Patent Publication Number: US-6989533-B2

Title: Permanent magnet ion trap and a mass spectrometer using such a magnet

Description:
FIELD OF THE INVENTION 
   The present invention relates to a magnetic trap for ions and to a mass spectrometer using such a trap. 
   BACKGROUND OF THE INVENTION 
   Ion traps are used in numerous applications in molecular physics, and in particular in the ion cyclotron resonance phenomena implemented, for example, in Fourier transform mass spectrometers or FTICRs. 
   Such magnetic traps for ions enable the ions to be held captive in a defined volume in order to perform various measurements such as detecting cyclotron movements. 
   Conventionally, magnetic traps for ions implement means for generating a uniform magnetic field of high intensity, said means comprising solenoids that are resistive or superconductive. 
   Such generator means enable magnetic fields to be obtained of high intensity that can be as great as 9.4 teslas (T) and they present great stability over time. 
   Nevertheless, such components are very bulky and can weigh several tons. In addition, they require complex power supply and cooling installations and they are therefore suitable for use only in fixed installations. 
   In order to enable mobile devices to be developed, certain magnetic traps for ions make use of permanent magnets (L. C. Zeller, J. M. Kennady, J. E. Campana, H. I. Kentamaa, Anal. Chem. 1993, 65, 2116–2118, U.S. Pat. No. 5,451,781 in the name of Dietrich). 
   However, such permanent magnets generate fields that are generally limited to about 0.4 T and/or that are of volumes that are too small. 
   The qualities of an ion trap are associated with the uniformity and the intensity of the magnetic field to which it is subjected. Certain performance features of a trap vary as a function of the square of the intensity of the magnetic field and a minimum value of about 1 T is recommended for a high performance application to mass spectrometry of the FTICR type. 
   Siemens&#39; “Advance quantra” mass spectrometer uses a permanent magnet generating a magnetic field of tesla order, but in order to do that, it requires a closed geometrical shape that is highly constraining. 
   OBJECT OF THE INVENTION 
   The object of the present invention is to remedy that problem by defining a magnetic trap for ions in which the trap is of reduced size and weight, while maintaining good performance and a practical shape. 
   SUMMARY OF THE INVENTION 
   To this end, the invention provides a vacuum ion trap, the trap comprising a gastight processing enclosure and a permanent magnet defining a cavity and creating a directed magnetic field in said cavity, said enclosure being disposed inside said cavity and containing a confinement cell comprising at least two mutually parallel trapping electrodes perpendicular to said directed magnetic field, said trapping electrodes being connectable to a voltage generator, the trap being characterized in that it includes at least one permanent magnet in the form of a hollow cylinder and structured with a Halbach cylinder type structure so as to generate said permanent magnetic field directed perpendicularly to the longitudinal axis of the cavity of said magnet. 
