Abstract:
The present invention relates to an on-line analyzer for VOCs in gas and method of using the same, comprising: a sampling device ( 1 ), an gas-supply device ( 2 ) and a refrigerating device ( 3 ) connected to the sampling device ( 1 ) and the gas-supply device ( 2 ) respectively; the on-line analyzer for VOCs in gas with two cryo-focuses in empty tubes and two detectors also comprises a gas flow control device ( 4 ) connected to the sampling device ( 1 ) and the gas-supply device ( 2 ) respectively, an analyzing device ( 5 ) connected to the gas flow control device ( 4 ), and a data acquisition and processing device connected to the analyzing device ( 5 ). Therefore, the present invention has a following advantage: the present invention runs automatically and continuously over a long period since it uses electronic refrigeration technique to achieve cryo-refrigeration, instead of using liquid nitrogen refrigeration technique which needs frequent addition of liquid nitrogen and limits application conditions.

Description:
BACKGROUND 
       [0001]    The present invention generally relates to an analyzer for VOCs (Volatile Organic Compounds) in gas and a method of using the same. More particularly, the present invention relates to an on-line analyzer for VOCs in gas and method of using the same. 
         [0002]    Volatile Organic Compounds (VOCs) refer to organic chemical compounds that have boiling points roughly in the range of 50° C.-260° C. and saturation vapor pressures above 133.322 Pa on room-temperature conditions. These compounds commonly exist in indoor and outdoor air, and play an import role in increasing tropospheric ozone and generating other oxidants, and may directly produce secondary pollutants, such as organic aerosols. More significantly, some compounds in components of VOCs, such as benzene, 1,3-butadiene etc, have potentially carcinogenic effect. Therefore, it is brought to the attention of the people and becomes an important point of domestic and foreign research, to study the existence, source, distribution regularity and transformation of VOCs in environment and their effect on human health. It is a necessary means for conducting this research to establish a simple and reliable method of sampling and measuring VOCs that have complicated components and extremely small content. 
         [0003]    Nowadays, the widely-used methods of analyzing VOCs in air include 1) field sampling-laboratory analysis system and 2) field on-line continuous automatic monitoring system. 
         [0004]    The field sampling method of field sampling-lab analyzing system mainly includes bag sampling, stainless steel canister sampling and adsorbent tube sampling. Bag sampling is rarely used now because its inner surface absorbs VOC; stainless steel canister sampling has a well sampling effect, but in this method, it is complex and costly to apply liquid nitrogen to freezingly preconcentrate the sample gas, which needs frequent addition of liquid nitrogen; absorbent tube sampling has some problems, such as low efficiency of absorption and desorption of VOCs, high disturbance of adsorbents and replacement of absorbent tubes and the like. Laboratory analysis methods mainly include gas chromatographic method (GC) (gas chromatographic/hydrogen flame ionization detector/photo-ionization detector) (GC/FID/PID), gas chromatographic/mass-spectrography (GC/MS). In field sampling-laboratory analysis system. The main disadvantage of this system in analyzing VOCs in gas is that it cannot realize real-time continuous automatic monitoring of VOCs in ambient atmospheres and cannot capture the rapid change of concentration of VOCs in air. 
         [0005]    At present, analysis principle of the field on-line continuous automatic monitoring system monitoring system mainly involves absorbent tube sampling (room-temperature/low-temperature)/thermal desorption gas chromatography, and the detectors therefor are hydrogen flame ionization detector (FID) and photoionization detector (PID). The main disadvantages of the on-line analyzing system involve problems existing in absorbent tube sampling, such as absorption/desorption efficiencies of different VOCs and replacement of absorbent tubes, both of the two detectors of FID and PID having no specificity for the qualitative analysis of VOCs and having selectivity for the detecting of different VOCs (FID is only used to detect hydrocarbons. PID cannot detect a portion of hydrocarbons C2 and C3, such as ethane and propane, a portion of halogenated hydrocarbons, such as dichloromethane, chloroform, ethylene dichloride and the like, and compounds containing oxygen (nitrogen, sulphur), such as methanol, acrylonitrile, carbonyl sulfide and the like). Therefore, currently, the on-line analyzer for VOCs is mainly used to analyze hydrocarbons C2-C12, and hydrocarbons C2-C5 and hydrocarbons C6-C12 are detected by two analyzers respectively, instead of simultaneously analyzing and detecting hydrocarbons C2-C12, VOCs containing oxygen (nitrogen, sulphur) and halogenated hydrocarbons. 
