GB2180687A - Method and apparatus for examining a gas mixture - Google Patents

Abstract

An apparatus for examining a gas mixture for the presence of at least one particular contaminant comprises a single quistor (quadrupole ion store) having electrodes (1,3,4) defining a chamber (2) which is fed with the gas mixture, in particular contaminated air, via a diaphragm (9) which exhibits greater permeability to the contaminant than to the molecules of the basic substance. In the chamber (2) all the steps of a double mass spectroscopic analysis are carried out successively in time, i.e. ionization of the gas mixture, elimination of all ions which have a mass/charge ratio different from that of the contaminant whose presence is to be determined, subsequent fragmentation of the remaining ions and, finally, mass-spectroscopic analysis of the fragments so obtained. Such an apparatus, which, due to the use of a quistor and a diaphragm for supplying the gas mixture, can do with a pump having a pumping capacity of only 10 l/s at a pressure of 10 mPa, permits contaminants to be detected even if they are present in concentrations as low as 0.01 ppm. <IMAGE>

Description

SPECIFICATION Method of examining a gas mixture, and apparatus for carrying out this method The present invention relates to a method of examining a gas mixture consisting of a basic substance and at least one contaminating substance present in low concentration, in particular contaminated air, in order to determine the presence of a specific contaminant, comprising thefollowing steps which are carried out in series in an ion-storing mass spectrometer.

a) ionizing the gas mixture, b) separating from the ions obtained, by massspectrometric methods, those ions which exhibit a charge/mass ratio different from that ofthe contaminantto be determined; c) subjecting the remaining ions to a chemical and/ or physical transformation; and d) examining by mass-spectrometric methods if transformation products characteristic of the contaminantto be determined have been produced.

Mass spectrometry and its applications have been described in detail in a book by Dawson entitled "Quadrupole Mass Spectrometryand its App- lications", Amsterdam-Oxford-New-York 1976. On pages 190, 191 ,the use of an ion trap as "storage source" has been described in which the usual ion source in the form of a mass filter has been replaced by an ion trap of the type normally used for mass spectroscopic examinations. This ion source is then followed by an analyzer, i.e. another massspectroscopic instrument for mass analysis. The arrangement is intended to examine the develop mentintimeofchemical and physical processes,for example the speed of chemical reactions.

EP Disclosure Document No.0113207 describes a specific design of an ion trap intended for mass spectroscopy which is described as a quadrupole ion store or ion trap or, shortly, quistor (Quadrupole ton Store). The q u istor com prises th ree mutually insulated electrodes, one ofwhich defines an annular closed wall of a chamberwhilethe others form covers closing the ends of the chamber. The surfaces of the electrodes facing the chamber are hyperbolic.

A sample contained within the chamber under low pressure and being ionized, for example, by irradiation by an electrode pulse can be analyzed by varying d.c. and a.c. voltages applied to the electrodes in such a mannerthatthe ions trapped in the chamber under the effect ofthe electric fields generated by the said voltages can escape from the chamberthrough an opening in one ofthe end electrodes, depending on their respective mass/charge ratios. A detector provided outside the chamber, behind the opening, then permits the mass/electron ratios ofthe ions to be determined and, thus, conclusions to be drawn regarding the substances contained in the chamber.

An ion trap is provided also in the case ofthe known "lon Cyclotron Resonance Spectrometers" (ICR) so that the method described at the outset can be carried out also using such ICR spectrometers.

Ifcomplicatedorganiccompoundsareto be examined, the mass/charge ratio does not provide sufficiently exact information on the ionized substance.

There a number of different organic compounds which present the same mass/charge ratio, for example becausetheyhavethesamemolecular weight, inspite of being differently structured, or because, though having a different molecularweight, they reach a correspondingly different ionization state. In orderto improve the accuracyofthe analysis results that can be obtained by mass spectroscopy, onetherefore makes use of a doublemassspectrosc opicexamination (MS), and accordinglytheseexaminations are described as MS/MS. In MS/MS spectroscopy,the gas mixture passes two mass spectrometers arranged in series.In the first mass spectrometer, one eliminates initially all ions which do not present the same mass/charge ratio. The substance is then introduced into a reaction chamber where the ions are subjected to chemical reaction or fragmentation by shock effects or the like. One ob tains inthis manner newproductswhich arechar- acteristic ofthe structure ofthe isolated products exhibiting the pre-determined mass/charge ratio. The reaction products or fragments are then analyzed in the second mass spectrometer. The mass/charge ratios of the reaction products orfragments provide information on the structure of the previously isolated substances with equal mass/charge ratio and permit in this manner, in particular,todetermine whether or not a pre-determined substance was present in the analyzed product.