   According to other characteristics: 
   the dimensions and the composition of the or each magnet are adapted to generate a uniform permanent magnetic field of intensity of at least 0.8 T; 
   the trap includes two permanent magnets in the form of hollow cylinders, both structured with a Halbach cylinder type structure, and of identical dimensions and composition, the magnets being disposed in axial alignment on the same longitudinal axis and being oriented in such a manner as to cause the magnetic fields they generate to be directed identically; 
   the two permanent magnets are spaced apart from each other along their longitudinal axis by a predetermined non-zero gap in order to increase the uniformity of said magnetic field; 
   said gap is less than 1 millimeters (mm); 
   the or each permanent magnet presents an inside diameter in the range 45 mm to 55 mm, an outside diameter in the range 180 mm to 220 mm, and a length in the range 90 mm to 110 mm; 
   the or each permanent magnet ( 30 ) is made up of individual segments of Nd—Fe—B; 
   said confinement cell further comprises two mutually parallel detector electrodes perpendicular to said trapping electrodes, said measurement electrodes being connectable to measurement means in order to transmit information relating to the movements of ions contained in said confinement cell; 
   said confinement cell further comprises two mutually parallel exciter electrodes perpendicular to said trapping electrodes, said exciter electrodes being connectable to an excitation signal generator in order to excite ions contained in said confinement cell; 
   said trapping, exciter, and detector electrodes are plane and rectangular in shape so that said confinement cell is generally in the form of a rectangular parallelepiped; 
   each of said exciter electrodes is constituted by four plates arranged generally in the form of a rectangular parallelepiped that is open via two opposite faces, said exciter electrodes being disposed on a common axis on either side of said trapping electrodes, said open faces facing each other so that said confinement cell is generally in the form of a tunnel; 
   said confinement cell that is generally in the form of a tunnel is placed on the longitudinal axis of said magnet; 
   said processing enclosure includes, at at least one end, a port-hole disposed on the axis of the cell that is generally in the form of a tunnel, and that allows photons to pass therethrough; 
   the processing enclosure includes means for connection to pump means and to means for injecting gas in order to control the density and/or the nature of the atmosphere inside the processing enclosure; and 
   it is associated with means for emitting electrons towards said enclosure in order to generate ions at least in said confinement cell. 
   The invention also provides a mass spectrometer comprising a magnetic trap for ions, a pump device, a trapping voltage generator, and measurement means suitable for performing Fourier transform analysis of the cyclotron movement of ions contained in the ion trap, the mass spectrometer being characterized in that said magnetic trap for ions is a trap as described above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the following description given purely by way of example and made with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagram showing the principle of a mass spectrometer fitted with an ion trap of the invention and shown partially in section; 
       FIGS. 2 and 3  are cross-sections of the permanent magnets used in the invention; 
       FIG. 4  is a diagram showing the principle of ion motion in a uniform magnetic field; 
       FIG. 5  is a fragmentary perspective diagram of trapping electrodes contained in an ion trap of the invention; 
       FIGS. 6 and 7  are views from above of the confinement cell of the ion trap of the invention; and 
       FIG. 8  is a fragmentary section view of a second embodiment of the ion trap of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The Fourier transform mass spectrometer or FTICR shown in  FIG. 1  is fitted with a magnetic trap  2  for ions of the invention. 
   This magnetic trap  2  for ions comprises a gastight processing enclosure  4  of generally cylindrical shape about a longitudinal axis XX′, and connected to a pump device  6 . 
   By way of example, the pump device  6  comprises an assembly of turbomolecular pumps, diaphragm pumps, and pipework for injecting and extracting gas in order to control the density and the nature of the atmosphere inside the enclosure  4 . 
   In operation, the pump  6  serves to create an ultrahigh vacuum inside the enclosure  4  at a pressure of about 10 −8  millibars. 
   Inside the enclosure  4 , the mass spectrometer includes a filament  7  for generating electrons, serving in particular to emit electrons in order to create ions inside the enclosure  4 . 
   A confinement cell  8  defining a processing volume in which the movement of ions can be analyzed is provided within the enclosure  4 . 
   The cell  8  comprises two trapping electrodes  10  of plane and square shape extending parallel to each other and parallel to the longitudinal axis XX′ of the enclosure  4 . 
   Each electrode  10  presents an opening  11  in its middle, and the electrodes  10  are disposed in such a manner that their openings are in alignment with the electron emission axis of the filament  7 . 
   The electrodes  10  are also electrically connected to a direct current (DC) trapping voltage generator  12 , in order to be electrically charged to a predetermined potential. 
   The cell  8  also includes two exciter electrodes  14  that are plane and square in shape, extending parallel to each other, perpendicularly to the trapping electrodes  10 , and perpendicularly to the longitudinal axis XX′ of the enclosure  4 . 
   The exciter electrodes  14  are electrically connected to an excitation signal generator  16 . 
   Finally, the cell  8  includes two detector electrodes that are plane and square in shape, extending parallel to each other and perpendicularly to the trapping electrodes  10  and also to the exciter electrodes  14 . 
