Abstract:
The invention relates to gas chromatography analysis of a sample having components to be investigated and water contained therein, which after thermodesorption is separated and analyzed, the thermodesorbed sample being transferred by a carrier gas into a first polar separation column which retains higher-boiling components and water and passes low-boiling components, the latter being led, past a branching point which leads, on the one hand, to a second polar or non-polar separation column and, on the other hand, to the non-polar separation column, to the non-polar separation column in a fashion excluding access to the second polar or non-polar separation column, after which higher-boiling components and water are transferred to the second polar or non-polar separation column in a fashion excluding access to the non-polar separation column, water being eliminated upstream of the second polar or non-polar separation column by cryofocussing.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for gas chromatography analysis of samples. 
     To use gas chromatography to investigate small quantities of components present in gases or liquids, such as foreign substances or pollutants or impurities, it is known firstly to enrich these in order then to feed them into a gas chromatograph via an appropriate feeding system. However, problems occur in this case when the collected samples contain moisture such as is the case, for example, when pollutants contained in the air are enriched, since the moisture contained in the air is then also enriched. 
     However, water severely disturbs a gas chromatography system, and likewise the analysis, in the case of which, for example, a substantial loss in sensitivity occurs in the mass spectrometer. The presence of water in separation columns alters the retention time, doing so, specifically, as a function of quantity and differently for different substances, thus creating the need to eliminate this as completely as possible in order to obtain reliable measurement results. 
     BACKGROUND OF THE INVENTION 
     It is known to eliminate the moisture which is present in samples to be chromatographically analyzed by osmosis. However, this has the disadvantage that polar components are also eliminated in the process, while non-polar components remain essentially uninfluenced. However, the elimination of polar components other than water falsifies the chromatogram. 
     Also known are packed capillary columns which exhibit a temperature-dependent adsorptivity with reference to water, so that given appropriate setting, low-boiling components are passed while higher-boiling components and water are retained. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for gas chromatography analysis of samples which permits reliable gas chromatograms to be obtained from samples containing water. 
     It is a further object of the invention to provide an apparatus for gas chromatography analysis of samples which permits reliable gas chromatograms to be obtained from samples containing water. 
     According to the invention a method for gas chromatography analysis of a sample after preceding thermodesorption, in which the components to be separated and water are contained, is provided, 
     wherein the thermodesorbed sample is transferred by means of carrier gas into a first polar separation column which retains higher-boiling components and water and passes low-boiling components, 
     said low boiling components being led, past a branching device which leads, on the one hand, to a second polar or non-polar separation column and, on the other hand, to a non-polar separation column, to the non-polar separation column in a fashion excluding access to the second polar or non-polar separation column, 
     after which the higher-boiling components and the water are lead to the second polar or non-polar separation column in a manner excluding access to the non-polar separation column, 
     the water being eliminated upstream of the second polar or non-polar separation column by means of cryofocussing. 
     According to the invention, further an apparatus for gas chromatography analysis of a sample is provided, comprising: 
     a thermodesorption device for holding a sampling tube; 
     a first polar separation column being connected downstream of the thermodesorption device; 
     a branching device being connected downstream of the first polar separation column; 
     a non-polar separation column; 
     a second separation column being of the group of a polar and a non-polar separation column; 
     wherein said branching device being switchable over between said non-polar separation column; and 
     a device for eliminating water which is connected upstream of the second separation column. 
     By virtue of the fact that according to the present invention use is made as a precolumn of a polar separation column with a stationary phase, which water does not initially have the effect of separating it preliminarily into two fractions, higher-boiling components and water can be retained at the beginning, while low-boiling components are passed. The low-boiling components are separated on the non-polar separation column via a pneumatically closeable bifurcation which leads, on the one hand, to a non-polar separation column for gases and, on the other hand, via a cryofocussing device, to a further polar or non-polar separation column, whereupon after pneumatically switching over the bifurcation the water with higher-boiling components is eliminated in the region of the cryofocussing device, whereupon the higher-boiling components are separated in the polar or non-polar separation column downstream of the cryofocussing device. In addition, in this case the water elimination with subsequent separation and analysis of a sample, and the separation on the further separation column with subsequent analysis of another sample, can be carried out simultaneously. 
     In this case, not only gaseous but also liquid samples which contain water can be taken automatically by means of the apparatus. 
