Patent Publication Number: US-2004051038-A1

Title: Ion guide

Description:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT  
       [0001] The invention relates to an ion guide for guiding ions produced at a low vacuum area to a high vacuum area.  
       [0002] A liquid chromatograph mass spectrometer and the like uses an electro-spray mass spectrometer, high frequency inductive coupling plasma mass spectrometer, atmospheric pressure chemical ionization mass spectrometer and the like as a mass spectrometry portion thereof. In these devices, a sample is ionized, and a quadrupole mass spectrometer or a time-of-flight mass spectrometer analyzes the ions for the mass spectrometry.  
       [0003] While the sample is ionized under a pressure near the atmospheric pressure, the mass spectrometry is required to be carried out under a high vacuum to suppress scattering of the ions. Therefore, it is necessary to transport the ions from a ion source in a low vacuum area to a mass spectrometer in a high vacuum area. It is important to suppress loss of the ions caused during the transportation thereof for attaining a high sensitivity of the mass spectrometer.  
       [0004] Hereunder, with reference to FIG. 1, a structure for transporting the ions from a low vacuum area to a high vacuum area will be explained by means of an electro-spray mass spectrometer as an example.  
       [0005] The mass spectrometer includes the first intermediate chamber  12  and the second intermediate chamber  13  between an ionizing chamber  11  provided with a nozzle  15  connected to an exit of a column of a liquid chromatograph device and a mass spectrometry chamber  14  provided with a quadrupole filter  17  and an ion detector  18 . The ionizing chamber  11  and the first intermediate chamber  12  are isolated by a partition provided with a desolvent pipe  16  having a small diameter. Between the first intermediate chamber  12  and the second intermediate chamber  13  and between the second intermediate chamber  13  and the mass spectrometry chamber  14  are provided partitions having apertures  19 ,  20  with a small diameter, respectively. The first intermediate chamber  12  and the second intermediate chamber  13  include ion guides  21  and  22 , respectively.  
       [0006] The ionizing chamber  11  is held in a substantially atmospheric pressure for spraying a sample liquid through the nozzle  15 . The first intermediate chamber  12  is exhausted to be a low vacuum area in the order of 10 +2  Pa through a rotary pump, and the second intermediate chamber  13  is exhausted to be a middle vacuum area in the order of 10 −1 -10 −2  Pa through a turbo molecular pump. The mass spectrometry chamber  14  is exhausted to be a high vacuum area in the order of 10 −4  Pa through a turbo molecular pump. In other words, the mass spectrometry chamber  14  is held under a high vacuum condition by gradually increasing the vacuum levels from the ionizing chamber  11  toward the mass spectrometry chamber  14  (differential exhaustion).  
       [0007] Incidentally, the shown example includes two intermediate chambers. Alternatively, the number of the intermediate chambers may be two or more, or in order to simplify the device, the intermediate chamber may be one.  
       [0008] The sample liquid is sprayed in the ionizing chamber  11  through the nozzle  15 , and sample molecules are ionized while a solvent in liquid drops is evaporated. The fine liquid drops containing ions are drawn into the desolvent pipe  16  by the pressure difference between the ionizing chamber  11  and the first intermediate chamber  12 . The solvent is further evaporated while passing through the desolvent pipe  16  to thereby promote the ionization. The sample is ionized as described above, and the ions reach the mass spectrometry chamber  14  through the ion guide  21 , aperture  19 , ion guide  22  and aperture  20 .  
       [0009] Conventionally, an electrostatic ion lens as shown in FIG. 2 has been used to transport the ions in the intermediate chamber. Different DC voltages are applied to the juxtaposed plural electrostatic ion lenses  23  to transport the ions with potential difference between the respective electrostatic ion lenses. However, with such an ion lens, when the ions change an advancing direction through collision with air molecules in the low vacuum area, it is impossible to return the ions to the original advancing direction, resulting in poor ion transport efficiency.  