   The measurement electrodes  18  are connected to a measurement device  20 , e.g. constituted by a microcomputer provided with appropriate electronic cards for acquisition purposes and with appropriate analysis software. 
   The trapping electrodes  10 , the exciter electrodes  14 , and the measurement electrodes  18  are disposed in such a manner that the cell  8  is generally in the form of a cube, or more generally in the form of a rectangular paralellepiped. 
   For example, the electrodes used are square plates having a side of 20 mm, made on the basis of an ARCAP AP4 material mounted on an insulating support of Macor and electrically connected using silver wires. 
   The ion trap  2  also comprises two identical permanent magnets  30  of cylindrical shape and hollowed out so as to present cavities on their longitudinal axes. Each magnet is thus in the form of a hollow cylinder or a tube. 
   The magnets  30 , described in greater detail below with reference to  FIGS. 2 and 3 , are structured permanent magnets having a structure of the type known as a Halbach cylinder. Such magnets are described in particular in document WO-A-00/62313. 
   Because of its structure, each magnet  30  generates a uniform magnetic field B that is oriented transversely across its longitudinal axis. 
   The magnets  30  present annular sections as shown in the section views of  FIGS. 2 and 3 . 
   Each magnet comprises a plurality of individual segments magnetized in different directions and distributed angularly around the axis, each generally extending along a longitudinal generator line of the magnet  30 . 
   A Halbach cylinder has a structure that is symmetrical about a plane of symmetry defined by the longitudinal axis of the cylinder and the direction of the uniform magnetic field B created by the cylinder. 
   The individual segments making up the cylinder thus correspond in pairs symmetrically on either side of the plane of symmetry, and they are magnetized in directions that are symmetrical relative to said plane. 
   In addition, the individual segments disposed on the same side of the plane of symmetry are magnetized in directions that vary progressively over a range of 360° as a function of the angular position of the segment around the half-cylinder defined beside the plane of symmetry. 
   In other words, the segments are disposed in a ring in a sequence such that the segments that are symmetrical about the longitudinal axis of the cylinder are magnetized with the same orientation. In addition, the change in angle between the directions of magnetization between two adjacent segments is constant. 
   This variation in magnetization direction differs from one segment to another by an angle corresponding to 360° divided by half the number of segments. 
   Thus, as described with reference to  FIG. 2 , the magnet  30  has eight segments, such that the magnetization direction of each segment is offset by 90° relative to the magnetization directions of the segments adjacent thereto. 
   Similarly, with reference to  FIG. 3 , the sixteen segments present magnetization directions that are offset relative to one another by 45°. 
   Each magnet  30  in the form of a hollow cylinder generates, inside its cavity and perpendicularly to its longitudinal axis, a magnetic field B that is uniform, permanent, and of high intensity. 
   For an infinite length, the theoretical magnetic field B obtained in this way in each cylinder satisfies the following formula: 
       B   =       B   r     ⁢           ⁢   ln   ⁢           ⁢       r   0       r   1             
 
In this formula, B r  is the remanent magnetic field due to the materials used, r 0  is the outside diameter of the cylinders  30 , and r 1  is the inside diameter.
 
   The length of the cylinder has an effect on the real intensity of the magnetic field and also on its uniformity. 
   By way of example, the magnets  30  are made of neodymium, iron, and boron (Nd—Fe—B), presenting an outside diameter of 20 centimeters (cm), and inside diameter of 5 cm, and a length of 10 cm. Each of them thus generates a permanent magnetic field of 1 T with uniformity of about 1 part in 100 within a central volume of about 1 cubic centimeter (cm 3 ). 
   In the embodiment described with reference to  FIG. 1 , the two magnets  30  are placed on the same axis and they are spaced apart axially by a gap δ. In addition, they are disposed in such a manner that the structure of their magnetic poles are directed identically so as to generate uniform magnetic fields oriented in the same direction. 
