Abstract:
A micro-machined back-flush injector that allows for a sample introduced into the injector to be properly injected into a gas chromatography apparatus in a short time period of between 10 and 100 milliseconds. A micro-machined injector having back-flushing capability that allows back-purging of unwanted components in the device and provides clean-up of channels in contact with the sample. Further, a method of operating an injector such that a sample is properly injected and purged from the system to which the injector is operably attached.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to micro-machined back-flush injectors for gas chromatography. The present invention also relates to methods for manufacturing and operating micro-machined back-flush injectors.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1 illustrates a back-flush injector  10  according to the related art. The injector  10  includes a carrier gas inlet  20  connected to a main carrier gas loop  30  that is terminated at a fore-flush valve  35 .  
           [0005]    Off-shooting from the main carrier gas loop  30  is a reference column loop  40  that terminates at a reference column inlet  50 . Also, off-shooting from the main carrier gas loop  30  is a pre-column back-flush loop  60  that terminates at a back-flush valve  70 . A gas chromatography reference column (not shown) is positioned external to the injector  10  and operably connected to the reference column inlet  50 . The reference column, typically used in conjunction with a thermal conductivity detector (not shown), enhances the detector signal and the overall sensitivity of the gas chromatography system.  
           [0006]    The back-flush valve  70  is connected to an analytical column inlet channel  80  and a pre-column outlet channel  85 . The analytical column inlet channel  80  leads to a gas chromatography analytical column (not shown) that is positioned externally to the injector  10 . The pre-column outlet channel  85  leads to a pre-column (not shown) that will be discussed below.  
           [0007]    A sample inlet  90  is also illustrated in FIG. 1. The sample inlet  90  is connected to an inlet channel  100  that, in turn, is connected to a sample valve  110 . The sample valve  110  connects the inlet channel  100  to a dead volume channel  120  that extends to an injection valve  130 .  
           [0008]    One function of the injection valve  130  is to control flow between a pre-column inlet channel  135 , that connects to the pre-column discussed above, and a fixed sample loop  140 , that extends to the fore-flush valve  35 . The fore-flush valve  35  regulates flow between the main carrier gas loop  30 , the fixed sample loop  140 , and a sample chamber  150 . The back-flush valve  70  controls flow from the pre-column back-flush loop  60  into the analytical column inlet channel  80  and the pre-column outlet channel  85 . The functions of these valves will be elaborated upon further when the operation of the injector  10  is discussed.  
           [0009]    The sample chamber  150  terminates at a sample chamber outlet  160  that itself is connected to a switch solenoid  170 , which is external to the injector  10 . The switch solenoid  170  can either be opened to a carrier gas pressure source  180  or a pump  190  that leads to a vent  200 . The pressure of gas in the carrier gas pressure source  180  is approximately the same as the pressure of the gas at the carrier gas inlet  20 . The carrier gas pressure source  180 , when allowed by the switch solenoid  170  to be connected to the sample chamber outlet  160 , delivers carrier gas into the injector  10 .  
           [0010]    During gas chromatography analysis, a carrier gas at a regulated gas pressure is delivered by an outside source to the injector  10  through the gas carrier inlet  20 . This carrier gas fills the main carrier gas loop  30 , the reference column loop  40  and the pre-column back-flush loop  60 . Carrier gas from the same outside source is also delivered to the carrier gas pressure source  180 .  
           [0011]    During operation, the injector  10  injects a gaseous sample to be analyzed via gas chromatography through the pre-column and analytical column discussed above. In order to properly inject the sample, the injector  10  uses five stages of operation. These stages include sampling, dwelling, sample compression, injection, and back-flushing.  