       [0010] Recently, an ion guide using a high-frequency electric field has been used. As shown in FIG. 3 and FIG. 4, such ion guides include a multipole ion guide wherein an even number of pole electrodes  24  is disposed in parallel and symmetrical relative to an ion beam axis; and an ion guide wherein a plurality of ring-shape poles  25  is disposed in the advancing direction. In any of these ion guides, high frequency voltages having different phases are applied to the adjacent electrodes to form a high frequency electric field. Even if the advancing direction of the ions is changed by collision with the gas molecules in the low vacuum area, the ions are reflected toward the central (ion beam axis) direction in the ion guide by the high frequency electric field to thereby proceed to the heading direction. Therefore, the transport efficiency of the ions in the ion guide is higher than that of the conventional electrostatic ion lens where the DC voltage is applied.  
       [0011] It is preferable that the ion guide has a large inner diameter in order to increase a quantity of the transported ions, while it is preferable that the aperture has a small inner diameter in order to maintain the vacuum levels in the respective chambers. Therefore, the ions passing through the ion guide tend to collide against a partition around the aperture, resulting in lower ion transport efficiency.  
       [0012] As shown in FIG. 5, in order to increase the number of the ions passing through the ion guide and the aperture as much as possible, there have been used a method wherein an electrostatic ion lens  26  is disposed between the ion guide and the aperture; and a method wherein an aperture itself has an electrostatic lens function. However, in any of these methods, a high frequency electric field is not created between the ion guide and the aperture (area  27  in an example shown in FIG. 5). Accordingly, it is possible that the ions collide against the gas molecules in the low vacuum atmosphere to thereby change their advancing direction.  
       [0013] As described above, although the ion transport efficiency of the mass spectrometer as a whole has been improved, the ion transport efficiency in the vicinity of the aperture has not been improved yet.  
       [0014] In view of the above problems, the present invention has been made, and an object of the invention is to provide an ion guide with improved ion transport efficiency in the vicinity of the aperture, thereby obtaining the high ion transport efficiency of the device as a whole.  
       [0015] Further objects and advantages of the invention will be apparent from the following description of the invention.  
       SUMMARY OF THE INVENTION  
       [0016] In order to solve the above problems, according to the present invention, an ion guide is installed in an intermediate chamber provided between a low vacuum chamber and a high vacuum chamber, and is located on a passage of transporting ions from the low vacuum chamber to the high vacuum chamber. The ion guide includes a plurality of plate-shape electrodes juxtaposed in a transport direction of the ions in the intermediate chamber and provided with ion passage holes around an ion beam axis, respectively; a plate-shape aperture electrode disposed as a part of partition for separating the intermediate chamber and an adjacent chamber and provided with an aperture around the ion beam axis; and high frequency power sources for applying high frequency electric voltages to the plate-shape electrodes and the aperture electrode, respectively. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1 is a block diagram showing a structure of a mass spectrometer;  
     [0018]FIG. 2 is a sectional view showing a structure of an electrostatic ion lens;  
     [0019]FIG. 3 is a perspective view showing an ion guide with a conventional multipole structure;  
     [0020]FIG. 4 is a perspective view showing a structure of an ion guide using conventional ring-shape electrodes;  
     [0021]FIG. 5 is a drawing showing an area of ion dispersion in a conventional ion guide;  
     [0022] FIGS.  6 ( a ) and  6 ( b ) are sectional views showing structures of ion guides according to the present invention;  
     [0023]FIG. 7 is a sectional view showing a structure where DC voltages are applied to the respective electrodes in the ion guide according to the present invention; and  
     [0024]FIG. 8 is a sectional view showing an embodiment of a mass spectrometer using the ion guide according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
     [0025] The invention is applicable to a device including one or more intermediate chambers held at an intermediate vacuum level between a low vacuum chamber, such as an ionizing chamber, and a high vacuum chamber, such as a mass spectrometry chamber.  
     [0026] According to the present invention, an ion guide is provided in the intermediate chamber. The intermediate chamber is isolated from both adjacent chambers by partitions. In the present specification, the “adjacent chamber” includes another intermediate chamber, low vacuum chamber, or high vacuum chamber. Also, while one intermediate chamber includes two adjacent chambers, the following explanation will focus on only one of the two adjacent chambers for explanation purpose. The structure described below is applicable to other adjacent chambers.  
