method for removing gas prior to sample detection

Abstract: Methods and devices for removing gas prior to detecting analyte ions are disclosed. In one aspect of the invention, gas by-products from the suppression of the mobile phase are removed prior to detecting analyte ions at a detector. In another aspect, the amount of dissolved carbonic acid is reduced in a suppressed carbonate/bicarbonate mobile phase by removing carbon dioxide gas.

Description: This application is a continuation-in-part application of Serial No. 09/365,496 filed August 2, 1999. FIELD OF THE INVENTION The present invention relates to the field of ion chromatography (IC), and, in particular, method of ion chromatography wherein gas is removed prior to detection of sample ions. BACKGROUND OF THE INVENTION Suppressed ion chromatography (SIC) is a commonly practiced method of ion chromatography which generally uses two ion-exchange columns in series followed by a flow through conductivity detector for detecting sample ions. The first column, called the analytical, chromatography or separation column, separates the analyte ions (e.g., the sample ions) in a sample by elution of the analyte ions through the column. The analyte ions are flowed through the analytical column via a mobile phase comprising electrolyte. Generally, a dilute acid or base in deionized water is used as the mobile phase. From the analytical column, the separated analyte ions and mobile phase are then flowed to the second column, which is called the suppressor or stripper. The suppressor serves two primary purposes: (1) it lowers the background conductance of the mobile phase by retaining (e.g., suppressing) the electrolyte of the mobile phase, and (2) it enhances the conductance of the analyte ions by converting the analyte ions to their relatively more conductive acid (in anion analysis) or base (in cation analysis). The combination of these two functions enhances the signal to noise ratio, and, thus, improves the detection of the analyte ions in the detector. Accordingly, upon exiting the suppressor, the analyte ions and suppressed mobile phase are then flowed to the detector for detection of the analyte ions. A variety of different types of suppressor devices and methods are discussed in U.S. Patent No. 3,897,213; 3,920,397; 3,925,019; 3,926,559; and U.S. Serial No. 08/911,847. As those skilled in the art will appreciate, both the mobile phase and the sample contain counterions of the analyte ions. A suppressor operates by ion exchange of suppressor ions, which are located in the suppressor, with both (1) the mobile phase electrolyte counterions and (2) the sample counterions. In anion analysis, for example, the suppressor ions normally comprise hydronium ions and the mobile phase comprises electrolyte such as sodium hydroxide or mixtures of sodium carbonate and sodium bicarbonate. In cation analysis, the suppressor ions normally comprise hydroxide ions, and the mobile phase may comprise electrolytes such as hydrochloric acid or methanesulfonic acid. The suppressor ions are located on a stationary phase, which may be an ion exchange membrane or resin or both. As the mobile phase and sample (which contains both analyte ions and counterions of the analyte ions) are flowed through the stationary phase of the suppressor, the electrolyte counterions in the mobile phase and the sample counterions are retained on the stationary phase by ion exchange with the suppressor ions. When the suppressor ions are either hydronium or hydroxide, ion exchange of the electrolyte counterions with suppressor ions converts the mobile phase to water or carbonic acid, which are relatively non-conductive. On the other hand, the ion exchange of sample counterions with suppressor ions (i.e., hydronium or hydroxide ions) converts the analyte ions to their relatively more conductive acid (in anion analysis) or base (in cation analysis). Thus, the analyte ions, which are now in their relatively more conductive acid or base form, are more sensitive to detection against the less conductive background of the mobile phase. However, unless the suppressor ions are continuously replenished during the suppression process, the concentration of suppressor ions on the stationary phase is reduced. Eventually the suppressor will become exhausted and its suppression capacity is either lost completely or significantly reduced. Thus, the suppressor must be either replaced or regenerated. The need to replace or regenerate the suppressor is inconvenient, may require an interruption in sample analysis, or require complex valving or regeneration techniques known in the art. Methods of electrochemically regenerating an at least partially exhausted suppressor are known in the art. See, for