   With the dimensions chosen for the magnets  30 , the gap δ is typically less than 1 mm, advantageously lying in the range 0.3 mm to 0.7 mm, and is preferably equal to 0.5 mm. 
   When aligned in this way, the magnets  30  form in their center a cavity  32 , and given their structure and their disposition, they generate throughout the cavity  32  a magnetic field that is uniform and of high intensity. 
   The magnetic field created by the magnets  30  in the cell  8  is not less than the magnetic field of each magnet  30 , such that the cell  8  is subjected to a magnetic field of at least 1 T. 
   It can also be seen that using two 1 T magnets  30 , the two-part structure taken by way of example makes it possible to obtain a magnetic field in the confinement cell  8  having an intensity of 1.25 T, which is a value equivalent to that which would be provided by a single magnet of the same material, length, and section. 
   In addition, by adjusting the gap δ, it can be seen that said two-part structure described with reference to  FIG. 1  makes it possible to obtain increased uniformity of the magnetic field along the longitudinal axis in a zone of much greater length than that which is obtained in the center of an equivalent single magnet. 
   For this purpose, the gap δ is adjusted to obtain a magnetic field of maximum uniformity in the cell  8 . Similarly, the dimensions of the magnets  30  are adjusted to within ±10%. 
   In operation, the processing enclosure  4  is disposed on the axis inside the cavity  32  defined by the magnets  30 , such that the axis XX′ represents the longitudinal axis of the enclosure  4  and of the magnets  30 . 
   The enclosure  4  is oriented in such a manner that the trapping electrodes  10  are perpendicular to the magnetic field B generated by the magnets  30 . 
   Thereafter, samples of gas are injected into the enclosure  4  by the pumping device  6 . 
   The filament  7  then emits electrons which penetrate into the cell  8  through the openings  11  in the trapping electrodes  10 . These electrons ionize the molecules of gas contained inside the enclosure  4 , and in particular inside the cell  8 . 
   The ions produced thereby are then trapped inside the confinement cell  8  and they can be excited in such a manner as to obtain a mass spectrum by so-called “fast Fourier transform (FTT)” analysis. 
   It can thus be seen that the ion trap  2  presents a cell  8  having a volume of about 8 cm 3  and a magnetic field of 1.25 T. 
   The magnetic trap  2  for ions is thus small in size while still enabling a uniform magnetic field of high intensity to be created in a cell that is of a size that is large enough to enable experiments to be performed. 
   In addition, the pump device  6 , the generators  12  and  16 , and the analysis means  20  are all small in size, such that the mass spectrometer described with reference to  FIG. 1  constitutes an installation of overall size of about one cubic meter and of weight of about one hundred kilograms. 
   Similarly, the mass spectrometer requires a standard power supply only and may optionally run on a battery so as to make it easily transportable. 
   With reference to  FIGS. 4 to 7 , there follows a description of operating details of the mass spectrometer described above. 
   The device  6  establishes an ultrahigh vacuum in the enclosure  4  into which samples for analysis are injected in gaseous form. By way of example, these injections are performed by a pulsed valve operating with open periods of about ten milliseconds. 
   Under the effect of excitation, the filament  7  generates electrons that are emitted towards the processing enclosure  4  in order to ionize the molecules contained therein. 
   These electrons  40  pass through one of the trapping electrodes  10  via the openings  11  and they penetrate into the cell  8 . They then ionize the molecules contained within the cell  8  by colliding with them, thereby causing ions  40  to appear. 
   As shown with reference to  FIG. 4 , these ions  40  are subjected to the magnetic field B and describe trajectories that are generally helical in shape. 
   In operation, the trapping electrodes  10  are charged to a constant potential V by the DC generator  12 . 