     Further objects, embodiments and advantages of the invention will become apparent from the following description and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained below in more detail with reference to preferred embodiment illustrated schematically in the attached illustrations. 
     FIG. 1 shows a diagram of a gas chromatography apparatus according to the invention, partially in section. 
     FIG. 2 shows the diagrammatic design of an embodiment of a thermodesorption device or cryofocussing device or a device for eliminating water for the gas chromatography device of FIG. 1, in section. 
     FIG. 3 shows a diagram of a design of a branching point for the gas chromatography device of FIG. 1, in section. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The gas chromatography apparatus illustrated in FIG. 1 comprises a thermodesorption device  1  for a sample contained in a sampling tube  2 , a carrier gas connection  3  and a gas exhaust line  4  being provided. A transfer capillary  7  leading from the thermodesorption device  1  to a feed head  5  of a cryofocussing device  6  can be heated by a transfer furnace  8  in order to avoid material losses upon transfer from the sampling tube  2  to the cryofocussing device  6 . The cryofocussing device  6  comprises a gas exhaust line  9 . A transfer capillary  10   a ,  10   b  downstream of the cryofocussing device  6  leads, if appropriate, via a switchover valve  11  to a column collecting piece  12  of a polar separation column  13  serving as capillary precolumn, the column connecting piece  12  comprising a gas exhaust line  14 . The switchover valve  11  also comprises several feed or discharge lines  11   a - 11   d  for flushing, calibration or automatic sampling. The transfer capillary  10   a ,  10   b  is arranged in a transfer furnace  15  which can, if appropriate, form a common furnace with the transfer furnace  8 . 
     A branching device  16  is arranged at the end downstream of the polar separation column  13 , which exhibits stable properties with regard to separation in the presence of water. Separation columns  17 , 18  are connected separately from one another to the branching device  16 , it being possible to exclude pneumatically the access to in each case one of the separation columns  17 , 18  via a gas line  19 , which can be charged with gas via a valve  20  or  21  and a controller  22 . 
     The separation column  17  is a non-polar separation column which, in particular, operates according to the principle of a micropacked column, and serves to separate low-boiling components. The separation column  17  is connected to an analyzer A 1 . 
     The separation column  18  is a polar or non-polar separation column with stable properties with regard to the separation of polar components. The separation column  18  is connected to an analyser A 2 . Connected upstream of the separation column  18  is a device  23  for eliminating water, which comprises a carrier gas connection  24  and a gas exhaust line  25  for the purpose of eliminating interfering water. In this case, a thermal conductivity detector  26  connected to the gas exhaust line  25  is used to monitor the completeness of the elimination. 
     The polar separation column  13  can be arranged in a furnace  27  which can, if appropriate, form a single furnace with the transfer furnace  8 . 
     The capillary separation columns  17 , 18  are preferably arranged in the furnaces  28  and  29 , respectively, but they can also be arranged in a common furnace, if appropriate together with the polar separation column  13 . 
     The device  23 , illustrated in FIG. 2, for eliminating water comprises a cooling device, which can be formed by a Peltier element, a cyrostat or a passage for liquefied gas such as liquid nitrogen. In the exemplary embodiment illustrated, a housing casing  30  is provided with coolant bores  31  which can be connected to a coolant source, the housing casing  30  accommodating a metal tube  33  which is surrounded by a heating winding  32  and for its part accommodates the sampling tube  2 . An annular gap  34  which is connected to the gas exhaust line  25  is located between the metal tube  33  and the sampling tube  2 . The carrier gas connection  24  opens into the sampling tube  2  in the region of a feed head  35 . The separation column  18  is plugged into the device  23  for eliminating water in such a way that it projects into the sampling tube  2 . Since the inside diameter of the sampling tube  2  is larger than the outside diameter of the separation column  18 , the interior of the sampling tube  2  is also connected to the annular gap  34 . 