The MS/MS method described last, which is excellently suited for many analytical purposes, requires, however, a very high apparatus input which limits its practical application quite considerably. It is of particular importance in this connection thatto achieve effective reaction yields, the pressure of the shock gas has to have a value at least three orders of magnitude higherthan that prevailing in the mass spectrometers. This means that high-capacity vacuum pumps are required to generate and maintain the pressure differences between the ionization and reaction chambers on the one hand and the chambers of the mass spectrometers on the other hand, and such pumps have a considerable size and a correspondingly high energy consumption.In addition, a considerable input of control systems is requiredforcontrolling the different potentials atthe mass spectrometers and the transmission lines in connection with the measures for ionizing, selecting, changing and transmitting the ions.

It is common practice in ICR spectrometry thatthe individual steps making up the MS/MS process are carried out in the ion trap of the spectrometer successively in time. From a paper read on the occasion ofthe 1985 Annual Meeting ofthe "American Society for Mass Spectrometry" it has also become known to carry out these steps successively in an ion trap formed by a quistor. However,thegasvolumeana- lyzed during such measurements is too small to permit contaminating substances to be detected even if they are present only in extremely small concentrations, as is frequently the case with highly toxic contaminants.

There is a demand for an apparatus which permits the presence ofcontaminations in a basic substance to be detected with a high degree of safety even if the contaminants are present only in extremely small concentrations, and which is sufficiently small to be suitableforfield application. An apparatus ofthis type should permit, for example, to detect con taminationsoftheairatanydesired place. Itwould be very advantageous for such an application if the dimensions and the weight of the apparatus were small enough to permit the apparatus to be carried. It is the object of the present invention to provide a method that allows an apparatus ofthistype to be constructed in this manner.

This object is achieved according to the invention by a method in which the gas mixture is supplied to the ion trap ofthe mass spectrometer through a diaphragm which offers a higher degree of permeability to the contaminant than to the basic substance ofthe gas mixture.

The use of diaphragms for enriching organicsubstances in anorganic basic gases for the purposes of mass spectrometry has been known already from German Disclosure Document No. 1673239. However, the use of such diaphragm arrangements which are described as "diaphragm separators", could not gain general acceptance in mass spectrometry, and this fortwo reasons. Due to their high dissolving powerfor organic substances, such diaphragms represent on the one hand a substance reservoir which isfilled and emptied again and which,therefore, leads to shifts in the concentration of the substances overtime which are very detrimental forthe pur- poses of high-precision GC separations.On the other hand, such diaphragms, which consist of polymerized organic substances and which have to be heated if they are to meet their purposes, give off into the mass spectrometer a continuous stream of decomposition products. Frequently, these decomposition products are formed exactly underthe effect of the passing substances so that they vary as to type and quantity, which makes it impossible to obtain clear spectra by deducting a constant background.

These are the reasons why diaphragm separators, in particular multi-stage types with large diaphragm surfaces, have practically vanished in mass spectrometry. Even the research activities directed at investigating the properties of diaphragms have been reduced considerably.

Inspite ofthese drawbacks connected with the known diaphragms, it is exactlytheir use in connection with the MS/MS method that leads to the solution ofthe problem of the invention. For, the unfavorable behavior overtime of the diaphragm and the factthat it gives off decomposition products are no longerof any importanceforthe analysis oftraces of organic substances in the air or other mixtures by the MS/MS method.The unfavorable behavior over time does no longer influence the analysis results because the measuring conditions can be maintained over extended periods of time and because one is in a position to wait until a stationary balanced condition has been reached. The influence of the decomposition products on the results is also excluded by this measuring technique.

There are diaphragms which, dueto their dissolv- ing power and diffusion properties, providedif- ferences in permeability to organic substances on the one hand, and for example, to the molecules of the air on the other hand which are easily in the range of two to three orders of magnitude, so that a corresponding enrichment ofthe contaminants in the gas mixture occurswhen the latter passesthediaphragm, whereby the sensibility of the method is increased by two to three orders of magnitude. This renders it possible to determine with the aid ofthe method according to the invention contaminations in concentrations as low as 1 02 ppm.

