Abstract:
A device for use in a mass spectrometer allows an ion-optical assembly to be removed, cleaned and reinserted with relatively high positioning accuracy. In particular, the device obviates the need for complex adjustments requiring special knowledge after the reinsertion. The objective is achieved by an arrangement comprising a receptacle and a mount for a removable ion-optical assembly in a mass spectrometer. Favorable implementations provide a mount and a receptacle with three pairs of complementary support elements, the three support elements on the receptacle form a support plane, and, when the mount is inserted into the receptacle, at least two pairs of support elements are engaged and the mount is aligned with respect to the support plane with the aid of the third pair of support elements.

Description:
PRIORITY INFORMATION 
     This patent application claims priority from German Patent Application 10 2011 109 397.8 filed on Aug. 4, 2011, which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to an arrangement comprising a receptacle and a mount for a removable ion-optical assembly in a mass spectrometer, and a mass spectrometer with a corresponding arrangement. 
     BACKGROUND OF THE INVENTION 
     The performance of a mass spectrometer can be reduced by contamination of its components, such as ion sources. During operation of a MALDI desorption ion source, for example, a sometimes visible coating of organic material can build up on the electrodes. In the prior art, such coatings on ion-optical devices in mass spectrometers are described by Girard et al. in the Journal of Chromatography Science, 2010 October, 48 (9), 778-779, and in the article by Kenneth L. Busch entitled “Ion Burn and the Dirt of Mass Spectrometry”, online publication, Sep. 1, 2010. The insulating organic coating becomes charged when the ion source is in operation and thus generates an electrical interference field which is superimposed onto the desired electric field between the electrodes and the MALDI sample support when the desorption ion source is in operation. This interferes with the acceleration process. Field changes, in particular, interfere with the focusing properties of the accelerating electrodes. Consequently, the ion beam is no longer focused accurately onto the detector. 
     A noticeable effect of such a coating can be a decrease in ion throughput to the mass analyzer connected to the ion source. The reduced ion throughput in turn requires the additional acquisition and summation of spectra in order to maintain a specific quality level for the mass spectra. The reduction in the ion throughput also limits the number of analyses which are possible per sample, and reduces the detection limit of the mass spectrometer. 
     Girard et al. describe a method whereby a simple reversal of the polarity of the ion source, which changes the polarity of the ions to be analyzed, can neutralize the charging effect. Since a MALDI method generates ions of both polarities, the polarity of the accelerating field would therefore have to be reversed for the analogous application of the method according to Girard et al. However, this method only addresses the symptoms of the loss of throughput in the ion source, and promises only a short-term effect. 
     Irrespective of the short-term solution mentioned above, there is therefore regularly a need to remove the coating and thus restore the performance of the mass spectrometer. Sometimes, if the cleaning is not able to restore something approaching the ideal state of an ion-optical device, it must be replaced by a new, clean one. Ion-optical devices can be taken to include all of the elements of a mass spectrometer and/or of an ion source on which deposits can form, for example, accelerating and/or ground electrodes of an ion source, and also injection capillaries, multipole rod systems, ion funnels comprising ring electrodes, ion deflectors (condensers) and similar. 
     Cleaning methods are known in the prior art with which the contamination can be at least partially removed. U.S. patent application U.S. 2004/0163673 A1 (Holle et al.) describes a sample support dummy with bristles, for example, which can remove interfering coatings by “scrubbing”, and a spray cleaning device which utilizes the low pressure in the vacuum chamber of the ion source in order to direct a jet of solvent onto the accelerating electrodes and to dissolve and remove deposits by its impact. German patent application DE 10 2008 008 634 A1 (Holle et al.) discloses a method where the coating is removed by local heating. 
     A further method for removing the coating, which is still used in practice, is to clean the electrodes manually after venting and opening the mass spectrometer. The cleaning is usually carried out with solvents such as ethanol or acetone, but when the contaminations are stubborn the electrodes can also be abraded with cleaners containing abrasive agents (including toothpaste, for example). Since the confined space makes it difficult to clean the ion-optical assembly while it is inside the mass spectrometer, and the aim is to avoid contaminating neighboring components with the dirt removed during the cleaning process, the ion-optical assembly is usually uninstalled. If the mass spectrometer is vented during the disassembly, it often takes some hours until the necessary operating vacuum is restored after the ion-optical assembly has been cleaned and re-inserted. 
     The removal and cleaning themselves are usually steps which can be carried out in a straightforward manner They require a certain degree of experience, but no special knowledge. These steps can thus also be carried out without any major difficulty by members of staff who have been given appropriate instruction. It becomes difficult, however, when the removed parts have to be reinserted in their position in the mass spectrometer. The necessary use of electromagnetic forces and fields to control and manipulate ions means that the ion-optical devices have to be positioned within narrow tolerances. These tolerances should be no larger than a few tens of micrometers. For example, the separation, perpendicular to the surface, between a MALDI sample support with sample applied to it and the first accelerating electrode essentially determines the acceleration path of ions on their way to the mass analyzer, and thus the kinetic energy they accumulate along the path. The control and correct adjustment of this kinetic energy is decisive for the operation of a time-of-flight mass spectrometer. Deviations of the separation, perpendicular to the surface, between the sample and the accelerating electrode (also indirectly via a lateral offset/shift) can therefore have a significant negative influence on a mass spectrometric analysis. 
     In addition, ion-optical devices have to be supplied with voltages. This means that electrical supply lines, which at present usually have screw connections, must be detached during removal of the ion-optical assembly and reconnected during reinstallation, which requires additional manual measures. 
