Abstract:
An analyte pre-concentrator for gas chromatography is disclosed generally comprising a tube having a restricted outlet and packed with an adsorbent, wherein the tube serves as the liner of a chromatographic injector, as an adsorbent trap coupled to a chromatographic column, and/or as an adsorbent trap coupled to a headspace sampler. Preferably, a heating device allows the tube to be heated. In a preferred embodiment, the analyte pre-concentrator further comprises a column isolating accessory so that a chromatographic column can be temporarily isolated from substances flowing through the tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation of copending U.S. patent application Ser. No. 10/954,784, filed Sep. 30, 2004, which is a continuation of U.S. patent application Ser. No. 10/625,259, filed Jul. 22, 2003, now U.S. Pat. No. 6,814,785, which is a continuation of U.S. patent application Ser. No. 10/202,147, filed Jul. 24, 2002, now U.S. Pat. No. 5,944,877. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method and apparatus for pre-concentrating analytes and removing moisture in a sample. More specifically, the invention relates to an adsorbent trap for use with chromatographic columns, chromatographic injectors, and headspace samplers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas or liquid are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern that, by calibration or comparison with known samples, indicates the constituents of the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw et al.  
         [0004]     The value of using a separate, heated device for receiving the sample and subsequently introducing it into the column has long been recognized. One such device is disclosed in U.S. Pat. No. 4,038,053 to Golay, which describes using a chromatographic injector for receiving the sample, heating it, and subsequently injecting it into a chromatographic column. Such a device is desired because higher sample equilibrium temperatures can result in much larger chromatographic peaks. A disadvantage of such devices, however, is that such temperature increases may also increase the concentration of other material that detrimentally affects the sensitivity of the system, such as water.  
         [0005]     To remedy this problem, numerous assemblies have been suggested to pre-concentrate analytes in a sample and remove moisture therefrom prior to introducing the sample into a chromatographic column. For example, U.S. Pat. No. 5,612,225 to Beccanti et al, U.S. Pat. No. 4,245,494 to Legendre et al, and U.S. Pat. No. 2,813,010 to Hutchins disclose a means for removing water from a sample prior to introducing the sample into a chromatographic column by first passing the sample through an anhydrous substance, which absorbs the water. However, because the anhydrous substance absorbs the water, rather than adsorbing the analyte and allowing the water to be purged from the system, repeated use of the anhydrous substance is likely to be limited and require frequent replacement.  
         [0006]     Several assemblies have been suggested which utilize an adsorbent trap, which adsorbs the analytes while allowing water to pass through. For example, U.S. Pat. No. 6,223,584 to Mustacich et al discloses the use of an adsorbent trap in a pre-concentrator assembly for pre-concentrating analytes in a sample prior to introducing the sample into a chromatographic column, which device comprises a tube containing an adsorbent material. However, a disadvantage of this arrangement is the dead volume that exists between the adsorbent bed and the chromatographic column, which is undesirable because, at typical column flow rates, excessive peak broadening results.  
         [0007]     U.S. Pat. No. 5,814,128 to Jiang et al discloses the use of an adsorbent trap in conjunction with a separate water management device, which device removes water from a sample prior to entry into a chromatographic column via the swirling motion caused by a threaded (or other non-smooth geometrically shaped) bore in the device. Similarly, U.S. Pat. No. 4,293,316 to Block discloses the use of an adsorbent trap in conjunction with a membrane separator device, which device removes water from a sample prior to entry into a gas analyzer. However, rather than optimizing the utility of the adsorbent trap itself as a means for analyte pre-concentration and moisture elimination, these assemblies each require a separate device in addition to the trap, which not only creates additional manufacture and maintenance costs, but also does not solve the aforementioned problem of excessive volume between the adsorbent bed and the chromatographic column.  