The fact that in the case of the method of the inventionthe procedural steps following each otherspati allyareconverted into proceduralstepsfollowing each other in time so that only a single ion trap with the relevant installations is required, makes the apparatus for carrying out the method very simple.A particularly important aspect is, however, seen in the factthatthe ionization times may be as long asdesired so that the working pressure may be the same during the shock and reaction phases as during the phases of mass-spectroscopic elimination and analysis, and no notable pumping efforts are required to maintain the low pressure required in mass spectro copy. Accordingly, pumps of low capacity, for example 10-3 Pa, will already be sufficient for carrying out the method ofthe invention.

Ionization ofthe gas mixture is advantageously achieved by generation of primary ions of the basic substance by electron bombardment, and subsequent ionization ofthe contaminant by charge exchange and/or chemical ionization. Considering that in the case of the method according to the invention, the contaminant is present only in very low concentration, only a fraction of the molecules of the contaminant contained in the ion trap will be ionized by such electron bombardment. Following this step, however, the contaminant is ionized more or less selectively by charge exchange and/or chemical ionization. To assist this process, it may be advantageous to add to the gas mixture, priorto supplying it into the ion trap ofthe mass spectrometer, a gas supporting the ionization by charge exchange and/orchemical ionization.It is sufficientforthis purpose to add this gas in very low concentrations.

It should be remembered that during the elimination phase the electric potentials present at the ion trap are adjusted in such a mannerthat all ions, ex ceptforthe ions of the contaminant whose presence is to be determined, are permitted to escape from the ion trap. Thereafter, these residual ions are converted in the pre-determined manner, preferablyfragmented, a process that can be carried out without any intervention from the outside, by collision with the molecules of the basic substance, i.e. in particularthe molecules ofthe air. The fragments can then be and lyzed in the manner described before which permits perfect identification ofthe contaminant.

Considering that the method according to the in vention is intended not so much to detect unknown substances, but rather to determine the presence of a given contaminant in a gas mixture, the method according to the invention providesthepossibilityto optimize the procedural parameters for every contaminantto be determined. Such optimization inclu des, for example, the selection of optimum potentials and frequencies forthe generation of a d.c. and a.c.

field which ensures that all undesired ions are re moved effectively and that the ions of the pre determined contaminant are perfectly trapped. In addition, the gas supporting the ionization process can be selected specifically with a view to assisting the ionization ofthe pre-determined contaminant.

The present invention further relates to an appar atusforcarrying out the method according to the invention, wh ich meets the requirementssetout above. The apparatus according to the invention comprises an ion-storing spectrometer provided with an inlet opening forthe gas mixture. According to the invention, the inlet opening is closed by a dia phragm which offers higher permeability to the contaminants than to the basic substance. The dia phragms in question may, in particular, consist of dimethyl silicone diaphragms.

The spectrometer of the apparatus according to the invention may, for example, be an ICR spectro meter. Preferably, however, a three-dimensional quadrupole ion trap (Quistor) comprising an annular electrode and two cover electrodes arranged coaxially therewith for generating an axially symmetrical radio-frequency field, is used as an ion-storing spect rometer.

Conveniently, the inlet opening has connected to it a quartz capillary which can be heated and which is coated with a chemically bonded, chromatographic phase. The use of quartz glass providestheadvan tageofchemical neutrality, and heating supports the diffusion process through the diaphragm. Coating the quartz capillary with a chemically attached chromatographic phase serves the purpose to protectthe contaminants against adsorption or decomposition.

Ifthe method ofthe invention makes use ofthe addition to the gas mixture of a gas supporting the chemical effect, then the vessel containing this gas may be connected with the inlet opening and, in part icular,with the quartz capillary via a diaphragm.

Such a diaphragm causes the reaction gas to be added automatically in the very small quantities required forthis purpose. Accordingly, a vessel having a capacity of only a few cubic centimeters would suffice in this case to supply the reaction gas during the whole normal service life of such an apparatus.

Finally, the ion-storing spectrometer may be provided, in addition to the inlet opening, with another opening connected to an electron source and further with an outlet opening forthe ions to which an ion detector may be connected. This ion detector serves to detect ions exhibiting specific mass/charge ratios which are selectively released after conversion ordecomposition of the contaminant by varying the electric fields in the ion trap.

The invention will be described and explained hereafterwith reference to one embodiment, shown in the drawing, ofan apparatus according to the invention and ofthe method to be carried out with the aid ofthis apparatus. The drawing shows the essential parts of an apparatus designed in accordance with the invention, partly in cross-sectional and partly in diagrammatic representation.

