Abstract:
The present invention provides a corona discharge device, comprising a first electrode including: a first substantially cylindrical inner chamber portion and a second substantially conical inner chamber portion in communication with the first inner chamber portion, wherein the second inner chamber portion has a cross sectional area that gradually enlarges in a direction away from the first inner chamber portion. The present invention also provides an ion mobility spectrometer comprising: an ionization region; and the corona discharge device disposed in the ionization region. With the above construction and structure, the ion mobility spectrometer of the present invention has the advantages that extraction of ions is facilitated and a life time of the corona electrode is lengthened. In addition, the focusing and storing electrode is used to effectively shield interference of a corona discharge pulse, and to push and focus sample ions. A designed voltage control solution is used to achieve mobility differentiating of ions, while a corona pulse is shielded to prevent variation in an ion quantity due to the corona pulse, thereby achieving an effect of stabilizing mobility spectrum lines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a corona discharge device and an ion mobility spectrometer having the corona discharge device. 
     2. Description of the Related Art 
     An ion mobility spectrometer achieves differentiating of ions based on the fact that different ions drift at different speeds in a uniform weak electric field. The ion mobility spectrometer has the following advantages. The ion mobility spectrometer is capable of differentiating ions quickly, has a high sensitivity, does not need a vacuum environment, and facilitates miniaturization. Therefore, the ion mobility spectrometer is widely used in the field of detection of drugs and explosives. A typical ion mobility spectrometer is generally composed of a sample feeding part, an ionization region, an ion door, a drift region, a collection region, a reading circuit, a data acquiring and processing part, a control part, and the like. The ionization part mainly functions to convert molecules of a sample into ions that can be drifted so as to be separated. Therefore, the effect of ionization affects performance of the spectrometer very directly. Among current techniques, the common and widely used ionization assembly is one which employs a Ni63 radiation source. The ionization assembly has a small volume and a high stability, and does not need any additional circuit, but brings about a narrow linear range, a lower concentration of converted ions, and radioactive contamination. Especially the radioactive contamination causes much inconvenience to the operation, transportation and management of the apparatus. 
     One solution for overcoming the radioactive contamination is to adopt corona discharge technology instead of radiation source technology. The corona discharge is a phenomenon in which ionization of molecules of gas is caused due to a local strong electric field in a spatial nonuniform electric field. Ions generated directly by the corona discharge are generally called reactant ions. When molecules of a sample having a higher proton or electron affinity pass through the ionization region, they capture electric charge of the reactant ions so as to be ionized. Generally, a structure for the corona discharge is relatively simple and thus has a low cost, while a concentration of electric charge generated by the corona discharge is far higher than that generated by a radiation source. Therefore, the corona discharge facilitates improvement of sensitivity of the ion mobility spectrometer and obtains a large dynamic range. 
     However, it has disadvantages to perform ionization by using corona discharge. The corona discharge needs a high-voltage power source for supplying electric power. Furthermore, because the corona discharge itself occurs in a pulsed process (a Trichel pulse), a disorder of spectrum lines will be caused such that a detection result will be seriously affected if ions are allowed to enter the ion mobility spectrometer directly through the ion door. In addition, ions in a region of the corona discharge will be accelerated by an electric field in the region to strike a corona electrode so as to be lost. As a result, improvement of the sensitivity of the ion mobility spectrometer is inhibited. How to effectively drag out the ions from the ionization region is still a serious problem to be solved. In addition, since the corona discharge will causes oxidation of the corona electrode, it is an important problem to lengthen a life time of the electrode. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a corona discharge device and an ion mobility spectrometer having the corona discharge device which can structurally lengthen a life time of an electrode. 
     Another object of the present invention is to provide an ion mobility spectrometer which can structurally facilitate extraction of ions. 
     A further object of the present invention is to provide an ion mobility spectrometer which can effectively shield interference of a corona discharge pulse by means of a focusing and storing electrode. 
