Abstract:
Methods and related apparatus are disclosed for selective removal of inorganic carbon from a fluid sample using selective membranes to minimize the loss of volatile organic compounds from the fluid sample prior to analysis for determination of the total organic carbon contents of the sample.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates particularly to methods and apparatus for facilitating the selective and sensitive detection of organic carbon compounds in water by first treating an aqueous solution for the selective removal of inorganic carbon (IC), which may be in the form of CO 2 , HCO 3 —, and/or CO 3   −2 , using a gas-permeable membrane having a relatively high permeability to carbon dioxide and a relatively low permeability to volatile organic compounds. More generally, the methods of this invention can be applied to separating IC from a fluid stream, which can be a gas or liquid stream, without substantially removing or affecting volatile organic components of that gas or liquid stream.  
       BACKGROUND OF THE INVENTION  
       [0002]     In recent years it has become increasingly important to be able to accurately and reproducibly determine to very low levels, on the order of a few parts per billion, the organic carbon content of an aqueous sample. Sensitive industrial applications, for example, semiconductor manufacture, require extremely pure water that is substantially free of organic carbon contamination. Another example is municipal drinking water systems where the presence of even very small amounts of organic carbon contaminants could, during conventional chlorination treatments, produce deleterious chlorinated hydrocarbons.  
         [0003]     Because carbon may be present in an aqueous sample either in an organic form or in an inorganic form (for example, as dissolved carbon dioxide or in ionic form as carbonate or bicarbonate), the prior art has long recognized the need for distinguishing these two forms of carbon in order to obtain accurate determinations of organic carbon. In general, two approaches have been developed for addressing this problem.  
         [0004]     Thus, conventional TOC (total organic carbon) analyzers either measure separately the concentrations of IC (inorganic carbon) and TC (total carbon) in the sample to obtain mathematically a measure of TOC; or, alternatively, the IC is removed from the sample prior to analysis for TOC. In the first case, the IC concentration is subtracted from the TC concentration to calculate the TOC concentration. However, the precision of this technique is low when the IC concentration is relatively large in comparison to the TOC concentration because even small percentage inaccuracies in the IC and TC determinations significantly impact the TOC calculation.  
         [0005]     For samples having a relatively high ratio of IC/TOC, better precision is obtained when the second approach is adopted. By substantially removing the IC from the sample before making a TC determination, TOC is determined directly thereby avoiding the need to calculate the TOC concentration from the difference between two larger values (TC and IC). Additionally, the time required for the measurement often is less when IC is removed first than when both IC and TC have to be separately measured.  
         [0006]     A number of prior art patents and literature references teach various approaches to addressing IC in a process for measuring TOC. Of these, some teach selective removal of IC from an aqueous sample prior to making a TC determination and others do not. For example, U.S. Pat. No. 4,209,299, “Method and Apparatus for Determination of Volatile Electrolytes,” (Robert M. Carlson) has no reference to selective removal of IC in the determination of volatile organic compounds. U.S. Pat. No. 5,567,388, “Apparatus for Measuring Total Organic Carbon,” (Morita et al.) describes use of a sample solution, to which base has been added, as removal/acceptor media specifically for carbon dioxide. This patent does not, however, teach use of a selective membrane to prevent removal of volatile organic compounds. U.S. Pat. No. 5,051,114, “Perfluorodioxole membranes,” (Nemser et al.) describes enrichment and removal of volatile components selectively in a gas/gas configuration. U.S. Pat. No. 6,248,157, “vacuum degassing,” (Sims et al.) describes vacuum degassing without selective removal of IC relative to volatile organics. U.S. Pat. No. 5,443,991, “Method for determination of dissolved carbon in water,” (Godec et al.) describes the use of a CO 2 -permeable membrane, but does not address the problem of possible loss of volatile organic compounds. The foregoing U.S. patents are incorporated herein by reference.  
