Patent Publication Number: US-6667474-B1

Title: Capillary tube assembly with replaceable capillary tube

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates generally to a capillary tube assembly which connects an atmospheric pressure ionization chamber to a lower pressure mass analyzer assembly, and more particularly to a capillary assembly having a removable capillary tube. 
     BACKGROUND OF THE INVENTION 
     When an atmospheric pressure ionization (API) source such as an electrospray (ES) or atmospheric pressure ionization (APCI) source is installed on a mass spectrometer system, the gas flow from the atmospheric pressure ionization chamber into the vacuum system of the mass spectrometer must be set to match the pumping capacity of the vacuum pumps used. Small diameter orifices and capillary tubes are the two methods most often used to limit the gas flow from the atmospheric pressure spray chamber into the vacuum system of the mass spectrometer. These capillary tubes are often heated to provide thermal energy to the solvated ions passing through them, thus desolvating these ions. 
     Non-volatile material from the samples being analyzed by the mass spectrometer system can accumulate at the entrance or on the inner bore of these capillary tubes. These non-volatile materials can be salts from the liquid mobile phase being sprayed into the atmospheric pressure ionization source. They can also be proteins, lipids or salts, remaining in the sample solution after extraction from biological fluids such as plasma or urine. The accumulation of non-volatile material on these capillary tubes can lead to a reduced transfer of ions and reduction of signal. This requires the user to clean the capillary tube or replace it with a new one. Capillary tube maintenance or replacement typically occurs every several months for complex samples, and yearly for clean samples. Whatever the frequency of maintenance, the fact remains that the system is vented. The time for maintenance and bringing the mass spectrometer system back online takes approximately one day. This means a low sample throughput which translates into loss of revenue for a laboratory. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a heated capillary assembly in which the capillary tube can be easily and quickly removed from its heater for inspection, cleaning and replacement. 
     It is another object of the present invention to provide a heated capillary tube assembly with a replaceable capillary tube in which venting of the mass analyzer system is restricted during capillary tube removal and replacement. 
     These and other objects of the invention are achieved by a system in which an atmospheric pressure ionization chamber is connected to a lower pressure region of a mass analyzer via a capillary tube assembly having a removable capillary tube and to a system which inhibits venting of the lower pressure region during removal and replacement of the capillary tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of the invention will be more clearly understood from the following detailed description when read in conjunction with the accompanying drawings in which: 
     FIG. 1 shows an API probe coupled to a mass spectrometer via a capillary tube assembly in accordance with the prior art. 
     FIG. 2 shows a capillary assembly in accordance with one embodiment of the present invention. 
     FIG. 3 is an enlarged view of the region  3 — 3  of FIG.  2 . 
     FIG. 4 is an enlarged view of the end portion of a capillary tube assembly including a flap for inhibiting venting of the mass analyzer system. 
     FIG. 5 is a partial view of the input end of a capillary assembly in accordance with another embodiment of the invention. 
     FIG. 6 is an enlarged view of the capillary assembly in accordance with still another embodiment of the invention. 
     FIG. 7 is an enlarged view of a capillary assembly in accordance with a further embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT(S) 
     Referring to FIG. 1, a prior art mass spectrometer with an ionization source having probe  11  is illustrated coupled to a mass analyzer  12  by an ion transmission assembly. It is apparent to one skilled in the art that the ion source can be operated at pressures ranging from below atmospheric pressure to above atmospheric pressure. Although a quadrupole mass analyzer  12  is illustrated, it will be apparent to those skilled in the art that the mass analyzer may include, and is not limited to, time of flight (TOF), quadrupole, Fourier transform (FTMS), ion trap, magnetic sector or hybrid mass analyzers. By way of example, the ion source may be an atmospheric pressure ion source (API). More particularly, the ion source may comprise an electrospray ion source (ES) or atmospheric pressure chemical ionization source (APCI). In any event, the source includes an ion probe  11  which forms an ion spray  13 . The ionization mechanism involves the desorption at atmospheric pressure of ions from the fine electrically-charged particles formed by the ES or APCI probe. 
     The sample liquid may be delivered to the probe  11  by, but is not limited to, liquid chromatography pumps, syringe pumps, gravity-feed vessels, pressurized vessels and/or aspiration-feed vessels. Samples may also be introduced using auto injectors, separation systems such as liquid chromatography or capillary electrophoresis, capillary electrophoresis chromatography and/or manual injection valves connected to the API probe. 
     The ion transmission assembly includes successive chambers  16 ,  17  and  18 , maintained at successively lower pressures with the mass analyzer  12  in the lowest pressure chamber. The first chamber  16  communicates with the atmospheric pressure ionization chamber  21  via a capillary tube  22 . Due to the potential at the end of the capillary tube, ions are caused to travel to the capillary tube where the difference in pressure between the chambers  16  and  21  cause ions and gases to enter the orifice  23  of the capillary tube and flow through the capillary passage into the chamber  16 . The other end of the capillary is opposite a skimmer  31  which separates the chamber  16  from the chamber  17  which houses an ion guiding octopole lens assembly  32 . A tube lens  36 , as described in U.S. Pat. No. 5,157,266 cooperates with the end of the capillary to force ions into the center of the expanding ion flow which leaves the capillary and travels toward the skimmer  31 . The octopole lens assembly  32  is followed by ion optics which may comprise a second skimmer  34  and lens  35  which direct ions into the analyzing chamber  18  and into a suitable mass analyzer  12 . The combination of capillary tube  22 , skimmer  31 , lens  32 , skimmer  34  and lens  35  form the ion transmission assembly. Although a particular ion transmission system is described, ions from the capillary can be guided into a mass analyzer by other ion guiding systems. 
