Patent Publication Number: US-2021172913-A1

Title: Micro gas chromatography system

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a micro gas chromatography system. More specifically, the present disclosure relates to a thermal desorption unit and a column module in a micro gas chromatography system. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Gas chromatography (GC) is widely used for separating and analyzing fluid compounds. A typical GC system may include a thermal desorption unit (also referred to as a “preconcentrator”) that concentrates a fluid sample, such as a volatile organic compound, a column module that separates the concentrated fluid samples into various fluid components, and a detector that analyzes the various fluid components. 
     Recently, miniaturized and portable GC systems, such as micro GC systems, have been developed for applications such as on-site environmental monitoring. In these micro GC systems, it is desirable for every component to be compact in size. Developing components that are compact in size can present unique design challenges. 
     SUMMARY 
     According to one aspect of the present disclosure, a thermal desorption unit is provided. The thermal desorption unit includes a tube, an adsorbent material including one material or a combination of several materials disposed inside the tube, holding members disposed inside the tube and configured to hold the adsorbent material in the tube, and a heating wire coiled around the tube and configured to generate heat along the tube. 
     According to another aspect of the present disclosure, a column module is provided. The column module includes a capillary column, a heating wire coiled around the capillary column, a temperature sensor configured to monitor the temperature of the capillary column, and an electrical insulating layer disposed around the capillary column and the heating wire. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate example embodiments and, together with the description, serve to explain the principles of the embodiments. In the drawings: 
         FIG. 1A  is a schematic illustration of a gas chromatograph (GC) system in a sampling operation, according to one embodiment of the present disclosure. 
         FIG. 1B  a schematic illustration of a GC system in an analyzing operation, according to one embodiment of the present disclosure. 
         FIG. 2  is a schematic illustration of a thermal desorption unit (TDU), according to some embodiments of the present disclosure. 
         FIG. 3  is a flow chart of a method of assembling a TDU, according to some embodiments of the present disclosure. 
         FIGS. 4, 5, and 6  are schematic illustrations of a TDU during various stages of assembly, according to some embodiments of the present disclosure. 
         FIGS. 7A, 7B, and 7C  are schematic illustrations of a column module, according to some embodiments of the present disclosure. 
         FIGS. 8A and 8B  are schematic illustrations of a column module in an assembled state, according to some embodiments of the present disclosure. 
         FIGS. 9A, 9B, and 9C  are schematic illustrations of a two-piece case of a column module, according to some embodiments of the present disclosure. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of drawings. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of devices and methods consistent with aspects related to the appended claims. 
     A micro GC system may include a micro thermal desorption unit (TDU) that concentrates a fluid sample, a column module that separates the concentrated fluid samples into various fluid components, and a detector that analyzes the various fluid components. 
     For example, the micro TDU may collect the fluid sample, such as one or more volatile organic compounds, onto an adsorbent material, while a high vapor pressure gas like oxygen, nitrogen, and carbon dioxide passes through it. After collecting the volatile organic compounds, the micro TDU may be heated by electrical resistive heating and a carrier gas may flow through the micro TDU to release the compounds and concentrate them into a smaller volume. 
     An increased collection time may be directly correlated to further increased performance of the micro GC. In other words, higher sensitivities may be achieved by collecting target vapors for a longer time in the micro thermal desorption unit. 
     Traditionally, two types of TDUs are used in a micro GC. A first typical TDU includes a metal or glass tube packed with adsorbent materials and is connected to external heating equipment. In order to analyze volatile organic compounds, the metal or glass tube packed with the adsorbent materials is taken to field sites to collect the analytes. Then the TDU is brought back to a laboratory and connected to external heating equipment to desorb the analytes into a GC system, such as a bench-top GC system. Because the first typical TDU needs to be connected to external heating equipment before analysis is performed using the bench-top GC system, it cannot be used in real-time for on-site (e.g., on the site where the volatile organic compounds to be analyzed) continuous sampling and analysis. 