     For this reason, specially trained staff from the manufacturer or its appointed dealers are often required to reinstall an ion-optical assembly which has been removed for cleaning. This may involve realigning the reinstalled ion-optical assembly in the mass spectrometer in order to ensure that it is reinserted with high positioning accuracy. If there are no alignment marks or similar in the mass spectrometer, it is often not just that the ion-optical assembly must be realigned with respect to the mass spectrometer. It may also be necessary to realign other components of the mass spectrometer, such as a reflector or a detector (in two planes), not to mention additional fine tuning of the supply voltages. The man-hours required for such maintenance work are considerable, and also costly for the user of the mass spectrometer, who must pay the travel expenses of the specialist personnel, for example. 
     U.S. patent application U.S. 2009/0242747 A1 (Guckenberger et al.) discloses a mass spectrometer in which an ion source and various ion-optical elements are integrated in a sub-unit. The sub-unit is removed from the mass spectrometer in order to clean away the contamination that builds up during operation, and reinserted, whilst maintaining the vacuum. 
     U.S. Pat. No. 7,601,951 B1 (Whitehouse et al.) describes an atmospheric pressure ion source which is designed so that all or some of the vacuum components, such as ion-focusing and ion-transporting electrostatic lenses and ion guides and two or more vacuum stages integrated into a unit are removed from an ion source or vacuum housing 
     U.S. Pat. No. 7,667,193 B2 (Finlay) discloses a mass spectrometer with modular design in order to provide a user with an appropriately personalized analytical device by inserting a personalized analytical module. High positioning accuracy when reinserting the modular components is assumed in all three documents, largely without providing design details 
     U.S. Pat. No. 6,797,948 B1 (Wang) and U.S. Pat. No. 4,745,277 A (Banar et al.) disclose assembly plans for a multipole rod set and a heated filament ion source in a mass spectrometer, respectively, in more detail. 
     At the beginning, MALDI ion sources were referred to specifically. The invention to be presented below should not, however, be limited to special types of generation or guidance of ions in a mass spectrometer. Similar considerations can also be made for electrospray ion sources, electron impact ion sources, ion sources with chemical ionization, and others. 
     There is a need to provide a device for use in a mass spectrometer which allows an ion-optical device or assembly to be removed, cleaned and reinserted with high positioning accuracy without the need for special knowledge. In particular, the device should obviate the need for complex adjustments requiring special knowledge after the reinsertion. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention an arrangement comprising a receptacle and a complementary mount for a removable ion-optical assembly in a mass spectrometer is provided. The mount and the receptacle have three pairs of complementary support elements; the three support elements on the receptacle form a support plane, and, when the mount is inserted into the receptacle, at least two pairs of support elements are engaged and the mount is aligned with respect to the support plane with the aid of the third pair of support elements. 
     The small spatial size of the points via which the support elements engage with each other, by a simple small contact area, for example, facilitates the removal and reinsertion of the mount into the receptacle, particularly because of the low friction resistances. The small number of contact points, furthermore, makes it possible to satisfy the high demands in terms of reducing material abrasion in a vacuum environment, which can be caused by a relative motion of two touching surfaces. 
     The paired interaction of two support elements on the mount and the receptacle respectively forms two stabilization points. The mount can then be rotated, within a predetermined angular range, about an axis which passes through the two pairs of support elements which are engaged with each other. It is preferable if the predetermined angular range is determined by the orthogonal separation between the axis and the third pair of support elements. 
     In one embodiment, the third pair of support elements has contact areas opposite each other. It can thus limit the predetermined angular range in one rotational direction, for example. 
     In a further embodiment, the receptacle is cylindrical. A cylindrical design makes it relatively easy to define an axis of the receptacle which can coincide with an ion path. The three support elements of the receptacle are preferably arranged in the region of one end face of the cylinder. The term ‘cylindrical’ is not to be construed restrictively as “circular cylindrical”. Various shapes and forms of cylinders, be they rotationally symmetric or asymmetric, can be adequate for the intended purpose. 
     In a further embodiment, two of the support elements on the receptacle are recessed, and the complementary support elements on the mount have a bulged configuration; they can, for example, protrude in the shape of a dome. A milled hemispherical contour can be used, for example. Alternatively, a metal sphere can be partially recessed on the mount. 
     In a further embodiment, the mount and the receptacle can be interlocked with each other. A disengaging and re-engaging locking mechanism is preferably arranged on the third pair of support elements and ensures that a holding force is applied to the third pair of support elements. The third pair of support elements can, for example, have a tapered contact head on the receptacle and a pre-tensioned catch on the mount. It is also possible for the engagement of the holding force and the alignment to the support plane to be separate. To this end, the third pair of support elements can have a partially recessed sphere at one end of the receptacle and a counter-surface on the mount for the alignment; and adjacent to this, the locking mechanism can comprise a tapered contact head on the receptacle and a pre-tensioned catch on the mount. 
     In a further embodiment, the mount has a ring and a radially projecting handle. The dimensions of the ring are preferably matched to the dimensions of the cylinder. 
     In a further embodiment, the ion-optical assembly is supported on the mount, for example by an adhesive or mechanical connection (such as for example screws, clips, frictional connection, positive engagement or similar). 
     In a further embodiment, the mount can be removed and reinserted in a plane roughly perpendicular to an axis of the receptacle, which may coincide with an ion path or an ion path axis in the mass spectrometer. 
     The mount preferably features electrodes, or is configured to accept electrodes, as are required for an ion source of matrix-assisted laser desorption and ionization. These electrodes, in particular accelerating and/or ground electrodes, can be fastened (detachably) on the mount with the aid of insulating parts. If the mount is made of a non-conducting material, the insulating parts can be omitted. 
