Abstract:
A suppressor which comprises: an ion-exchange membrane; an eluate channel which is in contact with one side of the ion-exchange membrane, serves as a channel through which an eluate discharged from a separation column flows, and has inside no obstacle to the flow; a regenerant channel which is in contact with the other side of the ion-exchange membrane, serves as a channel through which a regenerant for regenerating ionic functional groups of the ion-exchange membrane flows, has been disposed so that the regenerant channel has no region facing the eluate channel and extends in parallel to the eluate channel in such a nearby position that the ionic functional groups can move through the ion-exchange membrane, and has inside no obstacle to the flow; and an ion-exchange membrane support member which is in contact at least with that region on one side of the ion-exchange membrane which is opposed to the regenerant channel and with that region on the other side of the ion-exchange membrane which is opposed to the eluate channel to thereby support the ion-exchange membrane with wall surfaces.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ion chromatograph for separation and analysis of inorganic ions or organic ions contained in a sample solution and a suppressor for suppressing the background electrical conductivity of an eluate discharged from a separation column of such an ion chromatograph. 
     2. Description of the Related Art 
     In an ion chromatograph, a sample is introduced into a separation column to separate it into component ions, and then the component ions are detected by measuring the electrical conductivity of an eluate discharged from the separation column in an electrical conductivity measurement cell. In order to achieve high-sensitive measurement, a suppressor is provided between the separation column and the detector. The suppressor removes nontarget ions contained in an eluate discharged from the separation column to reduce the electrical conductivity of the eluate. 
     As such a suppressor, one using an ion-exchange membrane is conventionally used. An example of a conventional suppressor is shown in  FIGS. 7A and 7B .  FIG. 7B  is a sectional view of the conventional suppressor shown in  FIG. 7A  taken along the X-X line. A channel  104  through which an eluate discharged from a separation column flows, and a channel  106  through which a regenerant for regenerating ionic functional groups of an ion-exchange membrane  102  flows are arranged so as to be opposed to each other through the ion-exchange membrane  102 . Base bodies  108  and  110  are arranged so that the channels  104  and  106  are opposed to each other with the ion-exchange membrane  102  being interposed between them. 
     The channels  104  and  106  are merely hollow channels through which liquid flows. The ion-exchange membrane  102  has low stiffness, and therefore, when the pressure in the channel  104  and the pressure in the channel  106  are changed due to a change in the back pressure of the suppressor, and thus, a large difference is caused between the pressure in the channel  104  and the pressure in the channel  106 , the ion-exchange membrane  102  is displaced toward one of the channels due to pressure exerted on the ion-exchange membrane  102 . The displacement of the ion-exchange membrane  102  causes changes in the volumes of the channels  104  and  106 , and as a result, the amount of nontarget ions removed from an eluate fluctuates and therefore the base line of a chromatogram becomes unstable. 
     In order to prevent such displacement of the ion-exchange membrane, a suppressor obtained by filling the channels  104  and  106  with an ion-exchange resin as a filler (see JP-A-1-169353) and a suppressor obtained by filling the channels  104  and  106  with a cross-linked material (see JP-A-61-172057) have been proposed. 
     In a method for filling channels with a filler, such as the proposed suppressor, pressure losses in the channels are increased by the filler and therefore, liquid feed pressures need to be increased. This, however, increases the load on the ion-exchange membrane, which may cause disadvantages, such as leakage of liquid from parts fixing the ion-exchange membrane. 
     Further, in a case where the filler is an ion-exchange resin, there is a case where a difference in performance is caused among individual suppressors due to variations in the characteristics of the ion-exchange resin or the activity of the ion-exchange resin is changed due to repeated use so that changes in ion chromatogram occur with time. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a suppressor capable of preventing the displacement of an ion-exchange membrane without filling its channels with a filler and an ion chromatograph employing such a suppressor. 