         [0007]     In addition to the foregoing patents, other patents and technical literature address various aspects of this technical field. One well-established method of removing IC from an aqueous sample before making a TC determination involves acidification of the sample to a pH of about 2 or less to convert all HCO 3   −  and CO 3   −2  to carbon dioxide, followed by purging the sample with a gas stream for several minutes to remove the CO 2 . Many publications and patents have described methods and instrumentation that incorporate acidification and gas purging for IC removal. Such prior art includes Kaplan, L. A., “Comparison of Three TOC Methodologies,”  J. AWWA,  Vol. 92, Issue 4, pp. 149-156; April, 2000; Takahashi, Y., “Sparging Device,” U.S. Pat. No. 3,958,945 (Envirotech Corporation) May 25, 1976; and, Purcell, M. W.; Yang, S. S.; Martin, J. T.; Reckner, R. R. and Harris, J. L., “Liquid Sample Carbon Analyzer,” U.S. Pat. No. 6,007,777 (Tekmar Company) Dec. 28, 1999, each of which is incorporated herein by reference. A disadvantage of this procedure is that at least some indeterminate portion of volatile organic compounds originally present in the sample is likely to be lost from the sample during the gas purging step leading to inaccurate TOC measurement (see, e.g., American Water Works Association, “Total Organic Carbon (TOC),” Standard Method 5310C in Standard Methods for the Examination of Water and Wastewater, 19th Edition Supplement; 1996). The fraction of such volatile, purgeable organic compounds in a sample is commonly referred to as “Purgeable Organic Carbon” (POC); while the fraction of organic compounds that is not lost during purging is referred to as “Non-purgeable Organic Carbon” (NPOC). For some samples, gas purging may be acceptable and not a significant source of inaccuracy because the POC content of such a sample represents only a small proportion of the overall TOC content.  
         [0008]     However, many types of aqueous samples which are of particular interest in many modern, ultra high purity industrial and other applications have substantial concentrations of POC, (see, e.g., Barcelona, M. J, “TOC Determinations in Ground Water,”  Ground Water , Vol. 22(1), pp. 18-24; 1984). For these samples, gas purging is not an acceptable choice for IC removal. For such samples, alternative membrane-based techniques have been developed. In one early such membrane-based process, the sample is acidified to convert IC to carbon dioxide. This acidified solution is then flowed on one side of a nonporous, gas-permeable silicon rubber membrane, allowing the carbon dioxide to diffuse through the membrane. A basic solution on the other side of the membrane absorbs the carbon dioxide because the base converts the carbon dioxide to bicarbonate and carbonate ions. Such a process is taught by West, S. J.; Frant, M. S. and Ross, J. W., Jr., “Development of a Water Quality Monitor for Spacecraft Application,” SAE Paper 76-ENAs-10, Presented at International Conference on Environmental Systems, San Diego, Calif.; Jul. 12-15, 1976.  
         [0009]     This process was refined in subsequent developments, one of which involved splitting the sample into two parts, with one being acidified while the other is made basic. These two portions of the sample are then flowed on opposite sides of the same gas-permeable membrane, such that the IC from the acidified portion diffuses through the membrane into the basic portion. See e.g., West, S. J.; Frant M. S. and Franks S. H., “Preliminary Design of a Preprototype Water Quality Monitor,” SAE Paper 77-ENAs-36, Presented at International Conference on Environmental Systems, San Francisco, Calif., Jul. 11-14, 1977. See also, Lantz, J. B.; Davenport, R. J.; Wynveen, R. A. and Cooper, W. J., “Development of TOC/COD Analyzer for Process Applications,”  Chemistry in Water Reuse,  Volume 1, Cooper, W. J. (Ed.), Ann Arbor Science Publishers, Inc.; 1981. One advantage of this arrangement is that volatile organics in the acidified sample are not lost through the membrane because the partial pressure of those organics remains virtually the same on both sides of the membrane. A disadvantage is that both acid and base reagents are required.  
