EP0509986A1

EP0509986A1 - Generation of an exact three-dimensional quadrupole electric field.

Info

Publication number: EP0509986A1
Application number: EP90903006A
Authority: EP
Inventors: Yang Wang
Original assignee: Bruken Franzen Analytik GmbH
Current assignee: Bruker Daltonics GmbH and Co KG
Priority date: 1990-01-08
Filing date: 1990-01-08
Publication date: 1992-10-28
Anticipated expiration: 2010-01-08
Also published as: EP0509986B1; CA2033753A1; DE69019829D1; CA2033753C; DE69019829T2; WO1991011016A1

Abstract

Un champs quadripolaire à symétrie de révolution tridimensionnel exact ou un champs électrique à moments multipolaires plus élevés peut être généré dans des limites fermées par application d'un potentiel continuellement variable, plus particulièrement d'un potentiel linéairement variable, et dans le cas idéal, à l'intérieur de simples limites coniques par application d'un potentiel linéairement variable. Un tel champs trouve par exemple un domaine d'application dans le stockage de particules chargées à l'intérieur de limites fermées. A l'intérieur de ces mêmes limites coniques, on peut superposer un champs idéal homogène dirigé dans le sens de l'axe de symétrie. Ce champs peut être utilisé pour l'excitation de l'énergie synéthique, pour des opérations de trempe ou pour l'analyse de l'énergie des particules chargées ainsi stockées. Pour générer des spectres de masse, on éxcite les fréquences fondamentales spécifiques de la masse par rapport à la charge des particules chargées qui sont stockées dans la structure d'électrodes. Les courants d'image induits dans la structure d'électrodes sont analysés en fréquence (par exemple par transformée de Fourier).An exact three-dimensional rotationally symmetric quadrupole field or an electric field with higher multipole moments can be generated within closed boundaries by applying a continuously varying potential, more particularly a linearly varying potential, and in the ideal case, to within simple conical boundaries by application of a linearly varying potential. Such a field finds for example a field of application in the storage of charged particles inside closed limits. Inside these same conical limits, one can superimpose a homogeneous ideal field directed in the direction of the axis of symmetry. This field can be used for the excitation of synethic energy, for quenching operations or for the analysis of the energy of the charged particles thus stored. To generate mass spectra, the fundamental frequencies specific to mass are excited with respect to the charge of the charged particles which are stored in the electrode structure. The image currents induced in the electrode structure are analyzed in frequency (for example by Fourier transform).

Description

Generation of an Exact Three-Dimensional Quadrupole Electric Field

This invention relates to a method of generating a three-di¬ mensional rotationally symmetric quadrupole electric field or an electric field of higher multipole moments inside an electrode structure forming the boundary of the field by appli¬ cation of a resultant electric potential qo to the electrode structure. Up to now, three-dimensional rotationally symmetric quadrupol fields were generated by an array of metallic electrodes with hyperbolic isopotential surfaces (US-A 2 939 952 and US-A 3 527 939) . As an example in Fig. 1 the standard structu is shown, which consists of a ring electrode (1) of radius r two end caps (2) of distance 2zo . ro and Zo are characteristi dimensions, which are related to the spacings of the hyperbol surfaces from the center of the structure. The application of the three-dimensional rotationally symmetric quadrupole field to trap ions and charged particles and to study the propertie of the trapped species and to generate mass spectra is well reported in the literature (Quadrupole Mass Spectrometry and Its Applications, P.H. Dawson, Ed., Elsevier, Amsterdam, 1976, and D. Price and J.F.J. Todd, Int. Mass Spectrom. Ion Process 60 (1984) 3).

For the generation of mass spectra chiefly four methods are described:

Mass analyser method, disclosed in US-A 2 939 952,

The mass-selective storage method disclosed in

US-A 3 527 939,

The mass-selective instability method disclosed in

US-A 4 540 884,

Detection of image currents disclosed in US-A 2 939 952, published in E. Fischer, Z. Phys., 156 (1959) 26, employ

Fourier Transformation.

