Abstract:
The present invention relates generally to a gas sampling system, and specifically to a gas sampling system for transporting a hazardous process gas to a remotely located mass spectrometer. The gas sampling system includes a capillary tube having a predetermined capillary length and capillary diameter in communication with the supply of process gas and the mass spectrometer, a flexible tube surrounding and coaxial with the capillary tube intermediate the supply of process gas and the mass spectrometer, a heat transfer tube surrounding and coaxial with the capillary tube, and a heating device in communication the heat transfer tube for substantially preventing condensation of the process gas within the capillary tube.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention through an employer-employee relationship between the U.S. Department of Energy and The National Energy Technology Laboratory. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a gas sampling system, and specifically to a gas sampling system for transporting a hazardous process gas from a supply thereof to a remotely located mass spectrometer. 
     BACKGROUND ART 
     Mass spectrometry is an analytical method widely used to determine the atomic weight and structure of molecules. The basic technique is carried out at extremely low pressures, and consists of bombarding a gas-phase sample with an electron beam strong enough to fragment the molecules into their respective ions, after which the gas phase ions are separated and analyzed according to their mass-to-charge ratios (m/e). This technique is practiced using a mass spectrometer, which typically consists of five main components: a vacuum system; an inlet into which the sample is introduced; an ion source which separates molecules into their respective ions; an analyzer which separates those ions based on their mass-to-charge ratios; and a detector. 
     Mass spectrometers are often used to provide real-time analysis of process gases formed during a chemical reaction. However, the mass spectrometer employed is typically located at a remote distance from the process apparatus, typically at a distance of six feet or more. Therefore, a quantity of the process gas must be withdrawn from the process apparatus and transported to the mass spectrometer by way of an inlet system. 
     Conventional inlet systems accomplish this task by transporting a bulk quantity of sample process gas to the mass spectrometer. At or near the mass spectrometer, an aliquot of the bulk sample is removed and fed into the mass spectrometer. The remaining sample gas is then either returned to the process apparatus, or is simply directed to an exhaust stack, incinerator or the like. 
     However, conventional inlet systems are inadequate for transporting hazardous process gases (i.e.: gases which are toxic or explosive), because conventional inlet systems withdraw large quantities of the process gas from the process apparatus. The risk of leakage from the inlet system is ever-present. If the process gas is toxic or possibly explosive then possible leakage is unacceptable. Further, the mass spectrometer employed is usually located at a remote distance from the process apparatus, typically a distance of 6 feet or more, further increasing the risk of leakage. 
     One answer to the above problem is to withdraw smaller amounts of gas, but such conventional inlet systems are inadequate for transporting low flow rate process gasses on the order of less than 10 mL/min. Conventional inlet systems require a process gas flow rate of approximately 60 to 100 mL/min in order to maintain the internal pressure of the mass spectrometer, and to provide a sufficient amount of material to obtain a signal from the instrument. However, where the flow rate of the process gas is on the order of 1 to 5 mL/min, conventional inlet systems cannot be used. 
     Therefore, it is a first object of the present invention to provide an apparatus for safely transporting a hazardous process gas to a mass spectrometer. 
     A second object of the present invention is to provide an apparatus for transporting a low flow rate process gas to a mass spectrometer which is located at a remote distance from the process apparatus. 
     A third object of the present invention is to provide an inlet system which eliminates the need for a process gas return line from the inlet system to the process apparatus. 
     A further object of the present invention is to provide an inlet system which withdraws an aliquot from process apparatus rather than a gas stream introduced into the inlet system. 
     Another object of the present invention is to provide a heating device for substantially preventing condensation of the process gas within the inlet system. 
     Yet another object of the present invention is to provide an inlet system that is easy and economical to manufacture, yet durable. 
     Another object of the present invention is to provide a method for readily calculating the requisite length and diameter for the conduit which transports the gas from the process apparatus to the mass spectrometer. 
