Abstract:
An ion mobility spectrometer with a GC column and an internal circulation system is provided which can be used in trace gas analysis. Due to the special design of the gas circulation, the parameters: carrier gas velocity in the GC column, the flow rate of the gas to be analyzed and the flow rate of the drift gas can be varied extensively independently and without reaction. Additional pumps and gas splitters are arranged in the circulation system for this purpose.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to an ion mobility spectrometer (IMS) with gas chromatography (GC) column (GC-IMS) and internal controlled gas circulation, which can be used in trace gas analysis. 
     BACKGROUND OF THE INVENTION 
     A gas analyzer with internal gas circulation has been known from DE-OS 198 56 784. A circulation filter for water vapor and higher-molecular-weight constituents of the gas, a circulating pump, a metering means for the inlet for the gas to be analyzed as well as a gas-chromatographic separation column to a closed circulation system are additionally arranged in a gas circulation of a concentration-dependent gas detector. The air of the internal gas circulation is used as the carrier gas utilizing the separation column of a suitable low admission pressure to distinguish components with equal mobility but different retention time and to suppress cross sensitivities. The supply of an external carrier gas can be eliminated. 
     However, many measurement problems in industrial practice require defined analysis times in agreement with technological requirements such as the rhythm of the measurement, the accuracy of the measurement and the sensitivity of the measurement. 
     SUMMARY OF THE INVENTION 
     According to the invention an ion mobility spectrometer with GC column and internal controlled gas circulation is provided. A flow of gas to be analyzed from a sample gas outlet of the IMS cell is split via a splitter into two partial flows. One branch has a pump and a analytical circulation filter. The smaller partial flow is sent to a sample gas inlet of the IMS cell via a switchable sample loop device for passing on or sampling and subsequently via a GC column. The larger flow of the gas to be analyzed is sent back from a splitter to a branch with a further pump, a circulation filter to an additional gas inlet of the IMS cell. An additional gas outlet of the IMS cell provides the flow back to the further pump as well as a pressure sensor and a temperature sensor of the larger flow gas circulation. This circulating gas flow is split internally in the IMS cell in a splitter into a drift gas flow and the internal flow of the gas to be analyzed. 
     The circulating flow may be split via a splitter arranged outside the IMS cell into the drift gas flow, which is sent into the cell via the inlet, and the flow of gas to be analyzed, which is sent to the branch. 
     The Ion mobility spectrometer may be provided that a splitter is provided in the flow of gas to be analyzed. A partial flow may be sent as a make-up gas flow via another splitter to the carrier gas flow. This partial flow is used for diluting the sample. 
     The invention makes possible the independent control of defined analysis times in agreement with technological requirements such as the rhythm of the measurement, the accuracy of the measurement and the sensitivity of the measurement in the embodiment of an analysis system operating with a closed gas circulation. The gas flow to be analyzed, which leaves the IMS cell, is sent over an additional pump and an additional filter. The gas flow to be analyzed is split downstream of the additional pump and an additional filter into two partial flows. 
     The larger partial flow is returned in a closed circuit to the area upstream of the pump. The other partial flow is sent via the sample loop (in the solenoid valve (MV) block) to the GC column and then to the sample inlet of the IMS cell. It is ensured as a result that the admission pressure before the GC column can be set sensitively and varied by varying the output of the pump. Disturbing pump shocks are eliminated by the filter. The splitting of the gas flow is provided because the GC column is able to process, in principle, only very small gas flows and a sensitive control based on the output of the pump is possible at relatively large flows only. 
     At the same time, the additional possibility of controlling the admission pressure of the GC column ensures the absence of reaction of the gas flow to be analyzed on the closed drift gas system that is formed by the circulating pump, the circulating pump filter, the drift gas inlet of the IMS cell and the drift gas outlet of the IMS cell, and the independence of a variation of the gas flow to be analyzed. The parameters in this circulation can also be varied based on the output of the circulating pump independently from reactions on the circulation of the gas to be analyzed. Sensors for the pressure and temperature provide data used for the control of the circulation parameters and the compensation of the measured values of the IMS detection by calculation. These sensors may be additionally arranged in the drift gas circulation. 
     In particular, the following parameters and properties can be varied independently from one another due to this arrangement: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Velocity of the carrier gas in the GC column: 
                 time response of the 
               
               
                   
                 arrangement 
               
               
                 Flow rate of the gas to be analyzed, 
                 sensitivity 
               
               
                 which enters the IMS cell: 
               
               
                 Flow rate of the drift gas: 
                 accuracy, resolution 
               
               
                   
               
             
          
         
       
     