The generation of a three-dimensional electric quadrupole fie by hyperbolically shaped metallic electrodes generates severa severe problems: - The manufacturing of electrodes is complicated and costl Due to the finite size of the electrodes, field imper¬ fections are generated.

Since gaps exist between ring and cup electrodes the re¬ sulting quadrupolar field is easily influenced by charge accumulated on the surface of the electrodes. The detection of the image current signal generated by the ions is disturbed by other electric fields. The image current generated by the charged particles de¬ pends on their position in the trap, resulting in a nois signal.

Finally, there is one further important disadvantage in ■ generating a three-dimensional electric quadrupole field usin hyperbolically curved electrodes:

It is impossible to generate additional electric fields withi the same interior region of the electrodes without any inter¬ ference with the first electric field.

However, employing metallic electrodes of hyperbolic surface is not the only possibility of generating three-dimensional quadrupole fields, although up to now only electrode surfaces following the equipotential surfaces at the boundary of the electric field are commonly used because of prejudice.

Accordingly, it is an object of the invention to provide a method and the corresponding structures for generating a thre dimensional quadrupole electric field or an electric field of higher multipole moments which is much more exact, using no hyperbolically curved metallic electrodes and thus presenting the possibility of superimposing additional homogeneous elect fields without interference with the first electric field. This object is achieved according to the invention by continuously varying the resultant electric potential Φ_qo acro the electrode structure.

Since the electric potential applied to the electrode structur is not chosen to be constant, but varies continuously across the electrode structure, those surfaces of the electrode structure forming the boundary of the electric field do not have to be parallel to the equipotential surfaces of the elec¬ tric field at its boundary. In other words, those parts- of the electrode structure forming the boundary of the electric field do not necessarily have to be curved, but are only required to form contours corresponding to the boundary conditions of an implied resultant electric potential generating the quadrupole electric field or an electric field of higher multipole momen

In one embodiment of the invention the resultant electric po¬ tential is continuously varied with position on the surface of the electrode structure adjacent to the electric field. In another embodiment the resultant electric potential is compos of a plurality of single electric potentials being applied each to separate electrodes forming the electrode structure. In both cases as a result there will be an electric potential continuously varied across the electrode structure and generating a quadrupole field.

As a special case of a continuously varied resultant electric potential there can be chosen a linearly varied resultant po¬ tential. Even for this special choice there exists an infinit plurality of possible boundary conditions for the resultant electric potential generating the three-dimensional rotationa symmetric quadrupole electric field or an electric field of higher multipole moments. Amongst these boundary conditions there is again a special solution, namely the case of a double cone shaped boundary in which an applied linearly varied elec¬ tric potential generates a quadrupole field. Such a double- cone shaped structure can be manufactured very easily and with high precision.

By the choice of an adapt second potential applied to the electrode structure, a second electric field inside the electrode structure which is homogeneous in symmetry axis direction can be generated and superimposed to the quadrupole field without interaction. The possibility of creating such a homogeneous electric field not interfering with the quadrupole electric field is one of the major advantages of the method according to the invention.

The main application of this method will be the field of mass- spectro etry, especially the mass selected analysis of stored ions. In one variant of the method according to the invention the ions to be analyzed are generated outside the electrode structure. They could be e.g. components of an ion beam direct into the electrode structure. Another possibility is the creation of ions out of neutral particles inside the boundary of the quadrupole field. In this case the ionization may be performed by electron impact, ion-impact or resonant photon absorption. Accordingly for the generation of the ions an electron beam, a primary ion beam or a laser beam can be em¬ ployed. It can be of advantage, if the ionizing beams are pulsed. In this case it is possible to perform the ass-spectr metric analysis of the stored ions in a time-dependent mode by running a plurality of measuring cycles. In certain applications, it might be, on the other hand, desirable to use a c.w. ionizing beam, for example, if a scattering experimen with a primary ion beam shall be performed or, if charge ex¬ change processes are to be studied.