The apparatus shown in the drawing comprises an ion trap formed by a quistor consisting of a central annular electrode 1 delimiting a chamber 2, and two cover electrodes 3,4 closing the said chamber at the top and the bottom. The surfaces ofthe electrodes 1, 3 and 4facing the chamber 2 are hyperbolic. The electrodes 1,3 and 4 are interconnected in the areas of their peripheries by means of rings 5 of an insulating material which on the one hand hold the electrodes in spaced and electrically insulated relationship re lative to each other, while connecting them on the other hand in gas-tight manner so thatthe chamber2 is sealed hermeticallyfrom the outside.

The upper cover electrode 3, as viewed in the draw ing, comprises an inlet opening 6 to which a capillary 8 is connected by means of a threaded bushing 7. The capillary consists of quartz glass and has an inner diameter of approx. 0.3 mm. Its inside is coated, in a manner not shown indetail,withachemicallybon- ded chromatographic phase, for example with SE 54.

Adiaphragm consisting of dimethyl silicone and exhibiting a thickness of approx. 60 pm and a surface of approx. 4 m m2 is arranged between the said threa- ded bushing 7 and the inlet opening 6, in a corresponding recess 31 of the cover electrode 3 which is provided with an outlet 32. The diaphragm pres ents higher permeabilityto the molecules of organic su bsta nces than to the molecu les of air. The term "permeability" is meantto describe that the product of diffusion and solubility is much higherfororganic molecules than forthe molecules of air. Sincethe diffusion of gas molecules istemperature-dependent, the capillary is surrounded by a heating element 10.

In addition, a vessel 11 containing a reaction gas is connected to the capillary 8. Another diaphragm 13 is arranged in the connection line 12 between the vessel 11 and the capillary 8.

The upper cover electrode 3 comprises in addition a central opening 14 which is followed on its outside by a chamber 15 containing an electron gun 16. The chamber 15 is likewise sealed hermeticallyfrom the outside by a plate 17 carrying the electron gun 16.

The lower cover electrode 4 - as viewed in the drawing - is provided in its central portion with a gridlike section 18 separating a chamber 19 provided at the outside ofthe cover electrode 4from the chamber 2forming the ion trap. The outer chamber 19 houses a detector 20 for ions passing through the grid-like section 18. Here again, the detector20 is carried bya plate 21 sealing the chamber 19 in the lowercover electrode hermetically from the outside. A line 22 establishing the connection to a vacuum pump 23 ends in the carrying plate 21. The pump 23 is a small ion getter pump with a capacity of approx. 101/s art a pressureofapprox. 103Pa.

For operation of the quistor it is necessary to raise the potential atthe central annular electrode 1 to a pre-determined value, relative to the cover electrodes 3, 4which are kept at mass potential and, in addition, to apply an rffieldtothe annular electrode. The annular electrode 1 is connected to this end electrically with a d.c. voltage source 24 and an rfgenerator 25 which delivers a cycle with a frequency in the range of 1.4 MHz and a peakvoltagethatcan be adjusted between 0 and 6 kV. The electron gun 16 is con nected to a separate power pack, and a signal processor 27 which is conductively connected therewith is provided for processing and evaluating the output signals ofthe detector 20.

For carrying outthe method according to the invention the airwhich is to be examined to determine the presence of a specific contaminant is supplied into the recess 31 comprising the diaphragm 9 through the capillary 8 provided with an innercoat- ing in the form of a chemical Iy attached ch romato- graphic phase. When the gas mixture passes the diaphragm 9, it is greatly enriched with the contaminant because the latter passes the diaphragm 9 much more easily than the air molecules. The resulting excess air is removed from the recess 31 through the outlet 32.It is a generally known phenomenon that certain polysiliconesare passed byorganicmolec- ules having a molecularweightinthe range of 70to 300 considerably, i.e. 100 to 1000 times, more quickly than byairmolecules.Accordingly,theairvolume received in the chamber 2 ofthe quistor exhibits a degree of contamination 100 to 1000 times hig her thantheairoriginallyfed in.

The analysis ofthe airvolume received in the chamber 2 commences by releasing a short electron beam pulse bywhich predominantly ions of the air molecules, i.e. N2+ and 02+ are generated. These ions are trapped in the chamber 2 ofthe quistor by the voltages applied to its electrodes and the d.c. and a.c. fields generated thereby. The ions reactwith the neutral molecules present in the cham ber 2 by charge exchange (CE) and chemical ionization (Cl).