           [0012]    During the operation of gas chromatograph and of the injector  10 , a carrier gas such as, but not limited to, helium, hydrogen and argon, is delivered into the injector  10  through the carrier gas inlet  20  and fills the main carrier gas loop  30 , the reference column loop  40  and the pre-column back-flush loop  60 . The fore-flush valve  35  does not allow the carrier gas to flow into the fixed sample loop  140  or the sample chamber  150 . The reference column inlet  50  allows some carrier gas to flow into the reference column. The carrier gas that enters the reference column does not return to the injector  10 .  
           [0013]    The back-flush valve  70  is also normally open during the idling stage (before the sample is introduced into the injector  10 ) and allows the carrier gas in the pre-column back-flush loop  60  to enter and fill the analytical column inlet channel  80  and the pre-column outlet channel  85 . However, whether the back-flush carrier gas can travel into the fixed sample loop  140  is dependent on the status of the injection valve  130 . When the injection valve  130  is open to the pre-column inlet channel  135 , the carrier gas can then be delivered to the fixed sample loop  140  and the sample chamber  150 . This flow is known as back-flushing.  
           [0014]    The injector  10  can be set to allow back-flushing in the idling stage or can be set to not conduct back-flushing in order to reduce the consumption of the carrier gas. The carrier gas flow that passes through the analytical column inlet channel  80  proceeds to enter the analytical column, passes the detector (not shown), and does not return to the injector  10 .  
           [0015]    During the sampling stage, the sample valve  110  is opened and the pump  190  starts. Alternately, the pump  190  can be started earlier and the sample valve  110  can be opened subsequently. As another alternative, if the sample stream has a positive pressure, use of the pump  190  may not be needed.  
           [0016]    Regardless of the alternative chosen, an inflow of gaseous sample from the sample inlet  90  enters and fills the inlet channel  100 , passes through the sample valve  110  and fills the dead volume channel  120 . The injection valve  130  allows the sample to fill the fixed sample loop  140  but does not allow flow of the sample into the pre-column inlet channel  135 .  
           [0017]    After the gaseous sample has moved through the fixed sample loop  140 , it does not enter into the main carrier gas loop  30  because the fore-flush valve  35  is closed to this path. The sample can only travel into the sample chamber  150  and exits the injector  10  via the sample chamber outlet  160 . Further, because the switch solenoid  170  is opened to the pump  190  during the sampling stage, the sample then travels through the pump  190  and exits the gas chromatographic instrument via the vent  200 .  
           [0018]    After the sampling stage, the sample valve  110  closes and the pump  190  stops drawing the sample into the injector  10 . After approximately 100-500 milliseconds, the sample pressure in the fixed sample loop  140  and sample chamber  150  are set to be in equilibrium with the ambient pressure. This is known as the dwelling stage. Sample compression then follows.  
           [0019]    During the compression stage, the switch solenoid  170  is actuated to open to the carrier gas pressure source  180  and a stream of carrier gas is delivered to the sample chamber  150  via the sample chamber outlet  160 . Since the carrier gas has a higher pressure than the sample which has been set to be at ambient pressure during the dwelling stage, the carrier gas compresses the sample toward the fore-flush valve  35 , the fixed sample loop  140 , the injection valve  130 , the dead volume channel  120 , and the sample valve  110 . Furthermore, during the compression stage, the fore-flush valve  35  does not allow the compressing sample to enter the main carrier gas loop  30 .  
           [0020]    During the injection stage, the injection valve  130  allows flow of the sample into the pre-column inlet channel  135 . Also, the fore-flush valve  35  allows carrier gas from the carrier gas inlet  20  to travel from the main carrier gas loop  30  into the fixed sample loop  140  and sample chamber  150 . However, since carrier gas from the carrier gas pressure source  180  is still compressing the sample, the only direction in which the carrier gas from the main carrier gas loop  30  can move is in one which forces the sample that was in the fixed sample loop  140  to enter the pre-column inlet channel  135  and, ultimately, the pre-column.  
           [0021]    Also, during injection, the back-flush valve  70  closes and stops the back-flushing carrier gas in the pre-column back-flush loop  60  from entering into the analytical column inlet channel  80  and the pre-column outlet channel  85 . This reduces resistance to the injection stream from the fore-flush valve  35  and the main carrier gas loop  30 .  