         [0010]     Silicon rubber is deteriorated by prolonged contact with strong acids and bases, however, so more durable and inert membrane materials were needed. Microporous Teflon) has been found useful in this application. See e.g., West, S.; Chrisos, J. and Baxter, W., “Water Quality Monitor,” Final Report, NASA Contract NAS9-14229; Orion Research, Inc., Cambridge, Mass.; March 1979. U.S. Pat. No. 5,567,388 (Morita et al.) teaches that films of polytetrafluoroethylene, silicone rubber, cellulose acetate, or porous polyethylene, or a composite film made from those materials, can be used to remove IC from an acidified sample stream, with the carbon dioxide diffusing into a portion of the sample on the other side of the membrane that has been made basic.  
         [0011]     There are still several problems and limitations, however, with this approach. One problem is that strong acids and bases, as well as some water sample constituents, attack non-porous membranes made from many traditional materials. Silicon rubber and cellulose acetate are among this group.  
         [0012]     Non-porous polytetrafluoroethylene and polyethylene are typically compatible with strong acids, strong bases, and typical water sample constituents, but the rates at which carbon dioxide diffuses through these membranes are so low that an IC removal device based on such membranes would have to be very large to process a typical sample in a reasonable time. A large IC removal device, however, causes slow response of the TOC analyzer when a new sample is measured after measuring another sample that has a significantly different concentration. It has also been found that there can be problems with conventional porous membranes. One problem is that constituents of some water samples wet the surfaces of these porous membranes, thereby allowing the solutions on either side of a membrane to mix. This causes measurement errors and increases maintenance labor associated with the apparatus. Another problem is that conventional porous membranes allow volatile organics to rapidly diffuse from the acidified sample stream into the basic solution. To avoid such loss of volatile organics, thereby distorting the accuracy of a TOC measurement it is necessary to use two portions of the sample (one acidified and one made basic) on each side of a porous membrane, as discussed above. When this is done, however, the apparatus is more complex and costly, and an additional reagent is required to make the sample basic.  
         [0013]     These and other problems with and limitations of the prior art methods are overcome in whole or in part with the methods and apparatus of the present invention.  
       OBJECTS OF THE INVENTION  
       [0014]     Accordingly, a general object of the present invention is to provide improved methods and related apparatus for processing a sample fluid for the selective removal of inorganic carbon, as defined herein, while minimizing the removal or loss of volatile organic compounds.  
         [0015]     Another general object of the present invention is to provide a system, and methods of using such system, for passing a fluid sample along one side of a selectively gas-permeable membrane while passing an acceptor medium along the opposite side of the same membrane in order to selectively diffuse at least one component of the fluid sample through the membrane and into the acceptor medium without significantly altering the content of at least one other component of the fluid sample.  
         [0016]     It is a principal object of this invention to provide methods and apparatus for more efficient, simple, compact and accurate determinations of the total organic carbon content of a fluid sample by selective removal of inorganic carbon from the sample before analysis.  
         [0017]     A specific object of the present invention is to provide gas permeable membranes having a relatively high permeability to carbon dioxide and a relatively low permeability to volatile organic compounds as part of a system for the selective removal of inorganic carbon from a fluid sample prior to analysis without significant loss of volatile organic compounds together with methods for operating such a system.  
         [0018]     Another specific object of this invention is to provide a system and methods for acidifying or not acidifying an aqueous sample and thereafter passing it into contact with one face of a CO 2 -selective membrane, while contacting the opposite face of the membrane with a substantially CO 2 -free acceptor medium, to remove inorganic carbon from the sample to prepare the sample for analysis for total organic carbon content.  
         [0019]     Still a further specific object of this invention is to use membranes made of Teflon AF, PFA, Polyfluoropolymer and comparable materials as CO 2 -selective membranes in methods and apparatus for selective removal of inorganic carbon from a fluid sample stream without significantly removing volatile organic compounds.  