     The invention also overcomes the above-mentioned problems with the aid of a mass spectrometer with an arrangement as described above. A lock for introducing and extracting the mount without breaking the vacuum is preferably provided. With a MALDI ion source, a lock can be provided with a double function, which is designed for introducing and extracting the MALDI sample mounts and also introducing and extracting the removable ion-optical assembly, in this case a part of the ion source (screening, accelerating and ground electrodes). 
     In a further embodiment, sprung contact pins are supported on the receptacle and serve to touch appropriate counter-contacts, which are supported on the mount, in order to create an electrical connection when the mount is inserted into the receptacle. 
     In a second aspect, the invention presents an arrangement for a mass spectrometer that comprises a receptacle, a mount and an ion-optical assembly member. The mount and the ion-optical assembly member are configured such that the ion-optical assembly member can floatingly engage with the mount. The receptacle, the mount, and the ion-optical assembly member are further configured such that, upon insertion of the ion-optical assembly member into the receptacle with the aid of the mount, the ion-optical assembly member becomes at least partially disengaged from the mount and aligned towards the receptacle in at least one dimension. 
     In various embodiments, partial disengagement comprises releasing direct physical contact and maintaining pre-tension contact through a sprung member interposed between the mount and the ion-optical assembly member. A sprung member for exerting the pre-tension is mentioned in this context by way of example only. Pre-tension may also be created by actuators, such as for example stepper motors, for instance, which do not necessarily provide a restoring force Skilled workers in the field will find many alternatives with which to exert pre-tension on the ion-optical assembly member. 
     In further embodiments, the mount has an inner aperture, and the ion-optical assembly member comprises a body which is slightly undersized to fit into the inner aperture. This allows easy insertion of the ion-optical assembly member into the mount. The ion-optical assembly member may further comprise a flange portion protruding from the body as to contact a rim region of the inner aperture when engaging therewith. 
     In some embodiments, the flange portion is doubly stepped as to provide alignment surfaces for contacting corresponding counter-surfaces at the receptacle. Such design allows for accurate alignment of the ion-optical assembly member in relation to the receptacle without suffering from any mechanical tolerance at interfaces with the mount. The counter-surfaces may be located at a groove portion of the receptacle. Preferably, the groove portion has a beveled entrance to facilitate insertion of the counterpart at the ion-optical assembly member and to assist in the disengaging step. 
     In various embodiments, a direction of insertion and withdrawal is approximately perpendicular to an axis of the receptacle, which preferably coincides with an ion path (axis) in a mass spectrometer. Such direction generally represents the shortest way of withdrawing something from a mass spectrometer thereby facilitating compact construction of the whole assembly. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the following, the invention is described with the aid of example embodiments in conjunction with the attached drawings. The drawings illustrate the fundamentals of the invention and are often schematic in nature. The drawing shows: 
         FIG. 1  illustrates an example of a receptacle according to an aspect of the invention; 
         FIG. 1A  illustrates a modification of the receptacle from  FIG. 1  in an enlarged partial view; 
         FIG. 2  illustrates an example of a mount according to an aspect of the invention; 
         FIGS. 3A and 3B  illustrate a catch for a locking mechanism; 
         FIG. 4  illustrates a cross-sectional view of a mount according to an aspect of the invention with catch; 
         FIG. 5  illustrates a side view of the mount with catch from  FIG. 4 ; 
         FIG. 6  illustrates a further side view of the mount with catch from  FIG. 4 ; 
         FIG. 7  illustrates how a mount according to an aspect of the invention is held in a receptacle; 
         FIG. 8  illustrates the mount and receptacle from  FIG. 7  with rotated catch; 
         FIG. 9  illustrates a detail view of the engagement between an embodiment of a support element and the corresponding complementary element; 
         FIG. 10  illustrates an arrangement according to an aspect of the invention with an ion-optical assembly; 
         FIG. 11  illustrates an example of the locking out of the mount from a vacuum stage of a mass spectrometer without breaking the vacuum; 
         FIG. 12  illustrates a mount and an ion-optical assembly member according to aspect of the invention; 
         FIGS. 13A and 13B  illustrate a receptacle configured to interact with the ion-optical assembly member from  FIG. 12 ; 
         FIGS. 14A-14C  illustrate a method of inserting an ion-optical assembly into a receptacle with the aid of a mount in three steps; and 
         FIG. 15  illustrates an arrangement according to an aspect of the invention in a cross sectional isometric view wherein an ion-optical assembly is inserted into a receptacle with the aid of a mount. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an embodiment of a receptacle  2 . The receptacle  2  has a cylindrical basic structure. On one end  4  of the cylinder  6 , two angled elements  8   a ,  8   b  are arranged diametrically opposite each other in this example and serve as guide elements for the mount, which will be described in connection with another illustration. Roughly along a line which is at right angles to the line connecting the angled elements  8   a ,  8   b , there are three support elements  10   a ,  10   b ,  10   c , which are approximately diametrically opposite each other on the end  4  of the cylinder  6 . The three support elements  10   a ,  10   b ,  10   c  define a support plane, onto which a mount is aligned with the receptacle  2  when inserted. 
     One support element  10   a  is formed by a sunken hole; another support element  10   b  by a recessed pocket in a shoulder piece  12  arranged on the end face  4  so as to be open toward the cylinder axis. The cylinder axis (not shown) can preferably correspond to an ion path when used in the operation of the mass spectrometer. The chamfers of the sunken hole and the pocket are oriented toward the cylinder axis and serve to center and spatially fix a mating counterpart (on the mount) when it is inserted. The pocket partially extends in the direction of the circumference in order to allow the insertion of an appropriate counterpart. 