     A second typical TDU is formed by microfabrication on a substrate, such as a glass substrate or a silicon wafer. A resistance heater is formed on an opposite side of the substrate. The second typical TDU does not need any external heating equipment before analysis using the bench-top GC system, and therefore it can be used in real-time on-site continuous sampling and analysis. However, the microfabrication of the second typical TDU usually requires special equipment and complicated fabrication process. In addition, the material (glass or silicon wafer) of the substrate is fragile, which impairs the durability of the second typical TDU. A polymer adhesive is used to seal the second typical TDU, but such an adhesive is unreliable and may contaminate the gas line formed in the TDU. 
     Embodiments of the present disclosure address one or more disadvantages associated with the above-described typical TDUs. According to one embodiment, a compact TDU may include a tube packed with adsorbent materials and a heating wire coiled around the tube. Since the TDU integrates the tube and the heating wire into a compact device, there is no need to connect the TDU to external heating equipment before analysis. As a result, the TDU of the embodiments of the present disclosure may be integrated into a GC system, especially in a portable micro GC system, for on-site continuous sampling and analysis. 
     According to one embodiment, in the TDU, two gas flow columns may be coupled to opposite ends of the tube, respectively. In addition, two connectors may be disposed around the opposite ends of the tube, respectively, to seal a gap between the tube and the gas flow columns, such that no gas will leak through the gap between the tube and the gas flow columns. Therefore, the TDU according to the embodiments of the present disclosure has a relatively small dead volume. The relatively small dead volume may improve fluidic flow, thereby making the signal peak much sharper than those devices using conventional tube preconcentrators. 
     According to one embodiment, in the TDU, the heating wire, which is coiled around the tube, may be electrically connected to an electrical power source. When the electrical power source applies electrical power to the heating wire, the heating wire may generate heat rapidly. The TDU may have a relatively low thermal mass and a high heating efficiency. As a result, the tube and the adsorbent material disposed inside the tube may be heated rapidly. For example, the adsorbent material may be heated up to 270° C. within 0.3 seconds. Consequently, the adsorbent material disposed inside the tube may release the collected volatile organic compounds in a relatively short amount of time, such as, in less than 0.5 seconds. 
     Traditionally, a GC system may include a column part that separates a fluid sample. When the fluid sample to be analyzed enters the column part, the column part is heated by a traditional oven to increase temperature. This increase in temperature causes the fluid sample to separate into various fluid components. The fluid components may then successively emerge from the column part and enter a detector. However, the oven is usually large in size, making the column part unsuitable for use in a miniaturized, portable gas analytical system, such as a micro gas chromatography (GC) system. 
     In addition, the column part includes a capillary column which is configured to separate a targeted compound (e.g., a fluid sample) for a targeted application. When the application of the column part changes and the targeted compound changes to a different targeted compound, the capillary column needs to be replaced with a new capillary column configured to separate the different targeted compound. Because of the small dimension of the capillary column, the capillary column is fragile and not suitable for being handled. These factors make it difficult to replace the capillary column. 
     Embodiments of the present disclosure address one or more disadvantages associated with the above-described column part. According to one embodiment, a column module may include a capillary column and a heating wire coiled around the capillary column. The heating wire may be supplied with electrical power to heat the capillary column. In this manner, the traditional oven may be removed to save space and power consumption, without compromising the temperature control capability, making the column module suitable for use in a GC system, especially in a portable GC system. 
     According to one embodiment, the column module may include a case that encloses the capillary column. The case may be formed with standard connectors to be connected to other components in a GC system. The standard connectors may be commercially available connectors that are commonly used in GC systems or other gas analytic systems. The standard connectors may include standard gas flow connectors that fit most component in the GC system. When the capillary column needs to be replaced for a different application, the capillary column does not require direct handling. Instead, the standard connectors may be disconnected from other components in the GC system, and the entire column module may be replaced with a different column module. As a result, it is easy to replace the column module with one designed for different targeted compounds. In addition, the case may protect the capillary column, making the column module a reliable unit. 
       FIGS. 1A and 1B  are schematic illustrations of a gas chromatograph (GC) system  100 , according to one embodiment of the present disclosure.  FIG. 1A  illustrates the GC system  100  when it is performing a sampling operation, according to one embodiment of the present disclosure.  FIG. 1B  illustrates the GC system  100  when it is performing an analyzing operation, according to one embodiment of the present disclosure. 