example, U.S. Patent Nos. 5,633,171 and 5,773,615, which are directed to intermittent electrolytic packed bed suppressors. The assignee of this application also discloses, among other things, similar methods of intermittent electrochemical regenerating of a suppressor in U.S. Patent No. 5,759,405. A method of an intermittent, but "frequent," chemical regeneration of a suppressor is disclosed in U.S. Patent No. 5,597,734. One problem associated with such "intermittent" methods of electrochemically regenerating a suppressor is that the suppressor being regenerated must be taken "off-line", that is, while being regenerated the suppressor is not used in a sample or analysis run. An example of a known technique for continuously regenerating a suppressor by continuously replenishing suppressor ions is disclosed in U.S. Patent No. 5,352,360. Another problem associated with SIC is that a separate suppressor unit is usually required, and, therefore, the number of components in the system is increased over traditional IC systems. Traditional IC systems usually contain a mobile phase source, a pump, a sample injector, an analytical column and a detector for detecting the sample ions. In SIC, a separate suppressor unit is added to the system. This, in turn, increases the complexity of the system and also increases extra-column volume which may decrease chromatographic resolution and sensitivity. Therefore, it would be advantageous to have a system of ion suppression chromatography which reduced the number of system components in traditional SIC systems. Another problem associated with prior art SIC systems is that the mobile phase is converted to a weakly ionized form, which renders the mobile phase unsuitable for reuse. Thus, it would be advantageous if a system of SIC were developed in which the mobile phase is converted back to its strongly ionized form after suppression and, thus, may be reused. Another problem associated with SIC systems using sodium carbonate/bicarbonate mobile phases is that suppression of the mobile phase yields carbonic acid which interferes with the detection of the sample ions. More specifically, when a sodium carbonate/bicarbonate eluant is used, during suppression of the sodium electrolyte carbonic acid is formed. The carbonic acid is more conductive than water and creates "background noise" which interferes with detection of the sample ions. In order to avoid this problem, it is known from US-A-4 672 042 to add CO 2 gas to the eluant and to use a stripper column permeable to CO 2 gas. SUMMARY OF THE INVENTION In its various aspects, the present invention addresses one or more of the foregoing problems associated with SIC. According to a first aspect, the present invention relates to a method of suppressed ion chromatography as defined in the appended claims. Moreover, a method of continuous electrochemically suppressed ion chromatography is disclosed. Analyte ions in a mobile phase comprising electrolyte are separated in a chromatography column resulting in a chromatography effluent comprising electrolyte and separated analyte ions. The chromatography effluent is then split into a first chromatography effluent stream and a second chromatography effluent stream. Electrolysis ions selected from the group consisting of hydronium ions and hydroxide ions are generated by the electrolysis of water. The electrolysis ions having the same charge as the electrolyte and the second chromatography effluent stream, which contains electrolyte and analyte ions, are simultaneously flowed through a stationary phase thereby suppressing the electrolyte in the second chromatography effluent stream. In a preferred aspect of the invention, the electrolysis ions force the electrolyte away from the second chromatography effluent stream and into the first chromatography effluent stream thereby effectively suppressing the second chromatography effluent stream. The analyte ions in the suppressed second chromatography effluent stream are then detected. Furthermore, a suppressor adapted for use in a method of continuous electrochemically suppressed ion chromatography is disclosed. The suppressor comprises an inlet, a first outlet, a second outlet and a third outlet. A first stationary phase comprising ion exchange resin is positioned in the path of fluid flow through the suppressor from the inlet to the third outlet. A second stationary phase comprising ion exchange resin is positioned in the path of fluid flow through the suppressor from the inlet to the first outlet. A first regeneration electrode is positioned at the third outlet and a second regeneration electrode is positioned at the second outlet. In another aspect, the suppressor further comprises sensor electrodes positioned in the