Patent Publication Number: US-6661002-B2

Title: Mass spectrograph

Description:
This is a continuation-in-part of application Ser. No. 09/615,380 filed Jul. 13, 2000, now pending. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a mass spectrograph of the type for ionizing a sample under a relatively near atmospheric condition of pressure such as an inductively coupled plasma mass spectrograph (ICP-MS), an electro spray mass spectrograph (ES-IMS) or an atmospheric pressure chemical ionization mass spectrograph (APCI-MS). 
     A prior art ESI-MS is shown schematically in FIG.  4  and an portion thereof around its skimmer is shown enlarged in FIG.  5 . This mass spectrograph is provided with a first intermediate chamber  12  and a second intermediate chamber  15  between an ionization chamber  10  having a nozzle  11  connected to the outlet of the column of a liquid chromatographic apparatus and an analyzing chamber  18  with a quadrupole filter  19  and an ion detector  20 , each being mutually separated by a partition wall. The ionization chamber  10  and the first intermediate chamber  12  are connected only through a heated capillary of a small inner diameter serving as a solvent-removing pipe  13 . The first intermediate chamber  12  and the second intermediate chamber  15  are connected only through a conically shaped skimmer  16  having an orifice  16   a  of a small diameter at its tip. 
     The interior of the ionization chamber  10  is nearly in the atmospheric condition due to the gasified molecules of the sample liquid continuously supplied thereinto through the nozzle  11 . The interior of the first intermediate chamber  12  is at a low vacuum condition of about 10 2  Pa by means of a rotary pump (RP). The interior of the second intermediate chamber  15  is at a medium vacuum condition of about 10 −1 -10 −2  Pa by means of a turbo-molecular pump (TMP). The interior of the analyzing chamber  18  is at a high vacuum condition of about 10 −3 -10 −4  Pa by means of another turbo-molecular pump (TMP). In other words, the degree of vacuum increases as one moves from one chamber to the next, starting at the ionization chamber  10  towards the analyzing chamber  18  such that the interior of the analyzing chamber  18  is maintained at a high vacuum condition. 
     A sample liquid is sprayed (or electro-sprayed) through the nozzle  11  into the ionization chamber  10 , and the sample molecules are ionized while the solvent contained in the liquid drops is evaporated. Small liquid droplets with ions mixed in are pulled into the solvent-removing pipe  13  due to the pressure difference between the ionization chamber  10  and the first intermediate chamber  12 . As they pass through the solvent-removing pipe  13 , the solvent is evaporated and the process of ionization proceeds further. A pair of mutually facing planar electrodes or a ring-shaped electrode  14  is provided inside the first intermediate chamber  12 . The electric field generated by this electrode  14  serves not only to pull in the ions through the solvent-removing pipe  13  but also to converge the ions to a point (“backward focal point”) F near the orifice  16   a  of the skimmer  16 . 
     The converged ions are caused to pass through the orifice  16   a  of the skimmer  16  by the pressure difference between the first intermediate chamber  12  and the second intermediate chamber  15  and is directed into the analyzing chamber  18  after being converged and accelerated by means of an ion guide  17  (also referred to as the ion lens or the ion-transporting lens). Inside the analyzing chamber  18 , only those of the ions having a specified mass number (the ratio of mass m to charge z) are passed through the longitudinal space at the center of the quadrupole filter  19  and reach the ion detector  20  to be detected thereby. 
     The function of the ion guide  17  is to accelerate flying ions while causing them to be converged. Ion guides with many different shapes have been proposed. The so-called multi-pole type is one of known types, having a plurality of approximately cylindrically shaped rod electrodes arranged so as to circumscribe a circle of diameter d 1  and mutually separated and having a voltage difference superposing high-frequency voltages with phases mutually inverted by a same direct-current voltage applied between each mutually adjacent pair of these rod electrodes. Such a high-frequency electric field causes the ions introduced in the direction of the optical axis C to move forward while vibrating at a specified frequency. As a result, the ions can be converged more effectively and more ions can be sent into the analyzing chamber  18  on the downstream side. 