     As shown in  FIGS. 1A and 1B , the GC system  100  may include a thermal desorption unit (TDU)  110  (which may also be referred to as a “preconcentrator”), a six-port valve  120 , a pump  130 , a sample inlet  140 , a carrier gas inlet  150 , a column module  160 , and a photoionization detector  170 . The sample inlet  140  may be used to introduce a fluid sample into the GC system  100 . The fluid sample may include gases, vapors, liquids, and the like. For example, the fluid sample may be volatile organic compounds (VOCs). The carrier gas inlet  150  may be used to introduce a carrier gas into the GC system  100 . For example, the carrier gas may be an inert gas. 
     In the embodiment shown in  FIG. 1A , during the sampling operation, the six-port valve  120  may be configured to connect the thermal desorption unit  110  with the pump  130  and the sample inlet  140 , and to disconnect the TDU  110  from the carrier gas inlet  150  and the column module  160 . When the pump  130  starts pumping, a fluid sample may enter the TDU  110  through the sample inlet  140 . The TDU  110  may collect and concentrate the fluid sample. 
     In the embodiment shown in  FIG. 1B , during the analyzing operation, the six-port valve  120  may be configured to connect the TDU  110  with the carrier gas inlet  150  and the column module  160 , and to disconnect the TDU  110  from the pump  130  and the sample inlet  140 . A carrier gas may be introduced into the TDU  110  through the carrier gas inlet  150 . The carrier gas may carry the fluid sample collected in the TDU  110  into the column module  160 . The column module  160  may separate the fluid sample into various fluid components having different retention times. The fluid components may then successively emerge from the column module  160  and enter the PID  170  according to their respective retention times. 
     Descriptions related to a thermal desorption unit (TDU) according to embodiments of the present disclosure will be provided below with reference to  FIGS. 2-6 . 
       FIG. 2  is a schematic illustration of a thermal desorption unit (TDU)  200 , according to some embodiments of the present disclosure. The TDU  200  may be an example implementation of the TDU  110  included in the GC system  100  in the embodiment illustrated in  FIGS. 1A and 1B . 
     As shown in  FIG. 2 , the TDU  200  may include a tube  210 , an adsorbent material  220  disposed inside the tube  210 , holding members  230  disposed inside the tube  210  and at opposite ends of the adsorbent material  220 , an electrical insulating layer  240  disposed on an external surface of the tube  210 , a heating wire  250  coiled around the tube  210 , a housing  260  defining an inner space where the tube  210  is disposed, and two gas flow columns  270  coupled to opposite ends of the tube  210 . For example, the tube  210  may include a first opening and a second opening at opposite ends of the tube  210 , and the two gas flow columns  270  may include a first gas flow column  272  and a second gas flow column  274 . The first gas flow column  272  and the second gas flow column  274  may be inserted into the first and second openings of the tube  210 , respectively. Two connectors  280  may be disposed around the opposite ends of the tube  210  to seal the opposite ends of the tube  210  with the respective gas flow columns  270 , such that no gas will leak from the tube  210 . 
     When a fluid sample enters the TDU  200  via the first gas flow column  272 , the adsorbent material  220  may adsorb the fluid sample. When an electric power is applied to the heating wire  250 , the heating wire  250  may generate heat along the tube  210  to heat the tube  210  and the adsorbent material  220  contained in the tube  210 . As a result, the adsorbent material  220  may desorb the fluid sample, and release the fluid sample through the second gas flow column  274 . 
     The tube  210  may be formed of a heat conductive and corrosion resistive material. In one embodiment, the tube  210  may be formed of stainless steel. The tube  210  may be configured to have an inner diameter slightly larger than an outer diameter of the gas flow columns  270  in order to achieve gas-tight sealing and minimum dead volume. The tube  210  may be configured to have a relatively thin wall in order to achieve fast thermal conduction. In one embodiment, the tube  210  may have an inner diameter (ID) of 0.58 mm and an outer diameter (OD) of 0.81 mm. 