     As discussed above, non-volatile materials from the sample can accumulate at the entrance or the inner bore of the capillary tube. These non-volatile materials can be salts from the liquid mobile phase being sprayed into the atmospheric pressure chamber  21 . They can also be proteins, lipids or salts remaining in the sample solution after extraction from biological fluids such as plasma or urine. The accumulation of these non-volatile materials on or in the capillary tube can lead to reduced signal, which requires the user to clean the capillary tube or replace it with a new one. 
     In accordance with the present invention, there is provided a capillary tube assembly, for example a heated capillary tube assembly, which connects the source chamber  21  to the lower pressure region of a mass spectrometer such as the region  16 , FIG.  1 . The capillary tube assembly is constructed such that the capillary tube may be easily removed from its mount for inspection, cleaning or replacing, and then reinserted in an aligned position into the assembly. This can be accomplished with minimum venting of the lower pressure regions thereby considerably reducing the down time of the mass spectrometer system, and increasing the daily throughput. 
     Referring to FIGS. 2 and 3, a heated capillary tube assembly  22  is illustrated. The capillary tube assembly includes a cylindrical heater  41  which is electrically heated via the heater wire  42 . A capillary tube  43  extends axially through the heater. The end of the heater includes a threaded bore portion  44  which receives the cooperating threads of the nut  46 . More particularly, the nut includes a head which has wrench flats  47  to allow a tool to loosen or tighten the nut. A shoulder  48  which abuts against the mating face of the capillary heater precisely determines the distance between the nut and heater, and thus the amount of compression of the sealing o-ring  49 . A compressible fitting  51  is compressed by the nut and urged against a capillary tube and heater to form a seal. Thus, the capillary can be easily removed by unthreading the nut and sliding the capillary along the axial bore of the heater assembly for removal and inspection or replacement. The capillary tube may, for example, be a steel capillary tube which slides into the mating bore of the heater body. The heater body is preferably made of a different metal such as an aluminum or bronze alloy to prevent the capillary tube and heater body from galling or seizing. The threads on the nut may be plated with a different metal such as silver or nickel to prevent galling of the threads to the heater body. Although a steel capillary tube has been described, the tube may be titanium, nickel, coated or lined e.g. glass lined, glass or other type of capillary tube known in the art. Although a heater body has been described for heating the capillary, the capillary tube may be directly resistively heated by applying a current along the tube which is mounted in a cylindrical body. 
     Referring to FIG. 4, the end of the heater assembly may be provided with a flap or seal  52  secured to the heater assembly by, for example, a screw  53 , the flap being made of a resilient material so that when the capillary tube is withdrawn the flap closes the opening at the end of the heater assembly, thereby minimizing venting of high pressure gases into the low pressure adjacent chamber. 
     Referring to FIG. 5, another embodiment of the invention is illustrated. The body  41  is supported by the wall  54  between the atmospheric pressure chamber and the lower pressure chamber by a sleeve  56 . The end of the body is provided with an external thread  57  which is adapted to receive a nut  58 . The capillary tube  43  is inserted axially into the body  41 . A compression fitting or ferrule  61  slides over the capillary, and the inclined surfaces of the ferrule and nut cooperate to compress the ferrule against the capillary tube to seal the tube to provide a seal for the capillary tube. The capillary tube is removed and replaced by unthreading the nut  58 , sliding the capillary out of the body, and then re-inserting the cleaned capillary tube or a replacement capillary tube and tightening the nut. 
     FIG. 6 shows an alternate sealing assembly in which the interior bore of the heater body  41  is inclined  62  to receive the compressible sealing ring or ferrule  63  which is compressed against the capillary by tightening the nut  64 . 
     FIG. 7 shows another embodiment of the invention in which a wall  71  and shield  72  support a heated capillary assembly. The heater body  73  has one end  74  supported by the shield  72  and its central portion supported by the wall  71 . A temperature sensor  76  is held against the heater body by spring  77 . The heater body receives a capillary tube  78 . The capillary tube is pressed against the heater body by a spring assembly  79 . The end of the capillary tube is secured to a nut or fitting  81  having external threads which are received by the internal threads at the end of the heater  71 . Thus, the capillary tube with its integral fitting  81  is screwed into the heater body and compresses an o-ring  82  to provide a seal. The heater assembly includes a sealing ball  83  which is retained in a well  84  formed in the heater assembly by a spring-loaded fitting  86  secured to the heater by screws  87 . Thus, as the capillary tube is removed, the ball  83  seals the bore of the heater preventing venting of the lower pressure chamber. 
     Although several arrangements for securing the capillary to the end of a support assembly or a heated assembly have been described, it is apparent that other attachments such as bayonet-type fittings or snap-on fittings may be used. 
     There has been provided a capillary tube assembly in which the capillary tube can be easily removed for cleaning or replacement. 
     The foregoing descriptions of specific embodiments of the present invention are presented for the purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.