         [0008]     One means of introducing a sample containing an analyte into a chromatographic column is known as “headspace sampling.” In conventional headspace sampling, sample material is sealed in a vial and subjected to constant temperature conditions for a specified time. Analyte concentrations in the vial gas phase should reach equilibrium with the liquid and/or solid phases during this thermostatting time. The vial is subsequently pressurized with carrier gas to a level greater than the “natural” internal pressure resulting from thermostatting and equilibration. Then the pressurized vial is connected to the chromatographic column in such a way as to allow for the transfer of a portion of the vial gas phase into the column for a short period of time. Such a system is disclosed in U.S. Pat. No. 5,711,786 to Hinshaw, which describes using a chromatographic injector between the vial and the chromatographic column. However, the use of such devices presently known in the art, including chromatographic injectors, for headspace applications results in the same disadvantages previously mentioned for introducing samples into chromatographic columns generally.  
         [0009]     What is desired, therefore, is a method and apparatus for pre-concentrating analytes and eliminating moisture in a sample prior to introducing the sample into a chromatographic column. What is further desired is a method and apparatus for pre-concentrating analytes and eliminating moisture in a sample in a chromatographic injector. What is also desired is a method and apparatus for pre-concentrating analytes and eliminating moisture in a sample in connection with a headspace sampler.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, an object of the present invention is to provide an analyte pre-concentrator for gas chromatography that permits temperature increases without detrimentally affecting the sensitivity of the chromatographic system.  
         [0011]     It is a further object of the present invention to provide an analyte pre-concentrator for gas chromatography that provides a moisture trap permitting moisture to be purged from the chromatographic system so that the trap may be used for multiple injections.  
         [0012]     It is yet another object of the present invention to provide an analyte pre-concentrator for gas chromatography that provides a moisture trap resulting in little or no dead volume between the trap and a chromatographic column, thereby decreasing excessive peak broadening.  
         [0013]     It is still another object of the present invention to provide an analyte pre-concentrator for gas chromatography that provides a moisture trap without the addition of a separate device, thereby decreasing manufacturing and maintenance costs.  
         [0014]     It is still another object of the present invention to provide a method for pre-concentrating analytes in a gas chromatographic system that achieves the objects listed above.  
         [0015]     To overcome the deficiencies of the prior art and to achieve at least some of the objects and advantages listed, the invention comprises a chromatographic injector, a liner located inside the injector, the liner having an inlet and an outlet, wherein the inner diameter of the outlet is smaller than the inner diameter of the inlet, and an adsorbent located inside the liner. Preferably, the injector is temperature controllable.  
         [0016]     In another embodiment, the invention comprises a tube having an inlet and an outlet, wherein the inner diameter of the outlet is smaller than the inner diameter of the inlet, an adsorbent located inside the tube, and a gas chromatographic column coupled to the tube. Preferably, the invention further comprises a heating device for heating the tube.  
         [0017]     In another embodiment, the invention comprises a headspace sampler, a tube coupled to the headspace sampler, the tube having an inlet and an outlet, wherein the inner diameter of the outlet is smaller than the inner diameter of the inlet, and an adsorbent located inside the tube. Preferably, the invention further comprises a heating device for heating the tube.  
         [0018]     In a preferred embodiment, the invention further comprises a column isolating accessory, with which a chromatographic column can be temporarily isolated from the tube or liner.  
         [0019]     The invention also relates to a method comprising the steps of setting the pressures of a main carrier gas inlet and an auxiliary carrier gas inlet such that a column is isolated from substances flowing through a tube, introducing a sample mixture into the tube, such that the mixture passes through an adsorbent, which adsorbs the analytes, and is vented from the chromatographic system, introducing additional carrier gas into the tube, such that moisture is dry purged from the adsorbent, setting the pressures of the main carrier gas inlet and the auxiliary carrier gas inlet such that the column is no longer isolated from substances flowing through the tube, and heating the tube, such that the analytes adsorbed by the adsorbent are desorbed into the column.  
         [0020]     The invention and its particular features will become more apparent from the following detailed description when considered with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a perspective view of a gas chromatographic sampling system including a headspace sampler and a gas chromatograph in accordance with the invention.  
         [0022]      FIG. 2  is an isometric view of a chromatographic injector and a column isolating accessory connected to the bottom of the injector for use with the chromatographic sampling system of  FIG. 1 .  