second stationary phase for detecting the analyte ions in the suppressor. In yet another aspect, a method of suppressed ion chromatography is disclosed wherein the suppressed chromatography effluent is converted back to its strongly ionized state after suppression. Thus, the mobile phase is recycled and may be reused in a subsequent sample run. In a further aspect, a method of continuous electrochemically suppressed ion chromatography is disclosed where analytical column effluent, which contains separated analyte ions and electrolyte, is flowed to a first inlet of a suppressor. The suppressor comprises a stationary phase. The chromatography effluent is flowed through at least a portion of the stationary phase to suppress the chromatography effluent. The suppressed chromatography effluent is flowed to a detector where the analyte ions are detected. The detector effluent is then flowed back to the suppressor through a second inlet and out a second outlet to waste. In another aspect, a suppressor is disclosed wherein the same suppressor may be used in both anion and cation analysis. The suppressor has a first inlet, a first outlet and a second outlet. A first stationary phase is located in the path of fluid flow through the suppressor from the first inlet to the first outlet. A second stationary phase is located in the path of fluid flow through the suppressor from the first inlet to the second outlet. A pair of regeneration electrodes are further provided wherein the first and second stationary phases are located between the electrodes such that an electrical potential may be applied across the first and second stationary phases. The first and second stationary phases further comprise oppositely-charged ion exchange resin. In yet a further aspect a method of SIC is disclosed using a sodium carbonate/bicarbonate mobile phase. Analyte ions in a mobile phase comprising sodium carbonate/bicarbonates are chromatographically separated to yield a chromatography effluent comprising separated analyte ions and a sodium carbonate/bicarbonate mobile phase. The sodium carbonate/bicarbonate mobile phase is then suppressed to yield a suppressed chromatography effluent comprising carbonic acid, carbon dioxide gas and separated analyte ions. Prior to detecting the analyte ions, the carbon dioxide gas is removed. In yet another aspect, analyte ions in an aqueous mobile phase comprising electrolyte are chromatographically separated to form an aqueous chromatography effluent comprising separated analyte ions and electrolyte. The electrolyte is suppressed by ion exchange with electrolysis ion selected from the group consisting of hydronium ions and hydroxide ions which are generated by the electrolysis of water; the electrolysis of the water further forming gas by-products. The gas by-products are then removed from the separated analyte ions and the separated analyte ions are subsequently detected. DESCRIPTION OF THE DRAWINGS Figures 1 and 2 are schematic views of two systems using a suppressor adapted for use in a method of continuous electrochemically suppressed ion chromatography. Figure 3 is a schematic view illustrating the method of operation of a suppressor adapted for use in a method of continuous electrochemically suppressed ion chromatography. Figure 4 is an exploded perspective view of a suppressor adapted for use in a method of continuous electrochemically suppressed ion chromatography. Figure 4a is a cross-section view of the suppressor illustrated in figure 4 along line A-A. Figure 5 is an illustration of the method of operation of a suppressor adapted for use in a method of continuous electrochemically suppressed ion chromatography wherein the suppressor includes sensor electrodes for detecting analyte ions. Figure 6 is an exploded view of an integrated suppressor and detector that may be used according to another aspect. Figure 7 is an illustration of the operation of a suppressor according to another aspect. Figure 8 is an illustration of another suppressor configuration. Figure 9 is a chromatogram generated by the sample run discussed in Example 1. Figure 10 is a chromatogram generated by the sample run discussed in Example 2. Figures 11-14 are a chromatograms generated by the sample runs discussed in Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Figure 1 illustrates a system of continuous electrochemically suppressed ion chromatography. The system comprises a mobile phase source 10 comprising electrolyte, a pump 11, a sample injector 12 and a chromatography column 14, all in fluid communication. The pump 11, sample injector 12 and chromatography column 14 may be selected from the variety of types known by those skilled in the art. For example, preferred pumps include the ALLTECH 