           [0022]    After the sample has entered and traveled through the pre-column, the sample re-enters the injector  10  through the pre-column outlet channel  85 . Because the back-flush valve  70 , during the injection stage, is positioned to allow the sample to flow from the pre-column outlet channel  85  to the analytical column inlet channel  80 , the sample continues into the analytical column where the gas chromatographic analysis is conducted.  
           [0023]    The above-described injection or fore-flushing stage typically takes several seconds to finish, depending on the particular gas chromatographic analysis undertaken. According to one type of analysis, all components of a sample to be analyzed are moved by the carrier gas towards the analytical column. However, during the movement of the components in the pre-column, some components may travel faster and some may be slower. Hence, the injection or fore-flushing time is selected to allow those components that are important to the analysis to move into the analytical column while leaving behind unimportant components in the pre-column.  
           [0024]    During the back-flushing stage, which follows the fore-flushing stage, the unimportant components are purged away from the injector  10  so that they do not interfere with the analysis. In order to properly back-flush or “purge” all residual sample components in the pre-column from the injector  10 , the back-flush valve  70  is opened to allow carrier gas from the pre-column back-flush loop  60  to flow into both the analytical column inlet channel  80  and the pre-column outlet channel  85 . This causes carrier gas from the carrier gas inlet  20  to back-flush the pre-column on one hand, and to continue to move the components of interest into the analytical column, through the analytical column and towards the detector.  
           [0025]    Once the back-flushing carrier gas passes through the pre-column, the carrier gas travels through the pre-column inlet channel  135  and flows out of the injection valve  130 , through the fixed sample loop  140 , through the fore-flush valve  35  and into the sample chamber  150 . Because the switch solenoid  170  is open to the pump  190  during the back-flushing stage, the back-flushing carrier gas and any residual sample pushed by the carrier gas is released through the vent  200 .  
           [0026]    As can be seen from FIG. 1, a short-coming of the related art injector  10  illustrated has to do with the fact that there is sample trapped in the dead volume channel  120  during the injection process. To understand the problem that the trapped sample presents, one must take into account that the injection carrier gas from the fore-flush valve  35  only takes a small fraction of a second (10-100 millisecond) to move all sample in the fixed sample loop  140  into the pre-column inlet channel  135 . The rest of the injection time or fore-flushing is supposed to have only ‘pure’ carrier gas flowing.  
           [0027]    However, as there is no physical partition between the dead volume channel  120  and the fixed sample loop  140 , the sample in the dead volume channel  120  continuously diffuses into the moving carrier gas stream and get ‘injected’, trace amount by trace amount, into the pre-column and the rest of the device. Since sample components with higher volatility and concentration diffuse faster, the chromatograms of these components are interfered with and unwanted shoulders  33  are found on the gas chromatographic peaks obtained during analysis, as illustrated in the chromatogram shown in FIG. 2.  
           [0028]    Hence, what is needed is a back-flush injector  10  that allows for all of the sample introduced into the injector  10  to be properly injected into the pre-column and analytical column.  
           [0029]    What is also needed is an injector  10  that is capable of back-flushing all of the sample remnant in the injector  10  after sample components of analytical concern have entered the analytical column.  
         BRIEF SUMMARY OF THE INVENTION  
         [0030]    According to one embodiment, a micro-machined back-flush injector that includes a sample inlet, an analytical column inlet channel, and a plurality of channels that connect the sample inlet and the analytical column inlet channel, wherein the plurality of channels include a fixed sample loop connecting a sample valve and a fore-flush valve in the injector.  