     As the present invention, a suppressor for suppressing the background electrical conductivity of an eluate discharged from a separation column of an ion analyzer, including an ion-exchange membrane; an eluate channel, which is in contact with one side of the ion-exchange membrane, serves as a channel through which an eluate discharged from the separation column flows and has inside no obstacle to the flow; a regenerant channel, which is in contact with the other side of the ion-exchange membrane, serves as a channel through which a regenerant for regenerating ionic functional groups of the ion-exchange membrane flows, has been disposed so as to have no region facing the eluate channel and to extend in parallel to the eluate channel in such a nearby position that the ionic functional groups can move to the eluate channel through the ion-exchange membrane and has inside no obstacle to the flow; and an ion-exchange membrane support member, which is in contact with at least a region on one side of the ion-exchange membrane which is opposed to the regenerant channel and a region on the other side of the ion-exchange membrane, which is opposed to the eluate channel, to support the ion-exchange membrane with wall surfaces. 
     The region opposed to the eluate channel, which is on the other side of the ion-exchange membrane opposite to the one side of the ion-exchange membrane, which is in contact with the eluate channel, is supported by the wall surface of the ion-exchange membrane support member, and a region opposed to the regenerant channel, which is on the one side of the ion-exchange membrane opposite to the other side of the ion-exchange membrane which is in contact with the regenerant channel, is also supported by the wall surface of the ion-exchange membrane support member. This makes it possible to prevent the displacement of the ion-exchange membrane toward the eluate channel and the regenerant channel even when the pressure in the eluate channel and the pressure in the regenerant channel are changed, or a pressure difference between these channels is changed. 
     The eluate channel and the regenerant channel are arranged in such a nearby position that ionic functional groups can move through the ion-exchange membrane. This makes it possible for the suppressor to maintain its ion exchange function, that is, the function of exchanging nontarget ions with the ionic functional groups, and act as a suppressor. In a case where the ion-exchange membrane is a cation exchange membrane, the ionic functional groups are hydrogen ions (H + ). In a case where the ion-exchange membrane is an anion exchange membrane, the ionic functional groups are hydroxide ions (OH − ). 
     According to a preferable embodiment, the suppressor has a laminate structure in which the ion-exchange membrane is interposed between two base bodies. In this case, the eluate channel is provided in one of the two base bodies so as to have an inlet and an outlet and to be in contact with the ion-exchange membrane, the regenerant channel is provided in another base body so as to have an inlet and an outlet and to be in contact with the ion-exchange membrane, and the ion-exchange membrane support member is composed of part of the two base bodies, which are in contact with the ion-exchange membrane. 
     The regenerant channel may be provided on the same surface of the ion-exchange membrane so as to be located on opposite sides of the eluate channel. 
     According to another embodiment, the ion-exchange membrane is composed of two first and second ion-exchange membranes, the eluate channel is interposed between the two ion-exchange membranes so that two sides of the eluate channel are in contact with these ion-exchange membranes, and the regenerant channel is provided for each of the ion-exchange membranes. In this case, the regenerant channel may be provided on the same surface of at least one of the ion-exchange membranes so as to be located on opposite sides of the eluate channel. 
     According to a preferable embodiment, the suppressor having two ion-exchange membranes has a laminate structure in which the first ion-exchange membrane is in contact with one side of a first base body and is interposed between the first base body and a second base body, and the second ion-exchange membrane is in contact with the other side of the first base body and is interposed between the first base body and a third base body. In this case, the eluate channel is provided in the first base body as a groove penetrating in the thickness direction of the first base body, the regenerant channel is composed of a first regenerant channel provided in the second base body so as to have an inlet and an outlet and to be in contact with the first ion-exchange membrane, and a second regenerant channel provided in the third base body so as to have an inlet and an outlet and to be in contact with the second ion-exchange membrane, and the ion-exchange membrane support member is composed of part of the first, second, and third base bodies, which are in contact with the first or second ion-exchange membrane. 
     The suppressor may be used singly, but a multistage suppressor may be produced by connecting two or more suppressors to each other. In this case, the multistage suppressor is provided in a channel through which an eluate discharged from a separation column flows, and the outlet of the eluate channel of the upstream suppressor is connected to the inlet of the eluate channel of the downstream suppressor. 