     In accordance with an aspect of the present invention, the present invention provides a corona discharge device, comprising a first electrode including: a first substantially cylindrical inner chamber portion and a second substantially conical inner chamber portion in communication with the first inner chamber portion, wherein the second inner chamber portion has a cross sectional area that gradually enlarges in a direction away from the first inner chamber portion. 
     In accordance with an aspect of the present invention, the first electrode further includes: a first substantially cylindrical portion defining the first inner chamber portion; and a second substantially conical portion connected with the first portion and defining the second inner chamber portion. 
     In accordance with an aspect of the present invention, the first inner chamber portion has a shape of a substantially circular cylinder and the second inner chamber portion has a shape of a substantially circular cone, and the first inner chamber portion and the second inner chamber portion are substantially coaxially arranged. 
     In accordance with an aspect of the present invention, the first electrode further includes an opening passing through a wall of the first electrode; and the corona discharge device further comprises a second electrode inserted in an inside of the first electrode from an outside of the first electrode through the opening of the first electrode, wherein the second electrode has a shape of a needle. 
     In accordance with an aspect of the present invention, the second electrode is inserted in the first inner chamber portion. 
     In accordance with an aspect of the present invention, the second needle-shaped electrode is at least one pair of second needle-shaped electrodes disposed opposite to each other and extending on substantially the same straight line. 
     In accordance with an aspect of the present invention, the present invention provides an ion mobility spectrometer comprising: an ionization region; and the above corona discharge device disposed in the ionization region. 
     In accordance with an aspect of the present invention, the ion mobility spectrometer further comprises a focusing and storing electrode having a substantially conical skirt section, wherein at least a portion of the skirt section is inserted in the second inner chamber portion of the first electrode. 
     In accordance with another aspect of the present invention, the skirt section has a shape of a substantially circular cone. 
     In accordance with a further aspect of the present invention, the first inner chamber portion has a shape of a substantially circular cylinder, and an end of the skirt section close to the corona discharge device has a diameter smaller than a diameter of the first inner chamber portion. 
     In accordance with a further aspect of the present invention, the ion mobility spectrometer further comprises a first grid electrode, wherein the first grid electrode is electrically connected to an end of the skirt section of the focusing and storing electrode away from the corona discharge device. 
     In accordance with a further aspect of the present invention, the ion mobility spectrometer further comprises a second grid electrode, wherein the second grid electrode is separated from the first grid electrode by a predetermined distance. 
     In accordance with a further aspect of the present invention, the first electrode and the focusing and storing electrode are substantially coaxially arranged. 
     In accordance with a further aspect of the present invention, a carrier gas flows substantially in an axial direction of the first electrode in the first electrode. 
     In accordance with a still further aspect of the present invention, the present invention provides an ion mobility spectrometer comprising: an ionization region; a corona discharge device disposed in the ionization region, a cylindrical electrode of the corona discharge device having an inner chamber portion; and a focusing and storing electrode having a substantially conical skirt section, wherein at least a portion of the skirt section is inserted in the inner chamber portion of the electrode. 
     In accordance with another aspect of the present invention, the skirt section has a shape of a substantially circular cone. 
     In accordance with a further aspect of the present invention, the inner chamber portion has a shape of a substantially circular cylinder, and an end of the skirt section close to the corona discharge device has a diameter smaller than that of the inner chamber portion. 
     In accordance with a further aspect of the present invention, the ion mobility spectrometer further comprises a first grid electrode, wherein the first grid electrode is electrically connected to an end of the skirt section of the focusing and storing electrode away from the corona discharge device. 
     In accordance with a further aspect of the present invention, the ion mobility spectrometer further comprises a second grid electrode, wherein the second grid electrode is separated from the first grid electrode by a predetermined distance. 
     In accordance with a further aspect of the present invention, the electrode and the focusing and storing electrode are substantially coaxially arranged. 
     In accordance with a further aspect of the present invention, a carrier gas flows substantially in an axial direction of the electrode in the electrode. 
