Abstract:
A chromatograph includes an inlet for receiving a sample and a pressurized hydrogen gas flow and in response providing a sample/fluid mixture; a separation column located in a temperature-controlled zone for receiving the sample/fluid mixture and for providing a column effluent stream; a detector for receiving the effluent stream and for providing a detector output stream; and a gas storage system for receiving the detector output stream (and optionally a split flow and a septum purge flow in the instance of a split/splitless inlet) and for storing the received gas stream for subsequent reuse. In the preferred embodiment of the gas storage system, a plurality of metal hydride storage (MHS) systems are used.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is related to commonly-assigned U.S. patent application Ser. No. ______, filed on even date herewith, in the name of William H. Wilson, and entitled “CHROMATOGRAPH HAVING A GAS RECYCLING SYSTEM”.  
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to methods and apparatus for use in analytical instrumentation for the detection of an analyte in a carrier fluid, and in particular to analytical instrumentation having a closed-loop system for storage and reuse of hydrogen carrier gas.  
         BACKGROUND OF THE INVENTION  
         [0003]    A simplified schematic view of a conventional chromatograph  100  is shown in FIG. 1. The illustrated chromatograph  100  is representative of a Hewlett-Packard 6890 Gas Chromatograph. Analytical instruments such as the gas chromatograph  100  are known for use in determining the chemical composition of a sample which is typically injected at an inlet  112  into a carrier gas stream provided by a carrier gas source  111  through a manifold  113 . A fluid mixture of the sample and the carrier gas is directed through a separation column  114  located within an oven  116  and exposed to a controlled temperature environment provided by a heater  118 . The separation column  114  includes a stationary phase coating on the interior of the column. The interaction of the constituent compounds in the sample with the stationary phase cause differing chemical compounds in the sample to travel through the separation column at different rates and to leave the separation column at different times. The presence of compounds in the column effluent gas is sensed by a detector  124 . A detector output signal is provided to a controller  126  and a computer  122  on signal lines  128 ,  130 . The compound of interest typically called an analyte.  
           [0004]    A significant shortcoming in the conventional gas chromatograph is due to the loss of one or more gas streams that are typically vented to the atmosphere from the inlet  112  or the detector  124 . The majority of the composition of such streams is carrier gas; for example, if the inlet  112  is constructed as a split/splitless inlet, much of the carrier gas employed by the chromatograph  100  is vented away from the inlet  112 . Accordingly, a column with a 1 ml/min flow rate and a split ratio of 100:1 will vent 100 times the amount of gas actually required to carry a sample through the column  114  for an analysis. Six liters of carrier gas at inlet pressure are typically lost to the surrounding environment during one hour of analysis. However, if the carrier gas were to be conserved, such a volume of gas could easily supply a column flow for many more hours of continuous operation.  
           [0005]    The high rate of consumption of carrier gas observed in the conventional apparatus is one of the major factors that have inhibited the development of portable instrumentation, and has also limited the deployment of most bench top (i.e., non-portable) chromatographs in underdeveloped areas of the world where cylinders of carrier gas are in short supply.  
           [0006]    There thus exists a need for analytical instrumentation that employs a carrier gas storage system wherein, among other factors, the flow of the carrier fluid is conserved and reused to an extent satisfactory for most analytical applications.  
         SUMMARY OF THE INVENTION  
         [0007]    The advantages of the invention are achieved in a preferred embodiment of an analytical instrument, preferably provided in the form of a chromatograph, wherein a closed loop carrier gas storage system receives the gas streams that would be otherwise be vented to the atmosphere in a conventional apparatus, filters the received gas streams in order to remove the residual impurities, and stores the filtered gas stream in a gas storage system, such that the stored gas may thereafter be reused.  
           [0008]    The preferred embodiment includes an inlet for receiving a sample and a pressurized stream of carrier gas supplied from a carrier gas reservoir, and in response, providing a sample/fluid mixture and (in some embodiments hat operate a split/splitless inlet) an inlet output stream in the form of a combination of a spit flow and a purge flaw; a separation column located in a temperature-controlled compartment for receiving the sample/fluid mixture and for providing a column effluent stream; a detector for receiving the column effluent stream and providing in response a detector output signal and a detector output stream; and a gas storage system for filtering and storing the received gas streams for subsequent reuse. The detector generates an output signal, whereby one or more characteristics of the effluent stream that are related to the analyte of interest may be represented by the output signal.  