     A third support element  10   c  includes a contact head located on the end face  4 , with a neck (or holding rib) which is tapered in sections toward the shown end  4  of the cylinder  6 . The end of the contact head pointing away from the end face  4  is flat, and here has a dome  14  to provide as small a contact area as possible. The dome  14  is preferably spherically rounded toward the outside in order to allow a small, especially tangential, contact area. 
     The dome  14  can take the form of an added disk or a sphere partially recessed in the flat surface, whose curvature means that it provides a relatively small contact area. Certain embodiments also comprise a contact area  18 , facing away from the end  4  of cylinder  6 , on a partially recessed sphere  16 , and a separate head  10   c * (without dome), which can be a part of a locking mechanism to be described further below. A corresponding embodiment can be seen in  FIG. 1A  in a partial cross-section. Nickel-plated aluminum is preferably used as the material for the receptacle  2 . 
       FIG. 2  shows an embodiment of a mount  20  which has a form that is complementary to the receptacle  2  shown in  FIG. 1 . The mount  20  is also preferably made from nickel-plated aluminum The mount  20  has a flat ring  22  with a radially projecting handle  24 , which in turn has a flat end plate  26 . The ring  22  has an inside diameter which is preferably matched to the inside diameter of cylinder  6  of the receptacle  2 . The external circumference of ring  22  also agrees in an advantageous way with the external circumference of cylinder  6  (with a slight undersize) in order to fit into the space between the angled elements  8   a ,  8   b  of the receptacle  2 , for example. The available contact area of the ring  22  can be formed in such a way that it approximately matches the end  4  of the cylinder  6  of the receptacle  2 . The elements to be carried, an ion-optical assembly, for example, are preferably supported on the ring  22 , jointed, bonded or otherwise mechanically anchored. It is advantageous if the ion-optical assembly is connected to the mount  20  in such a way that it can be detached and reconnected (i.e., the mount is removable and replaceable), for example by a clamping mechanism. 
     Two complementary support elements  28   a  and  28   b , which in this example have the form of two protrusions, are located on the narrow outer edge of the ring  22 , at the end which is approximately diametrically opposite the handle  24 . These protrusions can be the result of milling off the originally larger dimensioned ring  22  (milling contour). The protrusions may have a rounded contour and can therefore easily engage in the chamfered recesses (sunken hole and pocket) on the receptacle  2 . The spacing of the protrusions on the narrow outer edge of the ring  22  corresponds to the spacing of the recesses on the shoulder piece  12  of the receptacle  2 . The elongated design of the pocket allows tolerances resulting from the manufacturing process or temperature-dependent material movements to be accommodated. 
     At the distal end of the handle  24 , in the example shown mainly in the flat end plate  26 , there is a T-shaped elongated hole  30 , the main part of which extends along the longitudinal axis of handle  24 . This elongated hole  30  serves to accept a compression spring described in connection with a locking mechanism, which can lock together the third support element  10   c  and a matching complementary element, with the aid of a further illustration. A drill hole  32  is located at the proximal end of the handle  24  to accept a guide pin, which is also part of the aforementioned locking mechanism. The drill hole  32  can take the form of a tapped hole, for example, into which a screw is inserted. The threaded body of the screw can then assume the guiding function of the pin. 
     The ring  22  of the mount  20  shown in the example is particularly suitable for mounting an ion-optical assembly. If the ion-optical assembly is a first accelerating electrode and/or a ground electrode, for example, as explained at the beginning in connection with a MALDI ion source, these are preferably arranged concentrically with the ring opening in order to ensure that the accelerated ions pass without hindrance on the ion path. An electrode can be located in the plane of the ring  22 . But it can also be displaced axially in order to space it from the plane of the ring. Such an arrangement also allows several electrodes or ion-optical devices to be connected to the ring  22 . 
       FIGS. 3A and 3B  illustrate a catch  34  in two different views as part of a locking mechanism, which here represents the complementary element matched to the third support element  10   e , and simultaneously makes locking possible. The catch  34  includes an angular structure, where an angled end  36  is provided for manual operation of the locking mechanism. In this example, the edge of the angled end  38  does not run perpendicular to the longitudinal direction of the catch  34 , but slants slightly. The reason for this form becomes clear in connection with illustrations to be described further below. The elongated end  40  has a lateral notch  42  at its distal end, which creates a kind of bow  44 . In this example, the bow  44  has a slightly undercut lateral section  46 , which is chamfered as a contact area on the side pointing inwards into the bow. Between the edge of the angled end  38  and the bow  44 , the catch has a guide slot  48 , which is provided to accept the guide pin. The guide slot  48  is divided into two guide sections  48   a  and  48   b , which have different angles with respect to the longitudinal axis of the catch  34 . The first guide section  48   a  runs essentially parallel to the longitudinal axis; the second guide section  48   b  is at a slight angle to it. A drill hole  50  is provided toward the proximal end of the catch  34 , with which the catch  34  can be connected to the mount  20  via a spring element so as to be movable, for example by a bolt and lock-nut. 
       FIGS. 4 to 6  show different views of the catch  34  connected to the mount  20 . A compression spring  52  is located in the elongated hole  30  with one end in contact with the interior wall of the mount  20 , and the other end in contact with a fastening device such as a screw  54 . The compression spring  52  is fastened to the catch  34  for example by the screw  54 . The elongated hole  30  can have a thin wall on one side of the mount  20  (at the bottom in the illustration) which extends over part of the longitudinal extension of the elongated hole  30 , supports the guiding of the compression spring  52  in the elongated hole  30 , and thus prevents the compression spring  52  bulging out of the plane of the mount  20  on one side. Bulging to the other, open side of the elongated hole  30  is prevented by the body of the catch  34  itself. The compression spring  52  generates a pre-tensioning or bias of the catch  34  in the direction of the distal end of the handle  24 , i.e., a position in which the guide pin  56  in the distal end of the guide slot abuts in the first guide section  48   a ; depending on the state of tension of the spring  52 , abutment in the sense of actual contact is not strictly necessary, however. In this state, the catch  34  and the handle  24  of the mount  20  are approximately congruent in the example shown (see  FIG. 5 ). 