     With the above construction and structure, the extraction of ions is facilitated and a life time of an electrode is lengthened. In addition, the focusing and storing electrode is used to effectively shield the electrical disturbance of a corona discharge pulse, and drag and focus sample ions. A designed voltage control solution is used to achieve well separated mobility spectrum, while a stable data line is achieved by minimizing the difference of charged particle number from each corona spark. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an ion mobility spectrometer according to an embodiment of the present invention. 
         FIG. 2  is a schematic cross sectional view of a corona discharge device according to an embodiment of the present invention. 
         FIG. 3  is a schematic left view of the corona discharge device according to the embodiment of the present invention. 
         FIG. 4  is a schematic perspective view of the corona discharge device according to the embodiment of the present invention. 
         FIG. 5  is a schematic cross sectional view of a focusing and storing electrode according to an embodiment of the present invention. 
         FIG. 6  is a schematic right view of the focusing and storing electrode according to the embodiment of the present invention. 
         FIG. 7  is a schematic graph of electric potentials of components of an ion mobility spectrometer according to an embodiment of the present invention in a positive ion mode. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An ion mobility spectrometer according to the present invention can be operated in either positive or negative ion mode. For the purpose of convenience, embodiments of the present invention will be described hereinafter based on only the positive ion mode. 
       FIG. 1  is a schematic view of an ion mobility spectrometer  100  according to an embodiment of the present invention. As shown in  FIG. 1 , the ion mobility spectrometer  100  composes a housing  20 , a sample feeding part  22 , an ionization region  24 , a focusing and storing electrode  14 , a drift region  28 , a collection region  30 , a reading circuit  40 , a data acquiring and processing device, a control part, and the like. The sample feeding part  22  comprises an inlet  221  for introducing a carrier gas and a sample. In addition, the ion mobility spectrometer  100  further comprises a gas outlet  201  and a drift gas inlet  202 . The ion mobility spectrometer  100  further comprises a corona discharge device  50  disposed in the ionization region  24 ; drift electrodes  16  which are disposed in the drift region  28  and are configured as coaxial circular rings arranged at equal intervals; a Faraday plate  18  disposed in the collection region  30 ; and an aperture grid  17  disposed between the drift electrodes  16  and the Faraday plate  18  for restraining ions from generating electrostatically induced charge on the Faraday plate  18 . In an example, the aperture grid  17  is configured to be a single screen. The Faraday plate  18  is a circular flat plate, and is coupled to a charge sensitive amplifier to read an ion signal. 
     As shown in  FIGS. 2 to 4 , the corona discharge device comprises a first electrode  12  including: a first substantially cylindrical inner chamber portion  530  and a second substantially conical inner chamber portion  550  in communication with the first inner chamber portion  530 . The second inner chamber portion  550  has a cross sectional area that gradually enlarges in a direction away from the first inner chamber portion  530 . The first electrode  12  is substantially tube-shaped. The first electrode  12  further includes: a first substantially cylindrical portion  53  defining the first inner chamber portion  530 ; and a second substantially conical portion  55  connected with the first portion  53  and defining the second inner chamber portion  550 . 
     As shown in  FIGS. 1 to 4 , the first inner chamber portion  530  may have a shape of a substantially circular cylinder and the second inner chamber portion  550  may have a shape of a substantially circular cone, and the first inner chamber portion  530  and the second inner chamber portion  550  are substantially coaxially arranged. The first portion  53  may have a shape of a substantially circular cylindrical surface and the second portion  55  may have a shape of a substantially circular conical surface, and the first portion  53  and the second portion  55  may be substantially coaxially arranged. The first inner chamber portion  530 , the second inner chamber portion  550 , the first portion  53  and the second portion  55  may also have any other appropriate shapes. 
     As shown in  FIGS. 2 to 4 , the first electrode  12  further includes an opening  51  passing through a wall of the first electrode  12 . The opening  51  may pass through the wall of the first portion  53  or the second portion  55 . The corona discharge device further comprises a second electrode  11  inserted in an inside of the first electrode  12  from an outside of the first electrode  12  through the opening  51  of the first electrode  12 . The second electrode  11  has a shape of a needle. The second electrode may be inserted in the first inner chamber portion  530  or the second inner chamber portion  550 . 