These processes are particularly effective for heavier organic molecules so that intensified ionization is obtained forthe contaminants contained in the air. This effect isfurther intensified by the fact that during admission ofthe air into the chamber 2 a reaction gas is added to the airvia line 12 and the diaphragm 13, which reaction gas supports the formation of secondary ions although it is present only in traces.

Reaction gases suitableforthis purpose are low aliphatic hydrocarbons, such as methane, butane, etc. The resulting secondary ions of both the reaction gas and the contaminants aretrapped by the electric fields in the quistor, as are the ions ofthe air molecules.

At the end of a given ionization time, the q u istor, acting as a mass spectrometer, is used for selecting ions of a pre-determined mass/charge ratio. By increasing the amplitude ofthe rf voltage, it is possible to eliminate ions with an excessively low mass/ charge ratio, and by increasing the d.c. voltage, it is possible to eliminate ions with an excessively high charge/mass ratio. Elimination ofthe undesired ions is effected in the course of approximately 100 cycles ofthe rfvoltage and, accordingly, in about 100 Fs.

Now,the ions remaining in the chamber 2 ofthe quistorarefragmented by collision with each other and withthe air molecules. This process,which isto be regarded as a shock-induced dissociation (CID), takes place in the chamber 2, and the fragments resulting therefrom are also trapped in the chambers Given the fact that the ions are prevented from escaping,thefragmentation time may be extended until the maximum degree offragmentation has been achieved. It is not necessary to feed in additional collision gas during this period oftime.

Finally, the ions generated by the fragmentation process, all of which originate from mother ions having a selected mass/charge ratio, are subjected to mass analysis. Tothis end,the d.c. voltage applied to the annular electrode is increased linearly whereby the ions are ejected through the grid-like section 18 of the cover electrode 4 - as viewed in the drawing.

The ejected ions are recorded by the detector 20. The heaviest ions are the first to be ejected. The signal processor 27 permits a mass spectrum to be recorded on the basis of the signals generated by the detector 20 in response to the respective values ofthe d.c. voltage. Finally, it can be determined precisely from the complete spectrum of the fragments if the pre-determined contaminant is present or absent.

As mentioned before, the diaphragm 9 at the inlet of the chamber 2 provokes a rise in concentration of thecontaminantto be determined intheairentering the chamber 2. This is achieved with the assistance of the outlet 32 which permits a multiple of the gas mixture entering the chamber 2, corresponding to the enrichment factor, to flow past the diaphragm 9. For known warfare agents, the enrichment factor is 300.

The narrowopenings in the grid-like section 18 further result in a pressure difference between the chamber2 andthe pump 23. Atthe narrowopenings of this section 18, an effect opposite to that occurring at the diaphragm 9 is encountered - known as Knudsen effusion - because the lower-mass molecules pass this section more easily than the heavier molecules of the contaminant. The values ofthe streams of lighter and heavier molecules exhibit a relation approximately equal to the square root ofthe ratio of their masses. It follows that in the case of molecules having a molecularweight of app- roximately 300, the stream of air has approximately three times the value of the stream of molecules so thatthe heavy molecules ofthe organic contaminants are enriched bythefactor3.Accordingly, the concentration of the contaminants is increased totally by the factor 1 & If, therefore, the contaminant is present in the air in a concentration of 10.2 ppm,the airvolumecontained in the chamber 2 will have a contamination content of 10 ppm.

A contaminant ofthe described type, which is present in the air in a concentration of 10 ppm, has a partial pressure of 1 04 mPa if the pressure prevailing in the chamber2 ofthe quistoris 10 mPa sothatthe concentration of the contaminant in the chamber is 2.109 molecules/cm3.

An ion beam pulse of a duration of approximately 1 ms produces a sufficient amount of ions to saturate the quistor. The saturation of the quistor is dependent on space-charge effects and commences in the area of 1 07 to 108 ions/cm3. If the chamber has a radius of 2 cm, it may contain 1 o8 ions withoutthe risk of saturation. The velocity constant for the formation of CI ions in the quistor chamber 2 is approximately 10-9. cm3.molec~1.s~1 or 30 mPal.This velocity constant means that 3 x 104 ions of a componentwith a partial pressure of 10-4 mPa will be generated in the course of a reaction time of 10 msfrom 108 primary ions. Assuming a loss of 90% by immediate fragmentation or elimination by elastic scattering, one finally obtains 3000 mother ionsfrom the 10.2 ppm component.