     If the catch  34  is now operated, for example by pressing the angled end  36  radially inwards (see  FIG. 6 ), the compression spring  52  is compressed and the slot  48  moves along the fixed guide pin  56  until the second guide section  48   b  makes contact with it. The catch is deflected sideways by the change of direction in the second guide section  48   b . From this view it becomes clear why the edge of the angled end  38  of the catch  34  slants slightly with respect to the longitudinal axis of the catch  34 . The angle of slant is adapted to the angle between the two guide sections  48   a ,  48   b  of the catch  34  so that the angled end  36  can make flush contact at the distal narrow edge of the end plate  26  in this state. By releasing the compression spring  52 , the guide pin  56  moves in the guide slot  48  in the opposite direction so that the catch  34  moves back into its position, partially covering the handle  24  of the mount  20 . 
       FIG. 7  shows how the mount  20  is held in the receptacle  2 , for example as a consequence of a lateral insertion movement. The third support element  10   c  and the corresponding complementary element are not yet engaging, however, because the catch  34  is still in the deflected position, as is shown. The angled elements  8   a ,  8   b  on the end  4  of the receptacle  2  provide a guiding surface for the ring  22  of the mount  20  for correct positioning when the mount  20  is inserted into the receptacle  2 . The two protrusions  28   a ,  28   b  can thus be introduced into the recesses provided (sunken hole  10   a  and pocket  10   b ) by a relatively simple forward movement of the mount  20 . There they are centered and spatially fixed. If the cylinder axis aligns with the direction of an ion path, the lateral forward movement corresponds to a lateral introduction or insertion movement, i.e. perpendicular to the ion path or an ion-optical axis. 
     In the state depicted in  FIG. 7 , the mount  20  can be rotated through a predetermined angular range about an axis AX running through the two protrusions  28   a ,  28   b  or recesses  10   a ,  10   b . This angular range is essentially determined by the perpendicular distance N of the third support element  10   c  from the axis AX which passes through the recesses  10   a ,  10   b . The larger the distance, the smaller the angular range available becomes. Further elements limiting the rotary movement are, for example, the angled elements  8   a ,  8   b  on the end  4  of the cylinder  6 , or in the other direction the third support element (contact head  10   c , for example). Here, the accessible angular interval is only a few degrees. 
     If the pressure on the angled catch end  36  is removed, this causes a release movement of the compression spring  52 , which means that the slot  48  is guided along the guide pin  56  until the guide pin  56  again takes up its position at the distal end of the first guide section  48   a . The catch  34  therefore rotates back from its deflection, and the lateral section  46  of the bow comes to rest on the tapered section (neck) of the contact head  10   c , as shown in  FIG. 8 . This pressurized contact means that the contact head  10   c  and the lateral section  46  of the bow are engaged with each other and prevent any further movement of the mount  20  relative to the receptacle  2 . Thus the position of the mount  20  relative to the receptacle  2  is fixed spatially, and particularly in a reproducible way (with a positioning accuracy of a few micrometers), with the aid of three stabilization points which form a support plane. 
     It is preferable if the contact head  10   c  projects axially from the end face  4  of the receptacle  2  so far that a proximal segment of the handle  24  is also in, preferably punctiform, contact with the dome  14  on the bottom of the contact head  10   c . However, it is also possible to select an arrangement with a partially recessed sphere  16 , as is shown in  FIG. 1A , which means that the point where force is applied (pressurized contact of the bow on the contact head) and the location of the alignment (contact between sphere and handle as third point of the support plane) are separate from each other. 
     In the example shown, the three pairs of support elements represent the only points at which the mount  20  comes into contact with the receptacle  2 . The small number of contact points means that relatively high positioning accuracy can be achieved in a reproducible way, i.e. after repeated removal and reinsertion of the mount  20 . The force application between catch bow and contact head ensures that positional tolerances are reduced. 
     In order to remove the mount  20  from the receptacle  2  again, for example in order to expose an ion-optical assembly attached to the mount  20  and, where necessary, to clean it in an ultrasonic bath, the above-mentioned steps can simply be run through in reverse order, i.e.: press the angled catch end  36  to deflect the catch  34  and disengage the locking between the lateral section  46  of the bow and the contact head  10   c ; and laterally extract the mount  20  from the receptacle  2 . 
     The invention is described here mainly with the aid of the example embodiment shown in the illustrations. Modifications of this embodiment are easily possible, however, and those skilled in the art can carry them out with knowledge of the inventive principle without leaving the scope of the present invention. For example, it is possible to form the first two support elements  10   a ,  10   b  so that they protrude in the faun of a dome, whereas the corresponding complementary support elements  28   a ,  28   b  then have the form of sunken holes (or pockets) on the narrow edge of the carrier ring  22  (see  FIG. 9  with partial cross-section, where the dome is a sphere  58  partially recessed in the material of the receptacle  2 ). It is also possible to have a mixed design, where one dome and one sunken hole  60  are provided on both the mount  20  and the receptacle  2 . 
     Furthermore, a compression spring is used to generate a pre-tension or bias. It is understood, however, other mechanisms can also be used to generate a pre-tension, for example a block of elastic material, an extension spring or a magnet with appropriate design adaptations. In certain embodiments, an actuator for providing pre-tension may be foreseen. 