         [0020]     Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises, but is not limited to, the methods and related apparatus, involving the several steps and the various components, and the relation and order of one or more such steps and components with respect to each of the others, as exemplified by the following description and the accompanying drawings. Various modifications of and variations on the method and apparatus as herein described will be apparent to those skilled in the art and all such modifications and variations are considered within the scope of the invention.  
       SUMMARY OF THE INVENTION  
       [0021]     In a first embodiment, the present invention comprises methods and related apparatus for facilitating the selective and sensitive detection of organic carbon compounds in a fluid by selective removal of inorganic carbon (IC) from the fluid using a gas-permeable membrane having a relatively high permeability to carbon dioxide and a relatively low permeability to volatile organic compounds. The present invention provides an easy and reliable way to remove IC from a sample gas or liquid stream without significantly impacting the accuracy of a subsequent analytical measurement for organic carbon. More specifically, this invention is useful in removing IC from a water sample without significantly changing the TOC content of that water sample.  
         [0022]     In another embodiment, this invention comprises methods and related apparatus for selective removal of Inorganic Carbon (IC defined as the sum of the concentrations of CO 2 , HCO 3   − , and CO 3   −2 ) from liquid analyte with minimal removal of volatile organic compounds.  
         [0023]     In still another embodiment, this invention relates to improving TOC measurements by the selective removal of substantially all or at least excess IC from a donor sample stream, to an acceptor stream, with minimal removal of volatile TOC from the donor sample stream. This invention therefore overcomes a problem with existing non-selective degassing and purging techniques in that they typically remove significant amounts of volatile organic compounds from samples that are intended to be subsequently analyzed for TOC.  
         [0024]     The Total Carbon (TC) content in water consists of two components, TOC and IC. TC and IC each can be measured, and TOC then can be calculated from the equation: TOC=TC−IC. However, when the concentration of TOC is significantly less than the IC concentration, it is essentially impossible to precisely determine the TOC concentration from the small difference between the large TC and IC concentrations. The precision of the TC and IC measurements is adversely affected by the normal “noise” associated with any analytical measurement and such effect may be substantial in relation to a relatively small, but still significant, TOC content.  
         [0025]     Thus, when the IC concentration is larger than the TOC concentration, the TOC measurement will be more precise if the IC is removed prior to the carbon measurement. When that is done, the TOC concentration becomes substantially if not exactly equal to the measured total carbon concentration TC. In the prior art, IC removal employs non-selective degassing of the acidified sample or so-called acid sparging treatment. Both methods involve acidification of the sample with an inorganic acid. Then, the sample is either degassed (with or without membrane separation), or it undergoes a sparging. The fist technique requires a vacuum pump and usually, a carbon dioxide scrubber to purify any gas that flows over the acidified sample. The second technique requires a supply of carbon dioxide-free gas. The efficiency, quality and purity of these consumables can often vary in ways that affect the efficiency of the IC removal process and/or the accuracy of subsequent TC measurement  
         [0026]     In contrast with the prior art, the methods and apparatus of this invention are based on the use of a non-porous gas- permeable membrane that has a relatively high permeability for carbon dioxide and a relatively low permeability for volatile organic compounds. The membrane separates the fluid analyte (which may also be called the donor stream) from a second fluid stream (which may also be called the acceptor stream), which initially contains essentially no IC. Carbon dioxide diffuses from the fluid analyte (which may in some embodiments be acidified to assist in converting carbonate and bicarbonate ions to carbon dioxide), through the membrane and into the acceptor stream. The carbon dioxide and bicarbonate ions which diffuse into the acceptor stream as a result of carrying out this method may be subsequently removed by externally provided carbon dioxide-free gas or with ion-exchange resin. These methods and related apparatus of this invention are ideal for removal of background IC from a water sample before Total Carbon (TC) is measured. With the prior removal of IC, TOC will be the same as the measured TC. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a schematic flow diagram of one embodiment of the present invention utilizing a planar membrane element.  