     An ion chromatograph employing the suppressor according to the present invention includes: a separation column; an eluent supply channel for supplying an eluent to the separation column; an injector provided in the eluent supply channel to inject a sample into the eluent supply channel; an electrical conductivity detector provided in an eluate channel through which an eluate discharged from the separation column flows; and the suppressor according to the present invention provided in the eluate channel from the separation column between the separation column and the electrical conductivity detector. 
     In a suppressor according to the present invention and an ion chromatograph employing such a suppressor, one side of an ion-exchange membrane opposite to the other side of the ion-exchange membrane, which is in contact with an eluate channel, is supported by an ion-exchange membrane support member, and the other side of the ion-exchange membrane opposite to the one side of the ion-exchange membrane, which is in contact with a regenerant channel, is also supported by the ion-exchange membrane support member, and therefore, the displacement of the ion-exchange membrane toward one of the eluate channel and the regenerant channel can be prevented without filling these channels with a filler. Further, since the eluate channel and the regenerant channel are not filled with a filler, it is not necessary to increase liquid feed pressures, thereby preventing liquid leakage caused by excessive pressure exerted on the ion-exchange membrane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the channels of an ion chromatograph according to one embodiment of the present invention. 
         FIG. 2A  is a sectional view of a suppressor according to one embodiment of the present invention, taken along a direction perpendicular to the longitudinal direction of channels. 
         FIG. 2B  is a sectional view of the suppressor shown in  FIG. 2A , taken along the Y-Y line. 
         FIG. 2C  is a partially cutaway plan view of the suppressor according to the embodiment. 
         FIG. 3  is a sectional view of a suppressor according to another embodiment of the present invention, taken along a direction perpendicular to the longitudinal direction of channels. 
         FIG. 4  is a sectional view of a suppressor according to another embodiment of the present invention, taken along a direction perpendicular to the longitudinal direction of channels. 
         FIG. 5  is a diagram showing the channels according to one embodiment in which two suppressors according to the present invention are connected to each other. 
         FIG. 6  is a graph showing the performance of the suppressor as one example. 
         FIG. 7A  is a sectional view of a conventional suppressor, taken along the longitudinal direction of channels. 
         FIG. 7B  is a sectional view of the conventional suppressor shown in  FIG. 7A , taken along the X-X line. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic diagram of one embodiment of an ion chromatograph according to the present invention. A liquid channel  7  equipped with a liquid pump  4  for supplying an eluent  6  is connected to a separation column  2 . The liquid channel  7  is equipped also with an injector  8  for injecting a sample. A sample is introduced into the separation column  2  and separated into individual ions. The eluate channel  9  is guided from the separation column  2  to an electrical conductivity measurement cell  10 . The electrical conductivity of the eluate is detected when the eluate passes through the cell  10 . Effluent from the cell  10  is discharged into a drain  12 . 
     The eluate channel  9  connecting the separation column  2  to the cell  10  is equipped with a suppressor  14  to remove nontarget ions causing an increase in the electrical conductivity of a column eluate to achieve high-sensitive measurement. 
     In a case where the ion chromatograph is intended to analyze anions, the suppressor  14  is used to remove cations contained in an eluate through ion exchange.  FIGS. 2A to 2C  show the suppressor  14  as a first embodiment of a suppressor according to the present invention. The suppressor  14  includes a cover  20  and a base  22  as base bodies. The cover  20  and base  22  are made of an inert material to which ions are not adsorbed and from which ions are not eluted. Examples of such an inert material include acrylic resins and PEEK (polyether ether ketone) resins. Between the cover  20  and the base  22 , an ion-exchange membrane  24  is interposed and fixed. The cover  20  has an eluate channel  30  formed therein. The eluate channel  30  has an inlet  26  and an outlet  28  and allows an eluate discharged from the separation column  2  to flow therethrough so that the eluate is brought into contact with the ion-exchange membrane  24 . The base  22  has a regenerant channel  32  formed therein. The regenerant channel  32  has an inlet  34  and an outlet  36  and allows a regenerant to flow therethrough so that the regenerant is brought into contact with the ion-exchange membrane  24 . The regenerant channel  32  extends in parallel to the eluate channel  30  at such a position that it is not opposed to the eluate channel  30 . The eluate channel  30  and the regenerant channel  32  are adjacent to each other, and the distance between the eluate channel  30  and the regenerant channel  32  is set to such a value that ionic functional groups can move from the regenerant channel  32  to the eluate channel  30  through the ion-exchange membrane  24 . 