         [0006]    At present, the research on VOCs in ambient atmospheres starts late and the sampling and analyzing of VOCs in air relay on foreign large analytical equipments and instruments, which are costly, complicated in analyzing process and unable to perform real-time on-line continuous automatic analysis, which has a large gap from the actual needs of research of air pollution. Therefore, it is important in a field of atmospheric environment monitoring and research, to seek a new sampling method and analyzing technique of VOCs, develop a analyzer of VOCs with high sensitivity, simple operation, cost-efficient and easy maintenance, and develop a on-line continuous automatic monitoring technique of VOCs. 
       SUMMARY 
       [0007]    One aspect of the present invention is directed to solve the problems in existing technology, such as frequent addition of liquid nitrogen required by liquid nitrogen refrigeration technique, limitation of application conditions and automation. The embodiments of the present invention provide an on-line analyzer for VOCs and a method of using the same, that apply electronic cryogenic refrigeration technique and method to simplify the operation procedures of analyzing system, achieve long-term and continuous operation, and facilitate automation, and method of thereof. 
         [0008]    Yet another aspect of the present invention is directed to solve the problems in existing technology that applies absorbent tubes for sampling, such as low absorption efficiency of a portion of VOCs, replacement of absorbent tubes and the like. The embodiments of the present invention provide an long life on-line analyzer for VOCs and a method of using the same, that apply empty capillary column to cryogenic condensation. 
         [0009]    Still another aspect of the present invention is directed to solve the problems in existing on-line monitoring technology that uses a single gas path to sample and uses two instruments to detect hydrocarbons C2-C12. The embodiments of the present invention provide an on-line analyzer for VOCs and a method of using the same, that use two gas paths to sample simultaneously and concentrate hydrocarbons C2-C5 and C5-C12 respectively, and a single instrument to simultaneously detect hydrocarbons C2-C12. 
         [0010]    Another aspect of the present invention is directed to solve the problems in existing on-line monitoring technology that uses selective detectors (FID and PID), such as less types of VOCs to be detected. The embodiments of the present invention provide an on-line analyzer for VOCs and a method of using the same, that utilize a detection technique and method using FID detector and MS detector to simultaneously analyze and detect VOCs such as hydrocarbons C2-C12, compounds containing O (N, S) and halogenated hydrocarbons. 
         [0011]    Yet another aspect of the present invention is directed to solve the problems in existing on-line analysis for VOCs in air, such as the incompatibility of field sampling-laboratory analysis and field on-line continuous automatic monitoring. The embodiments of the present invention provide an on-line analyzer for VOCs and a method of using the same that can be used in both of field sampling (canister sampling)-laboratory analysis and field on-line continuous automatic monitoring VOCs. 
         [0012]    Above problems of the prior art can be solved by the following solutions: 
         [0013]    An on-line analyzer for VOCs in gas comprising a sampling device; a gas-supply device; and a refrigerating device connected to the sampling device and the gas-supply device respectively; wherein the on-line analyzer for VOCs uses two cryo-focuses in empty tubes and two detectors, also comprises a gas flow control device connected to the sampling device and the gas-supply device respectively, an analyzing device connected to the gas flow control device, and a data acquisition and processing device connected to the analyzing device. 
         [0014]    In above on-line analyzer for VOCs in gas, the sampling device comprises: an 6-way valve; and a filter, an inlet tube for sample gas (continuous sample or single sample injection can be selected by setting the software), an inlet tube for internal standards, an inlet tube for external standards, a first solenoid valve and a second solenoid valve connected to the 6-way valve respectively, wherein the first solenoid valve is connected to the second solenoid valve. 