         [0028]      FIG. 2  is a schematic flow diagram of another embodiment of the present invention utilizing a tubular membrane element.  
         [0029]      FIG. 3  is a graph of IC removal efficiency plotted against residence time for three types of acceptor media in accordance with the present invention.  
         [0030]      FIG. 4  is a graph of IC removal efficiency plotted against residence time at three temperatures.  
         [0031]      FIG. 5  is a graph of IC removal efficiency plotted against residence time at three concentrations of inorganic carbon.  
         [0032]      FIG. 6  is a graph of before and after concentrations of various volatile organic compounds in a fluid sample showing, for each of two types of membrane material, the extent to which IC removal practiced in accordance with the present invention impacts the level of the volatile organic compound in the sample.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0033]     The present invention is generally directed to the use of particular types of membrane materials in connection with methods and apparatus for selective removal of inorganic carbon (IC), defined herein as the sum of the concentrations of CO 2 , HCO 3   −  and CO 3   −2 , from a fluid medium with little or no removal of volatile organic compounds so as not to adversely affect a subsequent analysis of the fluid medium for determination of total organic carbon (TOC).  
         [0034]     In accordance with this invention, two ways have been found to achieve selective removal of IC from a fluid medium. The first is based on the choice of the membrane material. Although porous materials provide very high permeation rates for gases, they do not provide any selectivity. Non-porous membranes, on the other hand, if properly selected, provide an opportunity to achieve selectivity because of differences in the permeability rates of different compounds through the membrane material. We have now found that at least relative to IC and TOC, it is possible to choose a membrane material with a higher rate of permeation for the compound of interest compared to other compounds in the analyte. In addition, optimization of residence time and temperature of the analyte can further enhance the selectivity of this separation step.  
         [0035]     A second way to provide or enhance selectivity is to change the pH. Thus, the fluid analyte stream (donor side) can be acidified to push “acidic gases” from sample solution. For example, in accordance with the present invention, the pH value of the fluid medium is generally reduced to less than about 7, preferably to less than about 4, for selective IC removal.  
         [0036]     Apparatus for practicing the methods of the present invention comprises an inorganic carbon (IC) transfer unit wherein a suitable membrane separates a first compartment or fluid region from a second compartment or fluid region whereby the sample medium contacts a first (donor) side of the membrane at the same time that an acceptor medium (which may, in some embodiments, be at least a partial vacuum) contacts the opposite (acceptor) side of the membrane. As discussed below with reference to  FIG. 1 , in one embodiment an IC transfer unit in accordance with this invention may be of a planar design As discussed below with reference to  FIG. 2 , in an alternative embodiment an IC transfer unit in accordance with this invention may be of a tubular or hollow cylindrical design Other membrane/transfer unit configurations, including hybrid designs, are also considered to be within the scope of this invention.  
         [0037]      FIG. 1  is a schematic illustration of an apparatus  10  in accordance with an embodiment of the present invention wherein the IC transfer unit  11  is of planar design. In  FIG. 1 , membrane element  12  of transfer unit  11  comprises a generally planar sheet or strip of the selective membrane material. An acidifying reagent  13  may or may not be added to sample fluid  14 , and the sample fluid is passed into a first compartment  15  of transfer unit  11  such that the sample fluid contacts the donor face  16  of membrane element  12 . A substantially carbon dioxide-free acceptor medium  17  is passed into a second (acceptor) compartment  18  of transfer unit  11  such that the acceptor medium contacts the acceptor face  19  of membrane element  12  resulting in acid gas, such as CO 2 , from sample fluid  14  diffusing through membrane  12  and into acceptor medium  17 , where it may be dissolved and/or ionized, e.g., into bicarbonate, if medium  17  is an aqueous fluid, or carried away as a gas if medium  17  is a gas stream. As shown in  FIG. 1 , an in-line pump  20 , or other suitable fluid circulation system, may be used to circulate acceptor medium  17  around a closed fluid loop, which preferably may include an IC removal system  22  for removing IC from the acceptor medium coming from acceptor compartment  18  before recycling the IC-free acceptor medium to compartment  18 . Also as shown in  FIG. 1 , in a preferred embodiment acceptor medium  17  is flowed through IC transfer unit  11  in a countercurrent direction relative to the fluid flow direction of sample fluid  14 .  