     The regenerant is pure water or an aqueous solution used to regenerate the ionic functional groups of the ion-exchange membrane  24 . The ionic functional groups are hydrogen ions (H + ) or hydroxide ions (OH − ). More specifically, in a case where the ion-exchange membrane  24  is a cation exchange membrane, the ionic functional groups are hydrogen ions (H + ), and in a case where the ion-exchange membrane  24  is an anion exchange membrane, the ionic functional groups are hydroxide ions (OH − ). 
     The eluate channel  30  and the regenerant channel  32  are provided on opposite sides of the ion-exchange membrane  24 , but the eluate channel  30  does not have a region opposed to the regenerant channel  32  and the regenerant channel  32  does not have a region opposed to the eluate channel  30 . A region opposed to the eluate channel  30 , which is on one side of the ion-exchange membrane  24  opposite to the other side of the ion-exchange membrane  24 , which is in contact with the eluate channel  30 , is supported by the wall surface of the base  22 . On the other hand, a region opposed to the regenerant channel  32 , which is on the other side of the ion-exchange membrane  24  opposite to the one side of the ion-exchange membrane  24 , which is in contact with the regenerant channel  32 , is supported by the wall surface of the cover  20 . Since a liquid feed pressure is always applied to each of the channels  30  and  32 , the ion-exchange membrane  24  is always pressed against the wall surface of the base  22  and the wall surface of the cover  20 , thereby preventing the displacement of the ion-exchange membrane  24 . 
     The eluate channel  30  and the regenerant channel  32  are merely hollow channels and are not filled with an obstacle such as a filler. 
     In a case where the ion chromatograph according to the one embodiment of the present invention is intended to analyze anions, the ion-exchange membrane  24  is a cation exchange membrane. In this case, in the suppressor  14 , nontarget cations contained in a col eluate flowing through the eluate channel  30  are selectively removed because they are exchanged for hydrogen ions by adsorption to the ion-exchange membrane  24  and dialysis through the ion-exchange membrane  24 . The hydrogen ions exchanged for nontarget cations react with hydroxide ions contained in the column eluate to form water. This reduces the electrical conductivity of the column eluate, thereby reducing noise detected in the electrical conductivity measurement cell  10 . The nontarget cations removed by adsorption to the ion-exchange membrane  24  and dialysis through the ion-exchange membrane  24  are exchanged for hydrogen ions contained in a regenerant flowing through the regenerant channel  32  and are discharged into the regenerant. 
     On the other hand, in a case where the ion chromatograph according to the one embodiment of the present invention is intended to analyze cations, the ion-exchange membrane  24  is an anion exchange membrane. In this case, nontarget anions contained in a column eluate flowing through the eluate channel  30  are selectively removed because they are exchanged for hydroxide ions by the ion-exchange membrane  24 . The hydroxide ions exchanged for nontarget anions react with hydrogen ions contained in the column eluate to form water. Therefore, also in this case, the electrical conductivity of the column eluate is reduced, thereby reducing noise detected in the electrical conductivity measurement cell  10 . The nontarget anions removed by adsorption to the ion-exchange membrane  24  and dialysis through the ion-exchange membrane  24  are exchanged for hydroxide ions contained in a regenerant flowing through the regenerant channel  32  and are discharged into the regenerant. 
       FIG. 3  shows a suppressor according to a second embodiment of the present invention. The suppressor according to the second embodiment is the same as the suppressor according to the first embodiment shown in  FIG. 2  in the structure of the eluate channel  30  but is different from that in that it has two regenerant channel  32   a  and  32   b  provided on the opposite side of the ion-exchange membrane  24  from the eluate channel  30 . 