In a variant of the method according to the invention, the a mentioned second, homogeneous electric field inside the boun of the quadrupole electric field or the electric field of hi multipole moments is used for a mass-to-charge specific exci tation of the fundamental frequencies of the ions to be ana¬ lyzed. This will cause a resonant movement of the excited charged particles in the direction of the symmetry axis. As result of this resonant movement image current signals are induced in the electrode structure which can be differential detected and processed into a frequency-analyzer. Employing Fourier Transformation techniques for the frequency analysis can be especially advantageous.

In another variant of the method according to the invention the excitation of the ions under investigation by the second homogeneous field is used for ejecting the ions out of the boundaries of the first electric field and detecting them wi a charge-sensitive detector. This can be, for example, desirable, if the number of ions under investigation inside the electrode structure is so small, that the image current induced by the ion movements has an amplitude below the nois signal level. In this case the detection of single ions by a adapt detector, like e.g. a secondary electron multiplier, a channeltron or multi-channel-plates, might be the only alter native to the image current method.

In an embodiment of the invention an electrode structure is operated according to the methods described above. This elec trode structure defines on the one hand the boundary of the electric quadrupole field or the electric field of higher mult pole moments, on the other hand the behaviour of the electric potential being applied to the electrode structure and generating the electric field.

In one embodiment those parts of the electrode structure facin the electric field and defining the boundary of the field con¬ sist of electrically resistive material. This can be accomplished either by coating a non-conductive substrate material with resistive material at those parts adjacent to the electric field, or one can use resistance wires for the construction of the electrode structure.

In both cases the operation of the electrode structure is similar to that of a continuous potentiometer and the construction consists substantially of a single part.

In embodiments of the invention the resistance wire can be helically wound or constructed to form a double umbrella frame work.

In a further embodiment of the invention the electrode structu is built of metallic material. In this case the electrode structure is constructed of a plurality of metallic sheets to which a plurality of single electric potentials is applied constituting a resultant electric potential which in turn generates the quadrupole electric field or an electric field of higher multipole moments. The spatial boundary of the rotationally symmetric quadrupole field can be defined by circular holes with successively varying radii whereby the metallic sheets are disposed with faces parallel in equal or unequal distances.

In an embodiment the metallic sheets are linked together by a resistance network. In this case it is not necessary to gener an adapt potential for each sheet but the negative and the positive output of a single voltage source is applied to the ends of the electrode structure and the resistances of the network are chosen such that the potentials and the single sheets form a resultant continuously varying potential.

In an embodiment the metallic sheets are equally spaced and the resistors are of the same resistance. This facilitates th manufacturing of the electrode structure.

In a further embodiment the metallic sheets with equal areas are equally spaced. Applying F-voltage to this electrode structure one can even omit the resistance network.

For the passing of the particles under investigation and, if necessary, the ionization means, like e.g. an electron beam, the electrode structures according to the invention comprise apertures. Especially when beams are employed, it is of ad¬ vantage to dispose the apertures at opposite points of the boundary surface with respect to the symmetry center of the electrode structure. In the case of an "airy" construction, like the helically wound resistance wire or the metallic shee the apertures are already built in by the construction principle.

In an embodiment of the invention with a double-cone shaped electrode structure two ring plane electrodes distant ± 1 z₀ 2 from the plane defined by the annular contact line of the two cones are provided for detecting the image currents of ions moving in symmetry axis direction inside the field boundary.