     SUMMARY OF THE INVENTION 
     The above-listed objects are met or exceeded by the present gas sampling system for transporting a low flow rate process gas from a remotely located process apparatus or other supply thereof to a mass spectrometer. The gas sampling system includes a capillary tube having a predetermined capillary length and capillary diameter in communication with the supply of process gas and the mass spectrometer, a flexible tube surrounding and coaxial with the capillary tube intermediate the supply of process gas and the mass spectrometer, a heat transfer tube, and a heating device in communication the heat transfer tube for substantially preventing condensation of the process gas within the capillary tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 is a schematic diagram of a prior art mass spectrometer inlet system; 
     FIG. 2 is a schematic diagram of the gas sampling system of the present invention; 
     FIG. 3 is a schematic diagram of an inlet adapter and mass spectrometer showing the relationship of the capillary tube and mass spectrometer in greater detail; 
     FIG. 4 is a schematic diagram of a coupler for attaching the gas sampling system to a mass spectrometer; 
     FIG. 5 is a schematic diagram of the connection of the gas sampling system to the supply or process apparatus. 
    
    
     DESCRIPTION OF THE INVENTION 
     While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail, one specific embodiment, with the understanding that the present disclosure is to be considered merely an exemplification of the principles of the invention and is not intended to limit the invention only to the embodiment illustrated. 
     FIG. 1 illustrates a conventional mass spectrometer inlet system  10 . A bulk quantity of sample gas is introduced from a remotely located process apparatus (not shown) into an inlet tube  12 . Contained within the inlet tube  12  is a capillary tube  14 . In general, capillary tubes  12  employed in conventional mass spectrometer inlet systems  10  do not exceed six inches in length, whereas the inlet tube  12  may exceed six feet in length. The capillary tube  14  extends through a commercial coupler  16  and into a mass spectrometer  20 . Mass spectrometers  20  operate below atmospheric pressure, often at an order of magnitude of approximately10 −6  torr. To reach the desired pressure, the mass spectrometer  20  is provided with a pump  22  for evacuating the mass spectrometer  20 . 
     As the bulk quantity of sample gas flows past the tip  15  of the capillary tube  14 , the capillary tube  14  skims an aliquot from the bulk sample. The excess sample gas, namely the gas which is not skimmed, flows through the coupler  16 , exits the inlet system  10  through an exhaust port  18 , and is returned to the process apparatus, or is incinerated or otherwise treated or disposed. 
     As shown in FIG.  2  and as previously mentioned, the gas sampling system  30  of the present invention includes a capillary tube  32  extending from a supply of process gas (not shown) to the mass spectrometer  20 . The capillary tube  32  is provided for transporting an aliquot of process gas to the mass spectrometer  20 . A flexible tube  34  surrounds and is coaxial with the capillary tube  32  for protecting the capillary tube  32 , and a heat transfer tube  36  surrounds and is coaxial with the flexible tube  34 . 
     A coupler subassembly  38  and an inlet adapter  42  are provided for connecting the sampling system  30  to the mass spectrometer  20 . The capillary tube  32  extends from within the mass spectrometer  20 , through the inlet adapter  42  and coupler subassembly  38 , through the flexible tube  34 , and into the process gas stream of the supply. 
     The gas sampling system  30  also includes a heating device  44  in heat transfer relationship with the capillary tube  32  for “substantially” preventing the process gas from condensing within the capillary tube  32 . The heating device  44  directly contacts and heats the heat transfer tube  36 , coupler  38  and inlet adapter  42 , thereby heating the capillary tube  32 . In the preferred embodiment, the heating device  44  is a conventional resistance heater (i.e.: heat tape/thermocouple combination). Alternate embodiments are contemplated wherein the heating device  44  includes a heated liquid or gas in heat transfer relationship with the gas sample system  30 . 
     The capillary tube  32  utilized in the preferred embodiment is manufactured from fused silica, and has a polyimide protective outer coating. However, capillary tubes  32  which are constructed from other materials (i.e.: brass) are commercially available. In general, the capillary tube  32  selected should be capable of withstanding the temperature of the process gas, and should be inert relative to the process gas. Capillary tubes  32  are readily available, and one with ordinary skill in the art could select a capillary tube which suits the particular application for which the system  30  is to be employed. 
     In addition, the capillary tube  32  utilized in the preferred embodiment has a circular cross section. However, the shape of the cross section of the capillary tube  32  is not critical. An alternate capillary tube  32  may be employed which has a square cross section or the like. 
     Because the capillary tube  32  employed is susceptible to damage (i.e.: hairline fractures, chipping and scoring), it is preferred that the flexible tube  34  have a smooth inner coating for substantially preventing damage to the capillary tube  32  upon insertion into the flexible tube  34 . In the preferred embodiment, the flexible tube  34  is manufactured from a polymeric material, and has a Teflon® inner coating. It has been found that such a flexible tube  34  can withstand temperatures of up to approximately 300° C. For higher temperature applications, alternate embodiments have been employed wherein the flexible tube  34  is manufactured from a metallic material such as brass. However, when metallic materials are employed, care must be take to ensure that no burrs are present at or near the ends of the flexible tube  34  to ensure the capillary tube  32  is not damaged upon insertion into the flexible tube  34 . 