     [0027] Hereunder, with reference to FIG. 6( a ), the ion guide of the invention will be described. A plurality of plate electrodes  32  having holes through which ions pass is juxtaposed around an ion beam axis in the ion transport direction in an intermediate chamber  30 .  
     [0028] A plate electrode (aperture electrode  33 ) having an aperture  33   a  around the ion beam axis is provided on a partition between an intermediate chamber  30  with the plate electrodes  32  and an adjacent chamber  31 . In a case that the partition is made of a conductive material, an insulating member  34  is provided between the partition and the aperture electrode  33 . The aperture  33   a  maintains a difference in vacuum levels between an intermediate chamber  30  and the adjacent chamber  31  while transporting the ions from the intermediate chamber to the adjacent chamber (or introducing the ion from the adjacent chamber  31  to the intermediate chamber  30 ).  
     [0029] As described above, it is preferable that the plate electrode  32  has a large ion transport hole in order to increase the quantity of the ions, while it is preferable that the aperture electrode  33  has a small aperture in order to hold the difference in the vacuum levels between the intermediate chamber  30  and the adjacent chamber  31 .  
     [0030] A high-frequency (HF) power source  36  is connected to these plate electrodes  32  and aperture electrode  33 . In the embodiment shown in FIG. 6( a ), two HF power sources are used and the adjacent electrodes are connected to different HF power sources. HF voltages having different phases by 180° are applied to the electrodes from the two HF power sources, respectively. The HF power sources are connected to both the plate electrodes  32  and the aperture electrode  33  in the same way. Therefore, a consistent continuous HF electric field is formed around the ion beam axis, so that the plate electrodes  32  and the aperture electrode  33  function as an ion guide in collaboration.  
     [0031] The ions passing through the ion guide are drawn back to the periphery of the ion beam axis  35  by the HF electric field when the ions collide with the gas molecules in the intermediate chamber. The HF electric field is formed up to the aperture electrode  33  disposed at the boundary of the adjacent chamber, so that a large number of the ions can reach the adjacent chamber.  
     [0032] In the embodiment described above, the description is focused only on one intermediate chamber. As shown in FIG. 6( b ), in a case that plural intermediate chambers are continuously provided, a plurality of plate electrodes  32  is juxtaposed in the respective intermediate chambers, and the aperture electrodes  33  are provided at the boundaries between the respective intermediate chambers. The HF power sources  36  are connected to the plate electrodes  32  and the aperture electrodes  33 . With this structure, the consistent continuous HF electric field is formed over the plural intermediate chambers, so that a great number of the ions can reach the adjacent chamber through the plural intermediate chambers.  
     [0033] Further, an electric field gradient may be formed in the travelling direction of the ion by superposing different DC voltages on the HF electric voltages applied to the respective electrodes, so that the ion can be accelerated in the travelling direction. Also, it is possible to control the kinetic energy of the ion by controlling a size of the electric field gradient.  
     [0034]FIG. 7 shows a structure for superposing the DC voltage as described above. Resistances R 1 , R 2 , . . . are connected between the adjacent electrodes, respectively, and the DC voltage is applied to the resistances from a DC power source  37 , so that different DC potentials are applied to the respective plate electrodes  32  and the aperture electrode  33 . As a result, different DC voltages are superposed on the HF voltages applied to the respective electrodes. It is possible to control the size of the DC voltage applied to the respective electrodes by controlling the DC voltages applied from the DC power source  37 .  
     [0035] According to the ion guide of the invention, the consistent continuous HF electric field can be formed around the aperture by structuring the aperture itself as one of the electrodes constituting the ion guide. Therefore, a large number of the ions can pass through the aperture since the loss of the ion due to collision with the gas molecules is suppressed, thereby obtaining the high ion transport efficiency as a whole device. The ion guide according to the embodiment is applicable to various devices for transporting the ions from the low vacuum area to the high vacuum area. For example, a mass spectrometer using the present ion guide can carry out analysis with a high sensibility due to the high ion transporting efficiency.  