The invention will now be described and explained in greater detail by way of the embodiments shown in the drawing, it bein understood that the features described in the specification and shown in the drawing may be used in other embodiments of the invention either individually or in any desired combinatio

In the drawing

fig. 1 shows a metallic structure with hyperbolic isopotential surfaces for generation of a three-di¬ mensional rotationally symmetric electric quadrupole field by application of the potentials ± Φqo to the ring (1) and end cap electrodes (2);

fig. 2 shows plane curves in symmetry axis coordinates cros section as a function of r and z with the applied potential varying linearly along these curves;

fig. 3 shows rhombic plane curves with linearly varied po¬ tential;

fig. 4 shows equipotential lines for the potential generate according to fig. 3 in the rz plane (fig. 4a) and in the xy plane (fig. 4b) ;

fig. 5 shows equipotential lines of a homogeneous electric field superimposed to the quadrupolar or higher multi pole electric field in the structure shown in fig. 3; fig. 6 shows a cone shaped surface of region in which exact three-dimensional quadrupole fields and additional homogeneous electric fields are generated;

fig. 7 shows an embodiment of the electrode structure com¬ prising densely placed equidistant metallic sheets with circular holes to form the inner surface of the cone;

fig. 8 shows an embodiment of the electrode structure com¬ prising a helically wound resistance wire;

fig. 9 shows an embodiment of the electrode structure com¬ prising an umbrella framework of resistance wires;

fig. 10 shows a block diagram of an advantageous realization of the invention;

fig. 11 shows the shape of excitation pulse for ion excitati in the electrode structure; and

fig. 12 show pulse sequences employed for generation of mass a and b spectra.

The invention provides a method and the corresponding structur of generating an exact three-dimensional quadrupole field or an electric field of higher multipole moments and a method and corresponding structures for superimposing further homogeneous electric fields in symmetry-axis direction on the first field. The application of the device to store charged particles and to generate mass spectra by simultaneous or consecutive detection of the image currents induced by the charged partic in the electrode structure or by charge detection is also presented.

Principles of Mass Analysis of Charged Particles Trapped in Electric Quadrupole Fields

If positive and negative voltages

± qo = ± (U - V cosωt) (1)

are imposed separately on a ring plane electrode and two end- plane electrodes of a cone shaped structure, described in deta later on, three-dimensional rotationally symmetric quadrupole fields are generated within the interior region of the electro structure. This field will be called trapping field. With ionizing radiation or an electron beam of sufficient energy passing the trap structure, neutral molecules inside the trap are ionized and a number of ions of different mass-to-charge ratio m/q is generated with certain initial conditions of motion.

The trajectories of the charged particles in the fields can be expressed by the canonical form of the linear Mathieu equation

(2) d² r fa

- (a₂ - 2_qz cos21 ) 2 = 0

^Λ ' > with parameters ar =

( 3 )

4 q V 8 q V qr = qz = _> = ^«f mro j ² mr ^zo co² The solution of the Mathieu equation leads to stable or un¬ stable trajectories of the charged particles, depending only on the selection of the parameters (3) . For a given set of parameters, U, V, r₀ , the charged particles of a certain m/ range have stable trajectories, the other charged particles have unstable trajectories. The charged particles of the same mass-to-charge ratio have the same motion regularities which can be considered as the sum of an infinite series of sinusoi oscillations with frequencies

The characteristic parameters β_r ,z satisfy 0 £ p 1 and have a known relationship with parameters a_r ,₂ and q_r ,₂. Therefore a relationship between the β values and the m/q ratios can be obtained

>r ,₂ (ar z . q^r.z ) f _ (5)

The component frequencies of i -on (moti)on are- unique and specifi for particular m/q ratios. According to the selected range of stable ions, in practical operation a_r and a₂ can be set to zero.

If some form of voltages ± Φ₂₀ (t) is imposed additionally on the two end plates of the cone-shaped electrode structure, a second electric field is superimposed in axial (z) direction on the first field. This second field will be called excitatio field. It acts on the stored charged particles as expressed by the even linear Mathieu equation

H_ + (a_r - 2q_r cos 2^.) r = 0

^d2z - (a₂ - 2q₂ cos 2 ) z = F (έ ) ⁽⁶⁾ d€,²- ^

The force F (fc^) depends only on time and not on position of the charged particles.

