Abstract:
A flow-through vacuum degassing unit for degassing a liquid includes a vacuum chamber adapted to be connected to a source for creating a vacuum in the chamber, an inlet and an outlet connection for admitting and discharging liquid to be degassed, a tube for conducting the liquid through the chamber, wherein the tube is a gas permeable polymeric resin material, and a control circuit for operating the source for creating a vacuum in the chamber responsive to a sensed vacuum level in the chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vacuum degassing system and more particularly pertains to a method and apparatus associated with removing gases from liquids in a flow-through relation in which an elongated gas-permeable tube addresses a vacuum chamber evacuated by a variable speed vacuum pump and gas is transferred by diffusion through the walls of the tube. The system is particularly suited to the removal of air or oxygen from the mobile phase associated with high performance liquid chromatographic equipment. 
     2. Description of the Prior Art 
     There are many chemical applications, particularly analytical applications, involving the use of liquid solvents, reactants or the like in which the presence of dissolved gases, particularly air, is undesirable. A prime example of such an application relates to the mobile phase in high performance liquid chromatography where the presence of even small amounts of dissolved gases, and in particular oxygen, interferes with the accuracy and sensitivity of the results obtained. For example, air dissolved in the mobile phase can manifest itself in the form of bubbles which causes noise and drift as the mobile phase passes through the detector. If the dissolved species be chemically active, as in the case of oxygen in air, it can additionally produce unwanted changes or deterioration in the mobile phase. Of course, the detrimental effect of the dissolved species is related to the relative concentration of the species in the mobile phase. These undesirable species usually are removed by a degassing process. It correspondingly follows that the more efficient the removal or degassing system is, the more desirable it will be. 
     The degassing of liquid materials has been necessary to the success of many processes and, consequently, the process has been pursued actively in several forms for a long period of time. Techniques have included heating or boiling the liquid to be degassed, exposing the material to a reduced pressure environment or vacuum and using combinations of heat and vacuum to reduce the amount of dissolved gases in the liquid. Ultrasonic energy has also been employed. As conventionally applied, however, these traditional techniques have generally fallen short of the desired degree of separation efficiency. Additionally, a means of degassing solvent involving the passing of a fine stream of bubbles of inert gas such as helium through the solution to be degassed has been shown by Bakalyar et al. in U.S. Pat. No. 4,133,767, and in apparatus such as that disclosed by Sims et al. in U.S. Pat. No. 4,994,180, co-invented by the co-inventor in the present application and assigned to the same assignee as the present invention. 
     Vacuum degassing through a membrane apparatus has long been known, and generally utilizes a length of relatively small diameter, thin-walled semi-permeable synthetic polymer resin material contained within an enclosed chamber held under a reduced pressure or vacuum in which the liquid to be degassified is caused to flow through the tube. One such apparatus is shown by Sims in U.S. Pat. No. 5,340,384, co-invented by the co-inventor in the present application and assigned to the same assignee as the present invention. Other such devices are shown in U.S. Pat. Nos. 5,183,486, 4,430,098, and 3,668,837. 
     While each of these devices employ a flow-through tube vacuum degassing approach, there remains a need, particularly with devices associated with high performance liquid chromatography instruments, to make degassing of solvents, and in particular the mobile phase, more efficient. One particular limitation or drawback associated with present devices concerns the efficiency of the degassification operation with respect to the composition of the tubing itself. Materials presently used in degassing applications include PTFE, PFA, and silicone rubber. These materials, while generally suitable for this application, require that the wall thickness be as thin as possible due to the gas permeability of materials typically utilized for these applications. A large internal diameter tube is disadvantageous as the gas must diffuse through a longer path from the center of the flow to the inner wall surface, thereby requiring a long tube. Additionally, a tube of greater length increases flow resistance through the overall system the resistance being a linear function of tubing length (assuming laminar liquid flow through the tubing). Liquid flow resistance is an inverse function of tubing internal diameter to the fourth power. 
     Amorphous perfluorinated copolymers reportedly have permeabilities of up to 2 or 3 orders of magnitude higher than those of PTFE. It has been found by the present inventors that by using amorphous perfluorinated copolymers, such as those marketed by Du Pont under the tradename Teflon AF that permeabilities of up to about 1 order of magnitude or greater are experienced. Nevertheless, in the fabrication of degassing tubes, greater gas mass transfer rates can be achieved with tubes of Teflon AF having increased wall thicknesses, thereby permitting the undertaking of applications requiring higher pressures. Advantageously, tubes of smaller internal diameter and shorter length offer reduced internal volumes. Low flow resistance is accomplished with multi-lumen tubing arrangements. 