     The adsorbent material  220  may be disposed inside the tube  210 . The adsorbent material  220  may include one material or a combination of several materials that are capable of adsorbing a fluid sample (e.g., VOC) at a room temperature and desorbing the fluid sample at a high temperature. In some embodiments, the adsorbent material  220  may be formed as adsorbent beads. In addition, the adsorbent material  220  may be a commercially available adsorbent material, such as activated carbon, carbon black, carbon molecular sieve, etc. For example, the adsorbent material  220  may be Carbopack X provided by Supelco, or Tenax TA™. 
     The holding members  230  may be disposed inside the tube  210  and at opposite ends of the adsorbent material  220 , respectively. That is, the holding members  230  may include a first member disposed at a first end of the adsorbent material  220  and a second member disposed at a second end of the adsorbent material  220 . The holding members  230  may be configured to hold the adsorbent material  220  in the tube  210  and prevent the adsorbent material  220  from entering the gas flow columns  270 . In one embodiment, the holding member  230  may be glass wool. 
     The electrical insulating layer  240  may be disposed on the external surface of the tube  210  between the tube  210  and the heating wire  250 . The electrical insulating layer  240  may be formed of an electrical insulating material. When the tube  210  is formed of stainless steel, if the electrical insulating layer  240  is not present between the stainless steel tube  210  and the heating wire  250 , a short circuit might occur between the stainless steel tube  210  and the heating wire  250 . In contrast, in the embodiments of the present disclosure, the electrical insulating layer  240  may electrically separate the stainless steel tube  210  and the heating wire  250 , thus preventing a short circuit between the stainless steel tube  210  and the heating wire  250 . In some embodiments, the electrical insulating material for forming the electrical insulating layer  240  may be ceramic adhesive, which may be coated on the external surface of the tube  210 . 
     The heating wire  250  may be coiled around the tube  210 . The heating wire  250  may be a resistance heating wire electrically connected to an external electrical power source. When the external electrical power source supplies an electrical power to the heating wire  250 , the heating wire  250  may generate heat along the tube  210  and heat the tube  210  and the adsorbent material  220  disposed inside the tube  210 . If a fluid sample is adsorbed in the adsorbent material  220 , the adsorbent material  220  may desorb the fluid sample and release the fluid sample through the second gas flow column  274 . For example, the heating wire  250  may be a commercially available product, such as Nichrome 80 resistance wire provided by K.Bee Vapor. The Nichrome 80 resistance wire is comprised of 80% Nickel and 20% Chromium, and has a diameter of 0.51 mm. 
     In an alternative embodiment, the heating wire  250  may be formed with an electrical insulating layer on its external surface. In this case, the heating wire  250  may be directly coiled around the tube  210  without the electrical insulating layer  240  disposed between the tube  210  and the heating wire  250 . 
     The housing  260  may be configured to enclose the tube  210 . The housing  260  may be formed of a metal, such as, for example, aluminum (Al). The housing  260  may have any shape and any size that provides an inner space in which tube  210  can be disposed. In one embodiment, the housing  260  may be a right prism (e.g., a square prism) or a cylinder extending in a direction parallel with an extending direction of the tube  210 . The housing  260  may include through holes. End portions of heating wire  250  may extend through the through holes and connect heating wire  250  with an external electrical power source. In addition, the housing  260  may include opening at opposite ends and the gas flow columns  270  may extend through the openings and connect to other components in a GC system. 
     The two gas flow columns  270 , including the first gas flow column  272  and the second gas flow column  274 , may be configured to fluidly connect the tube  210  with other components in a GC system (e.g., the GC system  100  illustrated in  FIGS. 1A and 1B ) in order to transfer a fluid sample into and out of the tube  210 . For example, during a sampling operation of the GC system, the first gas flow column  272  may be connected between the tube  210  and a sample inlet (e.g., the sample inlet  140  in the GC system  100 ) to transfer a fluid sample from the sample inlet into the tube  210 . In addition, during the sampling operation of the GC system, the second gas flow column  274  may be connected between the tube  210  and a pump (e.g., the pump  130  in the GC system  100 ), to transfer the fluid sample from the tube  210  to the pump. The gas flow columns  270  ( 272  and  274 ) may be formed of stainless steel and may be inserted into two opposite openings of the tube  210 , respectively. In some embodiments, the inner surface of the gas flow columns  270  may be treated or coated with a layer to prevent a reaction between the fluid sample with the stainless steel gas flow columns  270 . For example, the gas flow columns  270  may be formed of a commercially available GC metal column, such as a Hydroguard-Treated MXT Guard/Retention Gap Column provided by Restek. The gas flow columns  270  may have an inner diameter (ID) of 0.28 mm and an outer diameter (OD) of 0.56 mm. 