According to the theory of differential equations the solution of eq. (6) consists of one independent part with initial con¬ ditions and of a second part given in eq. (2) . When the exci¬ tation frequency matches the characteristic frequency of a charged particle with certain m/q or a subharmonic thereof, resonance occurs and the trapped particle moves with a frequen equal to the characteristic frequency. The amplitude of motion will grow linearly with time. The motion of the trapped particles is now coherent in z direction. If the characteristi frequencies of charged particles differ from the excitation frequency no resonance occurs.

In summary, the said quadrupole fields have two functions: to trap charged particles with a certain range of m/q ratios and to cause oscillations with frequencies characteristic for the different m/q ratios of the charged particles. With the aid of excitation fields the characteristic frequencies of the trappe charged particles can be excited, so that the motion is cohere in the z direction. Usually the above mentioned frequencies ar in the RF-range.

Generation of Potential Distribution

A Short Description of Theoretical Foundation

In the absence of space charge electrostatic potentials Φ obe the Laplace equation

γ² Φ = 0 (7)

with boundary conditions

Φ I s (8)

With given boundary conditions (e.g. the contours of a curved surface and the corresponding potential values on the surface unique electrostatic fields can be defined within the interio region of the boundaries. However, if once a definite electro static field has been defined according to eq. (7) a wide variety of corresponding boundary conditions according to (8) is still possible. If the potential values on each point of a curved surface correspond to the values of the definite elect static field at this point, the Laplace equation (7) and the boundary conditions (8) are also satisfied. If we apply this idea to three-dimensional quadrupole fields, we can select th ideal boundary conditions and the ideal electrode configurati for practical applications.

In cylinder coordinates r and z the potential constituting an exact three-dimensional rotationally symmetric quadrupole fie is expressed as

(9) It can be shown that the field resulting from the potential (9) can be generated within interior regions closed by curved surface which is formed by revolution of a plane curve by po¬ tentials varied along this plane curve. The equation of the plane curve in plane polar coordinates?, θ, in symmetry-axis coordinate cross section, is

where

b = dΦs (11) ds

For example, with b = 0 and Φs = constant, one obtains from eq. (10) that electrodes with hyperbolic isopotential surfaces expressed as

_r2 - 2 z² = constant (12)

yield the correct potential (cf. fig. 1).

The second, most important selection is b = constant, 18

b = dΦs = constant Φs = bs (1 ds

The potential values vary linearly along the plane curves.

In fig. 2 some of the corresponding plane curves in symmetry- axis coordinates cross section are shown with conditions

The outermost curve is for b = 0.3 V/cm, the next for b = 0.8 V/cm, the third is for b = 1.2 V/cm and the innermos curve is for b = 1.633 V/cm.

As a special case there exist simple rhombic closed-plane cu on which the potential varies linearly. This is shown in fig.

Let the expression of one rhombic line AB be

Zo.r z = - + Zo 0 < r < r₀ (1

With the aid of eq. (9) one obtains

Obviously, the potential values on line AB vary with r. Ther fore exact three-dimensional quadrupole fields can be genera within an interior region with boundaries revolved in symmet axis by plane rhombic curves: r ² - 2 ( z - zo ) ² = 0 2 > 0

(17 )

- 2 ( z + Zo ) ² = 0 z < 0

The corresponding contours of the equipotential lines are show in fig. 4a for the zr plane and in fig. 4b for the xy plane.

In addition a homogeneous field can be generated in the same interior region by applying a second potential which varies ■ linearly along the rhombic boundaries in a way different from the first, for example along the line AB, given in fig. 3

Φ -zs _= - ,r. j+. _AΦ₂1 (M18ot)

This generates the homogeneous field with equipotential lines as shown in fig. 5.

