Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/42—Stability-of-path spectrometers, e.g. monopole, quadrupole, multipole, farvitrons
  • H01J49/4205—Device types
  • H01J49/424—Three-dimensional ion traps, i.e. comprising end-cap and ring electrodes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/02—Details
  • H01J49/025—Detectors specially adapted to particle spectrometers
  • H01J49/027—Detectors specially adapted to particle spectrometers detecting image current induced by the movement of charged particles

Definitions

• 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.
• 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) .
• 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).
• This object is achieved according to the invention by continuously varying the resultant electric potential ⁇ qo acro 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.
• 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
• the resultant electric po ⁇ tential is continuously varied with position on the surface of the electrode structure adjacent to the electric field.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a charge-sensitive 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.
• 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.
• 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.
• the operation of the electrode structure is similar to that of a continuous potentiometer and the construction consists substantially of a single part.
• the resistance wire can be helically wound or constructed to form a double umbrella frame work.
• the electrode structu is built of metallic material.
• 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.
• the metallic sheets are linked together by a resistance network.
• a resistance network 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.
• the metallic sheets are equally spaced and the resistors are of the same resistance. This facilitates th manufacturing of the electrode structure.
• the metallic sheets with equal areas are equally spaced. Applying F-voltage to this electrode structure one can even omit the resistance network.
• 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.
• an "airy" construction like the helically wound resistance wire or the metallic shee the apertures are already built in by the construction principle.
• two ring plane electrodes distant ⁇ 1 z 0 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.
• 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 ⁇ ⁇ qo 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
• 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.
• 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 characteristic parameters ⁇ 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 ⁇ values and the m/q ratios can be obtained
• 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 a 2 can be set to zero.
• the force F (fc ⁇ ) depends only on time and not on position of the charged particles.
• the solution of eq. (6) consists of one independent part with initial con ⁇ ditions and of a second part given in eq. (2) .
• 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.
• 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.
• the characteristic frequencies of the trappe charged particles can be excited, so that the motion is cohere in the z direction.
• frequencies ar in the RF-range.
• 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.
• boundary conditions e.g. the contours of a curved surface and the corresponding potential values on the surface.
• the potential values vary linearly along the plane curves.
• the corresponding contours of the equipotential lines are show in fig. 4a for the zr plane and in fig. 4b for the xy plane.
• 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
• ⁇ qo an d - ⁇ q o are the applied potentials to generate a quadrupole fiel d
• ⁇ 2 1 and ⁇ 2 are the applied potentials to generate an additional electric field.
• 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 ⁇ eneration of conrinuousl varied potentials cn the corresponding boundaries.
• Such a continuously varied potential can be realized by a pot ⁇ ntio- m ⁇ ter-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 k ⁇ .
• the electrode structure consists of a nonconductive substrate material, e.g. ceramics, with an electrically resistive coating.
• the electrode structure consists of a polymeric halogeniz ⁇ d polyol ⁇ fin, especially of polytetra- flouorine-ethylene (PTFE) like Teflon, having a high share of carbon ranging especially between 10 and 30% wt.
• PTFE polytetra- flouorine-ethylene
• the resistive material in the electrode structure comprises semiconductor material like Si, Ge or GaAs .
• 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) .
• 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) .
• 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.
• 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.
• 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.
• 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.
• the ions to be studied inside the cell can be injected into the cell already in form of a pulsed or continuou ion beam.
• 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.
• the structure unde consideration is equivalent to a capacitor consisting of a pair of parallel plates.
• 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.
• 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.
• 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.
• a charge-sensitive detector like Secondary Electron Multiplier or channel plate.
• the spectrometer is operated in a pulsed mode, as shown in fig. 12.
• the RF trapping voltage 20 is applied constantly during the experiment.
• all ions being possib in the trap are quenched by a pulse 21 starting at a time ti .
• ions are generated with a pulse 22, e.g. an electron beam pulse of electrons having kinetic energy sufficient for ion formation.
• At t ⁇ ions are excited with pulse 23 and detect with detection pulse 24 starting at t .
• At the time ts a measuring cycle is completed.
• 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.
• 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.

Landscapes

• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Electron Tubes For Measurement (AREA)

Applications Claiming Priority (1)

Publications (2)

Family

ID=8165455

Family Applications (1)

Country Status (4)

Cited By (2)

Families Citing this family (2)

Family Cites Families (4)

• 1990
  • 1990-01-08 WO PCT/EP1990/000030 patent/WO1991011016A1/en not_active Ceased
  • 1990-01-08 EP EP90903006A patent/EP0509986B1/de not_active Expired - Lifetime
  • 1990-01-08 DE DE69019829T patent/DE69019829T2/de not_active Expired - Lifetime
• 1991
  • 1991-01-08 CA CA002033753A patent/CA2033753C/en not_active Expired - Lifetime

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events