     Because of the enhanced gas permeability property of materials utilized in accordance with the present invention, the diffusion rate of atmospheric gases from the liquid being degassed through the tubing wall is significantly increased. It appears likely that the increased gas permeability enhances the function of free (void) volume in the polymer component. 
     As a further feature of this invention, it has been found that very stable reduced pressure or vacuum is achieved within the vacuum chamber. This feature is possible due to the operational characteristics of the vacuum pump. In initial operation, the pump (typically operated @ 400 RPM) reduces pressure inside the vacuum chamber. When the partial pressure inside the chamber begins to asymptotically approach a maximum differential value (typically around 60 mm Hg absolute) the speed is substantially reduced, such as to about 60 RPM. The pump is run continuously at this reduced rate, with the vacuum then slowly descending to a “constant vacuum level” with the pressure remaining constant for so long as the pump is running. This “constant vacuum level” provides significant advantages in that it eliminates vacuum hysteresis (pressure) which typically is in the range of 15-25 mm Hg as the result of cycling the pump on and off in other systems. Through this operational feature, variations in remaining atmospheric gas in the mobile phase exiting the degassing apparatus to the liquid chromatograph are also eliminated. This feature provides technical advantages because of the resulting HPLC detector base line stability. Superior vacuum level, typically in the range of 30 mm Hg or less, also reduces the absolute concentration of dissolved gases in the mobile phase, which improves the flow rate precision of the HPLC pump. In addition, longer life expectancy is available because of low RPM. 
     Accordingly, it is a principal object of the present invention to provide a more efficient vacuum degassing system of the flow-through type using a tube or multiple tubes formed from an amorphous perfluorinated copolymer. 
     A further object of the present invention is to reduce the required inside diameter and length of the degassing tube. 
     A still further object of the present invention is the provision of a tube having a single lumen. 
     A yet further object of the present invention is the provision of a tube having multiple lumens. 
     Another object of the present invention is the provision of a variable speed pump and run continuously for evacuating the vacuum chamber, with the effects being a reduction or elimination of hysteresis and increased vacuum pump life expectancy. 
     Still further object of the present invention is the utilization of a flow restrictor in the system to facilitate bleeding or flushing of the head with atmosphere to prevent solvent vapor build up. 
     Still another object of the present invention is to provide a means for interconnecting liquid chromatography instrument components which simultaneously degasses the mobile phase in transit between the components. 
     It is yet a further object of the present invention to provide an improved vacuum pump head bleed or flush system in the form of a vent frit including a sintered metal plug, thereby eliminating the necessity of the solenoid operated vent valve typically utilized in current systems. 
     A still further object of the invention is to provide an improved connection system for the tube associated with a flow-through vacuum degassing apparatus. 
     SUMMARY OF THE INVENTION 
     By means of the present invention, the efficiency of a flow-through vacuum degassing system utilizing an elongated gas-permeable tube is improved by reducing the required inside diameter and length of the tube. This is achieved by forming the tube from an amorphous perfluorinated copolymer such as Teflon AF. Amorphous perfluorinated copolymers have been reported to have permeabilities of up to 2 or 3 orders of magnitude greater than other semipermeable polymeric resins utilized in degassing applications. By using such copolymers, it has been found that it is possible to significantly reduce the length of tubing utilized, which correspondingly and proportionally reduces the internal volume. All of this is achieved without either reduction or compromise in degassing performance. 
     Gas mass transfer rates are further improved by the present invention through the use of a tube having multiple lumens. The multiple lumen tubing arrangement offers greater degassing efficiency by providing a greater surface area through which the gas may travel and a smaller tubing diameter. A multiple lumen tubing of smaller diameter provides for reduced internal volume and lower flow resistance to the mobile phase sought to be degassed. 
     The degassing chamber of the invention includes an injection-molded plastic housing which is preferably sealed with O-rings or other sealing devices. The chamber is provided with a vacuum connection and liquid inlet and outlet connections for a coil of gas-permeable tubing. The coil may be either a single lumen tube or a multiple lumen tube. A pair of interface grids each positioned between a bulkhead fitting and an inlet or outlet nut is fabricated of TEFZEL, KEL-F, PTFE or PEEK for use with the multiple lumen tube and includes a center bore and a plurality of radially spaced bores for sealingly receiving the tubes in an adhesiveless manner when they are pulled through during assembly of the degassing chamber. Teflon AF tubing may be advantageously utilized, when placed through a slightly over-sized hole, with the tubing being pushed through and compressed on the ferrule with a nut. Compression seals utilizing TEFZEL ferrules have been found highly useful and are preferred in creating seals without requiring adhesives. The bores have a diameter slightly smaller than the nominal diameter of the tubes providing a seal between the TEFZEL, KEL-F, PTFE or PEEK composition of the grid and the material of the tubes. 