_{φz =} Φ»ι - φ_{22 z +} zi + Φ_{22 (ig)}

2z₀ 2

It can be shown that two or more definite electrostatic fields can be obtained within the same interior regions. Each of these fields can be generated by imposing the corresponding continuously varying potential values to the boundary surface. In this way exact three-dimensional quadrupole fields and ad¬ ditional exact excitation fields in symmetry-axis direction can be superposed without interference within the same interior region closed by the cone-shaped surface, shown in fig. 6. The potential constituting the resultant field is given in eq. (20) Φ^{q o} Φz 1 - Φz2 Φ₂₁ + Φ₂

Φ⁽r,z⁾ = Φ^q + Φz = (r² - 2z²) + z + ₍ r₀ ² 2zo 2

÷Φ^qo an^d - Φ^qo are the applied potentials to generate a quadrupole fiel^d, Φ₂1 and Φ∑2 are the applied potentials to generate an additional electric field.

Realizations

The realization of the exact three-dimensional quadrupole fiel or an electric field of higher multipole moments according to the new method depends on the way of σeneration of conrinuousl varied potentials cn the corresponding boundaries. Such a continuously varied potential can be realized by a potεntio- mεter-type structure employing electrodes made of electrically restistive material, with the voltage needed for generation of the required surface potential applied on the two ends of the electrode- structure situated on the z-axis. Typical values of resistance between the two ends of the electrode are ranging from 1 to 100 kΩ.

In an embodiment of the invention the electrode structure consists of a nonconductive substrate material, e.g. ceramics, with an electrically resistive coating.

In a preferred embodiment the electrode structure consists of a polymeric halogenizεd polyolεfin, especially of polytetra- flouorine-ethylene (PTFE) like Teflon, having a high share of carbon ranging especially between 10 and 30% wt.

In a special embodiment the resistive material in the electrode structure comprises semiconductor material like Si, Ge or GaAs .

In another embodiment of the invention, a plurality of metallic sheets is employed as electrode structure, the sheets having circular holes with successively varying radii to form the inner surface of the rotationally symmetric field boundary and being densely placed parallel to each other and in equal or unequal distances. These sheets are linked together by a re¬ sistance network dimensioned such that applying a voltage according to eq. (1) to the ends of the network results in a potential according to eq. (9) . In the case corresponding to equal sheet distances, all resistors have equal resistance and the network can even be omitted if the areas of each metallic sheet are equal and radio frequency is supplied (cf. fig. 7) .

Also other structures to generate the fields can be employed, especially in the case of cone-shaped boundaries. Among these are a structure with a helically-wound resistance wire, as shown in fig. 8, or a double-umbrella framework of resistance wires, shown in fig. 9.

The electrode structures according to the invention comprise apertures disposed at opposite points on the boundary surface with respect to the symmetry center of the cell. The particles to be studied inside the electric field and/or means for ionizing these particles can pass through those apertures. An embodiment of the electrode structure comprises sample beam inlets in the symmetry axis of the electrode structure coaxia with the ionizing electron beam or laser beam discussed later.

Now, as an example, the practical realization of a mass spect meter incorporating the electrode structure which consists of metallic sheets of equal surface areas arranged in equal distances, and connected by a network of equal resistors, wil be discussed in detail, as applied to the simultaneous image current detection and frequency analysis of mass-selectively stored charged particles, positive or negative ions in this example. A block diagram is shown in fig. 10. The three-dimensional quadrupole or higher multipole RF field is generated by the potential of the RF supply 10 connected to an electrode structure as shown in fig. 7. The additional homo geneous electric field is generated by the excitation waveform generator 11.

Ions are generated by a pulsed electron beam. The filament supply 12 operates the filament 13, the gate voltage supply pulses 14 the electron beam.

Instead of electron-impact any other ionization techniques can be applied. It is, for example, possible to use an ion beam for secondary ionization of particles inside the cell, especially if one wants to study scattering and charge transfer processes.

Also photoionization can be employed, preferably using a laser beam which can be c.w. or pulsed. Because of the high frequency selectiveness of photoinization processes the masses of the particles under investigation inside the quadrupole field can be preselected by the choice of the proper excitation frequency leading to photoionization which can in turn be performed using a tuneable laser.