     The regenerant channels  32   a  and  32   b  are provided on the same surface of the ion-exchange membrane  24  so as not to have a region opposed to the eluate channel  30 , and extend in parallel to the eluate channel  30  on opposite sides of the eluate channel  30 . A region opposed to the eluate channel  30 , which is on one side of the ion-exchange membrane  24  opposite to the other side of the ion-exchange membrane  24 , which is in contact with the eluate channel  30 , is supported by the wall surface of the base  22 . Regions opposed to the regenerant channels  32   a  and  32   b , which are on the other side of the ion-exchange membrane  24  opposite to the one side of the ion-exchange membrane  24 , which is in contact with the regenerant channels  32   a  and  32   b , are supported by the wall surface of the cover  20 . 
     A regenerant flows through the regenerant channels  32   a  and  32   b  in the same direction, which is opposite to the direction in which an eluate flows through the eluate channel  30 . 
     In the suppressor according to the second embodiment, ionic functional groups of the ion-exchange membrane  24  are supplied from a regenerant flowing through both the regenerant channels  32   a  and  32   b , and nontarget ions contained in an eluate flowing through the eluate channel  30  are removed by exchanging them for ionic functional groups supplied from a regenerant flowing through the regenerant channels  32   a  and  32   b.    
       FIG. 4  shows a suppressor according to a third embodiment of the present invention. The suppressor according to the third embodiment has an ion-exchange membrane  44  in addition to the ion-exchange membrane  24  so that these two ion-exchange membranes  24  and  44  are in contact with two different surfaces of the eluate channel  30 . The eluate channel  30  is provided as a through-groove in a base body  20   a  interposed between the two ion-exchange membranes  24  and  44 , and has a flat rectangular sectional shape. One of the two opposed sides of the eluate channel  30  is in contact with the ion-exchange membrane  24  and the other side of the eluate channel  30  is in contact with the ion-exchange membrane  44 . 
     On each of the ion-exchange membranes  24  and  44 , two regenerant channels are provided in the same manner as those of the suppressor shown in  FIG. 3 . More specifically, one set of the regenerant channels  32   a  and  32   b  is provided in a base body  22   a  so as to be in contact with one of the two ion-exchange membranes, that is, the ion-exchange membrane  24 . The regenerant channels  32   a  and  32   b  are provided on the opposite side of the ion-exchange membrane  24  from the eluate channel  30  and extend in parallel to the eluate channel  30  on opposite sides of the eluate channel  30 . The other set of regenerant channels  46   a  and  46   b  is provided in a base body  22   b  so as to be in contact with the other ion-exchange membrane  46 . The regenerant channels  46   a  and  46   b  are provided on the opposite side of the ion-exchange membrane  44  from the eluate channel  30  and extend in parallel to the eluate channel  30  on opposite sides of the eluate channel  30 . 
     A region opposed to the eluate channel  30 , which is on one side of the ion-exchange membrane  24  opposite to the other side of the ion-exchange membrane  24 , which is in contact with the eluate channel  30 , is supported by the wall surface of the base body  22   a . Regions opposed to the regenerant channels  32   a  and  32   b , which are on the other side of the ion-exchange membrane  24  opposite to the one side of the ion-exchange membrane  24 , which is in contact with the regenerant channels  32   a  and  32   b , are supported by the wall surface of the base body  20   a . The other ion-exchange membrane  44  is provided in the same manner as the ion-exchange membrane  24 . More specifically, a region opposed to the eluate channel  30 , which is on one side of the ion-exchange membrane  44  opposite to the other side of the ion-exchange membrane  44 , which is in contact with the eluate channel  30 , is supported by the wall surface of the base body  22   b . Regions opposed to the regenerant channels  46   a  and  46   b , which are on the other surface of the ion-exchange membrane  44  opposite to the one side of the ion-exchange membrane  44 , which is in contact with the regenerant channels  46   a  and  46   b , are supported by the wall surface of the base body  20   a.    
     A regenerant flows through the regenerant channels  32   a ,  32   b ,  46   a , and  46   b  in the same direction, which is opposite to the direction in which an eluate flows through the eluate channel  30 . 
     The mechanism of removing nontarget ions from an eluate by the suppressor shown in  FIG. 4  is the same as those by the suppressors according to the embodiments shown in  FIGS. 2 and 3 . 