         [0015]    In the above on-line analyzer for VOCs in gas, the refrigerating device comprises: a 12-way valve; and a first dewatering tube, a second dewatering tube, a first concentrating tube and a second concentrating tube connected to the 12-way valve respectively, wherein the first dewatering tube and the second dewatering tube are connected to the second solenoid valve simultaneously. 
         [0016]    In the above on-line analyzer for VOCs in gas, the refrigerating device also comprises a trapping tube for CO2, the second dewatering tube is connected to the 12-way valve through the trapping tube for CO2, and the first dewatering tube, the second dewatering tube, the first concentrating tube and the second concentrating tube are disposed in a cryotrap. 
         [0017]    In the above on-line analyzer for VOCs in gas, the gas flow control device comprises: a third solenoid valve; a forth solenoid valve; a first mass flow controller and a second mass flow controller connected to the third solenoid valve and the forth solenoid valve respectively; a third mass flow controller connected to the third solenoid valve and the forth solenoid valve simultaneously; a fifth solenoid valve connected to the first mass flow controller, the second mass flow controller, the first solenoid valve and a sampling pump simultaneously; and a pressure reducing valve connected to the third solenoid valve and the forth solenoid valve through the third mass flow controller simultaneously. 
         [0018]    In the above on-line analyzer for VOCs in gas, the analyzing device comprises: an oven; a first capillary column and a second capillary column, disposed in the oven and connected to the 12-way valve respectively; a first detector connected to the first capillary column; and a second detector connected to the second capillary column. 
         [0019]    In the above on-line analyzer for VOCs in gas, the gas-supply device comprises: a nitrogen-supply device; a helium-supply device; an air-supply device; and a hydrogen-supply device; wherein the nitrogen-supply device is connected to the pressure reducing valve, the helium-supply device is connected to the 12-way valve, both of the air-supply device and the hydrogen-supply device are connected to the second detector; the first detector is MS (mass spectrum) detector and the second detector is hydrogen FID detector. 
         [0020]    In the above on-line analyzer for VOCs in gas, both of the first dewatering tube and the second dewatering tube are empty glass tube or empty silica tube with an inner diameter of 1.0-1.5 mm and a length of 30-40 cm; the outer walls of both of the first dewatering tube and the second dewatering tube are bound with temperature-adjustable and temperature-controllable heating wire; both of the first concentrating tube and the second concentrating tube are empty capillary column with an inner diameter of 0.53 mm and a length of 30-40 cm, the first concentrating tube is activated silica capillary column and the second concentrating tube is PLOT capillary column; the outer walls of both of the first concentrating tube and the second concentrating tube are bound by temperature-adjustable and temperature-controllable heating wire. 
         [0021]    In the above on-line analyzer for VOCs in gas, the trapping tube for CO2 is polytetrafluoroethylene tube or glass tube having an inner diameter of 4 mm and a length of 15 cm, and the tube is filled with alkali asbestos. 