         [0038]     In a preferred embodiment of the method and apparatus illustrated in  FIG. 1 , membrane element  12  has a high permeability for CO 2  and a low permeability for volatile organic compounds. In another preferred embodiment of the invention as illustrated in  FIG. 1 , acceptor medium  17  is deionized (DI) water, and IC removal system  22  comprises an ion exchange system. In an alternative embodiment, acceptor medium  17  comprises a second portion of the sample fluid which has been made basic (for example, a pH of about 8 or higher) by the addition of alkali, if necessary. In still another embodiment, acceptor medium  17  is a substantially carbon dioxide-free gas. If the carbon dioxide-free gas is purified air, acceptor medium  17 , with carbon dioxide picked up in acceptor compartment  18 , may be vented downstream of compartment  18  instead of being recycled in a closed loop. In yet another embodiment, acceptor medium  17  may comprise at least a partial vacuum in acceptor compartment  18 .  
         [0039]      FIG. 2  is a schematic illustration of an apparatus  30  in accordance with an embodiment of the present invention wherein the IC transfer unit  31  is of a generally tubular design. In  FIG. 2 , membrane element  32  of transfer unit  31  comprises a hollow tube or conduit of the selective membrane material. An acidifying reagent  33  may be added to sample fluid  34 , and the acidified or not acidified sample fluid is passed through an inlet manifold  35  into a first (donor) compartment  36  of transfer unit  31 . First compartment  36  is a hollow tubular region bounded by a length of membrane element  32  extending between the IC transfer unit inlet manifold  35  and the outlet manifold  37 . The sample fluid inlet conduit carrying the sample fluid to IC transfer unit  31  connects to an inlet end of first compartment  36  at inlet manifold  35 . The sample fluid outlet conduit carrying the sample fluid leaving IC transfer unit  31  connects to an outlet end of first compartment  36  at outlet manifold  37 . Seals or bushing elements  43  and  44 , associated respectively with inlet manifold  35  and outlet manifold  37 , prevent fluid leakage from the interior regions of the manifold.  
         [0040]     Acidified or not acidified sample fluid inside compartment  36  contacts the donor face  38  of tubular membrane element  32 . A substantially carbon dioxide-free or molecular acid gas-free acceptor medium  39  is passed into a second (acceptor) compartment  40  of transfer unit  31  such that the acceptor medium contacts the acceptor face  41  of the tubular membrane element  32  resulting in acid gas, such as CO 2 , from sample fluid  34  diffusing through membrane  32  and into acceptor medium  39  where it may be dissolved and/or ionized, e.g., into bicarbonate, if medium  39  is an aqueous liquid, or carried away as a gas if medium  39  is a gas stream.  
         [0041]     As seen in  FIG. 2 , acceptor compartment  40  is an annular region surrounding membrane element  32 , the annular region being defined on the inside by acceptor face  41  of membrane element  32  and on the outside by the inner wall of a sleeve or conduit  42  of larger diameter than membrane element  32  and positioned substantially concentric relative to membrane element  32  between inlet manifold  35  and outlet manifold  37 . As shown in  FIG. 2 , in a preferred embodiment an acceptor medium inlet conduit carrying substantially carbon dioxide-free acceptor medium to IC transfer unit  31  connects to an inlet end of acceptor compartment  40  at outlet manifold  37 . Correspondingly, the acceptor medium outlet conduit carrying acceptor medium leaving IC transfer unit  31  connects to an outlet end of acceptor compartment  40  at inlet manifold  35 . This configuration results in a preferred embodiment wherein acceptor medium is flowed through IC transfer unit  31  in a countercurrent direction relative to the flow of acidified sample fluid through IC transfer unit  31 .  