Alternatively, the ions to be studied inside the cell can be injected into the cell already in form of a pulsed or continuou ion beam.

To generate the image current corresponding to the ions of a certain m/q range with stable trajectories, stored in the trap, a pulse of excitation frequencies including all the character- istic frequencies of the ions under investigation is applied, well distributed as shown in fig. 11. The resonant ions absor power and a coherent motion in z axis direction is generated.

In regard to the working mode and function the structure unde consideration is equivalent to a capacitor consisting of a pair of parallel plates. After the excitation pulse, the imag current signal induced by the coherent motion of the ions in axis direction can be detected on the boundary of the structu as if it were a capacitor with parallel plates.

An especially important technique is to employ differential detection of the image current signal at two ring plane electrodes at z = ± 1/2 Zo (2 z₀ being the distance from apex to apex of the double cone structure) where the trapping volt difference is always zero in order to substantially reduce trapping voltage interference with image current detection. Furthermore, a lock-in detector can be used to further reduce this interference in signal detection.

The image current signal is amplified with a high gain broad band amplifier 15. The resulting transient signal can be sub¬ jected to digital data processing after digitation with an analog-to-digital converter 16. The frequency spectrum of the characteristic frequencies of the stored ions can be obtained by any frequency analysis technique. Fourier transformation i especially well suited. The frequency analysis and the contro is performed by a scan and acquisition computer 17. The timin sequences are referenced to the master clock 18.

Instead of detecting the image current, alternatively the sto ions after mass-to-charge selective ejection by excitation of the fundamental frequencies with the homogeneous electric fie can be detected by a charge-sensitive detector like Secondary Electron Multiplier or channel plate. In this case the above mentioned ring electrodes are unnecessary and can even be omitted.

The spectrometer is operated in a pulsed mode, as shown in fig. 12.

In the case of fig. 12a the RF trapping voltage 20 is applied constantly during the experiment. First, all ions being possib in the trap are quenched by a pulse 21 starting at a time ti . At t₂ ions are generated with a pulse 22, e.g. an electron beam pulse of electrons having kinetic energy sufficient for ion formation. At tβ ions are excited with pulse 23 and detect with detection pulse 24 starting at t . At the time ts a measuring cycle is completed.

In the pulse sequence of fig. 12b the quenching pulse 21 is not activated. Instead, the RF trapping voltage 20 is not constantly applied, but is started at time ti and disconnected at time ts . Ions being in the cell after the time ts will, due to their finite kinetic energy, drift to the electrode structu and become neutralized or even pass the field boundary, if they, by chance, find the above mentioned apertures in the electrode structure. At the beginning of the next measuring cycle, with a great probability, there will be no more charged particles inside the field boundary.

The spectral resolution depends on the observation time of the transient signal generated by the coherently moving ions. In the described electrode structures the trapping quadrupole or higher multipole field and the z axis excitation fields are both exact and without mutual interference, the trajectories of the ions are exactly described by the even linear Mathieu equation. This is a major advantage of the described electrode structure compared to any other trap techniques known. The excitation of the ions is independent of their position in the trap. The image current is proportional to the number of ions in the trap. The m/q ratios of the ions correspond to their characteristic frequencies.

Claims

1. Method of generating a three-dimensional rotationally symmetric quadrupole electric field or an electric field of higher multipole moments inside an electrode structure forming the boundary of said field by application of a resultant electric potential Φqo to said electrode structure characterized in that said resultant electric potential Φqo is continuously varied across said electrod structure.

2. Method as claimed in claim 1, characterized in that the resultant electric potential is continuously varied with position on the surface of said electrode structure ad¬ jacent said quadrupole or higher multipole field.

3. Method as claimed in claim 1, characterized in that a plurality of single electric potentials being applied each to separate electrodes forming said electrode structure constitute said resultant electric potential continuously varied across said electrode structure.

4. Method as claimed in claim 2 or 3, characterized in that said resultant electric potential is linearly varied along the curve of any center cross section plane of said electrode structure.