     [0036] As an embodiment of the present invention, a liquid chromatograph mass spectrometer provided with the ion guide according to the present invention will be explained with reference to FIG. 8. In the same manner as in the mass chromatograph spectrometer having a conventional configuration, the liquid chromatograph mass spectrometer includes the first intermediate chamber  12  and the second intermediate chamber  13  between an ionizing chamber  11  for ionizing a liquid sample and a mass spectrometry chamber  14 . The respective chambers are separately pumped so that a vacuum level of every chamber is gradually increased from the ionizing chamber  11  toward the mass spectrometry chamber  14 . A quadrupole mass spectrometer is provided in the mass spectrometry chamber  14  shown in FIG. 8. Alternatively, other mass spectrometer such as a time-of-flight type mass spectrometer may be provided in the mass spectrometry chamber  14 .  
     [0037] The ion guide of the invention is provided in the first intermediate chamber  12  and the second intermediate chamber  13  from right after a desolvent pipe  16 . A plurality of the plate electrodes  32  having the ion transit holes with a diameter of 5 mm is provided in the first intermediate chamber  12  and the second intermediate chamber  13 , respectively. An aperture electrode  331  is provided between the first intermediate chamber  12  and the second intermediate chamber  13 . Also, an aperture electrode  332  is provided between the second intermediate chamber  13  and the mass spectrometry chamber  14 . The aperture electrodes  331  and  332  are provided with apertures  331   a  and  332   a  having a diameter of 3 mm or 5 mm in order to hold a difference in the vacuum levels between the adjacent chambers on both sides and to allow the ions to pass therethrough.  
     [0038] The high frequency power sources are connected to the plate electrodes  32  and the aperture electrodes  331  and  332 . In the present embodiment, two high frequency power sources  361  and  362  are alternately connected to the adjacent electrodes to apply the electric power. The high frequency power sources  361 ,  362  apply the high frequency voltages having different phases by 180° to the electrodes, respectively. With application of the high frequency voltages, the consistent continuous high frequency electric field is formed from right after the desolvent pipe  16  to the aperture  332   a.    
     [0039] Further, in the present embodiment, resistances are connected between the adjacent electrodes, and the DC voltage is applied to the resistances from the DC power source  37  so that the different DC voltages are applied to the respective plate electrodes  32  and aperture electrodes  331 ,  332  to thereby provide the electric field gradient in the ion travelling direction.  
     [0040] The ions are transported in the mass spectrometer having the structure as described above in a way described below. A sample liquid is atomized in the ionizing chamber  11  and further passed through the desolvent pipe  16  to thereby ionize the sample molecules. The first plate electrode  32  is provided right after the desolvent pipe  16 . The ion passage hole of the plate electrode  32  has a diameter (5 mm) larger than the inner diameter (0.3 mm) of the desolvent pipe  16 . Therefore, the ions passed through the desolvent pipe  16  are introduced into the ion passage hole of the first plate electrode  32  while only a small quantity of the ions spreads in a vertical direction with respect to the ion beam axis  35 . As a result, the loss of the ions between the desolvent pipe  16  and the first plate electrode  32  is sufficiently small.  
     [0041] The consistent continuous high frequency electric field is formed from the ion passage hole of the first plate electrode  32  to the aperture  332   a . Thus, the ions are drawn back to the periphery of the ion beam axis  35  even if the ions collide with the gas molecules or the like in the first intermediate chamber  12  and the second intermediate chamber  13  including the vicinities of the apertures where the ions are typically lost, so that a large quantity of the ions can reach the mass spectrometry chamber  14 .  
     [0042] Further, the ions are accelerated in the heading direction of the ion beam axis  35  through the electric field gradient formed by superposing the DC voltage on the HF voltage applied to the respective electrodes. It is possible to transport the ions from the ionizing chamber  11  to the mass spectrometry chamber  14  only by using the difference in the vacuum levels of the respective chambers without providing the electric field gradient. However, it is possible to further accelerate the ions in the advancing direction by the electric field gradient. It is also possible to control the kinetic energy of the ions by controlling the size of the electric field gradient.  
     [0043] While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.