       FIG. 10  shows a further example embodiment of an arrangement according to principles of the invention. The mount  20  is inserted here into the receptacle  2 , and the three support elements  10   a ,  10   b ,  10   c  and corresponding complementary support elements are engaged with each other. In this example, three electrodes  62 ,  63 ,  64  are attached to the mount  20 , and connected with the ring  22  via electrical insulation pieces  66 . The electrode  62  can be a screening electrode, electrode  63  an accelerating electrode, and electrode  64  a ground electrode of a MALDI ion source, for example. Their arrangement means the electrodes  62 ,  63 ,  64  extend to one side from the plane of the mount  20  so that the mount  20  equipped in this way can nevertheless be inserted into the receptacle  2  without being obstructed by the cylinder  6 . The alternative alignment and locking device, in accordance with  FIG. 1A , is indicated in the dashed circle. 
     The receptacle  2  also has an extension  72  supported by a screw connection  70 . This extension projects beyond the end face  4  of cylinder  6  and is equipped with sprung contact pins  74 , which are accessible in the radially inward direction in relation to the cylinder axis  76 . The purpose of the contact pins  74 , which are preferably manufactured from a material which is a good electrical conductor, such as gold-plated beryllium copper, is to create the electrical connection of the electrodes  62   63 ,  64  to the power supply/supplies when the mount  20  is inserted into the receptacle  2 . For this purpose, the contact pins  74  (along the cylinder axis) are at the same level as the contact counterparts which are correspondingly provided on the electrode holder; and when the mount  20  with the connected electrodes  62 ,  63 ,  64  in this example is inserted from the side, the pins are touched by the radial narrow side of the contact counterparts and pushed in slightly so that the electrical contact can be reliably created. In the representation shown, the contact pin which contacts the center electrode  63  is outside the area represented and therefore cannot be seen. 
       FIG. 11  is a schematic representation of how the mount  20  with ion-optical assembly attached to it (not shown) can be inserted into and withdrawn from the mass spectrometer, in which the corresponding receptacle is located, without breaking the vacuum. This example embodiment must not be seen as limiting. The removable ion-optical assembly can also be provided in designs of mass spectrometers which have no device for inserting and removing it without breaking the vacuum. 
     As indicated in the illustration, the receptacle  2  is located in a first chamber, which is maintained at a first pressure level p 1  below atmospheric pressure p(atm) with the aid of a suitable pumping device. A lock chamber, in which the pressure level p 2  is variable, is arranged adjacent to the first chamber. In this example, the pressure outside the two chambers described is to be atmospheric pressure p 3 =p(atm). In the top part A of the illustration, the mount  20  has been inserted into the receptacle  2 , and is thus in an operating position of the mass spectrometer. In this state, ions on an ion path (in the illustration from the bottom to the top or vice versa, for example) can be transported through the open areas of the mount  20  and the receptacle  2  into further sections of the mass spectrometer. 
     The first chamber and the lock chamber are separated from each other by a lock gate, which can be opened and closed as required. The lock gate can take the form of a combined swinging/sliding door, for example. When the lock gate is open (broken line), the first chamber and the lock chamber form a joint large chamber with approximately the same pressure level. The path is now clear for the mount  20  to leave the receptacle  2  and be moved into the lock chamber. Since it is difficult to access evacuated chambers manually, the mount  20  is preferably moved by an automatic, computer-controlled transport unit (not shown) in conjunction with the control of the first lock gate. The transport unit may be configured to actuate the locking mechanism at the third pair of support elements. Once the mount  20  has reached its position in the lock chamber, the first lock gate can be closed again (solid line) so that the pressure regime in the first chamber and the lock chamber are separated from each other again. Now the lock chamber can be vented, which means that the pressure level p 2  becomes equal to the external pressure level p 3  (bottom illustration B). The mount  20  with the possibly contaminated ion-optical devices can be removed and cleaned. 
     Reinsertion essentially proceeds with the previously described steps in reverse order. After inserting the mount  20  into the lock chamber and closing the lock chamber, the pressure in the lock chamber p 2  is lowered in order to equalize it to the pressure in the first chamber p 1 . 
       FIG. 12  shows another implementation of individual elements of an arrangement according to the second aspect of the invention. On the top left, a mount  1220  is presented that resembles the one shown in  FIG. 2  in that it has a ring  1222  with a circular inner aperture  1280  and a flat (handle-like) member  1224  that radially protrudes therefrom. The flat member  1224  ends in a fixing extension  1282  having holes  1284 , with which it can be attached to a support structure, such as a door in a vacuum housing wall (not illustrated). The mount  1220  further features a flat rectangular groove  1286  extending approximately radially from an inner rim of the circular aperture  1280  into a center portion of the flat member  1224 . At a distal end, the flat groove  1286  may have a through-hole  1288  for attaching a sprung member, the function of which will become apparent from the description further below. Attachment may be effected by inserting a screw from one side of the through-hole  1288  and drawing tight a lock nut on the screw thread at the other side of the through-hole  1288 , for instance. 