     The thermodesorption device  1  and the cryofocussing device  6  can be designed in a fashion corresponding to the device  23  for eliminating water, and so reference is made to FIG. 2 in each case in connection with these devices. The design can be selected, for example, to accord with DE 44 19 596 C1, but it is also possible here to provide cooling by a Peltier element or a cryostat, while consideration may be given respectively in this connection to a heating cartridge for example in accordance with DE 198 17 017 A1. However, if appropriate, the annular gap  34  and the gas exhaust line  4  or  9  can be dispensed with, if appropriate, in the case of the thermodesorption device  1  and the cryofocussing device  6  when split-mode operation is not desired. The thermodesorption device  1  can be designed as in the case where sampling tubes  2  are to be used such as described, for example, in DE 195 20 715 C1. Each of DE 44 19 596 C1, DE 198 17 017 A1, and DE 195 20 715 C1 is incorporated herein by reference, as are any English-language equivalents thereof. 
     In the embodiment of the branching device  16  of FIG. 3, a central branching piece  36  is connected to two further branching pieces  37 , 38  via capillary adapters  39  which, for their part, are connected via the valve  20  or  21  and the controller  22  to the gas line  19  or to the separation column  17  or  18 , it being possible, if appropriate, to connect the central branching piece  36  to a monitor detector  40 , in particular a thermal conductivity detector. 
     A sample contained in the sampling tube  2  is thermodesorbed in the thermodesorption device  1  by controlled heating of the sampling tube  2  by means of the heating winding  32 . During thermodesorption, carrier gas is fed into the sampling tube  2  via the carrier gas connection  3 , and led into the cryofocussing device  6  via the heated transfer capillary  7  for the purpose of transporting desorbed substances, including water which is present. Uniform feeding of carrier gas is maintained constant in this case in each method step via a flow sensor with a controller. Since thermodesorption is performed without splitting, the gas exhaust line  4  remains closed and thereby pneumatically closes the access to the annular gap  34 . 
     Initially, the cryofocussing device  6  is closed off at the end, if appropriate by means of the switchover valve  11 , from the column connecting piece  12 , its gas exhaust line  9  is opened, for example via a valve (not illustrated), and its sampling tube  2  is cooled down to minus 150° C. by appropriate cooling, for example with liquid nitrogen, such that all the components of the sample which are to be investigated, including the water contained, are collected in the sampling tube  2  and thus enriched. Thereafter, the gas exhaust line  9  is closed, while the sampling tube  2  is heated up, while being monitored, to a temperature of, for example, 350° C., by means of the heating winding  32 , all the enriched components leaving the sampling tube  2  of the cryofocussing device  6  and now being led into the separation column  13  by means of carrier gas because of the open switchover valve  11  via the column connecting piece  12 . 
     The preliminary separation into two fractions of the separation column  13  is initially not influenced by water which is present, and higher-boiling components and water are retained there by interaction forces of different strength for a longer time than low-boiling, essentially non-polar components. 
     In the first phase of the separation by the polar separation column  13  in which the furnace  27  is at ambient temperature, the low-boiling non-polar components, i.e. those with one to approximately four or more carbon atoms, flow through the polar separation column  13  virtually without a separation effect, and subsequently through the branching device  16 . The valve  20  is opened in this case, and so the branching device  16  is pneumatically closed towards the polar or non-polar separation column  18 , and the low-boiling non-polar components are permitted to pass to the non-polar separation column  17  by means of a controlled carrier gas flow. These components are separated in the non-polar separation column  17  and analyzed in the analyzer A 1 . 
     In a second phase of the separation by the polar separation column  13 , the valve  20  is closed and the valve  21  is opened such that the branching device  16  is now pneumatically closed off from the non-polar separation column  17 . The valves  20 , 21  are switched over in principle as a function of time, the switch over being calibrated to a retention time of a specific compound, which is low boiling by comparison with water, in the non-polar separation column  17 , for example to the retention time of toluene, but it can also be performed earlier, if appropriate, when the monitor detector  40  which reacts to water outputs a signal on the basis of incoming water which has the effect of permitting access by higher-boiling components and water on the basis of the now reversed direction of the overall gas flow to the polar separation column  18  via the device  33  for eliminating water, the polar separation column  13  then being additionally heated via the furnace  27  in order to release all higher-boiling components and/or water. 
     The device  23  for eliminating water permits higher-boiling components to be separated from water in three phases. 