     As described above, in the third embodiment shown in  FIG. 4 , two regenerant channels are in contact with each of the two ion-exchange membranes  24  and  44 . However, one regenerant channel may be in contact with one of the two ion-exchange membranes and the other two regenerant channels may be in contact with the other ion-exchange membrane. 
       FIG. 5  shows a two-stage suppressor according to another embodiment of the present invention in which the two suppressors according to any one of the above embodiments are connected in series along the flow of a column eluate. More specifically, a suppressor  14   a  and a suppressor  14   b  are arranged along an eluate channel  9  on the upstream side and the downstream side, respectively. The eluate outlet of the upstream suppressor  14   a  is connected to the eluate inlet of the downstream suppressor  14   b  through a channel  50 . An outlet  28  of the downstream suppressor  14   b  is connected to the electrical conductivity measurement cell  10 . 
     Hereinbelow, the characteristics of the suppressor according to the present invention will be more specifically described with reference to the suppressor shown in  FIGS. 2   a  to  2   c . As the ion-exchange membrane  24 , an anion exchange membrane having a thickness of 0.01 to 1 mm is used. More specifically, Nafion (registered trademark) is used. The thickness of the Nafion used as the ion-exchange membrane  24  is about 0.2 mm and has sulfonic acid groups to exchange cations contained in an eluate for hydrogen ions. The eluate channel  30  and the regenerant channel  32  each have a width of 1 mm and a depth of 0.1 mm. The length of part of each of the eluate channel  30  and the regenerant channel  32 , which is in contact with the ion-exchange membrane  24 , is 50 mm. The ion-exchange membrane  24  is interposed between the base  22  and the cover  20  and fixed by interposing these three stacked members between jigs and fixing the jigs by screws. 
     An ion exchange ratio was measured by changing the distance d between the longitudinal center axis of the eluate channel  30  and the longitudinal center axis of the regenerant channel  32  among three values. The measurement results are shown in  FIG. 6 . It is to be noted that when the distance d is 1 mm, the distance s between the inner sidewall of the eluate channel  30  and the inner sidewall of the regenerant channel  32  is 0, when the distance d is 2 mm, the distance s is 1 mm, and when the distance d is 3 mm, the distance s is 2 mm. 
     An alkaline aqueous solution containing 1.8 mmol/L of Na 2 CO 3  and 1.7 mmol/L of NaHCO 3  was used in place of a column eluate allowed to flow through the eluate channel  30 . As a regenerant allowed to flow through the regenerant channel  32 , 25 mmol/L H 2 SO 4  was used. The ion exchange ratio was measured under conditions where the flow rate of the regenerant was fixed to 0.2 mL/min and the flow rate of the solution allowed to flow through the eluate channel  30  was changed among three values, 0.05 mL/min, 0.1 mL/min, and 0.2 mL/min. The ion exchange ratio was expressed as the percentage of an ion concentration removed by the suppressor to the ion concentration of the aqueous solution allowed to flow through the eluate channel  30 . Since a current value detected by the electrical conductivity measurement cell  10  is proportional to an ion concentration, the ion concentration of the aqueous solution discharged from the suppressor can be determined from a current value detected by the cell  10  based on a previously-prepared calibration curve showing the relationship between the ion concentration of the aqueous solution and a current value detected by the cell  10 . An ion concentration removed by the suppressor can be determined by subtracting an ion concentration detected by the electrical conductivity measurement cell  10  from the known ion concentration of the aqueous solution allowed to flow through the eluate channel  30 . 
     As can be seen from the result shown in  FIG. 6 , a smaller distance between the eluate channel  30  and the regenerant channel  32  increases the ion exchange ratio and a smaller flow rate of the aqueous solution flowing through the eluate channel  30  increases the ion exchange ratio. This is because a smaller flow rate of the aqueous solution flowing through the eluate channel  30  increases the retention time of the aqueous solution in the suppressor, and therefore, the ratio of cations removed through ion exchange is increased in proportion to the retention time. 
     As described above, the length of each of the channels of this suppressor used is 50 mm, but it is apparent that a higher ion exchange ratio is achieved by a larger channel length. An ideal channel length is about 300 mm.