           [0031]    According to another embodiment, a method of operating a back-flush injector that includes introducing a sample into the injector, injecting the sample into an analytical device, and purging substantially all of the sample from the injector.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    The invention will be described by way of example, in the description of exemplary embodiments, with particular reference to the accompanying drawings in which:  
         [0033]    [0033]FIG. 1 illustrates a silicon, micro-machined, fixed-volume, back-flush injector according to the related art;  
         [0034]    [0034]FIG. 2 is a gas chromatogram of a sample analyzed using a back-flush injector according to the related art wherein shoulders are present adjacent to the main peaks of the sample;  
         [0035]    [0035]FIG. 3 illustrates one embodiment of a micro-machined, fixed-volume, back-flush injector according to the present invention;  
         [0036]    [0036]FIG. 4 is a gas chromatogram of a sample analyzed using a back-flush injector according to an embodiment of the present invention wherein no shoulders are present adjacent to the main peaks of the sample; and  
         [0037]    [0037]FIG. 5 illustrates another embodiment of a micro-machined, fixed-volume, back-flush injector according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    [0038]FIG. 3 illustrates one embodiment of a micro-machined back-flush injector  10  according to the present invention. As shown in FIG. 3, the fixed sample loop  140  is positioned between the fore-flush valve  35  and the sample valve  110 . Hence, although the injector  10  shown in FIG. 3 undergoes the sampling, dwell, sample compression, injection, and back-flushing stages described above, the configuration of the injector  10  carries out these stages in a more efficient manner.  
         [0039]    During the idling stage of the instrument and injector  10 , carrier gas enters through the carrier gas inlet  20  and fills the main carrier gas loop  30 , the reference column loop  40 , the pre-column back-flush loop  60  and the analytical column inlet channel  80 . When sampling starts, the pump  190  starts. Then, the sample valve  110  is opened, the sample to be analyzed enters the injector  10  through the sample inlet  90  and the sample fills the inlet channel  100 .  
         [0040]    Once the sample reaches the sample valve  110  at the end of the inlet channel  100 , a small portion of the sample fills the dead volume channel  120 . However, this portion of the sample cannot flow into the pre-column inlet channel  135  because the injection valve  130  is closed. The rest of the sample flows through the fixed sample loop  140 , through the fore-flush valve  35 , into the sample chamber  150 , and out through the vent  200  via the switch solenoid  170  that is open to the pump  190 . The fore-flush valve  35  does not allow the sample to flow into the main carrier gas loop  30 .  
         [0041]    The closure of the sample valve  110  and the shutting off of the pump  190 , if it is used, end the sampling stage and start the dwell stage that helps to equilibrate the sample pressure to that of the ambient pressure. The dwell stage takes about 100-500 milliseconds.  
         [0042]    During the sample compression stage, the switch solenoid  170  is opened to the carrier gas pressure source  180  that delivers carrier gas into the sample chamber  150  through the sample chamber outlet  160 . The carrier gas from the carrier gas pressure source  180  compresses the sample in a portion of the sample chamber  150 , in the fixed sample loop  140 , and in the dead volume channel  120  towards the injection valve  130 . During the compression state, the sample valve  110  does not allow flow of the sample into the sample inlet channel  100 .  
         [0043]    During the injection stage, the fore-flush valve  35  allows carrier gas in the main carrier gas loop  30  to flow into the sample chamber  150  and into the fixed sample loop  140 . The carrier gas that flows from the main carrier gas loop  30  into the fixed sample loop  140  pushes the sample in the fixed sample loop  140  towards the now-open injection valve  130 . Because the injection valve  130  is opened during the injection stage, the sample in the fixed sample loop  140  travels through the sample valve  110 , through the dead volume channel  120 , and into the pre-column inlet channel  135 . The sample then flows through the pre-column and pre-column outlet channel  85 .  
         [0044]    When injection or fore-flushing starts, the back-flush valve  70  closes and ceases to deliver carrier gas to the analytical column inlet channel  80  and the pre-column outlet channel  85 . This minimizes resistance to the inflow of sample in the fixed sample loop  140  into the injection valve  130 , the pre-column and the rest of the device. It is important to note that, during this stage, all sample from the fore-flush valve  35  to the injection valve  130  is injected into the pre-column in a short instant, leaving no residual sample that can escape into the pre-column during the rest of the analytical process.  