526 pump available from ALLTECH ASSOCIATES, INC. (Deerfield, IL). Preferred chromatography columns include the ALLTECH ALLSEP or UNIVERSAL CATION COLUMNS. Preferred sample injectors include the RHEODYNE 7725 injection valve. A suppressor 15 in fluid communication with the chromatography column 14 is further provided. The suppressor 15, which contains electrodes (not shown), is discussed in further detail below. The suppressor 15 is connected to a power source 18. Preferred power sources include the KENWOOD PR36-1.2A. The system also preferably includes a gas permeable tubing or membrane 17 in liquid communication with the suppressor 15 and a detector 21. The gas permeable tubing is preferably TEFLON AF 2400 (DUPONT) tubing available from BIOGENERAL of San Diego, CA. By flowing the mobile phase and sample ions through tubing 17 prior to detection of detector, gas may be removed before the detector thereby improving the detection of the sample ions. A preferred detector for use in the invention is the ALLTECH MODEL 550 CONDUCTIVITY DETECTOR. Other suitable detectors for use with the present invention are electrochemical detectors. The detector 21 measures or records the analyte ions detected by the detector. Finally, back pressure sources 21a, 21b and 21c are preferably included to control operating pressure in the system. By manipulating the operating pressure, gas bubbles from the electrolysis may be controlled. In operation, the direction of fluid flow is as follows. The mobile phase is flowed from mobile phase source 10 by pump 1 through injection valve 12 to chromatography column 14 to suppressor 15 and then to detector 21. Upon exiting the detector 21, the mobile phase is flowed through a cross 21b through back pressure regulator 21a and then to recycling valve 19, which directs fluid flow either to waste or back to mobile phase source 10 as discussed below. The recycling valve 19 is preferably a three-way valve. Figure 2 illustrates another system for use in the method of continuous electrochemically suppressed ion chromatography according to the present invention. This system differs from the system of Figure 1 in that the suppressor and detector are integrated to give an integrated suppressor and detector 16. The integrated suppressor and detector 16 has sensor electrodes (not shown) for detecting analyte ions and is discussed in further detail below. Additionally, a measuring device 20 is in electrical communication with the integrated suppressor and detector 16 for recording analyte (or sample) ions. A preferred measuring device is the OAKTON ¼ DIN CONDUCTIVITY AND RESISTIVITY CONTROLLER (OAKTON 100 SERIES). Also in electrical communication with the integrated suppressor and detector 16 is power source 18. The path of fluid flow through the system of Figure 2 is as follows. Fluid flow is from mobile phase source 10 by pump 11 through injection valve 12 to chromatography column 14 to integrated suppressor and detector 16. Upon exiting the integrated suppressor and detector 16, the mobile phase is flowed through recycling valve 19, which directs fluid flow either to waste or back to mobile phase source 10 as discussed below. The recycling valve 19 is preferably a three-way valve. With reference to figure 1, the mobile phase comprising electrolyte and analyte ions (e.g., sample ions that are to be detected) are flowed to chromatography column 14 where the analyte ions are separated. The separated analyte ions and electrolyte exit the chromatography column 14 as chromatography effluent and flowed to suppressor 15 where the electrolyte is suppressed. The operation of suppressor 15 is described with reference to Figure 3 for anion analysis and a mobile phase consisting of an aqueous solution of sodium hydroxide. As those skilled in the art will quickly appreciate, the invention may easily be adapted for cation analysis and/or different electrolytes. Referring to Figure 3, the suppressor 15 comprises first stationary phase 31 and second stationary phase 31a. By stationary phase, it is meant chromatography material comprising ion exchange functional groups in either free resin form or in any matrix that permits liquid flow therethrough. The stationary phase is preferably a strong cation exchanger, such as sulfonic acid cation exchanges such as BIORAD AMINEX 50WX8. The stationary phase may also comprise a solid polymer structure that permits liquid flow therethrough. The suppressor may also include end filters, 26a and 26b, comprising strong cation exchange resin encapsulated in a TEFLON filter mesh located at both ends of the suppressor 15. These end filters limit the amount of gas, which is generated at the regeneration electrodes during electrolysis, from entering the suppressor 