     The heating tube  36  employed in the preferred embodiment is manufactured from a flexible metallic material such as brass or the like. By using brass, the heating tube  36  can withstand direct contact with the heating device  44 , it can be soldered, brazed or welded to the coupler  38  and process gas supply to provide additional protection against leakage of the process gas, and provides additional rigidity to the gas supply system  30 . However, it should be noted that the heating tube  36  is not critical to the invention, and could be combined with the flexible tube  34 . 
     FIG. 3 a schematic diagram of an inlet adapter  42  and mass spectrometer  20  showing the relationship of the capillary tube  32  and mass spectrometer  20  in greater detail. The inlet adapter  42  sealingly connects the capillary tube  32  to the mass spectrometer  20 , thereby providing a pathway for the process gas into the mass spectrometer  20 . The capillary tube  32  extends from within the mass spectrometer  20 , and into and through the inlet adapter  42 . A ferrule  46  in combination with a ferrule compression nut  48  located inside the mass spectrometer  20  sealingly engages the outer surface of the capillary tube  32 . Because the mass spectrometer  20  typically operates below atmospheric pressure, often at an order of magnitude of approximately 10 −6  torr, this seal is required so as to maintain the pressure within the mass spectrometer  20 . 
     An inlet adapter compression nut  52  sealingly connects the inlet adapter  42  to the mass spectrometer  20 . Male threads  54  are provided on one end of the inlet adapter  42 , for a purpose hereafter set forth. Referring to FIGS. 3 and 4 in combination, an inlet opening  56  is provided within the coupler  38  for receiving the inlet adapter  42 . Female threads  58  within the inlet opening  56  mate with or engage the male threads  54 . 
     A heating tube opening  62  extends into the coupler  38 , and terminates at edge  63 . The heating tube opening  62  has a larger diameter than the inlet opening  56 . The heating tube  36  is inserted into the heating tube opening  62  and initially secured by two opposing set screws  64  (see FIG.  2 ). The set screws  64  extend into the heating tube opening  62  through a first and second set screw opening  66  and  68 , respectively. Although two set screws  64  are shown in the figures, alternate embodiments are contemplated wherein only one set screw  64  is utilized, or wherein more than two set screws  64  are utilized. The heating tube  36  is further secured within the heating tube opening  62  by welding, soldering or brazing the heating tube  36  to the coupler assembly  38 . 
     FIG. 5 is a schematic diagram of the connection of the gas sampling system  30  to a gas stream line  72  from the process apparatus (not shown). A t-shaped fitting  74  is inserted into the gas stream line  72  for connecting the gas sampling system  30  to the gas stream line  72 . The flexible tube  34  is inserted into one of the three openings of the fitting  74 . The capillary tube  32  is inserted into and through the fitting  74  so that the tip  76  of the capillary tube  32  extends into the process gas stream. The heating tube  36  is secured to the fitting  74  by welding, soldering or brazing the heating tube  36  to the fitting  74 . 
     Referring to FIGS. 2 through 5 in combination, assembly of the gas sampling system  30  will now be described with reference to a specific example. In this example, a gas sampling system  30  was constructed for transporting a process gas from a process apparatus exhaust pipe to an Extrel Quester GP mass spectrometer located approximately 6 feet from the process apparatus. 
     First, a capillary tube  32  was selected capable of withstanding the temperature and process gas to which the capillary tube  32  would later be subjected. Next, the requisite length and inner diameter for the capillary tube  32  was determined. The capillary tube  32  inner diameter and length are related by the general equation wherein len is the length of the capillary tube  32  in inches, D is the capillary        len   =         (           1        ,          520   ·   len   ·     in   3     ·     torr     -   1       ·     sec     -   1                 )            (   D   )     4          (       P   1   2     -     P   2   2       )         QP   2                              
     tube  32  diameter in inches, P 1  is the pressure in torr of the process gas at the supply, P 2  is the pressure in torr maintained in the mass spectrometer  20 , and Q is the pumping speed of the pump  22  of the mass spectrometer  20  in liters per second. 