     In a first phase, the cryofocussing, the higher-boiling components and water are collected and enriched—as in the case of enrichment in the cryofocussing device  6 . In a second phase, the sampling tube  2  of the device  23  for eliminating water is heated by means of its heating winding  32 , the water being eliminated via the open gas exhaust line  25 . This heating is performed to a temperature above the freezing point of water and below the boiling point of water, preferably to a relatively low temperature of, for example, 10 to 20° C., this temperature being selected in such a way that as little loss of components as possible results in this case, but an adequate water vapor partial pressure is present. The monitoring of the water content in the sample is performed in this case by means of the thermal conductivity detector  26 , which reacts to the presence of water and is connected to the gas exhaust line  25 . Once the water has been completely eliminated, the gas exhaust line  25  is closed on the basis of a signal output by the thermal conductivity detector  26 , whereupon in the third phase the sampling tube  2  of the device  23  for eliminating water is heated further in a programmed fashion by means of the heating winding  32 , and the individual components are released again one after another and are then led into the polar or non-polar separation column  18  in which they are successively separated and analyzed in the analyzer A 2 . 
     Water is eliminated in the device  23  for eliminating water by virtue of the fact that its sampling tube  2  is heated by means of the heating winding  32 , and that, with the gas exhaust line  25  open, the carrier gas flowing past a fed sample containing water flows to the polar or non-polar separation column  18  at the end, averted from the feed head  35 , of the sampling tube  2  of the device  23  for eliminating water, back to the gas exhaust line  25  through the annular gap  34 , and is thereby eliminated. This form of elimination of individual components, also termed split-mode operation, can also take place in the cryofocussing device  6  by means of a gas exhaust line  9 , which is open here, and in the thermodesorption device  1  by means of a gas exhaust line  4 , which is open here. The gas exhaust lines  4 , 9  and  25  each have a valve which is opened, preferably pneumatically, by means of pressure control during split-mode operation. 
     It is expedient for the sample to be introduced quickly in the column connecting piece  12  on the basis of operation as a consequence of a continuously open gas exhaust line  14  by means of the flow velocity, thereby increased, in order in this way to achieve a defined peak end (avoidance of peak tailing) with a defined sharpness of separation. Thinning of the sample resulting therefrom is generally acceptable. 
     A sample can be introduced into the thermodesorption device  1  by means of an exchangeable sampling tube  2 . Instead of this, the sample can, however, also be collected in the sampling tube  2  of the thermodesorption device  1  by the sucked-in ambient atmosphere during split-mode operation with the gas exhaust line  9  open, the gas being eliminated via the annular gap  34  and the gas exhaust line  9 . If appropriate, the switchover valve  11  can, also be arranged upstream of the cryofocussing device  6  in the region of the transfer capillary  7 . 
     The switchover valve  11  is adjusted after a passage of the sample in such a way that firstly, with the aid of the now connected feed line  11   a  and  11   b  the sample inlet is flushed up to the outlet, and secondly, with the aid of the likewise connected transfer capillary  10   a , the feed line  11   a  and  11   b , and also the connected feed line to the carrier gas connection  3 , the thermodesorption device  1  and the cryofocussing device  6  are flushed, while because of the closed exhaust line  14  the sample is led further to the column interface  12  via the polar separation column  13 . Consequently, on the basis of the above circuit it is possible to take a new sample in parallel with the sample to be analyzed or to carry out a calibration of the thermodesorption device  1  and of the cryofocussing device  6 . 
     In a preferred embodiment, the separation columns  13 , 17  and  18  are likewise arranged in individual furnaces  27 , 28  and  29  such that after passage of the respective sample the separation columns  13 , 17 , 18  are cooled down individually and prepared for the subsequent sample, the temperature intervals being selected to be smaller by the furnace  27 , 28 , 29 , which is to be assigned respectively to only one separation column  13 , 17 , 18 , and cooling taking place more quickly. 
     The pneumatic exclusion from the polar or non-polar separation column  18  via the device  23  for eliminating water, or from the non-polar separation column  17  is achieved on the basis of switching over the valves  20 , 21  and on the basis of the controller  22 , which sets a higher flow velocity of the gas from the gas line  19  than is prescribed by the carrier gas flow which flows through the polar separation column  13 . The capillary adapters  39 , which have a diameter of 50 μm to 100 μm, for example, are to be dimensioned in this case in terms of length and diameter and as a function of the gas pressure used in such a way that no diffusion takes place from the central branching piece  36  up to that one of the two branching pieces  37 , 38  which leads to the separation column  17 , 18  respectively not to be used. 
     While the invention has been shown and described with reference to a preferred embodiment, it should be apparent to one of ordinary skill in the art that many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the claims.