         [0042]     As shown in  FIG. 2 , an in-line pump  46 , or other suitable fluid circulation system, may be used to circulate acceptor medium around a closed fluid loop, which preferably may include an IC removal system  45  for removing IC from the acceptor medium coming from acceptor compartment  40  before recycling the IC-free acceptor medium back to compartment  40 . In a preferred embodiment of the method and apparatus illustrated in  FIG. 2 , membrane element  32  has a high permeability for CO 2  and a low permeability for volatile organic compounds. In another preferred embodiment of the invention as illustrated in  FIG. 2 , acceptor medium  39  is DI water, and IC removal system  45  comprises an ion exchange system. Other variations in the practice of this invention as described above for  FIG. 1  can be adapted for use with the apparatus configuration of  FIG. 2 .  
         [0043]     It will be understood that the apparatus illustrated in  FIG. 2 , with obvious minor modifications, could also be utilized to practice an alternative embodiment of this invention wherein acidified or not acidified sample fluid is passed through the outer annular-shaped compartment  40  and acceptor medium  39  is passed through the inner tubular compartment  36  of IC transfer unit  31 . Of course, in this modified configuration, compartment  40  would be the sample fluid (donor) compartment and compartment  36  would be the acceptor medium compartment. Also, in this modified configuration, sample fluid would contact outer face  41  of membrane  32 , whereas acceptor medium  39  would contact inner face  38  of membrane  32 .  
         [0044]     Also, while  FIGS. 1 and 2  illustrate preferred countercurrent flow configurations, it will be understood that both of these embodiments of the present invention can be practiced using cocurrent flows of the sample fluid and the acceptor medium through the respective IC transfer units.  
         [0045]     The membrane materials in accordance with this invention theoretically can be any gas-permeable materials depending on the chemical structures of the material that is to be passed through the membrane and the materials that are to be retained in the sample fluid. It is preferred to use a membrane material which has a relatively high permeation rate for the volatile compound that is to be removed from the fluid medium relative to low permeation rates for the material(s) that is(are) not to be removed from the sample fluid. The amount of volatile compound transferred from sample (donor) stream to the acceptor side, M acc , is given by the following equation: 
 
 M   aco   ≈M   s (1−exp(− P   0   t   res  exp(− A/T ))), 
 
 where: 
 
         [0046]     P 0 =Permeation at 25° C.  
         [0047]     A=Activation energy  
         [0048]     T=Temperature of the membrane  
         [0049]     M s =Initial concentration of volatile compound in the sample stream  
         [0050]     t res =Residence time of the sample stream in the IC removal module  
         [0000]     Therefore, the relative amount of removed volatile compound, RM, is: 
 
 RM=M   acc   /M   s ≈1−exp(− P   0   t   res  exp(− A/T )). 
 
 Relative removal of the volatile component increases, at any given permeation rate, as the time the sample spends in the module increases and also as the membrane temperature increases. The choice of membrane material affects the sizing of the IC removal module and the selectivity of the IC removal process. These aspects of the present invention can be optimized for a particular application by routine trial-and-error and/or by computerized modeling or similar techniques. 
 
         [0051]     A particularly preferred membrane material in accordance with the present invention is a DuPont Chemical Co. polymer product marketed under the trade name Teflon AF 2400. It has been found in accordance with this invention that use of Teflon AF 2400 as the membrane for the methods and apparatus of the present invention shortens the residence time required to remove the same amount of carbon dioxide by a factor of about 200 to 300 compared to membranes of comparable dimensions made of PFA or PTFE. The rate at which carbon dioxide diffuses through the fluid medium can become the limiting factor for the removal of IC in such applications.  
         [0052]     In general, the acceptor medium on the acceptor side of the membrane can be anything that is essentially free of the molecular acid gas compound that is being removed from the sample stream. Fluid acceptor media in accordance with this invention include: 
    (a) Alkaline sample fluid, or other substantially carbon dioxide-free water solutions (i.e., deionized water).    