           [0009]    Certain embodiments may further include a control system including a computer for sensing the volumetric flow rate of the fluid mixture entering the column and for generating a respective flow rate signal and for sensing the column input pressure and generating a respective input pressure signal, and an electronic pneumatic controller including means for receiving the flow rate signal and input pressure signal, for controlling the valve so as to control the input pressure and the volumetric flow rate of the carrier fluid.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a simplified block diagram of an analytical instrument constructed as a chromatograph in accordance with the prior art.  
         [0011]    [0011]FIG. 2 is a simplified schematic view of a preferred embodiment of an analytical instrument constructed according to the present invention.  
         [0012]    [0012]FIG. 3 is a simplified schematic view of first preferred embodiment of a gas storage system operable in the instrument of FIG. 2.  
         [0013]    [0013]FIG. 4 is a simplified schematic view of second preferred embodiment of a gas storage system operable in the instrument of FIG. 2.  
         [0014]    FIGS.  5 A- 5 C are simplify schematic representations of the valve sequence operable in the second preferred embodiment of the gas storage system of FIG. 4.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    The present invention will find useful application in a variety of analytical systems that operate with use of hydrogen gas as a carrier gas for detection of an analyte present in one or more carrier gas streams. Gases are the preferred fluids according to the practice of the present invention, and hydrogen gas is the preferred carrier gas, and therefore the following description of the invention will include a description of the arrangement, construction, and operation of a gas chromatographic analytical system (hereinafter, a chromatograph).  
         [0016]    For the purposes of the following description, the terms “fluid” and “pneumatic” will be considered to pertain to all types of fluids. “Fluid-handling function” refers to at least one of the following functions with respect to one or more fluid streams: initiation; distribution; and redirection; termination; control of temperature, pressure or flow rate; and sensing of temperature, pressure, or flow rate. “Fluid-handling functional device” refers to a device that provides one or more fluid-handling functions with respect to one or more fluid streams. “Electronic pneumatic control” and “EPC” refers to programmed electronic control of fluids and fluid handling functions, among which are included the control of volumetric flow rate and pressure of a fluid stream in a chromatograph, as for example in accordance with the invention disclosed by U.S. Pat. No. 4,994,096, and U.S. Pat. No. 5,108,466 in the names of Klein, et al., the disclosures of which are incorporated herein by reference, and to subsequent advances known in the art for programmed electronic pneumatic control of pressure, temperature, and/or flow rate of fluids in a chromatograph.  
         [0017]    In a significant departure from the prior art, the present invention will be understood to overcome a major problem of chromatographic systems that employ a conventional carrier gas source, and also will be understood to provide improved detection of a wide range of analytes present in a fluid stream.  
         [0018]    In the embodiments illustrated in the Figures and described below, like nomenclature and numerical identifiers refer to identical or equivalent structures; a single line indicates an electronic signal line capable of transmitting an electronic signal; and double parallel lines indicate a pneumatic flow path capable of bearing a fluid stream.  
         [0019]    A first preferred embodiment of a analytical instrument is shown in FIG. 2 and is generally designated as a chromatograph  200 . The illustrated chromatograph  200  is designed to operate using hydrogen gas as its carrier fluid. Accordingly, a hydrogen gas storage system  150  is initially charged with hydrogen gas. The manifold  113  is especially configured to include selectable pneumatic pathways for receiving the combine hydrogen gas flows that would otherwise be vented from the chromatograph  200  (such as from the detector  124  and/or the inlet  112 ). For example, in an embodiment wherein the inlet  112  is optionally provided in the form of a split/splitless inlet, the carrier gas flow from the storage system  150  is divided by the manifold  113  into three streams. A first stream is directed to a septum purge line, a second stream is directed to a split line, and a third stream is directed to the column  114 . A chemical trap (not shown) may be employed in the manifold  113  on the split line to prevent most of the sample material from passing through the split line, such that the compounds capable of traversing the chemical trap are restricted to volatile compounds. The output of the column  114 , septum purge line, and the split line may then be combined in manifold  113  and directed under the control of the controller  126  as an “incoming” gas stream to the gas storage system  150 . As will now be described, the gas storage system  150  then filters the incoming gas stream and stores the hydrogen gas component of such incoming gas stream for subsequent reuse as the carrier gas needed by the chromatograph  100 .  
         [0020]    As illustrated in FIG. 3, a first preferred embodiment of a gas storage system  300  may be constructed to include first and second valves V 1 , V 2 ; flow restrictors R, a filter F, and first and second metal hydride storage (MHS) systems S 1 , S 2  each having heaters H. The fitter F may be provided in the form of a chemical filter, an adsorbent trap, or a noble metal membrane that passes only hydrogen. In certain applications, the gas storage system  150  may also incorporate a pumping system constructed in the form of an electro-chemical pump (not shown) operable to boost the pressure of the incoming gas flow so as to better charge the metal hydride storage systems S 1 , S 2 .  