     On the bottom right, a cross section of a member  1290  of the ion-optical assembly is shown in an isometric view. Ion-optical devices such as electrodes (not shown) may be assembled together with the member  1290  to yield an ion-optical assembly. Additionally or alternatively, the ion-optical assembly member  1290  may also serve as an ion-optical device on its own. Here, the ion-optical assembly member  1290  has a generally annular cylindrical body  1291  with a doubly stepped outer flange portion  1292  and a recessed inner portion  1293  at one end of the cylindrical body  1291 . The ion-optical assembly member  1290  depicted is rotationally symmetric; however, other asymmetric forms are also conceivable. The recessed radial inward portion  1293  may serve as support for other parts of the ion-optical assembly to be assembled with the body  1291 , as will become apparent from the description further below. The other end of the cylindrical body  1291  shows a tapering body wall  1294  and further has a radially inward flange portion  1295  with an inner circular aperture  1296 . The body  1291  of the ion-optical assembly member  1290  is intended to be inserted into the circular aperture  1280  of the mount  1220  so that a lower surface  1297  of the second step  1292 A of the flange portion  1292  rests upon a rim around the circular aperture  1280 . A rotationally symmetric design comes in handy at this point since no special alignment of the ion-optical assembly member  1290  toward the mount  1220  has to be observed. The (optional) tapering portion  1294  also serves to facilitate easy insertion of the ion-optical assembly member  1290  into the mount  1220 . An outer diameter of the body wall  1294  is slightly undersized in relation to the inner diameter of the circular aperture  1280  of the mount  1220  so that the ion-optical assembly member  1290  can floatingly engage with mount  1220 . 
       FIGS. 13A and 13B  show an implementation of a receptacle  1302  according to principles of the invention in an isometric view (top left) and a straight axial view (bottom right). The receptacle  1302  generally has a circular cylindrical body  1306  with two front ends. The lower end  1304  visible in  FIG. 13A  shows an almost semi-circular axially protruding member or contact head  1310   c  with a flat lower surface, and opposite thereto at angles of approximately 130° (from center to center) two concave members  1310   a ,  1310   b . The concave members  1310   a ,  1310   b  have the shape of a milling contour at a shoulder piece  1311  axially protruding from a certain angular section of the front end  1304  of the cylinder body  1306 . This design allows simple manufacturing; however, it can be changed if considered practicable. For example, the concave members  1310   a ,  1310   b  can be provided at separate shoulder pieces which, in turn, themselves do not have to be integral with the cylinder body  1306  but can be attached thereto, for instance. In other embodiments, it can be useful to merge the two depicted concave members  1310   a ,  1310   b  into one which would then cover a certain minimum portion of the annular circumference at the front end  1304  of the cylindrical body  1306  as to provide stable contact surfaces for holding the ion-optical assembly member  1290 , as will become clear from the description further below. In this example, the concave members  1310   a ,  1310   b  comprise circumferential grooves  1313  the entrance of which, in these embodiments, features beveled surfaces  1315 . Beveled surfaces facilitate insertion of a complementary protruding portion of the ion-optical assembly member  1290 . The grooves  1313  and the beveled surfaces  1315  may be the result of milling away a part of an initially rectangular shoulder piece  1311 . 
       FIG. 14A  shows a cross sectional view that shall be representative of the three afore-described elements. The cross section generally follows line Y-Y in  FIG. 13B  as far as the receptacle is concerned. However, by not keeping the scale and omitting many details the illustration has been partially simplified as to facilitate focusing on the relevant parts and steps. The ion-optical assembly member  1490  is inserted with its cylindrical body  1491  into the circular aperture  1480  of the mount  1420 , and thus rests floatingly on the rim around the aperture  1480 . A sprung member  1452 , such as a curved leaf spring, is attached to the flat member ( FIG. 12 :  1224 ) of the mount  1420  by the through-hole ( FIG. 12 :  1288 ) and engages a space between an upper flange portion  1492 B and a mount surface and forces the ion-optical assembly member  1490  against a radially inward facing contour of the circular aperture  1480  generally opposite the circumferential position of the sprung member  1452 . Due to the undersize of the outer diameter of the ion-optical assembly member  1490  in relation to the inner diameter of the circular aperture  1480 , there remains a gap  1417  between the ring and the cylindrical body  1491  at the side where the sprung member  1452  is located. In principle, in this position freedom to move for the ion-optical assembly member  1490  is possible but restricted in radial directions, unlimited in the upward axial direction, and not possible in the downward axial direction. It goes without saying that the leaf spring  1452  is shown by way of example only. Many other mechanisms of exerting pre-tension, such as actuators, are known to those skilled in the art and can be employed as the specific implementation desired requires and/or allows. 
     On the left, the simplified cross section of the receptacle  1402  shows parts of the cylindrical body  1406  featuring a concave member  1410   a  with groove  1413  and beveled surfaces  1415  at the entrance thereto. In the figure, two beveled surfaces are arranged on both sides of the groove entrance; however, at least the upper beveled surface may be dispensed with as will become apparent from the description further below. On the right, there can be seen parts of the cylindrical body  1406  as well as the contour of the protruding contact head  1410   c  in the background. Insertion of the ion-optical assembly member  1490  and the mount  1420  proceeds laterally in a direction approximately perpendicular to an axis  1419  of the receptacle  1402  (dash-dotted line). This axis  1419  may also be an axis of an ion path in the mass spectrometer (not shown) wherein the herein-described arrangement is employed. 
       FIG. 14B  now shows a point in time during the insertion at which an outer edge of the upper flange portion  1492 B contacts the lower beveled surface  1415  at the entrance of the groove  1413  at the receptacle  1402 . At this point, when proceeding with the lateral motion of insertion, the ion-optical assembly member  1490  starts to be disengaged from the mount  1420  in that it is lifted up by the upward gliding motion of the flange edge on the beveled surface  1415 , whereas the mount  1420  remains roughly on the same height level. For facilitating the gliding motion, the edge of the flange shown rectangular here can be rounded in some design variants. Furthermore, the interacting surfaces can be treated as to further promote gliding. During the lifting of the ion-optical assembly member  1490 , the space  1421  between the upper step of the flange  1492 B and the mount surface, which accommodates the sprung member  1452 , gradually increases so that it can more spaciously accommodate the leaf spring  1452  in this case. The force component exerted by the leaf spring  1452  in an axially upward direction then becomes more pronounced thereby assisting the lifting up at the side of the arrangement approximately opposite the concave member  1410   a.    