   Because of the combination of the magnetic field B and the repulsion generated by the trapping electrodes  10  charged to potentials V, and as shown with reference to  FIG. 5 , the ions  40  are maintained inside the cell  8  between the trapping electrodes  10 . The other electrodes that are not shown in  FIG. 5  also contribute to this trapping by generating a potential well between the electrodes  10 . 
   Thereafter, and as shown with reference to  FIG. 6 , the generator  16  delivers excitation signals to the exciter electrodes  14 , which signals are at a mutual phase offset of 180°. 
   Depending on the frequency of the excitation signals applied to the electrodes  14 , the circular movement of the electrodes  40  maintained within the cell  8  is modified, and in particular the radii of their trajectories vary. 
   Thus, as a function of the frequency of the excitation signals delivered by the generator  16  to the electrodes  14 , the ions enter into resonance, and they can be ejected from the cell  8  by enlarging their trajectories, or they can be excited coherently so as to describe stable trajectories of large radius. 
   Ions are thus obtained inside the cell  8  that are driven with cyclotron movement of large amplitude. 
   As shown with reference to  FIG. 7 , it is then possible to perform various measurements on these ions. 
   When the ions  40  are in-phase, their coherent movement induces an electrical signal in the detector electrodes  18 . 
   This electrical signal is applied to the measurement means  20  which amplify it by means of an amplifier  42  prior to processing it in processor means  44 . By way of example, the processor means enable the induced signal to be sampled prior to being digitized, and then serves to perform a fast Fourier transform so as to obtain a frequency spectrum for the cyclotron resonance. 
   Using conventional calibration relationships, this frequency spectrum makes it possible to determine accurately the mass of the ions  40  contained in the cell  8 . 
   With reference to  FIG. 8 , there follows a description of a second embodiment of the invention. 
   This figure is a fragmentary section view of a magnetic trap  2  for ions having an axis XX′. 
   As above, the ion trap  2  includes the enclosure  4  integrated inside the cavity  32  of the structured cylindrical magnets  30 . 
   As described above with reference to  FIG. 1 , the confinement cell  8  placed inside the processing enclosure  4  comprises two plane and square trapping electrodes  10  that are parallel to each other and that extend perpendicularly to the magnetic field B. 
   The two detector electrodes  18  are disposed perpendicularly to the electrodes  10  and parallel to the longitudinal axis of the magnets  30 . 
   In this embodiment, each of the excitation electrodes  14  is constituted by four square plates that are electrically interconnected, and that together define a structure in the form of a cube that is open via two opposite faces. 
   The openings in the two cubes constituting the electrodes  14  face towards each other along the longitudinal axis of the magnets  30 . 
   The set of electrodes thus defines, inside the enclosure  4 , a confinement cell  50  that is generally in the form of a tunnel extending along the longitudinal axis XX′ of the magnets  30 . 
   Such a structure can be defined as being an open structure and presents numerous implementation advantages, in particular for ionizing the molecules present inside the enclosure  4  and for characterizing the ions by means of interaction with beams of photons or with other molecules. 
   For this purpose, the enclosure  4  includes means for connection to gas injection means  51  and includes port-holes  52  at its ends so as to make it possible to project gases directly into the cell  50 , or to cause photons to pass through the cell via the port-holes  52 , said photons being emitted by a laser beam, for example. 
   It can thus be seen that the magnetic trap  2  for ions of the invention is small in size and compact while enabling high quality processing to be performed on a large quantity of samples. 
   In other embodiments of the invention, the structured cylindrical magnets constituted by Halbach cylinders are integrated inside the processing enclosure. 
   Similarly, it is possible to make an ion trap of the invention from a single magnet or with electrodes of other shapes, such as, for example, electrodes that are cylindrical or rectangular. 
   Furthermore, the excitation voltage generator, the trapping voltage generator, and the measurement means may be constituted by a single device, such as a microcomputer fitted with electronic input/output cards that are suitable for generating excitation signals and trapping voltages. 
   Finally, it is also possible to perform processing on ions that are positive or negative, by inverting the polarities of the trapping electrodes.