     In an alternative embodiment of the invention, a degassing transfer line for interconnecting liquid chromatography system components includes a length of Teflon AF tubing extending between opposed ends of the transfer line and disposed within an elongate tube formed of an adhesive-lined, heat shrinkable material, with this alternative embodiment being described in detail herein. In this alternative embodiment, opposed ends of the tube sealingly surround a PTFE/FEP dual-shrink tubing through which the Teflon AF tubing extends. Distally of each opposed end of the elongate tube, a nut is sealingly attached to the PTFE/FEP tubing. Distally of the nut, ferrules are provided for connection to the various LC components. A vacuum adapter is provided for communication between an interior of the elongate tube and a vacuum source to evacuate the interior of the elongate tube and thereby degassing the mobile phase as it flows through the Teflon AF tubing. 
     Another feature of the present invention provides for a variable speed vacuum pump which evacuates the vacuum chamber. In a first preferred mode of operation, electronic control means responsive to a sensed vacuum level are operable to drive a brushless DC stepper motor which in turn drives an eccentric shaft coupled to a two-stage diaphragm pumping mechanism at a high speed to quickly evacuate the vacuum chamber (400 RPM) and at a low speed, such as about 60 RPM, for continuous operation and long life of the degassing system. In an alternative second mode of operation, a vacuum setpoint is set and the pump is intermittently driven at the high speed when the sensed pressure rises above the setpoint, with the rate dropping once the desired pressure reduction has been achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, wherein like numerals designate like parts throughout the same: 
     FIG. 1 is a schematic diagram showing the components of the present invention; 
     FIG. 2 is a cross sectional view of the vacuum chamber in accordance with the present invention showing a single lumen tube; 
     FIG. 3 is a cross sectional view of the vacuum chamber of FIG. 2; 
     FIG. 4 is a cross sectional view of the vacuum chamber in accordance with the present invention showing a multiple lumen tube; 
     FIG. 5 is a cross sectional view of the vacuum chamber of FIG. 4; 
     FIG. 6 is a cross sectional view of a transfer line having a single lumen according to the present invention; 
     FIG. 7 is a cross sectional view of a transfer line having a multiple lumen according to the present invention; 
     FIG. 8 is a cross sectional view of the vacuum pump of the present invention; and 
     FIGS. 9A,  9 B and  9 C are a flow chart showing the two modes of operation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The objects and advantages enumerated above together with other objects, features and advances represented by the present invention will now be presented in terms of a detailed embodiment described with reference to the attached drawing Figures which are intended to be but representative of many possible configurations of the invention. Other embodiments and aspects of the invention are recognized as being within the grasp of those having ordinary skill in the art. With particular initial reference to FIG. 1, there is shown generally at  10  a vacuum degassing system having a vacuum chamber  12 , a vacuum pump  14  and a vacuum sensor  20  operably coupled to the vacuum chamber  12 , an electronic control means  16  operably coupled to the vacuum pump  14  and to the vacuum sensor  20 , and an operator interface  18  operably coupled to the control means  16 . 
     The vacuum chamber  12  is preferably made of high-impact polymer material, such as high-density polyethylene or polypropylene, which can be readily assembled with sealing O-rings or heat welded together to form a strong, relatively inert, non-metallic housing  21 . A single lumen degassing tube is shown schematically in FIG. 2 at  22  as being loosely constrained by a central shaft or spool member  24  to form a coil. In the preferred embodiment the degassing tube is made from an amorphous perfluorinated copolymer such as Teflon AF. The tube  22  is connected between inlet and outlet connections  26  and  28 . The vacuum chamber  12  further contains a connection as at  30  for a vacuum line  32 , which is designed to be connected to the vacuum pump  14 . Additionally, a connection as at  33  for a vacuum line  35  operably coupled to the vacuum sensor  20  is shown. 
     The inlet and outlet connections as at  26  and  28  include a short length of interface tubing  34  which may be high strength, high density, relatively inert material, such as PEEK or, if metal, titanium or stainless steel and having an end as at  36  over which the degassing tube  22  is fitted. The interface tube  34  is further connected using an appropriate sealing ferrule  38  which may be of TEFZEL or other high impact inert material used in conjunction with a nut  40  to connect to the bulkhead union  42 . 