         [0022]    A method of on-line analyzing VOCs by using the on-line analyzer for VOCs in gas with two cryo-focuses in empty tubes and two detectors, comprises the steps of: 
         [0023]    Step 1, gathering the sample gas, wherein when a sampling pump is turned on, the sample gas flows into a sampling gas path through a filter, enters an inlet (S 1 ) of a 6-way valve and exits from a common outlet (S 6 ); the sample gas is divided into two branches after going through the second solenoid valve: 
         [0024]    the sample gas in the first gas path passes through a first dewatering tube to enter port  12  of 12-way valve, and then enters a first concentrating tube through port  11 ; the VOCs of hydrocarbon C4-C12, organic compounds containing O (N, S) and halogenated hydrocarbons are coldly concentrated in the first concentrating tube; other gases enter port  8  of the 12-way valve and exit from port  7 , flow through a forth solenoid valve to a fifth solenoid valve though a second mass flow controller, and are discharged by the sampling pump; 
         [0025]    the sample gas in the second gas path enters a second dewatering tube, enters port  1  of the 12-way valve through a trapping tube for CO2, and then enters a second concentrating tube through port  2 ; hydrocarbons C2-C5 are coldly concentrated in the second concentrating tube quantificationally; other gases enter port  5  of the 12-way valve and exit from port  6 , flow through a third solenoid valve, a first mass flow controller and the fifth solenoid valve, and are discharged by the sampling pump; 
         [0026]    in this step, the temperature of the first dewatering tube is −20° C. and the temperature of the second dewatering tube is −80° C. Both of the temperatures of the first concentrating tube and the second concentrating tube are −155° C.; 
         [0027]    Step 2, conducting thermal desorption, qualitative and quantitative analysis of the sample gas gathered in step 1, wherein the first concentrating tube and the second concentrating tube are quickly heated with a speed of 40° C./s simultaneously, and after they are heated from −155° C. to 100° C., VOCs in the two concentrating tubes are subjected to quick desorption; detailed operations are as follows: 
         [0028]    in the first gas path, helium gas from a helium-supply device flows from port  9  of the 12-way valve, exits from port  8  of the 12-way valve, then enters into the first concentrating tube; with helium gas purging, the desorbed hydrocarbons C5-C12, compounds containing O (N, S) and halogenated hydrocarbons and the like enter an oven through ports  11  and  10  of the 12-way valve, are separated in a first capillary column, and then enter a first detector for qualitative and quantitative analysis; the first capillary column has a inner diameter of 0.25 mm and a length of 60 m; 
         [0029]    in the second gas path, nitrogen gas from a nitrogen-supply device enter the second concentrating tube through ports  4  and  5  of the 12-way valve; with nitrogen gas purging, the desorbed hydrocarbons C2-C5 enter the oven through ports  2  and  3  of 12-way valve, are separated in a second capillary column and then enter a second detector for qualitative and quantitative analysis; the second capillary column has a inner diameter of 0.32 mm and a length of 15 m; during thermal desorption of the first concentrating tube and the second concentrating tube, the refrigeration continues and the temperature of cryotrap remains about −155° C.; 
         [0030]    during the analysis, the data are collected by a data acquisition systems; and after the completion of analysis, the collected data are processed by a data processing device. 
         [0031]    Step 3, conducting heating and back purging purification, wherein the first dewatering tube, the second dewatering tube, the first concentrating tube and the second concentrating tube are heated to a temperature of 100° C. or higher, and the temperature of cryotrap remains about −155° C. The purging gas of nitrogen gas from the nitrogen-supply device is divided into two branches through a pressure reducing valve and a third mass flow controller: 
         [0032]    nitrogen gas flows through the forth solenoid valve, ports  7  and  8  of the 12-way valve to backpurge the first concentrating tube, enters the first dewatering tube through ports  11  and  12  of the 12-way valve to purge the water in tubes, flows through the second solenoid valve and the first solenoid valve, and finally flows through the fifth solenoid valve to be discharged by the sampling pump; 
         [0033]    nitrogen gas flows through the third solenoid valve, ports  6  and  5  of 12-way valve to backpurge the second concentrating tube, enters the trapping tube for CO2 through ports  2  and  1  of the 12-way valve, enters the second dewatering tube, flows through second solenoid valve and the first solenoid valve, and finally flows through the fifth solenoid valve to be discharged by the sampling pump. 
         [0034]    Therefore, the present invention has the following advantages: 1. The present invention runs automatically and continuously over a long period since it uses electronic refrigeration technique to achieve cryo-refrigeration, instead of using liquid nitrogen refrigeration technique which needs frequent addition of liquid nitrogen and limits application conditions; 2. The present invention has high concentration/adsorption efficiency, runs automatically and continuously over a long period, since it applies empty capillary column for cold concentration of VOCs, avoiding the low absorption/desorption efficiency of a portion of organic compounds in the absorption and concentration and replacement of absorbent tubes, achieving a long-term, continuous and convenient operation and automation; 3. The present invention can concentrate hydrocarbons C2-C5 and hydrocarbons C5-C12, compounds containing O (N, S) and halogenated hydrocarbons respectively since it use cold concentration technique with two empty capillary columns; 4. The present invention can simultaneously analyze hydrocarbons C2-C12, compounds containing O (N, S) and halogenated hydrocarbons and the like since it uses the analytical technique with two analytical columns and two detectors and combines the selective detector (FID) with universal detector (MS), improves the ability of qualitative and quantitative analysis of VOCs; 5. The present invention provides two functions of sampling: continuous sampling and single sampling, that is, the present invention can perform singe sampling operation in the laboratory to analyze the sample gathered in the field sampling canisters, and can be installed in automatic monitoring station to perform on-line continuous automatic monitoring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: 
           [0036]      FIG. 1  illustrates the principle according to an embodiment of the present invention, wherein 
           [0037]    1 ( 101 - 106 ) represents the sampling device. 