         [0045]    The injection stage or fore-flushing takes several seconds, depending on the particular gas chromatographic analysis performed. Back-flushing then follows.  
         [0046]    During the back-flushing stage of operation, the back-flush valve  70  allows carrier gas in the pre-column back-flush loop  60  to flow both into the analytical column inlet channel  80  and into the pre-column outlet channel  85 . The back-flushing carrier gas travels through the pre-column, the pre-column inlet channel  135 , the injection valve  130 , the dead volume channel  120  and the fixed sample loop  140 . The carrier gas effectively pushes any remaining sample through the fore-flush valve  35 , out of the sample chamber  150  and, because the switch solenoid  170  is opened to the pump  190  during the back-flushing stage, out through the vent  200 . The carrier gas flow in the direction of the analytical column will continue to move the captured components towards the detector and, during the movement, the components are further separated by the analytical column.  
         [0047]    According to the processes described above, no appreciable amount of sample remains in the dead volume channel  120  during the injection process. Hence, as is seen in FIG. 4, a chromatogram of a sample analyzed via gas chromatography using the injector  10  illustrated in FIG. 4 shows no shoulders  33  on the sides of the peaks. Further, during back-flushing of the injector  10 , carrier gas will clean up all channels that have been in contact with the sample. This minimizes sample carry-over to future gas chromatography analyses that will be performed on other samples using the same apparatus.  
         [0048]    In other words, when using the injector  10  illustrated in FIG. 3, carrier gas typically pushes the sample completely into the analytical column in a short instant (below 100 msec) during the injection stage and leaves no residual sample that can escape into the pre-column during the rest of the injection or fore-flushing time. During the back-flushing stage, the carrier gas pushes residual sample that might be present in the sample chamber  150  out of the injector  10  and cleans up all channels previously in contact with the sample flow. Hence, the chromatograph shoulders  33  and sample residue shortcomings of the injector  10  according to the related art, as illustrated in FIG. 1, are avoided.  
         [0049]    [0049]FIG. 5 illustrates yet another embodiment of the present invention wherein an added channel  210 , that should be, according to certain embodiments, as short as possible, connects the sample valve  110  and the injection valve  130 . The “dead volume” channel  120  now is part of the fixed loop  140  and detached from the injection valve  130 . According to this embodiment, carrier gas can also push the sample quickly and completely into the pre-column during the injection stage while leaving no residual sample. Further, the back-flushing stage ensures that the carrier gas removes residual sample out of the injector  10  and allows subsequent chromatography analyses to be conducted using the same apparatus.  
         [0050]    More specifically, the sample that enters the injector illustrated in FIG. 5 is allowed, by proper opening and shutting of valves, to fill the fixed loop  140  and the added channel  210 . During sampling, the sample valve  110  allows sample flow from the sample inlet channel  100  into the fixed loop  140 . The injection valve  130  is also closed to the added channel  210  during this stage.  
         [0051]    When injecting sample, the injection valve  130  is opened and carrier gas that flows through the fore-flush valve  35  forces sample in the fixed loop  140  and in the added channel  210  to flow into the pre-column inlet channel  135 . This also ensures that no residual sample is allowed to diffuse from the dead volume channel  120 .  
         [0052]    When back-flushing the embodiment illustrated in FIG. 5, the valves are set such that carrier gas flows into the pre-column inlet channel  135 , flows through the added channel  210  and forces all residual sample out of the injector  10  via the added channel  210 . This allows for future analyses to be conducted on other samples, without residual interference, using the same apparatus.  
         [0053]    The foregoing detailed description has been given for understanding exemplary implementations of the invention only and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the art without departing from the scope of the appended claims and their equivalents.