     In accordance with an important aspect of the invention, a multiple lumen tubing  44  made from Teflon AF is shown schematically in FIG. 4 at  44  as being loosely constrained by a central shaft or spool member  46  to form a coil. A pair of interface grids  48  preferably in a form of a TEFZEL reverse or inverted ferrule, each positioned between a bulkhead fitting as at  42  and an inlet or outlet nut as at  40  is fabricated of stainless steel, KEL-F or PEEK for use with the multiple lumen tube  44  and includes a center bore  50  and a plurality of radially spaced bores  52  for sealingly receiving the tubes in an adhesiveless manner with nut  40  compressing reverse or inverted ferrule  48  when the tubes are pulled through during assembly of the degassing chamber  12 . 
     With reference to FIG. 6, an alternative embodiment of the present invention generally designated  60  is shown. The alternative embodiment  60  comprises a degassing transfer line in the form of an elongate tube  61  for use in interconnecting liquid chromatography system components. A length of gas permeable tube  62 , preferably Teflon AF, extends between opposite ends  64  and  66  of the transfer line. An enclosed interior portion  68  of the elongate tube  61  is formed by sealing opposed ends  70  and  72  of an adhesive-lined, heat shrinkable material about spaced sections of a PTFE/FEP dual-shrink tubing  71  and  73  disposed in surrounding relationship to the tube  62 . Distally of the opposed ends  70  and  72 , a pair of nuts  74  and  76  in conjunction with a pair of ferrules  78  and  80  are formed in surrounding relationship to the tubing  71  and  73  for connecting the transfer line between liquid chromatography system components. A vacuum adapter  82  is provided for communication between the interior portion  68  of the elongate tube  61  and a vacuum source to evacuate the interior portion  68  and thereby degass the mobile phase as it flows through the tube  62 . 
     An alternative embodiment of the transfer line is shown in FIG.  7  and generally designated  90 . The degassing transfer line  90  is similar to the transfer line  60  but is provided with a multiple lumen tube  92  in place of the single lumen tube  62 . 
     In accordance with another important aspect of the present invention, a variable speed vacuum pump  14  is operable in a first mode of operation to continuously evacuate the vacuum chamber  20  or to intermittently evacuate the vacuum chamber  20  in a second mode of operation. The vacuum pump  14  is shown in FIG.  8  and includes a two stage, series diaphragm pumping mechanism. A manifold  100  includes a first stage head  102  and a second stage head  104 . The manifold further includes an intake duckbill check valve  106  associated with the first stage and an outtake duckbill check valve  108  associated with the second stage. The first and second stages are in fluid flow communication with each other through a tube  109  attached to the first stage through a duckbill check valve  110  and attached to the second stage through a barbed fitting  112 . A vent frit including a sintered metal plug  114  provides a precise vent or bleed (precision flow restrictor from Mott metallurgical) to vent solvent vapors that may build up. Furthermore, the “precision leak” of the sintered metal plug advantageously replaces the solenoid operated vent valve which has been typically utilized. This bleed feature is normally placed in the first stage during the intake stroke thereof to prevent the buildup of vapor and to reduce the exposure of the pump diaphragm to degassed vapors entering the first stage and thereby reduce diaphragm and check-valve swelling and prolong the useful life of the vacuum pump  14 . As further described hereinbelow, during the exhaust stroke of the first stage, the second stage is on its intake stroke which ensures very little leakage to the outside. 
     A unitary diaphragm  116  extends from the first stage to the second stage. In the first stage, the diaphragm  116  is affixed to a rod  118  by means of a press fit pin or screw  120  received in a bore  122  formed in the rod  118 . A washer  124  and O-ring  126  seal the diaphragm  116  to the rod  118 . The diaphragm  116  is affixed to a rod  128  in the second stage in a similar fashion. The diaphragm  116  is preferably formed of PTFE which is inert and tolerant to exposure to common liquid chromatography solvents and vapors thereby insuring the longevity of the diaphragm  116 . 
     Rods  118  and  128  are shown connected to a shaft  130  coupled to a motor  132  at one end thereof. The shaft  130  is rotatably supported in a frame by means of spaced ball bearings  125  and  127 . The rods  118  and  128  are connected to the shaft  130  by means of needle bearings  134  and  136  operably coupled to opposed eccentric portions  138  and  140  fixedly and spacedly attached to the shaft  130 . By this arrangement, rotation of the shaft  130  results in reciprocal motion of the first and second stages  180  degrees out of phase with one another. 