       FIG. 14C  now shows the end position of the ion-optical assembly member  1490  within the receptacle  1402 . Therein, the upper flange portion  1492 B engages with the groove  1413 . A distance between upper and lower groove wall is preferably adapted to the thickness of the upper flange portion  1492 B so that it snugly fits therein. In this manner, the ion-optical assembly member  1490  is well-aligned with the receptacle  1402  in the axial direction. In certain favorable embodiments, some contact surfaces at the groove  1413  can also mechanically support the ion-optical assembly member  1490 . Under the pre-tension exerted by the leaf spring  1452 , the lower flange portion  1492 A, or second step of the flange, with a radially outward facing surface  1423  (see  FIG. 14A ) contacts a radially inward facing counter-surface  1425  (see  FIG. 14B ) of the receptacle  1402  below the beveled surface  1415  and thereby aligns the ion-optical assembly member  1490  with the receptacle  1402  in a radial direction. By virtue of the second concave member at the receptacle ( FIGS. 13A and 13B :  1310   b ), which provides another contact surface for the lower flange portion  1492 A, the ion-optical assembly member  1490  can be aligned with the receptacle  1402  in three spatial dimensions (a first radial direction; a second radial direction different from the first radial direction; and the axial direction). The leaf spring  1452  also forces an upper side of the ion-optical assembly member  1490  approximately opposite the concave member  1410   a  to contact the lower flat surface of the protruding contact head  1410   c , which preferably is located at the same level as the upper groove wall  1427  (see  FIG. 14B ). In this position, the ion-optical assembly member  1490 , and all elements not shown in these schematic illustrations that can be attached to it, such as accelerating electrodes, ground electrodes or screening electrodes in a MALDI ion source, for example, have a common axis with the receptacle  1402 . The contact head  1410   c  is shown with a flat contact surface. However, in variants of the assembly shown, the contact head may also have a protruding element, such as a dome, which allows an almost punctiform or tangential contact with a counterpart at the ion-optical assembly member  1490 . Preferably, a point of contact between the contact head  1410   c  and a counter-surface at the ion-optical assembly member  1290  may lie in the same plane as the upper wall of groove  1413 . This plane can be perpendicular to axis  1419  as illustrated in  FIG. 14A-14C . 
     It is to be understood from the foregoing description that alignment in three spatial dimensions is not strictly necessary for realizing embodiments according to principles of the invention. By omitting one of the concave members  1310   a  or  1310   b  shown in  FIGS. 13A and 13B , for example, and by reducing the circumferential extension of the remaining one, basically an alignment in two spatial dimensions, and a certain freedom of motion in a third spatial dimension, can be achieved. Moreover, if no contact between radially outward facing surface at the lower flange portion  1492 A and radially inward facing surface at the receptacle  1402  below the beveled surface  1415  is provided at a point of maximum insertion, alignment in only one spatial dimension, in this example in the axial direction owing to the snug fit of the upper flange portion  1492 B within the groove  1413 , can be provided. Thus, a rotational degree of freedom can be retained. Hence, it becomes apparent that certain modifications on the specific implementation presented here can easily be made without leaving the scope of the invention. 
     As can be seen from the gaps between the ion-optical assembly member  1490  and the mount  1420  in  FIG. 14C , any physical contact between the two has been released so that alignment is effected solely by contact of the ion-optical assembly member  1490  with the receptacle  1402 . This configuration allows decreasing the number of interfaces between the elements to be aligned to the minimum number of one thereby reducing the impact on positioning accuracy of any tolerances due to, for example, the mechanical tolerances during manufacturing or due to any response of the material to temperature changes. 
     Withdrawing the ion-optical assembly member  1490  from the receptacle  1402 , such as for the purpose of inspection, maintenance and/or cleaning, can be achieved by just pulling out the mount  1420  in a lateral direction generally opposite the direction of insertion (see dotted arrow). Then, the inner rim contour of the mount ring  1422  contacts with the outer contour of the cylindrical body of the ion-optical assembly member  1490  below the lower flange portion  1492 A, and the leaf spring  1452  is gradually disengaged from its position within the space between mount surface and upper flange portion  1492 B. From that point on, the ion-optical assembly member  1490  is pulled out of the groove  1413 , again gliding into the floating engagement position it had prior to insertion into the receptacle  1402  (see position in  FIG. 14A ). 
       FIG. 15  shows an arrangement according to principles of the invention in a final state of insertion in a cross sectional isometric view with slightly more details. A conductive central electrode  1529  is attached to the ion-optical assembly member  1590  via an electrically insulating annular member  1531  made, for instance, from a non-conductive plastic or ceramic. The insulator  1531  has several annular grooves  1533  in order to impede creeping currents, is supported at the recessed inner portion ( FIG. 12 :  1293 ) of the upper end of the cylindrical body  1591  of the ion-optical assembly member  1590  and fits flush therewith. Furthermore, also the receptacle  1502  shows some more structural features, such as two recessed steps  1535  in an upper portion of its cylindrical body  1506 , which may serve as further support for other ion-optical elements, such as other electrodes, which are dispensed with in the illustration for the sake of clarity. As described above, in the end position, the upper flange portion  1592 B engages with the groove  1513  at the receptacle  1502 , and the lower flange portion  1592 A contacts the inner circumference of the receptacle  1502  below the beveled surface  1515 . 
     The invention is described above with the aid of the embodiments shown in the illustrations. Modifications of these embodiments are easily possible, however, and those skilled in the art can carry them out with knowledge of the inventive principle without leaving the scope of the present invention. 
     Although the present invention has been illustrated and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.