     The two connectors  280  may be disposed around opposite ends of the tube  210 , respectively, to connect and tightly seal a gap between the tube  210  and the gas flow columns  270 . In some embodiments, each one of the connectors  280  may be a commercially available internal union connector that includes three parts: a body, a nut, and a ferrule. An inner diameter (ID) of the internal union connector may be larger (e.g., slightly larger) than an outer dimeter of the tube  210 . For example, the ID of the internal union connector may be 0.82 mm. The connectors  280  may have any other appropriate structure as long as they can seal the gap between the tube  210  and the gas flow columns  270  such that no fluid leaks out of the tube  210 . The present disclosure does not limit the structure of the connectors  280 . 
       FIG. 3  is a flow chart of a method  300  of assembling the TDU  200  illustrated in  FIG. 2 , according to some embodiments of the present disclosure.  FIGS. 4, 5, and 6  are images of an exemplary TDU during various stages of assembly, according to some embodiments of the present disclosure. 
     As shown in  FIG. 3 , in step  310 , the adsorbent material  220  and the holding members  230  may be loaded into the tube  210 . Next, the tube  210  loaded with the adsorbent material  220  and the holding members  230  may be connected with the gas flow columns  270 . Then, the tube  210  may be sealed by the connectors  280 .  FIG. 4  is a schematic illustration showing the tube  210  connected with two columns  270  at opposite ends of the tube  210 , and that the tube  210  is sealed by two connectors  280 . 
     Referring back to  FIG. 3 , in step  320 , an external surface of the tube  210  may be coated with the electrical insulating layer  240 , and then the heating wire  250  may be coiled around the tube  210 .  FIG. 5  is a schematic illustration showing the tube  210  in  FIG. 4  coiled with the heating wire  250 . 
     In an alternative embodiment, the heating wire  250  may be formed with an electrical insulating layer on its external surface. In this case, heating wire  250  may be directly coiled around the tube  210  without the electrical insulating layer  240  disposed between the tube  210  and the heating wire  250 . Thus, in this case, step  320  may only include coiling the heating wire  250  around the tube  210 . 
     Referring back to  FIG. 3 , in step  330 , the tube  210  coiled with the heating wire  250  and connected with the gas flow columns  270  may be placed in the housing  260 . End portions of the heating wire  250  may extend through the through holes formed on the housing  260  and may be connected to the electrical power source via power cables  610 . In addition, end portions of the gas flow columns  270  may extend through the openings formed at opposite ends of the housing  260 . The end portions may be connected with nuts and ferrules  620  in order to connect the end portions to other components in a GC system.  FIG. 6  is a schematic illustration showing a housing in which the tube in  FIG. 5  is disposed. 
     Descriptions related to a column module according to embodiments of the present disclosure will be provided below with reference to  FIGS. 7-9 . 
       FIGS. 7A, 7B, and 7C  are schematic illustrations of a column module  700 , according to some embodiments of the present disclosure.  FIG. 7B  is a schematic illustration of certain components of the column module  700 .  FIG. 7A  is an enlarged schematic illustration of a portion of the column module  700 .  FIG. 7C  is a schematic illustration of a case of the column module  700 . The column module  700  may be an example implementation of the column module  160  included in the GC system  100  in the embodiment illustrated in  FIGS. 1A and 1B . 
     As shown in  FIG. 7 , the column module  700  includes a capillary column  710 , a heating wire  720 , an electrical insulating layer  730 , a gas inlet  740 , a gas outlet  742 , a temperature sensor  750 , one or more sensor cables  760 , one or more power cables  770 , and a case  780 . 