     As shown in  FIGS. 2 and 4 , the second electrode  11  is at least one pair (such as one pair, two pairs, three or more pairs) of second electrodes  11  disposed opposite to each other and extending on substantially the same straight line. Alternatively, each pair of second electrodes  11  may also be disposed to be in a staggered manner rather than being opposite to each other. The second electrode  11  may be referred to as a corona needle, while the first electrode  12  may be referred to as a corona target electrode. 
     As shown in  FIGS. 2-4 , the second needle-shaped electrode  11  may be connected to the first portion  53  through a circularly cylindrical insulating piece  13 . The second electrode  11  may be extended in a radial direction of the first circularly cylindrical portion  53 , and a length of a part of the second electrode  11  which is inserted in the first inner chamber portion  530  is adjustable. The second electrode  11  is made of oxidation resistant metal such as stainless steel, tungsten, nickel, and platinum. The first electrode  12  may be made of common metal and plated with nickel. 
     As shown in  FIGS. 1-4 , an inside of the first electrode  12  serves as a gas path, and a carrier gas entering the ion mobility spectrometer  10  from an inlet  221  for the carrier gas and a sample flows through the gas path. The carrier gas flows substantially in an axial direction of the first electrode  12  in the first electrode  12 . In other words, a direction A in which the carrier gas flows into the ion mobility spectrometer is substantially parallel to the axial direction of the first electrode  12 . The second electrode  11  enters the gas path. A direction of an electric field generated by the first electrode  12  and the second electrode  11  is orthogonal to the direction A of the carrier gas flowing in the first electrode  12  so that interference from the electric field to the electric field region located downstream from the gas path can be avoided. 
     As shown in  FIGS. 1 and 5 , the focusing and storing electrode  14  has a substantially conical skirt section  141 , and the skirt section  141  may has a shape of a substantially circular conical surface. At least a portion of the conical skirt section  141  is inserted in the second inner chamber portion  550  of the first electrode  12 . The skirt section  141  is not in contact with the first electrode  12 . If the first electrode which is a conventional electrode and has an inner chamber is used, at least a portion of the conical skirt section  141  may be similarly inserted in the inner chamber of the first conventional electrode. The second portion  55  can enable the skirt section  141  of the focusing and storing electrode  14  to be located as close to the corona region as possible and form a focusing electric field together with the focusing and storing electrode  14 . An end  143  of the skirt section  141  close to the corona discharge device  50  has a diameter smaller than a diameter of the first inner chamber portion  530  of the first portion  53  of the first electrode  12 . For example, the end  143  of the skirt section  141  close to the corona discharge device  50  has a diameter about 1-3 mm smaller than a diameter of the first inner chamber portion  530  of the first portion  53  of the first electrode  12 . 
     Alternatively, the first electrode  12  may comprise only the first substantially cylindrical inner chamber portion  530  without the second substantially conical inner chamber portion  550 . In this case, at least a portion of the skirt section  141  of the focusing and storing electrode  14  may be inserted in the first inner chamber portion  530  of the first electrode  12 . 
     The focusing and storing electrode  14  may comprise only the skirt section  141 . Alternatively, the focusing and storing electrode  14  may further comprise a flange  149 , and the flange  149  is formed at a larger-diameter end  147  of the conical skirt section  141 . The first electrode  12  and the focusing and storing electrode  14  may be substantially coaxially arranged. 
     As shown in  FIG. 1 , the ion mobility spectrometer  10  further comprises a first grid electrode  145 . The first grid electrode  145  is electrically connected to an end  147  of the skirt section  141  of the focusing and storing electrode  14  away from the corona discharge device  50 . The first grid electrode  145  is in contact with the end  147  of the skirt section  141  of the focusing and storing electrode  14  away from the corona discharge device  50 , or the flange  149  of the focusing and storing electrode  14 . The first grid electrode  145  has a grid shape, and the lattices of the first grid electrode may have various shapes such as a hexagonal shape and a rectangular shape. A substantially equipotential region is formed inside the skirt section  141  near the end  147  or the first grid electrode  145 , and the region is used for storing ions. 