     For the purpose of passing ions as efficiently as possible through the first intermediate chamber  12  and the second intermediate chamber  15 , it is desirable to reduce the distance as much as possible between the orifice  16   a  and the space surrounded by the rod electrodes of the ion guide  17 . For this reason, the end surface of the ion guide  17  facing the skimmer  16  is formed with a slope so as to match the sloped surface of the skimmer  16  and the ion guide  17  is disposed such that its sloped end surface protrudes into the conically shaped portion of the skimmer  16 . This makes it time-consuming to fabricate the rod electrodes, affecting the production cost adversely. 
     Another problem is that the orifice  16   a  of the skimmer  16  and its neighboring parts become contaminated with sample ions that stick to them, and the skimmer  16  must therefore be designed to be detachable. With the skimmer  16  and the ion guide  17  as formed above, either of them should be made slidable in the direction of the aforementioned optical axis C or the skimmer  16  must be attached to be rotatable by means of a hinge. This causes the attachment mechanism of the skimmer  16  and the ion guide  17  to be complicated. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention in view of the problems described above to provide a mass spectrograph having an ion guide with a simplified structure and a simplified attachment mechanism for the skimmer while maintaining a high level of efficiency in passing ions. 
     A mass spectrograph embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as being of the kind having an ionization chamber for ionizing a sample, a skimmer in a conical shape with an orifice, an analyzing chamber at a lower pressure than inside the ionization chamber such that the generated ions are pulled through the orifice into the analyzing chamber, and a multi-pole ion guide which is disposed immediately behind the skimmer and comprised of an even number of cylindrically shaped electrodes all elongated in an axial direction but also wherein these electrodes are disposed so as to circumscribe an inscribed circle and the bottom surface of the conically shaped skimmer has a smaller diameter than the inscribed circle of the ion guide such that the ions can reach the analyzing chamber more efficiently. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a drawing for showing the structure around the skimmer of a mass spectrograph embodying this invention; 
     FIG. 2 is a schematic longitudinal view of the skimmer and the ion guide of the mass spectrograph of FIG. 1 for showing their positional relationship; 
     FIG. 3 is a graph showing the relationship between the density distribution of ions which have passed through the skimmer and the positional relationship of the skimmer with respect to the ion guide of the mass spectrograph of FIG. 1; 
     FIG. 4 is a schematic structural diagram of an example of conventional electro spray mass spectrograph (ESI-MS); and 
     FIG. 5 is a view of a portion of FIG. 4 around the skimmer shown enlarged. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described next by way of an example with reference to FIGS. 1-3. Since the general structure of this exemplary mass spectrograph to be described is as shown in FIG. 4 except the design and positional and dimensional relationship of its ion guide with respect to the skimmer, only the aspects which are different from what has been described above with reference to FIGS. 4 and 5 will be described for the convenience of disclosure. 
     As shown in FIG. 1, the mass spectrograph according to this example is characterized as having rod electrodes  171  and  172  (and also  173  and  174  shown in FIG. 2) of its ion guide  17  which are nearly perfectly cylindrical in shape with their end surfaces opposite the skimmer  16  cut perpendicularly to the axial direction. The diameter d 1  of the inscribed circle  17   a  of this ion guide  17  is uniquely determined by the diameter of rod electrodes and other factors. On the other hand, the opening angle θ at the top of the skimmer  16  is determined by taking into account the efficiency with which ions can pass through, and it is usually 40-60°. The diameter d 2  of the bottom opening  16   d  of the conically shaped part  16   b  of the skimmer  16  is selected to be sufficiently smaller than d 1  in view of how close the rod electrodes  171 - 174  are disposed to the skimmer  16 . Explained more in detail, the conical surface of the skimmer  16  (that is, the surface defining the conically shaped part  16   b  of the skimmer  16 ), when extended towards the downstream side towards the ion guide  17 , intersects the internally facing side surfaces of the rod electrodes  171 - 174 , rather than their front surface facing the skimmer  16 , as shown by broken lines in FIG.  1  and more clearly in FIG.  3 . The height d 4  of the conical part  16   b  is determined automatically from the opening angle θ and the diameter d 2  of the bottom opening  16   d . From FIG. 3, it is clear that d 1  is necessarily larger than d 2 , according to this invention. 