 
         [0054]     This technique is specific to removal of acid gases. 
    (b) Carbon dioxide-free gas stream.    
 
         [0056]     The gas stream can be air purified by a carbon dioxide scrubber in combination with a circulation system, e.g., a pump, or a compressed carbon-dioxide-free gas. This method is typically relatively costly, and it is specific to carbon dioxide removal. 
    (c) Vacuum.    
 
         [0058]     This method requires a vacuum pump and, therefore, is also relatively costly, in addition to which there can be a vacuum reliability problem. Liquid loss from the sample stream can also be a possibly significant problem with this embodiment The graph in  FIG. 3  plots IC removal efficiency against residence time of the sample in the IC transfer unit for the three types of acceptor media discussed above, namely DI water, CO 2 -free air, and vacuum.  FIG. 3  shows that there are no significant differences in removal efficiency between the different acceptor media described above.  
       EXAMPLE 1  
       [0059]     The following tests were performed to demonstrate the practice and efficiency of this invention in the removal of a volatile electrolyte, such as carbon dioxide, from an aqueous sample at varying temperatures.  
         [0060]     A tubular design gas transfer module similar to that illustrated in  FIG. 2  was used in this example. Carbon dioxide, at a concentration of 28 ppm C in water, was passed through the inside of a tubular Teflon AF membrane. The temperature was changed by heating the acceptor stream, which was deionized water.  
         [0061]     The results are illustrated in  FIG. 4  which plots IC removal efficiency against residence time of the sample in the IC transfer unit at three temperatures, 30° C., 50° C. and 70° C.  FIG. 4  shows that, at each temperature, essentially 100% removal of IC from the sample fluid was achieved in under three minutes (less than 180 seconds).  
       EXAMPLE 2  
       [0062]     Another set of experiments was performed using an IC transfer module containing a flat or planar PFA membrane similar to the membrane configuration illustrated in  FIG. 1 . For this set of experiments, the sample residence time in the IC transfer module was changed by varying the sample flow rate. The results are shown in  FIG. 5  which plots IC removal efficiency against residence time of the sample in the IC transfer unit at three concentrations of C and IC in the sample, 5 ppm C, 25 ppm C and 50 ppm C.  FIG. 5  shows that removal efficiency for any given residence time does not vary significantly with IC concentration over a broad range.  
       EXAMPLE 3  
       [0063]     In general, a solution containing a volatile organic compound will lose a very small amount of this compound while going through the IC module of the present invention. However, the amount that is lost in this way can be minimized through the choice of the membrane material and by minimizing the sample residence time.  
         [0064]     In this example, two different membranes were tested for IC removal in accordance with the present invention. The concentration of various volatile organic compounds in an aqueous sample stream was determined before and after the sample was passed through each of two IC transfer units according to the present invention, one such unit utilizing a Teflon AF membrane, the other utilizing a Gortex membrane. In addition, the sample stream was tested for IC (as CO 2 ) concentration both before and after passing through each of the two IC transfer units. A relatively short residence time of 15 seconds in the IC transfer units was used for these tests.  
         [0065]     The results of this Example are shown in  FIG. 6 .  FIG. 6  shows that the Gortex membrane achieves about 62% removal of IC, compared to about 48% for Teflon AF. However, the loss of organic compounds was more significant with the Gortex membrane in comparison to the Teflon AF membrane. When the Teflon AF membrane was used, a majority of the volatile organic compounds remained in the sample (e.g., removal of toluene was only 16%).  
         [0066]     It will be apparent to those skilled in the art that other changes and modifications may be made in the above-described methods and apparatus for selective removal of IC from a sample fluid using selective membranes without departing from the scope of the invention herein, and it is intended that all matter contained in the above description shall be interpreted in an illustrative and not a limiting sense.