If the pressure prevailing in the quistor chamber 2 is 10 mPa, it can be expected that approximately 50% ofthe 3000 mother ions will be fragmented within a few milliseconds so that 1000 daughter ions belong ing to only a fewspecieswill be obtained.

If one assumes an efficiency of 1 0%forthe extra ction of ions from the quistor chamber 2, 100 ions of a contaminant are generated if the original concentra tion ofthe contaminant in the airwas 10-2 ppm.

Actually, the number of ions extracted from the chamber2 may be higherwhen a channel electron multiplier is used as detector 20 and arranged close to the grid section 18 with its strong field. The before mentioned 100 or more ions are ejected within only a few milliseconds and generate a signal farabovethe noise level ofthe electron multiplier. Further, the en- tire detecting cycle requires less than 50 ms so that averagingthesignal overa period of 1 secwould lead to a number of ions 20 times higher and, thus, increase the sensibility ofthe method by more than 4 times.

Altogether, it can be noted that the application of the method according to the invention makes it pos sibleto determine contaminations in the air in the form of organic molecules with a molecularweight of 200 and over within a very short period oftimeand with high accuracy by means of an apparatus which comprises only a single ion trap of the quistortype and which can be operated with a pump of low cap acity, if a diaphragm is arranged atthe inlettothe quistor chamber. Accordingly, an apparatus des igned in accordance with the invention is excellently suited for determining specific contaminants in the air, in particularfor detecting chemical warfare agents, atany place in the field, because itcan easily be constructed as a portable unit with small dim ensions, due to its structure and low power require ments.

Claims (12)

1. Method of examining a gas mixture consisting of a basic substance and at least one contaminating substance present in low concentration, in particular contaminated air, in orderto determine the presence ofaspecificcontaminant,comprisingthefollowing steps which are carried out in series in an ion-storing mass spectrometer.

a) ionizing the gas mixture, b) separating from the ions obtained, by mass spectrometric methods, those ions which exhibit a charge/mass ratio different from that ofthe contami nantto be determined; c) subjecting the remaining ions to a chemical and/ or physical transformation; and d) examining by mass-spectrometric methods if is transformation products characteristic of the con taminantto be determined have been produced, wherein the gas mixture is supplied to the ion trap ofthe mass spectrometerthrough a diaphragm which offersa higherdegree of permeabilityto the contami nantthan to the basic substance of the gas mixture.

2. Method according to claim 1,wherein ionization of the gas mixture is achieved by generation of primary ions ofthe basic substance by electron bombardment, and subsequent ionization ofthe contaminant by charge exchange and/or chemical ionization.

3. Method according to claim 2, wherein a reaction gas supporting the ionization by charge ex changeand/orchemical ionization is added to the gas mixture, priorto supplying it into the ion trap of the mass sepctrometer.

4. Method according to anyofthe preceding claims, wherein the residual ions are fragmented by collision with the molecules of the basic substance.

5. Method according to any of the preceding claims, wherein the procedural parameters are optimized with a view to maximum ionization, selection and fragmentation ofthe moleculesofthecontami- nantto be determined.

6. Apparatus for carrying out the m eth od accord- ing to any ofthe preceding claims, comprising an ion-storing spectrometer provided with an inlet opening forthe gas mixture,wherein the inlet opening (6) is closed by a diaphragm (9) which offers higher permeability to the contaminants than to the basic substance.

7. Apparatus according to claim 6, wherein the ion-storing spectrometer is an ICR spectrometer.

8. Apparatus according to claim 6, wherein the ion-storing spectrometer is a three-dimensional quadrupole ion trap comprising an annular electrode (1 ) and two cover electrodes (3, 4) arranged coaxially therewith for generating a radio-frequency field, having rotation symmetry.

9. Apparatus according to any of claims 6 to 8, wherein the inlet opening (6) has connected to it a quartz capillary (8) which can be heated and which is coated with a chemically bonded, chromatographic phase.

10. Apparatus according to any of claims 6 to 9, wherein a vessel (11), which contains a reaction gas supporting the chemical reaction, is connected with the inlet opening, in particular with a quartz capillary (8), via a diaphragm (13).

11. Apparatus according to any of claims 6 to 10, wherein the ion-storing spectrometer is provided, in addition tothe inlet opening (6),withanotheropen- ing (14) connected to an electron source (16) and further with an outlet opening (18)forthe ionsto which an ion detector (20) is connected.

12. Method and Apparatus, substantially as herein described with reference to and as illustrated in the accompanying drawing.