           [0038]    2 ( 201 - 204 ) represents the gas-supply device. 
           [0039]    3 ( 301 - 306 ) represents the refrigerating device. 
           [0040]    4 ( 401 - 408 ) represents the gas controlling device. 
           [0041]    5 ( 501 - 505 ) represents the analyzing and detecting device. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    For a fuller understanding of the nature and advantages of the present invention, reference should be made to the ensuing detailed description and the accompanying drawings. Other aspects, objects and advantages of the invention will be apparent from the drawings and the detailed description that follows. 
       Embodiments 
       [0043]    An on-line analyzer for VOCs in gas comprises a sampling device  1 ; a gas-supply device  2 ; and a refrigerating device  3  connected to the sampling device  1  and the gas-supply device  2  respectively; wherein the on-line analyzer for VOCs uses two cryo-focuses in empty tubes and two detectors, also comprises a gas flow control device  4  connected to the sampling device  1  and the gas-supply device  2  respectively, an analyzing device  5  connected to the gas flow control device  4 , and a data acquisition and processing device connected to the analyzing device  5 . 
         [0044]    The sampling device  1  comprises: a 6-way valve  101 ; and a filter  102 , an inlet tube for internal standards  103 , an inlet tube for external standards  104 , a first solenoid valve  105  and a second solenoid valve  106  connected to the 6-way valve  101  respectively, wherein the first solenoid valve  105  is connected to the second solenoid valve  106 . 
         [0045]    The refrigerating device  3  comprises: a 12-way valve  301 ; and a first dewatering tube  302 , a second dewatering tube  303 , a first concentrating tube  304  and a second concentrating tube  305  connected to the 12-way valve  301  respectively, wherein the first dewatering tube  302  and the second dewatering tube  303  are connected to the second solenoid valve  106  simultaneously. Both of the first dewatering tube  302  and the second dewatering tube  303  are empty glass tube or empty silica tube having an inner diameter of 1.0-1.5 mm and a length of 30-40 cm; the outer walls of both of the first dewatering tube  302  and the second dewatering tube  303  are bound with temperature-adjustable and temperature-controllable heating wire; both of the first concentrating tube  304  and the second concentrating tube  305  are empty capillary column having an inner diameter of 0.53 mm and a length of 30-40 cm, the first concentrating tube  304  uses a deactivated silica capillary column and the second concentrating tube  305  uses PLOT capillary column; the outer walls of both of the first concentrating tube  304  and the second concentrating tube  305  are bound with temperature-adjustable and temperature-controllable heating wire. 
         [0046]    The refrigerating device  3  also comprises a trapping tube for CO2  306 , the second dewatering tube  303  is connected to the 12-way valve  301  through the trapping tube for CO2  306 . The first dewatering tube  302 , the second dewatering tube  303 , the first concentrating tube  304  and the second concentrating tube  305  are disposed in a cryotrap. The trapping tube for CO2  306  is polytetrafluoroethylene tube or glass tube having an inner diameter of 4 mm and a length of 15 cm, and the tube is filled with alkali asbestos, with each end filled with glass wool of 1 cm to fix and prevent the alkali asbestos being blew away by gas. 