     The present invention shall be described in greater detail below. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing an exemplary embodiment of the device according to the present invention with a gas splitter arranged within the IMS cell; 
     FIG. 2 is a diagram showing an exemplary embodiment of the device according to the present invention with a gas splitter arranged outside the IMS cell; 
     FIG. 3 is a diagram showing an exemplary embodiment of the device according to the present invention with a gas splitter arranged within the IMS cell and with a splitter in the flow of gas to be analyzed splitting a partial flow used for diluting the sample. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in particular, the invention comprises an ion mobility spectrometer with GC column and internal controlled gas circulation. These embodiments of the invention are shown in FIGS. 1,  2  and  3 . 
     The embodiment of FIG. 1 includes a MV block  2 . This has a normal segment from inlet  1  via the sample loop portion  2 A to  2 C and pump  3  to the outlet  14 . This may be switched over so that the sample volume located in the region between  2 A and  2 C is transferred in between  2 D and  2 B with the portion between  2 D and  2 B thereby providing a sample for analysis. The embodiment of FIG. 1 also includes a GC column  8  and an IMS cell  9 . The cell  9  has an inlet  9 A, an outlet  9 B, an outlet  9 C and an inlet  9 D. In the embodiment of FIG. 1 the outlet  9 C is connected back to the inlet  9 D through the circulation pump  11  and circulation filter  10 . Pressure sensor  12  and a temperature sensor  13  are operatively connected to the gas flow q 1 . In the embodiment of FIG. 1 the IMS cell  9  includes an IMS cell splitter  14 A which splits the incoming flow from  9 D q 1  into two portions q 1 ( 1 ) and q 1 ( 2 ). The flow q 1 ( 2 ) exits exit  9 B as to flow q 2 . The branch of the circuit with flow q 2  includes a branch element  7  feeding the flow q 2  through additional pump  6  and additional filter  5  to splitter  4 . By means of pump  6  including filter  5  and splitters  4  and  7  a pressure increase is realized thus providing a suitable flow q 2  through GC-column  8 . 
     The embodiment of FIG. 2 is similar to the embodiment of FIG.  1 . The portion of the circuit or loop with flow q 2  proceeds in a manner similar to that of the embodiment of FIG.  1 . However, unlike the embodiment of FIG. 1 the flow q 1  is directed to a splitter  14  which is external of the IMS cell  9 . Splitter  14  breaks the flow q 1  into the flow q 1 ( 1 ) which proceeds back to the IMS cell  9  via inlet  9 D. The other branch of splitter  14  forms flow q 1 ( 2 ) which proceeds as flow q 2  as described above. 
     The embodiment of FIG. 3 is identical to the embodiment of FIG. 1 except an additional splitter  16  is provided which is connected to the outlet  9 B of the IMS cell  9 . This splitter  16  branches off a partial flow q 2 ( 3 ) from flow q 2  which is sent as a make-up gas flow via branch element  15  to provide a make-up gas flow which is used for diluting the sample. 
     In the stand-by mode, the sample gas flow q 3  is delivered from the inlet  1  via the sample loop ( 2 A to  2 C) in the MV block  2  and the pump  3  to the outlet  14 . Sampling is not performed. The apparatus operates in a circulation mode circulating around flow portions q 2 ( 1 ), q 1 , q 2  and purifies itself. 
     The connection  2 B- 2 D is in parallel to the connection  2 A- 2 C within the MV block  2 . The MV block  2  is briefly switched over with the portion (loop) between  2 B- 2 D and the portion (sample loop) between  2 A- 2 C switching positions for the sampling and the start of a measurement cycle. The sample volume located in the sample loop between  2 A and  2 C is moved to the location between  2 D and  2 B upon switching. The sample volume is conveyed in the circulation in the carrier gas flow q 2 ( 1 ) to the GC column  8 . At the GC column  8  a preliminary gas-chromatographic separation of the constituents of the sample takes place according to their different retention times. 
     After the sample has been introduced into the circulation system, the MV block  2  is immediately reset to the connection configurations  2 A- 2 C and  2 B- 2 D. 
     The preliminarily separated sample volume is conveyed farther to the sample inlet  9 A of the IMS cell  9 . The ion mobility spectrometric analysis of the constituents of the sample is performed at the IMS cell  9 . The analytical circulation q 2  is completed via the sample outlet  9 B of the IMS cell  9 , the branch (flow combiner)  7 , the pump  6 , the filter  5  and the splitter  4  for the gas to be analyzed. The gas flow q 2  for the gas to be analyzed is split in the splitter  4  into the two components. The carrier gas flow q 2 ( 1 ) is directed to the MV block  2  and the bypass flow q 2 ( 2 ) back to the branch  7 . The circulation q 2  of the gas to be analyzed can be controlled on the basis of the output of the pump  6 . At the same time, the bypass flow q 2 ( 2 ) ensures the necessary pump load for the pump  6 , which would not be guaranteed by the carrier gas flow q 2 ( 1 ) alone. 
     The basic circulation with the circulating gas flow q 1  is formed by the pump  11 , the circulation filter  10 , the inlet  9 D and the gas outlet  9 C of the IMS cell  9 . This basic circulation is controlled by the output of the pump  11  on the basis of the parameters from the sensors arranged in the circulation, namely, the pressure sensor  12  and the temperature sensor  13  without reaction on the analysis circulation q 2 . 
     In the embodiments of FIGS. 1 and 3 the splitting of the circulating gas q 1  is performed internally in a cell splitter  14 A. The cell splitter  14 A splits the flow q 1  into the drift gas flow q 1 ( 1 ) and the flow q 1 ( 2 ) of the gas to be analyzed. In the embodiment of FIGS. 1 and 2 the flow q 1 ( 2 ) is equal to the flow q 2  of the gas to be analyzed. 
     This arrangement of FIG. 1 ensures that the flows can be varied very extensively independently from one another in both the circulating gas flow q 1  and the flow q 2  of the gas to be analyzed. 
     According to the embodiment of FIG. 2, the splitting of the circulating gas flow into the drift gas flow q 1 ( 1 ) and the gas flow to be analyzed q 1 ( 2 ), q 2  may also take place in an externally arranged splitter  14 . Splitter  14  directs drift gas flow q 1 ( 1 ) to inlet  9 D and gas flow to be analyzed q 1 ( 2 ), q 2  to branch  7 . 
     According to the embodiment of FIG. 3 an additional splitter  16  is provided receiving gas flow to be analyzed q 1 ( 2 ). The additional splitter  16  branches off parts of the gas to be analyzed q 1 ( 2 ) to form diluting gas flow q 2 ( 3 ). Diluting gas flow q 2 ( 3 ) mixes with the carrier gas flow q 2 ( 1 ) for diluting the sample by additional branch (flow combiner)  15  arranged in the flow q 1 ( 2 ), q 2  of the gas to be analyzed in FIG.  3 . 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.