15 during electrolysis. Preferred end filters are ALLTECH NOVO-CLEAN IC-H Membranes. The suppressor 15 further comprises first regeneration electrode 22 and second regeneration electrode 23. In this embodiment, the first regeneration electrode 22 is the cathode and the second regeneration electrode 23 is the anode. The first and second regeneration electrodes are preferably flow-through electrodes that are connected to power source 18 (not shown). The preferred electrodes are made of a titanium housing with flow-through titanium frits, 26c and 26d. The electrodes are platinum plated to provide an inert, electrically-conductive surface. The suppressor 15 further comprises an inlet 24 for receiving the chromatography column effluent and a first outlet 25 for flowing suppressed chromatography effluent (which contains analyte ions) to the detector 21. The suppressor also comprises second and third outlets 28 and 30, respectively, through regeneration electrodes 23 and 22, respectively. During a sample run power is continuously applied to activate regeneration electrodes 22 and 23 while providing water to the suppressor 15. The water source may be the chromatography effluent or a separate water source may be provided. In any event, electrolysis of the water occurs at the regeneration electrodes generating electrolysis ions selected from the group consisting of hydronium ions and hydroxide ions. In the present embodiment, hydronium ions are generated at the anode (second regeneration electrode 23) and hydroxide ions are generated at the cathode (first regeneration electrode 22). The hydronium ions are flowed from the second regeneration electrode 23 across second stationary phase 31a and first stationary phase 31 to first regeneration electrode 22. The hydronium ions eventually combine with the hydroxide ions generated at first regeneration electrode 22 to form water, which may exit the suppressor at third outlet 30. In operation, the chromatography effluent is introduced into the suppressor 15 at inlet 24. In this embodiment, the chromatography effluent comprises separated anions in an aqueous sodium hydroxide eluant. Upon entering the suppressor at inlet 24, the chromatography effluent is split into two chromatography effluent flow streams; namely a first chromatography effluent flow stream and a second chromatography effluent flow stream. The first chromatography effluent flow stream flows in a first chromatography effluent flow path from the inlet 24 through the first stationary phase 31 positioned between the inlet 24 and the first regeneration electrode 22. Thus, the first chromatography effluent flow path is defined by the flow of the first chromatography effluent flow stream from inlet 24 to first regeneration electrode 22. The first chromatography effluent flow stream may exit the suppressor 15 through the first regeneration electrode 22 and third outlet 30. The second chromatography effluent flow stream flows in a second chromatography effluent flow path from the inlet 24 through second stationary phase 31a, which is positioned between the inlet 24 and the second regeneration electrode 23, to the second regeneration electrode 23. Preferably, a portion of the second chromatography effluent exits the suppressor 15 at first outlet 25 and another portion at second outlet 28 through second electrode 23. The second chromatography effluent stream exiting at first outlet 25 is flowed to the detector where the analyte ions are detected. In the suppressor, the sodium ion electrolyte in the chromatography effluent preferably migrates from the second chromatography effluent flow stream into the first chromatography effluent flow stream by the combined action of the hydronium ion flow from the second regeneration electrode 23 to the first regeneration electrode 22 and the negative charge at the first regeneration electrode 22. The second chromatography effluent flow stream thus comprises separated anions which combine with the hydronium electrolysis ions to create the highly conductive acids of the analyte anions. The second chromatography effluent flow stream further comprises water that is generated, at least in part, by the hydroxide ions from the sodium hydroxide eluant combining with the hydronium electrolysis ions. A portion of the second chromatography effluent flow stream exits the suppressor at second and first outlets 28 and 25, respectively. The suppressed second chromatography effluent comprises an aqueous solution of the separated analyte anions in their acid form along with oxygen gas generated at the second regeneration electrode from the hydrolysis of water. Because the oxygen gas may interfere to some extent with the detection of the analyte anions at the detector, the