     As shown in  FIG. 1 , the ion mobility spectrometer  10  further comprises a second grid electrode  15 . The second grid electrode  15  is separated from the first grid electrode by a predetermined distance. The second grid electrode  15  has a grid shape, and the lattices of the second grid electrode may have various shapes such as a hexagonal shape and a rectangular shape. 
     The first grid electrode  145  and the second grid electrode  15  constitute the ion door. A voltage exerted across the first grid electrode  145  and the second grid electrode  15  generates a periodically-varying electric field. The periodically-varying electric field forms an ON state and OFF state of the ion door. 
     Referring to  FIGS. 1 and 7 ,  FIG. 7  is a schematic graph of electric potentials of components of an ion mobility spectrometer  10  according to an embodiment of the present invention in a positive ion mode. In  FIG. 7 , the axis of abscissas P denotes positions of the components, the axis of ordinate V denotes electric potentials of components, the reference numeral  110  denotes an electric potential of the second electrode  11 , the reference numeral  120  denotes an electric potential of the first electrode  12 , the reference numeral  140  denotes an electric potential of the focusing and storing electrode  14  and the first grid electrode  145 , the reference numeral  150  denotes an electric potential of the second grid electrode  15 , the reference numeral  160  denotes an electric potential of the drift electrode  16 , the reference numeral  170  denotes an electric potential of the aperture grid  17 , and the reference numeral  180  denotes an electric potential of the Faraday plate  18 . 
     As shown in  FIGS. 1 and 7 , when the ion mobility spectrometer  10  operates, the electric potential  110  of the second electrode  11  is around 700-3000V higher than the electric potential  120  of the first electrode  12  (depending upon a radius of a tip of the second electrode  11  and a length of the second electrode  11  since different geometric sizes will result in different corona-starting voltages) to generate the corona so as to produce ions. The electric potential  140  of the focusing and storing electrode  14  periodically jumps. The focusing and storing electrode  14  is in a storage state when the focusing and storing electrode  14  is at a low electric potential (as shown by the solid line indicated by the reference numeral  140  in  FIG. 7 ), while the focusing and storing electrode  14  is in a pushing state when the focusing and storing electrode  14  is at a high electric potential (as shown by the dashed line indicated by the reference numeral  140  in  FIG. 7 ). When the focusing and storing electrode  14  is in the storage state, the electric potential  140  of the focusing and storing electrode  14  is 60-150V lower than the electric potential  120  of the first electrode  12 , and around 5-60V lower than the electric potential  150  of the second grid electrode  15 . After ions enter the focusing and storing electrode  14 , they receive a weak electric field force and mainly perform thermal motion in the chamber of the focusing and storing electrode  14 . Ions accumulate in the focusing and storing electrode  14  to a certain quantity after a period of time, and then the electric potential of the focusing and storing electrode  14  jumps to the pushing state. The ions generated by corona discharge at the first electrode  12  stops entering the focusing and storing electrode  14  to prevent a fluctuation in the quantity of the ions in the focusing and storing electrode  14  due to the corona pulse. The ions in the focusing and storing electrode  14  quickly enter the drift electrode  16  through the second grid electrode  15  under the action of an electric field force between the focusing and storing electrode  14  and the second grid electrode  15 . In the drift electrode  16 , the ions reach a stable motional state under the action of both the drag force of the electric field and a drift gas flow moving in a reverse direction. After experiencing a long drift distance, the ions with different mobility are separated from each other due to their different speeds, and finally are received by the Faraday plate  18  after passing through the aperture grid  17 . 