5. Method as claimed in any of the preceding claims, characterized by application of a second resultant electri potential to said electrode structure for generating a second, homogeneous electric field in symmetry axis direction superimposed to said three-dimensional rotationally symmetric quadrupole electrical field or th electric field of higher multipole moments without inter action.

6. Method as claimed in any of the preceding claims, characterized by application of the method to mass spectro etric analysis of stored ions.

7. Method as claimed in claim 6, characterized in that the ions to be analyzed are generated outside said electrode structure.

8. Method as claimed in claim 6, characterized in that the ions to be analyzed are generated inside said boundary o said quadrupole or higher multipole field.

9. Method as claimed in claim 8, characterized in that said ions are generated by a pulsed electron beam.

10. Method as claimed in claim 8, characterized in that said ions are generated by a pulsed laser beam.

11. Method as claimed in claim 8, characterized in that said ions are generated by a primary ion beam.

12. Method as claimed in claim 5 and 6, characterized in tha the ions to be analyzed are mass-selectively stored insi said boundary of said quadrupole or higher multipole ele tric field and that the mass-to-charge specific fundamen frequencies of the ions to be analyzed are excited by said second, homogeneous electric field.

13. Method as claimed in claim 12, characterized in that the image current signals in said electrode structure resulti from the movements of said ions due to resonant excitatio by said second electric field are differentially detected

14. Method as claimed in claim 13, characterized in that a mass spectrum of said ions is generated by application of frequency analysis to said image current signals.

15. Method as claimed in claim 14, characterized in that Fourier transformation techniques are employed for said frequency analysis.

16. Method as claimed in claim 12, characterized in that said ions are ejected out of the boundaries of said quadrupole field and detected with a charge-sensitive detector.

17. Electrode structure operated according to a method as claimed in any of the preceding claims, characterized in that those parts of said electrode structure facing said quadrupole or higher multipole electric field and thus defining the boundary thereof consist of electrically resistive material.

S. Electrode structure as claimed in claim 17, characterized in that said electrode structure comprises a nonconductive substrate material coated ith resistive material at the parts facing said quadrupole or higher multipole field.

19. Electrode structure as claimed in claim 17, characterized in that said electrode structure consists of one or more resistance wires defining said boundary of said quadrupol or higher multipole field.

20. Electrode structure as claimed in claim 19, characterized in that said resistance wire is helically wound.

21. Electrode structure as claimed in claim 19, characterized in that said resistance wires are forming a double umbrel framework.

22. Electrode structure operated according to a method as claimed in any of the claims 1 to 6, characterized in that said electrode structure consists of metallic mate¬ rial.

23. Electrode structure as claimed in claim 22, characterized in that said electrode structure consists of a plurality of metallic sheets having each a circular hole defining the boundary of said quadrupole or higher multipole field whereby the radius of said hole varies successively from sheet to sheet, the sheets being densely placed with face parallel in equal or unequal distances.

24. Electrode structure as claimed in claim 23, characterized in that said metallic sheets are equally spaced in distance.

25. Electrode structure as claimed in claim 23, characterized in that said metallic sheets are linked together by a resistance network.

26. Electrode structure as claimed in claims 24 and 25, characterized in that all resistors of said resistance network have the same resistance.

27. Electrode structure as claimed in claim 23, characterized in that the areas of each metallic sheet are equal.

28. Electrode structure as claimed in any of the claims 17 to

27, characterized in that said electrode structure comprises apertures disposed at opposite points on said boundary surface with respect to the symmetry center of said electrode structure.

29. Electrode structure as claimed in any of the claims 17 to

28, characterized in that said electrode structure define a double-cone shaped boundary of said quadrupole field with a distance 2z₀ from apex to apex and a radius r₀ of the annular contact lines of the two cones.

30. Electrode structure as claimed in claim 29, characterized in that said electrode structure comprises two ring plane electrodes distanced ± 1 z₀ from the plane defined by

2 said annular contact line of said two cones.