     If the dimensional relationship between the skimmer  16  and the ion guide  17  is thus determined, the ions which pass through the orifice  16   a  of the skimmer  16  and advance forward in a diverging way nearly entirely enter the space inside the inscribed circle  17   a  of the ion guide  17 . The ions which enter this space are appropriately converged by the electric field formed by the voltages applied to the rod electrodes  171 - 174  and thereafter sent into the analyzing chamber on the downstream side. The efficiency of the ions passing through the ion guide  17  is thus improved. 
     In reality, however, those of ions which are introduced inside the inscribed circle  17   a  but closer to its outer periphery have a low probability of being properly made to converge and their efficiency is not necessarily high for passing through the ion guide  17 . In FIG. 2, the dotted circle with diameter d 3  around the optical axis C indicates the so-called acceptance area  17   b  where the passing efficiency for ions is extremely high. FIG. 3 shows the ion density distribution in the radial direction with respect to the position of the skimmer  16  as well as that of the ion guide  17 . As can be seen, the ion density is the largest near the ion optical axis C, quickly becoming smaller as the outer periphery is approached but there are some ions, although few, even near the peripheral wall of the skimmer  16 . With the structure as shown in FIG. 1, the ions emitted from areas close to the peripheral wall of the conically shaped part  16   b  of the skimmer  16  reach the space outside the acceptance area  17   b , having an extremely small probability of passing through the ion guide  17 . For improving the efficiency for passing the ions through, therefore, it is preferable to make the diameter d 2  of the bottom surface of the conically shaped part  16   b  of the skimmer  16  smaller than the diameter d 3  of the acceptance area  17   b . If the size relationship is so chosen, almost all of the ions which pass through the orifice  16   a  of the skimmer  16  enter the acceptance area  17   b , are appropriately converged by the ion guide  17  and reach the analyzing chamber  18  with a high probability. 
     If the height d 4  of the conically shaped part  16   b  of the skimmer  16  is too low, however, gasified solvent traveling slightly off the ion optical axis cannot be eliminated satisfactorily even where the opening angle θ at the top satisfies the condition given above. In reality, it is difficult to make the diameter d 2  of the bottom surface of the conically shaped part  16   b  of the skimmer  16  much smaller than the diameter d 3  of the acceptance area  17   b . It is appropriate to make the diameters d 2  and d 3  nearly equal to each other. 
     Although the invention was described above by way of only one example, this example is intended to be considered illustrative, not as limiting. It goes without saying that many modifications and variations are possible within the scope of this invention. With a mass spectrograph embodying this invention, ions pass through the orifice of the skimmer towards the analyzing chamber due to the pressure difference and even those of the ions entering divergently along the inner peripheral wall of the conically shaped part of the skimmer can be dependably directed into the space surrounded by the ion guide. As a result, more ions can be converged by the ion guide and directed into the mass spectrometer and hence the sensitivity and accuracy of analysis can be improved. 
     With a mass spectrograph embodying this invention, furthermore, the end part of the ion guide does not penetrate the conically shaped part of the skimmer and hence the skimmer can be moved sideways (perpendicularly to the axial direction) without first retracting the ion guide. Thus, the mechanism for detaching and attaching the skimmer can be simplified. Since the rod electrodes of the ion guides can be produced simply by cutting the rods perpendicularly to form the end surfaces, the manufacturing process is simpler and the production cost can be reduced.