         [0023]      FIG. 3  is an exposed side view of the chromatographic injector and column isolating accessory of  FIG. 2 .  
         [0024]      FIG. 4  is a perspective view of a tube used as a liner inside chromatographic injectors known in the prior art.  
         [0025]      FIG. 5  is a perspective view of an empty tube used as a liner inside the chromatographic injector of  FIG. 3 .  
         [0026]      FIG. 6  is a side view of the tube of  FIG. 5  containing an adsorbent.  
         [0027]      FIG. 7  is an exposed side view of the gas chromatographic system of  FIG. 1  during the vial pressurization stage.  
         [0028]      FIG. 8  is an exposed side view of the gas chromatographic system of  FIG. 1  during the analyte adsorption stage.  
         [0029]      FIG. 9  is an exposed side view of the gas chromatographic system of  FIG. 1  during the dry purge stage.  
         [0030]      FIG. 10  is an exposed side view of the gas chromatographic system of  FIG. 1  during the analyte desorption stage  
         [0031]      FIG. 11  is an exposed side view of a gas chromatographic system of  FIG. 1  during the cleanup stage. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     The basic components of one embodiment of a gas chromatographic sampling system  10  in accordance with the invention are illustrated in  FIG. 1 . As used in this description, the terms “top,” “bottom,” “upper,” and “lower” refer to the objects referenced when in the orientation illustrated in the drawings, which orientation is not necessary for achieving the objects of the invention.  
         [0033]     In the embodiment depicted in  FIG. 1 , a headspace sampler  12  holds a plurality of vials  14  that contain the sample to be analyzed. The headspace sampler  12  is connected to a gas chromatograph  16  via a transfer line  18 . The basic components of the gas chromatograph are an injector  20 , a chromatographic column  80 , and a detector (not shown).  
         [0034]     The basic components of one embodiment of the injector  20 , an example of which is the Programmed-Temperature Split/Splitless Inlet System (PSS) Injector manufactured by PerkinElmer Instruments LLC, are illustrated in  FIGS. 2 through 3 . A metal sleeve  22  creates a chamber  24  in which a removable tube  26  that serves as the “liner” of the injector  20  is inserted. The sleeve  22  is located inside a heater block  28 , which is governed by a heating element  30 , for heating the chamber  24 , including the liner  26 . The heater block  28  is provided with cooling fins  32 , and a fan  34  may be coupled to a housing  36  for subsequently cooling the chamber  24  and liner  26 .  
         [0035]     Referring to  FIGS. 5 through 6 , the main features and components of the liner  26  are illustrated. Preferably, the liner  26  is glass. The liner  26  has an inlet  70  located at its top and an outlet  72  located at its bottom. The outlet  72  is restricted, such that the bottom of the liner  26  has an inner diameter smaller than that of the top. A glass disk  74  rests at the top of the restricted bottom of the liner. Above the disk  74 , the liner is packed with an adsorbent material  76  capable of adsorbing analytes. Preferably, the adsorbent  76  is hydrophobic. One such adsorbent is graphitized carbon black, such as Carbopack B, manufactured by Supelco. The restricted outlet  72  serves the dual purpose of firmly retaining the adsorbent  76  and significantly reducing the dead volume between the adsorbent  76  and the column  80 . Preferably, a glass/quartz wool plug  78  is located above the adsorbent  76  both to facilitate mixing of the sample vapor and the carrier gas and to serve as a surface on which nonvolatile residues from the sample vapor are deposited, thereby permitting easier cleaning of the liner  26 . As shown in  FIG. 6 , the column  80  may be inserted directly into the bottom of the liner  26 .  
         [0036]     Referring again to  FIGS. 2 through 3 , a threaded collar  40  secures a septum assembly  42  to the housing  36  at the top of the metal sleeve  22 . A septum cap  44  secures a septum  45  to the top of the septum assembly  42 , through which septum a microsyringe  82  may inject the sample.  