     A fluid sample carried by a carrier gas may be introduced into the capillary column  710  via the gas inlet  740 . As the fluid sample traverses the capillary column  710 , the fluid sample may be separated into various fluid components having different retention times. The fluid components may then successively emerge from the capillary column  710  according to their respective retention times. 
     The capillary column  710  may be configured to separate the fluid sample into various fluid components having different retention times. An inner surface of the capillary column  710  may be coated with a thin coating layer, and a chemical reaction may occur between the fluid sample and the coating layer. The length of the capillary column  710  may be from 0.1 m to 30 m. The inner diameter (ID) of the capillary column  710  may be from 0.15 mm to 0.53 mm. The thickness (df) of the coating layer may be from 0.1 μm to 10.0 μm. For example, the capillary column  710  may be formed from a Rtx-VMS column provided by Restek, the length of the capillary column  710  may be 6 m, the inner diameter of the capillary column  710  may be 0.25 mm, and the thickness (df) of the coating layer may be 1.4 μm. The column type, length, ID, and df may be determined by many factors including targeted compounds, concentrations, interference compounds, analysis time, and so on, and the present disclosure is not limited to the example described above. 
     The heating wire  720  may be coiled around an outer surface of the capillary column  710 . In some embodiments, the heating wire  720  may be coiled around the entire length of the capillary column  710 . In some alternative embodiments, the heating wire  720  may be coiled around a portion of the capillary column  710 . The heating wire  720  may be connected to an external power source via the power cables  770 . The external power source may be controlled by a temperature controller to supply electric power to the heating wire  720 . The temperature controller may execute a preset heating program to control the power source to supply desired power, such that the heating wire  720  may heat the capillary column  710  to reach a desired temperature. The heating wire  720  may be formed of a resistance heating wire. For example, the heating wire  720  may be formed of Ni200 tempered Nickel wire (32 AWG). 
     The electrical insulating layer  730  may formed around the capillary column  710  and the heating wire  720 . Due to the limited space in a GC system, the combination of the capillary column  710 , the heating wire  720  coiled around the outer surface of the capillary column  710 , and the electrical insulating layer  730  formed around the capillary column  710  and the heating wire  720 , may be wound in several turns to form an assembly. Since there are many rounds of the capillary column  710  and the heating wire  720  wound together, if the electrical insulating layer  730  is not present, a short circuit may occur between different sections of the heating wire  720 . Thus, the electrical insulating layer  730  prevents a short circuit between different sections of the heating wire  720 . 
     The gas inlet  740  and gas outlet  742  may be disposed at opposite ends of the capillary column  710 , respectively. The gas inlet  740  and gas outlet  742  may be configured to connect the capillary column  710  to other components in a GC system (e.g., the GC system  100  illustrated in  FIGS. 1A and 1B ) and allow the fluid sample pass through the capillary column  710  to be separated. For example, the gas inlet  740  may be connected to a valve (e.g., the six-port valve  120  in the GC system  100 ), and the gas outlet  742  may be connected to a detector (e.g., the photoionization detector  170  in the GC system  100 ). 
     The temperature sensor  750  may be disposed contiguous to an outer surface portion of the assembly formed by winding the capillary column  710 , the heating wire  720 , and the electrical insulating layer  730  together. The temperature sensor  750  may be configured to measure the temperature of the capillary column  710 . The temperature sensor  750  may be formed of various type of sensors, such as a thermal couple, a thermistor, and a resistance temperature sensor. For example, the temperature sensor  750  may be formed of a K type thermal couple. The temperature sensor  750  may generate a temperature signal representing the temperature of the capillary column  710  and may transmit the temperature signal to the temperature controller via the sensor cables  760 . Based on the temperature signal, the temperature controller may execute a preset heating program to control the external power source to supply the desired electrical power to the heating wire  720  via the power cables  770 , such that the heating wire  720  may heat the capillary column  710  to reach a desired temperature. 