         [0047]    The gas flow control device  4  comprises: a third solenoid valve  401 ; a forth solenoid valve  402 ; a first mass flow controller  404  and a second mass flow controller  405  connected to the third solenoid valve  401  and the forth solenoid valve  402  respectively; a third mass flow controller  406  connected to the third solenoid valve  401  and the forth solenoid valve  402  simultaneously; a fifth solenoid valve  403  connected to the first mass flow controller  404 , the second mass flow controller  405 , the first solenoid valve  105  and a sampling pump  408  simultaneously; and a pressure reducing valve  407  connected to the third solenoid valve  401  and the forth solenoid valve  402  through the third mass flow controller  406  simultaneously. 
         [0048]    The analyzing device  5  comprises: an oven  501 ; a first capillary column  502  and a second capillary column  503  disposed in the oven  501  and connected to the 12-way valve  301  respectively; a first detector  504  connected to the first capillary column  502 ; and a second detector  505  connected to the second capillary column  503 . The oven has the functions of heating, programmed temperature raising and controlling, and can be equipped with two capillary columns of 60 m and two injectors, and connected with two detectors. The injectors and detectors have the functions of heating and temperature controlling. 
         [0049]    The gas-supply device  2  comprises: a nitrogen-supply device  201 ; a helium-supply device  202 ; an air-supply device  203 ; and a hydrogen-supply device  204 . The nitrogen-supply device  201  is connected to the pressure reducing valve  407 , the helium-supply device  202  is connected to the 12-way valve  301 , and both of the air-supply device  203  and the hydrogen-supply device  204  are connected to the second detector  505 . The first detector  504  may be a MS detector and the second detector  505  may be a hydrogen FID detector. 
         [0050]    The present invention provides a method of analyzing VOCs by using the on-line analyzer for VOCs in air, comprising the steps of: 
         [0051]    Step 1, gathering the sample gas, wherein when a sampling pump  408  is turned on, the sample gas flows into a sampling gas path through a filter  102 , enters an inlet S 1  of a 6-way valve  101  and exits from a common outlet S 6 ; the sample gas is divided into two branches after going through the second solenoid valve  106 ; 
         [0052]    the sample gas in the first gas path passes through a first dewatering tube  302  to enter port  12  of 12-way valve  301 , and then enters a first concentrating tube  304  through port  11 ; the VOCs of hydrocarbon C4-C12, organic compounds containing O (N, S) and halogenated hydrocarbons are coldly concentrated in the first concentrating tube  304 ; other gases enter port  8  of the 12-way valve  301  and exit from port  7 , flow through a forth solenoid valve  402 , flow though a second mass flow controller  405 , and flow through a fifth solenoid valve  403 , and are discharged by the sampling pump  408 ; 
         [0053]    the sample gas in the second gas path enters a second dewatering tube  303 , enters port  1  of the 12-way valve  301  through a trapping tube for CO2  306 , and then enters a second concentrating tube  305  from port  2 ; hydrocarbons C2-C5 are coldly concentrated in the second concentrating tube  305  quantificationally; other gases enter port  5  of the 12-way valve  301  and exit from port  6 , flow through a third solenoid valve  401 , a first mass flow controller  404  and the fifth solenoid valve  403 , and are discharged by the sampling pump  408 ; 
         [0054]    after completing the sampling procedure of sample gas, a data acquisition and processing device analyzes the data in the analyzing device; 
         [0055]    in this step, the temperature of the first dewatering tube  302  is −20° C. and the temperature of the second dewatering tube  303  is −80° C. Both of the temperatures of the first concentrating tube  304  and the second concentrating tube  305  are about −155° C. 