     It is prefered that the above-noted equation be solved by first determining the two pressures P 1  and P 2 , the distance between the supply and the mass spectrometer  20 , and the pumping speed Q of the mass spectrometer pump  22 . By knowing the distance between the supply and the mass spectrometer  20 , a capillary tube  34  can be selected which has an inner diameter D corresponding to a sufficient length [[L]] len of capillary tube  34 . 
     In this example, a Pfeiffer TMU turbo pump was provided having a pumping speed Q of 56 liters/sec. The requisite internal pressure P 2  of the Extrel Model Quester GP was 3×10 −6  torr. The pressure of the process gas at the supply P 1  was        len   =                 (           1        ,          520   ·   len   ·     in   3     ·     torr     -   1       ·     sec     -   1                 )                     (     5   ×     10     -   5                     m     )     4          [         (     760                 torr     )     2     -       (     3   ×     10     -   6                     torr     )     2       ]               56                 liters        /        sec   ×         10     -   3                       m   3         1                 liter       ×   3   ×     10     -   6                     torr       =     78                 in                              
     determined to be 760 torr, the inner diameter D of the available capillary tube  34  was 50 μm. From these values, the following capillary tube length len was determined. 
     It should be noted that the above-noted general equation is derived from the equation obtained from  Perry&#39;s Chemical Engineers&#39; Handbook , 6 th  Edition, McGraw-Hill Publishers, pp. 5-16, and wherein P 1  is the absolute pressure at the inlet, P 2  is the absolute pressure at the outlet, f is the Fanning friction factor (dimensionless), len is the length of the capillary tube in feet, G is the mass velocity in lb./(sec.)(sq.ft.), R is the gas constant which equals 1546 (ft.)(lb.force)/(lb.-mole), T is the absolute pressure or degrees Fahrenheit plus 460, g c  is the critical mass velocity in lb./(sec.)(sq.ft.), R H  is the hydraulic radius in feet, and M is the molecular 
     Once a capillary tube  32  having the requisite length and diameter was obtained, a length of heating tubing  36  was cut to length. The heating tube  36  was secured within the coupler  38  by the set screws  64 . The heating tube  36  was further secured within the heating tube opening  62  by welding the heating tube  36  to the coupler  38 . 
     The flexible tube  34  is then cut to length and inserted within the heating tube  34  so as to firmly abut the coupler edge  63 . In the preferred embodiment, the opposite end of the flexible tubing  34  (the end which will be attached to the supply) extends approximately 1 inch from within the heating tube  36 . By inserting the flexible tube  34  into the heating tube  36  prior to inserting the capillary tube  32  into the flexible tube  32 , scoring and chipping of the capillary tube  32  was reduced. Next, the inlet adapter  42  was then inserted into the coupler inlet opening  56 . 
     After the capillary tube  32  was inserted through the flexible tube  34 , and through the coupler  38 , the capillary tube  32  was then secured to the mass spectrometer  20  using the inlet adapter  42  as follows. First, the capillary tube  32  is first inserted into the inlet adapter  42 , and then through the ferrule  46 . The ferrule  46  was inserted into the inlet adapter  42 , and secured within the adapter  42  by the ferrule compression nut  48 . The inlet adapter  42 /capillary tube  32  combination was secured to the mass spectrometer  20  by the inlet adapter compression nut  52 . It is preferred that the capillary tube  32  extend from the inlet adapter  42  and into the ion source (not shown) of the mass spectrometer  20 . 
     Next, the gas sampling system  30  was connected to the exhaust line  72  of the process apparatus (not shown). The t-shaped fitting  74  was inserted into the exhaust line  72  prior to assembly of the gas sampling system  30 . The flexible tube  32  and capillary tube  32  were both inserted into one of the openings in the fitting  74 . Next, the capillary tube  32  was positioned so that its tip  76  extended into the process gas stream. The heating tube  44  was then secured to the fitting  74  by welding the heating tube  36  to the fitting  74 . 
     Finally, the heating tube  36  was wrapped with heating tape (generally discussed above as the heating device  44 ), and a thermocouple/temperature controller combination (not shown) were attached to the heating tape. The thermocouple/temperature controller (not shown) were provided for maintaining the process gas above its boiling point temperature as it passes through the gas sampling system  30 . An insulation tape was wrapped over the heating tape to reduce heat loss. 
     The foregoing description of an embodiment of the invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The description was selected to best explain the principles of the invention and practical application of these principles to enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention not be limited by the specification, but be defined by the claims as set forth below.