     In some embodiments, after the assembly including the capillary column  710 , the heating wire  720 , and the electrical insulating layer  730  is wound together, and the temperature sensor  750  is disposed adjacent to the assembly, a thermal conductive layer, such as an Aluminum foil or tape, may be wrapped around the assembly and the temperature sensor  750  to fix the assembly and the temperature sensor  750 , to improve the temperature uniformity, thus forming a composite assembly. 
     The case  780  may enclose the composite assembly to provide protection to the composite assembly. The case  780  may be formed in various shapes and sizes depending on the actual application of the column module  700 . The case  780  may be formed with one or more cooling holes  782  to release the heat of the column module  700  to the outside of the case  780  during a cooling step. An external cooling fan may be directly attached onto the case  780  or may be installed close to the cooling holes  782 . 
     Although not shown in  FIGS. 7A, 7B, and 7C , the case  780  may be formed with a thermal insulating layer on an inner surface of the case  780 . The thermal insulating layer may be configured to maintain the heat of the column module  700  during a heating step, thus saving energy. 
       FIGS. 8A and 8B  are schematic illustrations of a column module  800  in an assembled state, according to some embodiments of the present disclosure.  FIG. 8A  illustrates a front view of an exterior of the column module  800 .  FIG. 8B  illustrates a front view of an interior of the column module  800 . 
     The column module  800  in the embodiment illustrated in  FIGS. 8A and 8B  may include various components of the column module  700  illustrated in  FIGS. 7A-7C , including the capillary column  710 , the heating wire  720 , the electrical insulating layer  730 , the gas inlet  740 , the gas outlet  742 , the temperature sensor  750 , the sensor cables  760 , and the power cables  770 . The properties and the arrangement of these components may be the same as those in the embodiment illustrated in  FIGS. 7A-7C . Therefore, detailed descriptions of these components are not repeated here. 
     In the embodiment illustrated in  FIGS. 8A and 8B , the column module  800  may further include a case  880  that encloses the above-mentioned components, including the capillary column  710 , the heating wire  720 , the electrical insulating layer  730 , the gas inlet  740 , the gas outlet  742 , the temperature sensor  750 , the sensor cables  760 , and the power cables  770 . 
     The case  880  may be formed with standard connectors connected with the gas inlet  740 , the gas outlet  742 , the sensor cables  760 , and the power cables  770 . The standard connectors may be configured to connect various components of the column module  700  to a power source, a controller, and other components, in a GC system, thus allowing for quick installation and replacement of the column module  700 . Specifically, the case  880  may be formed with a first gas flow connector  882  connected with the gas inlet  740 , a second gas flow connector  884  connected with the gas outlet  742 , and a temperature control connector  886  connected with the sensor cables  760  and the power cables  770 . The first gas flow connector  882  may be connected to a component in a GC system that is upstream to the column module  800  (e.g., the six-port valve  120  in the GC system  100 ). The second gas flow connector  884  may be connected to a component in a GC system that is downstream to the column module  800  (e.g., the photoionization detector  170  in the GC system  100 ). The temperature control connector  886  may be connected to an external temperature controller. 
     In some embodiments, the case may be formed in two pieces and may be assembled when other components are disposed inside the case.  FIGS. 9A, 9B, and 9C  are schematic illustrations of a two-piece case  980  of a column module, according to such an embodiment of the present disclosure.  FIG. 9A  illustrates a perspective view of a base part  984  of the case  980 .  FIG. 9B  illustrates a perspective view of a shield part  986  of the case.  FIG. 9C  illustrates a perspective view of the case  980  when the base part  984  and the shield part  986  are assembled together. 
     While illustrative embodiments have been described herein, the scope of the present disclosure covers any and all embodiments having equivalent elements, modifications, omissions, combinations (e.g., of aspects across various embodiments), adaptations and/or alterations as would be appreciated by those skilled in the art based on the present disclosure. For example, features included in different embodiments shown in different figures may be combined. The limitations in the claims are to be interpreted broadly based on the language employed in the claims and not limited to examples described in the present specification or during the prosecution of the application. The examples are to be construed as non-exclusive. It is intended, therefore, that the specification and examples be considered as illustrative only, with a true scope and spirit being indicated by the following claims and their full scope of equivalents.