         [0037]     The septum assembly  42  has a main carrier gas inlet  46 , the pressure of which can be regulated, which is located above, and separated from the chamber  24  by, an internal seal  48 , such as an o-ring. Additionally, septum assembly  42  has a septum purge outlet  50 . The septum assembly mechanically defines the path of the septum purge flow to prevent cross contamination with the sample flow path.  
         [0038]     Preferably, a “split” vent  52 , for splitting the gas mixture, is located below the internal seal  48  and is in fluid communication with the chamber  24 .  
         [0039]     Preferably, a column isolating accessory  60 , such as the PreVent™ System manufactured by PerkinElmer Instruments, LLC, is interposed between the bottom of the injector  20  and the chromatographic column  80 . The accessory  60  has an auxiliary carrier gas inlet  62 , the pressure of which can be regulated. Additionally, the accessory may have a restrictor tubing (not shown) which is inserted into the bottom of the liner  26  and the top of the column  80  when the accessory is coupled to the bottom of the injector  20 .  
         [0040]     Operation of the above described gas chromatographic system  10  is illustrated stepwise in  FIGS. 7-11 . Beginning in  FIG. 7 , a sampling needle  82  descends into a vial  14 . A headspace sampler carrier gas inlet  84  is open to allow carrier gas to pressurize the vial  14  (indicated by arrows A). The main carrier gas inlet  46  is open and set at a pressure appropriate to introduce carrier gas into the injector  20  (indicated by arrows B), and the auxiliary carrier gas inlet  62  is open and set at a slightly lower pressure than the main carrier gas inlet  46  in order to introduce carrier gas into the injector  20  to isolate the chromatographic column  80  from the injector  20  (indicated by arrows C).  
         [0041]     Referring to  FIG. 8 , the headspace sampler carrier gas inlet  84  is closed, and headspace vapor elutes from the vial  14 , down the transfer line  18 , through the septum  45 , into the injector  20 , and into the liner  26  (indicated by arrows D). Carrier gas entering the injector from the main carrier gas inlet  46  mixes with the sample vapor, which mixing is further facilitated by the glass/quartz wool plug  78 , and helps to carry the sample vapor through the adsorbent  76  and out the restricted outlet  72  (indicated by arrows E), at which point it mixes with carrier gas entering the injector  20  from auxiliary carrier gas inlet  62  (indicated by arrows C), and out the split vent  52  (indicated by arrows Z).  
         [0042]     Referring to  FIG. 9 , the sampling needle  82  is removed from the vial  14  and the headspace sampler carrier gas inlet  84  is re-opened. The carrier gas supplied by the inlet  84  and the main carrier gas inlet  46  flow through the adsorbent  76  and dry purge the water therefrom (indicated by arrows F). Because the auxiliary carrier gas inlet  62  is still set at a pressure appropriate to isolate the chromatographic column  80  (indicated by arrows C), the water and other unwanted substances that exit the restricted outlet  72  are carried along the outside of the liner  26  and out the split vent  52  (indicated by arrows G). A small additional flow passes over the exposed surface of the septum  45 , sweeping volatile contaminants away from the main carrier gas flow, and passes through the septum purge outlet  50  (indicated by arrows H).  
         [0043]     Referring to  FIG. 10 , after the water has been removed, the split vent  52  is closed and the pressure of the main carrier gas inlet  46  is increased to permit flow into the column  80  (indicated by arrows I). The chamber  24 , including the liner  26 , is heated with the heater block  28  so that the analytes adsorbed by the adsorbent  76  are desorbed into the column  80  (indicated by arrows J).  
         [0044]     Referring to  FIG. 11 , after the analytes have been transferred to the column  80 , the pressure of the main carrier gas inlet  46  is decreased to again allow the carrier gas introduced into the injector  20  by the auxiliary carrier gas inlet  62  to isolate the column  80  from the injector  20  (indicated by arrows C). The liner  26  is further heated to drive any remaining unwanted less volatile residue off of the adsorbent  76  and out through the split vent  52  (indicated by arrows K).  
         [0045]     It should be understood that the foregoing is illustrative and not limiting, and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, reference should be made primarily to the accompanying claims, rather than the foregoing specification, to determine the scope of the invention.