         [0056]    Step 2, conducting thermal desorption of the sample gas gathered in step 1, wherein the first concentrating tube  304  and the second concentrating tube  305  are quickly heated simultaneously, and after they are heated from −155° C. to 100° C. with a speed of 40° C./s, VOCs in the two concentrating tubes are subjected to quick desorption; detailed operations are as follows: 
         [0057]    in the first gas path, helium gas from a helium-supply device  202  flows from port  9  of the 12-way valve  301 , exits from port  8  of the 12-way valve  301 , then enters into the first concentrating tube  304 ; with helium gas purging, the desorbed hydrocarbons C5-C12, compounds containing O (N, S) and halogenated hydrocarbons and the like enter an oven  501  through ports  11  and  10  of the 12-way valve  301 , are separated in a first capillary column  502 , and then enter a first detector  504  for qualitative and quantitative analysis; the first capillary column is a capillary column with a inner diameter of 0.25 mm and a length of 60 m; 
         [0058]    in the second gas path, nitrogen gas from a nitrogen-supply device  201  enter the second concentrating tube  305  through ports  4  and  5  of the 12-way valve  301 ; with nitrogen gas purging, the desorbed hydrocarbons C2-C5 enter the oven  501  through ports  2  and  3  of 12-way valve  301 , are separated in a second capillary column  503  and then enter a second detector  505  for qualitative and quantitative analysis of the desorbed hydrocarbons C2-C5; the second capillary column is a capillary column with a inner diameter of 0.32 mm and a length of 15 m; during thermal desorption of the first concentrating tube  304  and the second concentrating tube  305 , the refrigeration continues and the temperature of cryotrap remains −155° C. to −150° C. 
         [0059]    Upon start of the thermal desorption, a data acquisition and processing device begins to gather, process and analyze the data synchronously. 
         [0060]    Step 3, conducting heating and back purging purification, wherein the first dewatering tube  302 , the second dewatering tube  303 , the first concentrating tube  304  and the second concentrating tube  305  are heated to a temperature of 100° C. or higher, and the temperature of cryotrap remains −155° C. to −150° C. The purging gas of nitrogen gas from the nitrogen-supply device  201  is divided into two branches through a pressure reducing valve  407  and a third mass flow controller  406 ; 
         [0061]    nitrogen gas flows through the forth solenoid valve  402 , ports  7  and  8  of the 12-way valve  301  to backpurge the first concentrating tube  304 , enters the first dewatering tube  302  through ports  11  and  12  of the 12-way valve  301  to purge the water in tubes, flows through the second solenoid valve  106  and the first solenoid valve  105 , and finally flows through the fifth solenoid valve  403  to be discharged by the sampling pump  408 ; 
         [0062]    nitrogen gas flows through the third solenoid valve  401 , ports  6  and  5  of 12-way valve  301  to backpurge the second concentrating tube  305 , enters the trapping tube for CO2  306  through ports  2  and  1  of the 12-way valve  301 , enters the second dewatering tube  303 , flows through second solenoid valve  106  and the first solenoid valve  105 , and finally flows through the fifth solenoid valve  403  to be discharged by the sampling pump  408 . 
         [0063]    The present invention is used to automatically and continuously monitor volatile hydrocarbons C2-C12, organic compound containing O (N, S) and halogenated hydrocarbons in air. In these embodiments, the cryotrap is described in Chinese Patent No. 200810118237.4. 
         [0064]    It is understood that the examples and embodiments described herein are for illustrative purposes and that various modifications or changes in light thereof will be suggested to a person skilled in the art and are to be included within the spirit and purview of this application and the scope of the appended claims. 
         [0065]    Although specific terms are employed herein, such as, sampling device  1 , 6-way valve  101 , filter  102 , inlet tube for internal standards  103 , inlet tube for external standards  104 , first solenoid valve  105 , second solenoid valve  106 , gas-supply device  2 , nitrogen-supply device  201 , helium-supply device  202 , air-supply device  203 , hydrogen-supply device  204 , refrigerating device  3 , 12-way valve  301 , first dewatering tube  302 , second dewatering tube  303 , first concentrating tube  304 , second concentrating tube  305 , trapping tube for CO2  306 , gas flow control device  4 , third solenoid valve  401 , forth solenoid valve  402 , fifth solenoid valve  403 , first mass flow controller  404 , second mass flow controller  405 , third mass flow controller  406 , pressure reducing valve  407 , sampling pump  408 , analyzing device  5 , oven  501 , first capillary column  502 , second capillary column  503 , first detector  504 , second detector  505  and the like, they are used in a generic and descriptive sense only and not for purposes of limitation. It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope or spirit of the present invention.