     As shown in  FIGS. 1 and 7 , the focusing and storing electrode  14  and the first grid electrode  145  of the ion door form a combinatorial electrode. The electric potential  140  of the focusing and storing electrode  14  and the first grid electrode  145  of the ion door periodically jumps. The focusing and storing electrode  14  and the first grid electrode  145  of the ion door may be in the storage state and the pushing state depending upon the electric potential  140 . When the focusing and storing electrode  14  and the first grid electrode  145  of the ion door are in the storage state (as shown by the solid line indicated by the reference numeral  140  in  FIG. 7 ), the direction of the electric field between the first electrode  12  and the focusing and storing electrode  14  and the first grid electrode  145  of the ion door, and a moving direction of ions are the same. Since a diameter of the end  143  of the focusing and storing electrode  14  is less than the diameter of the first inner chamber portion  530  of the first electrode  12 , a drifting and focusing electric field region is formed. With the drifting and focusing electric field region, the ions generated by corona are effectively dragged away from the corona region and focused into a smaller beam spot to enter the focusing and storing electrode  14 . Because the focusing and storing electrode  14  and the first grid electrode  145  of the ion door are at the same electric potential and the internal electric field of the focusing and storing electrode  14  is weak, plus a weak backward electric field is exerted across the second grid electrode  15  of the ion door and the focusing and storing electrode  14  and the first grid electrode  145 , a substantially equipotential region is formed at least inside the focusing and storing electrode  14  near the first grid electrode  145  of the ion door. The ions are not subjected to an electric field and main action of the ions is thermal motion in the region after the ions enter the focusing and storing electrode  14  and travel a distance. The large chamber at the end  147  of the focusing and storing electrode  14  also ensures the ions perform thermal motion and do not collide with the focusing and storing electrode  14 , thereby the loss of the ions is decreased. After the ions in thermal motion accumulate to a certain quantity, however, the electric potential of the focusing and storing electrode  14  jumps to a drift state (as shown by the dashed line indicated by the reference numeral  140  in  FIG. 7 ). As a result, the electric field between the first electrode  12  and the focusing and storing electrode  14  has a direction opposite to the moving direction of ions to prevent the ions generated by corona from entering the focusing and storing electrode  14 , while the electric field between the second grid electrode  15  of the ion door and the focusing and storing electrode  14  and the first grid electrode  145  of the ion door is changed to have the same direction as the moving direction of ions. The electric potentials  160  of the circular ring-shaped electrodes  16  in the drift region  28  vary with an equal difference to form a drag electric field. The ions accumulated in the focusing and storing electrode  14  are quickly dragged into the drift region  28  by exerting a strong forward electric field by the ion door, so that the ions move towards the Faraday plate  18  at an electric potential  180 , through the aperture grid  17  at an electric potential  170 . As a result, the influence of variation in an ion quantity, which is caused by the corona discharge pulse, on mobility spectrum lines is weakened by an accumulation process of ions before the ion door so that the mobility spectrum lines can remain substantially stable under the corona discharge pules. 
     Referring to  FIGS. 1 and 7 , in the corona discharge device  50  as a corona discharge ion source, generally a voltage of about 700-3000V can be exerted across the first electrode  12  and the second electrode  11  to generate corona discharge. In other words, generally an electric potential difference between the electric potential  110  of the second electrode  11  and the electric potential  120  of the first electrode  12  may be about 700-3000V. Since the second electrode  11  is inserted in the first electrode  12  perpendicularly to the gas flow direction A of the carrier gas, the corona electric field is perpendicular to the gas path within the first electrode  12 , thereby weakening interference (especially impulsive interference) from the corona electric field to the electric fields of the following components. In addition, several electrodes are inserted in a view to improving an ion concentration. After one of the second electrodes  11  is degraded in performance due to oxidation, the ionization property will not be caused to remarkably decrease. The first portion  53  of the first electrode  12  may have a circular cylinder shape to achieve symmetric high electric field between the first portion  53  and the second electrode  11 . The second portion  55  of the first electrode  12  has a trumpet shape to allow the focusing and storing electrode  14  to be located closer to the corona ionization region, and to form a focusing electric field between the second portion  55  and the focusing and storing electrode  14 . An electric potential difference between the second electrode  11  and the first electrode  12  retains close to a corona-starting voltage to avoid increasing of charge density in the ionization region, avoid generation of a great deal of molecular fragments of a sample, and lengthen a lifetime of the second electrode  11 .