     The motor  132  is preferably a brushless dc stepper motor responsive to control means providing for closed loop control thereof. A pressure sensor  20  is operable to sense the vacuum level inside the vacuum chamber  12  and generate a voltage output which is ratiometric to the vacuum level and a supply voltage. The sensor output is amplified by an instrumentation amplifier and is then converted to a pulse width modulated signal which is sent to a microcontroller. A high-current pulse width modulated, uni-polar controller chip operably coupled to the microcontroller drives the motor  132  in such manner that in a first continuous mode of operation, the motor  132  runs at a high speed to quickly evacuate the vacuum chamber and at a low speed for continuous operation of the degassing system. A second intermittent mode of operation is provided wherein a vacuum setpoint is set and the pump is intermittently driven at the high speed when the sensed vacuum falls below the setpoint. 
     Firmware embedded in the microcontroller of the present invention provides for a user interface allowing for selection of either mode of operation as shown at  170  in FIG.  9 . In the first continuous mode of operation ( 210 ), the vacuum level is optionally displayed ( 220 ). To indicate that the vacuum degassing system is not yet operable, “not ready” is displayed ( 230 ). A setpoint is then obtained for comparison to the vacuum level in the vacuum chamber  12  ( 240 ). The setpoint may be entered by the user or optionally programmed in the firmware. A pump down timer is next set for five minutes ( 250 ). The pump is then driven at the high speed to evacuate the vacuum chamber  12  ( 260 ). The vacuum sensor value is read ( 270 ) and if after a one second delay ( 280 ) it is less than the set point ( 290 ), the pump is driven at the high speed if the user has not changed the mode of operation ( 300 ). If the sensed vacuum is greater than the setpoint then the vacuum pump is driven at the lower speed and a value for a leak condition setpoint is read ( 310 ). The sensed vacuum value is then read ( 320 ) and if the value is less than the leak condition setpoint ( 330 ) “leak” is displayed to the user ( 360 ). If the sensed vacuum value is greater than the leak condition setpoint then a change in mode of operation is checked for ( 340 ). If the user has selected the second intermittent mode of operation, then the program jumps to ( 200 ). If the user has not changed the mode of operation then after a one second delay ( 350 ) the sensed vacuum value is again compared to the leak condition setpoint ( 330 ) and the loop is repeated until either the user selects a different mode of operation or the sensed vacuum level falls below the leak condition setpoint. 
     In the second intermittent mode of operation ( 400 ) the vacuum level option selected is displayed ( 410 ) and to indicate that the vacuum degassing system is not yet operable, “not ready” is displayed ( 420 ). A setpoint is then obtained for comparison to the vacuum level in the vacuum chamber  12  ( 430 ). The setpoint may be entered by the user or optionally programmed in the firmware. A pump down timer is next set for five minutes ( 440 ). The pump is then driven at the high speed to evacuate the vacuum chamber  12  ( 450 ). The vacuum sensor value is read ( 460 ) and if after a one second delay ( 470 ) it is less than the set point ( 980 ), the pump is driven at the high speed if the user has not changed the mode of operation ( 490 ). If the sensed vacuum is greater than the setpoint then the vacuum pump is turned off and a “ready” indication is displayed ( 500 ). A hysteresis value and a leakdown time are also read from memory ( 500 ). The sensed vacuum value is then read ( 510 ) and if the value is greater than the setpoint plus the hysteresis value ( 520 ) then the leakdown time is compared to a maximum leakdown time ( 550 ). If the sensed vacuum value is greater than the setpoint plus the hysteresis value a change of mode of operation is checked for ( 530 ). If the user has changed modes, then the system jumps to ( 170 ). If the user has not changed modes, then after a one second delay ( 540 ), the system loops back to ( 520 ) and the sensed vacuum value is again compared to the setpoint value plus the hysteresis value. If the sensed vacuum value is less than the setpoint plus the hysteresis value the leakdown time is compared to the maximum leakdown time ( 550 ). If the leakdown time is less than the maximum leakdown time a “leak” indication is displayed ( 560 ). If the leakdown time is greater than the maximum leakdown time the pump down time is set for one minute ( 570 ) and the system jumps to ( 450 ) to drive the pump at the high speed to evacuate the vacuum chamber  12  ( 450 ). 
     The invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the example as required. However, it is to be understood that the invention can be carried out by specifically different devices and that various modifications can be accomplished without departing from the scope of the invention itself.