         [0021]    A storage cycle may be understood as follows. The system S 1  is initially fully charged with hydrogen provided by an external source (not shown), and the system S 2  is initially evacuated. The system S 1  is heated to approximately 60 degrees Centigrade while the system S 2  remains at ambient temperature. The heated environment in system S 1  generates approximately 100 PSI of hydrogen pressure in system S 1  and allows valve V 2  to be actuated to provide hydrogen gas to the storage system output, such that the inlet  112  receives the hydrogen gas at a selectable hydrogen gas pressure. Meanwhile, an “incoming” gas stream is developed from the gas streams from the inlet  112  and/or detector  124  that would ordinarily be vented to the atmosphere. The incoming gas stream is first passed through filter F and the resulting filtered gas stream is directed to the evacuated system S 2  through a restrictor R. The system S 2  then begins to charge with hydrogen while the system S 1  continues to be depleted due to the demands of the chromatograph  200  for hydrogen gas. When the system S 1  is nearly depleted, the system S 2  is heated to 60 degrees Centigrade and the vales V 1 , V 2  are rusted to provide hydrogen gas to the system output from the system S 2 , and to redirect the incoming flow to the system S 1 . As the system S 1  drops in temperature, it begins to absorb hydrogen from the incoming flow thus received, thereby storing the hydrogen for subsequent re-use. This storage cycle may be repeated so as to provide a sufficient and uninterrupted supply of carrier gas for repeated use of the chromatograph  200 .  
         [0022]    As illustrated in FIGS.  4 - 5 , a second preferred embodiment of a gas storage system  400  may be constructed to include certain components of like nomenclature and accordingly of identical construction to those already described with reference to FIG. 3, but with the addition of a third metal hydride storage (MHS) system S 3  and third and fourth valves V 3 , V 4 . As shown in FIG. 5A, the system S 1  is initially charged with hydrogen and systems S 2  and S 3  are evacuated. Valve V 1  directs the incoming flow to valve V 2  so as to bypass system S and thereafter into the system S 2 . Meanwhile, system S 1  is heated to generate an elevated hydrogen gas pressure therein. Valve V 3  directs a hydrogen gas flow from system S 1  to valve V 4  which then directs the hydrogen gas flow to the storage system output. System S 2  continues to collect hydrogen gas while system S 3  is idle. As shown in FIG. 5B, when system S 1  is sufficiently depleted of hydrogen gas, valve V 1  is actuated so as to direct the incoming flow received at the storage system input to system S 3 . Valve V 3  is actuated to isolate systems S 1  and S 3 . Valve V 4  is actuated to direct a flow of hydrogen gas from the newly charged system S 2  to the storage system output. As shown in FIG. 5C, valves V 1  and V 2  may be actuated to direct incoming flow to the system S 1 . Valves V 2  and V 4  are actuated to isolate system S 2  and valve V 3  is set to allow hydrogen gas to flow from system S 3  through valves V 3  and V 4  to the storage system output.  
         [0023]    In the above described embodiments, the hydrogen gas is circulated in a closed loop storage system. Accordingly, the chromatograph  200  can operate for an indefinite period. For example, the only path by which hydrogen gas would be lost in chromatograph  200  would be by leaks present in the pneumatic lines, fittings, connectors, and other hydrogen bearing portions of the chromatograph  200 . The propensity for such leaks may be reduced by techniques and practices known to those skilled in the art.  
         [0024]    Storage of hydrogen gas is an advantage for the construction of portable or hand-held chromatographs, remotely situated “online” chromatographs for environmental monitoring or process control, and bench top chromatographs for use in underdeveloped countries where a supply of conventional gas cylinders is unreliable or simply not feasible.  
         [0025]    Although hydrogen permits the fastest chromatography, its use has heretofore been avoided because of the potential exit hazard. Accordingly, because the carrier gas is conserved, the illustrated chromatograph  200  may be constructed for safe operation with hydrogen carrier gas. That is, the hydrogen gas may be maintained in a closed loop such that much of the hazard presented by the use of hydrogen gas is contained. This would permit the chromatograph  200  to be placed in an adverse environment without compromising most safety requirements.  
         [0026]    Furthermore, the storage of the hydrogen gas in a closed loop system minimizes the chances for contamination of Wee hydrogen gas stream. The analytical performance of the illustrated chromatograph  200  is accordingly enhanced.  
         [0027]    While the invention has been described and illustrated with reference to specific embodiments, those skilled in the art will recognize that modification and variations may be made without departing from the principles of the invention as described herein above and set forth in the following claims.