma ion source employed could probably be further improved. Ions could be detected both in positive and negative mode, and with the invention the opportunity is offered of detecting all elements of the periodic table.

Claims

PATENT CLAIMS

1. A method for element-selective detection of chromatographically or electrophoretically separated compounds, wherein for the detection there is employed a micro plasma mass spectrometer with a plasma ion source (10), a mass analyser (3) and an ion detector (4), characterized in that the plasma ion source (10) is located in the mass spectrometer's high vacuum chamber (1), and in that a plasma gas carrying one or more of the separated compounds, which are to be detected in the mass spectrometer, is introduced into the micro plasma ion source (10), whereby the separated compound(s) are atomized with the creation of atomic ions which are subsequently expelled from the micro plasma ion source (10) into the mass spectrometer's high vacuum chamber (1) for separation in the mass analyser (3) and detection in the ion detector (4).

2. A method according to claim 1, characterized in that the plasma in the plasma ion source (10) is formed capacitively by discharges between one or more radio-frequency electrodes (1 1) connected to a radio-frequency generator (2) and one or more earth connections (12).

3. A method according to claims 1-2, characterized in that the radio-frequency potential has any frequency between 100 kHz and 100 MHz, and in that the frequency is preferably 350 kHz.

4. A method according to claims 1-3, characterized in that helium, neon, argon, hydrogen or nitrogen are employed as plasma gas.

5. A method according to claims 1-4, characterized in that in the plasma ion source, any gas flow rate between 1 and 50 ml/min is employed.

6. A micro plasma mass spectrometer, especially for element-selective detection of chromatographically or electrophoretically separated compounds, wherein the micro plasma mass spectrometer comprises a plasma ion source (10), a mass analyser (3) and an ion detector (4), characterized in that the plasma ion source (10) is located in the mass spectrometer's high vacuum chamber (1), and in that a plasma gas carrying one or more of the separated compounds, which are to be detected in the mass spectrometer, is introduced into the micro plasma ion source (10), whereby the separated compound(s) are atomized with the creation of atomic ions which are subsequently expelled from the micro plasma ion source (10) into the mass spectrometer's high vacuum chamber (1) for separation in the mass analyser (3) and detection in the ion detector (4).

7. A micro plasma mass spectrometer according to claim 6, characterized in that the plasma in the plasma ion source (10) is formed capacitively by discharges between one or more radio-frequency electrodes (1 1) connected to a radio-frequency generator (2) and one or more earth connections (12).

8. A micro plasma mass spectrometer according to claims 6-7, characterized in that the radio-frequency potential has any frequency between 100 kHz and 100 MHz, and in that the frequency is preferably 350 kHz.

9. A micro plasma mass spectrometer according to claims 6-8, characterized in that helium or argon are employed as plasma gas.

10. A micro plasma mass spectrometer according to claims 6-9, characterized in that in the plasma ion source, any gas flow rate between 1 and 50 ml/min is employed.

11. A plasma ion source (10) for use in a plasma mass spectrometer, especially a micro plasma mass spectrometer for element-selective detection of chromatographically or electrophoretically separated compounds, wherein the mass spectrometer comprises the plasma ion source (10), a mass analyser (3) and an ion detector (4), wherein the plasma ion source (10) has an inlet (14) at one end and an outlet (15) at the opposite end, characterized in that the plasma ion source is arranged in the mass spectrometer in such a manner that the inlet (14) and the outlet (15) are located in the mass spectrometer's high vacuum chamber (1), in that between inlet (14) and outlet (15) the plasma ion source (10) is equipped with a capillary channel (13), in that a plasma gas which carries one or more of the compounds which are to be detected is introduced at the inlet (14) of the channel (13), and in that the plasma ion source (10) is provided with means for employing the plasma gas in the channel (13) to create and maintain a plasma inside the capillary channel (13), with the result that the compound or compounds are atomized into atomic ions.

12. A plasma ion source according to claim 11, characterized in that the means for forming the plasma in the capillary channel (13) comprises one or more radio-frequency electrodes (11) connected to a radio-frequency generator (2) and one or more earth connections (12), and in that the electrodes are located outside of the capillary tube (13), around its circumference.

13. A plasma ion source according to claim 11-12, characterized in that the radio-frequency potential imposed on the one or more radio-frequency electrodes (12) has any frequency between 100 kHz and 100 MHz, and in that the frequency is preferably 350 kHz.

14. A plasma ion source according to claims 1 1-13, characterized in that the capillary channel (13) is designed in the form of a tube with an internal diameter of less than 2 mm, thus permitting a gas flow rate of 25 ml/min or less to be employed.

15. A plasma ion source according to claims 1 1-14, characterized in that the tube (13) is a fused silica capillary tube.

16. A plasma ion source according to claims 1 1-14, characterized in that the inlet (14) is connected to an earthed metal tube (16) which contains a supply line (17) for the plasma gas, and in that the earthed metal tube (16) constitutes the earth electrode or one of the earth electrodes (12).

17. A plasma ion source according to claims 1 1-14, characterized in that the radio-frequency electrode (11) is provided at the outlet (15).

18. A plasma ion source according to claims 1 1-14, characterized in that the earth electrode or earth electrodes (12) are provided between the radio-frequency electrode (11) and the inlet (14).

19. A plasma ion source according to claims 1 1-14, characterized in that the earth electrode or earth electrodes (12) are provided at the outlet (15).

20. A plasma ion source according to claims 1 1-19, characterized in that the capillary channel (13) at the outlet ( 15) is provided with a narrowing (18).

21. A plasma ion source according to claim 1 1 -20, characterized in that the supply line (17) is formed in one piece with the plasma ion source (10), with the result that the capillary channel (13) also acts as a supply pipe.

22. A plasma ion source according to claim 21, characterized in that the capillary tube (13) is encapsulated by an outer tube (25), in that the outer tube (25) is provided with a narrowing (18) at the outlet 15, and in that it is tightly attached to the earthed metal tube (16) in the other end, and thus the outer tube (25) provides a satisfactory tight encapsulation of the capillary tube (13) such that the plasma ion source (10) can maintain the plasma with a plasma gas flow rate of less than 4 ml/min.

23. A plasma ion source according to claim 22, characterized in that in the plasma ion source, any gas flow rate between 1 and 4 ml/min is employed, preferably the gas flow rate is 2.25 ml/min.

24. A plasma ion source according to claim 22 or 23, characterized in that in the plasma ion source, the gas flow rate equals the rate of the effluent from any conventional gas chromato graph.

25. Application of the method according to claims 1-6, a micro plasma mass spectrometer according to claims 7- 10 and a plasma ion source according to claims 1 1-24 for selective detection of all elements in the periodic table, especially the halogens and non-metals.

26. Application of a plasma ion source according to claims 1 1-22 for installation in existing, commercial mass spectrometers.