suppressed second chromatography effluent exiting first outlet 25 is preferably flowed through a gas permeable membrane such as gas permeable tubing 17 where the oxygen gas is removed prior to detecting the analyte ions. In this respect, a back pressure source 21a (see Figure 1) may also be included in the system to create sufficient back pressure to efficiently force oxygen gas through gas permeable tubing 17 and out of first suppressor effluent. Similarly, back pressure sources 21b and 21c are likewise provided (see Fig. 1) to provide further pressure control in the system. As can be ascertained from figure 3, increasing the backpressure in the suppressed second chromatography effluent stream exiting at outlet 25 could disturb fluid flow through the suppressor 15. Therefore, it is preferable to apply counterbalancing pressure in the second chromatography effluent stream exiting at second outlet 28 and first chromatography effluent stream exiting at third outlet 30. The suppressed second chromatography effluent flow stream exiting suppressor 15 at first outlet 25 is then flowed through the gas permeable tubing 17 to the detector 21 where the analyte ions are detected. Because power is applied while analyte ions are flowed through the suppressor 15, that is, the regeneration electrodes are continuously activated and an electrical potential is continuously applied across the first stationary phase 31 and second stationary phase 31a, there is a continuous flow of hydronium ions from the second regeneration electrode 23 to the first regeneration electrode 22. It is believed that this continuous flow of hydronium ions allows the second stationary phase 31a in the second chromatography effluent flow path to continuously remain in its substantially unexhausted form. Thus, in the present embodiment, a hydronium form ion exchange resin will remain substantially in its unexhausted or hydronium form in the second chromatography effluent flow stream because sodium ions are substantially precluded from entering the second chromatography effluent flow stream (and thus they are unavailable to exhaust the second stationary phase 31a) and are driven into the first chromatography effluent flow stream. Additionally, although the first stationary phase 31 in the first chromatography effluent flow path may become at least partially exhausted by ion exchange of the sodium ions with hydronium ions, a continuous supply of hydronium ions is available to continuously regenerate the first stationary phase 31 by ion exchange with retained sodium ions. The first chromatography effluent flow stream will exit the suppressor 15 at third outlet 30 as a third suppressor effluent and will comprise hydroxides of the sample countercations and an aqueous sodium hydroxide solution which is formed from the hydroxide ions generated at the first regeneration electrode 22 combining with, respectively, the sodium ion electrolyte and the hydronium electrolysis ions generated at the second regeneration electrode 23. The third suppressor effluent flow stream further comprises hydrogen gas generated by the electrolysis of water at the first regeneration electrode 22. The third suppressor effluent, in this embodiment, also may contain a portion of the analyte anions. By removing the hydrogen gas through known methods in the art (as, for example, by gas permeable tubing) and removing the analyte anions by known methods, the aqueous sodium hydroxide solution may be reused by flowing it back to the eluant source 10 and using it as the mobile phase in a subsequent sample run. Alternatively, the third suppressor effluent flow stream exiting the suppressor at exit 30 may be flowed to waste. As those skilled in the art will recognize, the suppressor discussed above may be used in methods for continuous electrochemically suppressed ion chromatography for both anion and cation analysis. Moreover, various eluants may be used such as hydrochloric acid or methanesulfonic acid for cation analysis and sodium carbonate/bicarbonate, sodium hydroxide, or sodium phenolate for anion analysis. The first stationary phase 31 and the second stationary phase 31a may be different or the same. Moreover, within either the first or second chromatography effluent flow paths, the stationary phase may be the same or a combination of free ion exchange resin, ion exchange resin encapsulated in a membrane matrix, or a solid polymer structure. The stationary phase, however, must permit fluid flow therethrough and the ion flow as discussed above. Examples of suitable stationary phases for anion analysis include DOWEX 50WX8 and JORDIGEL SO 3 . Examples of suitable stationary phases for cation analysis include AMINEX AG-X8 and ZIRCHROM RHINO PHASE SAX. As illustrated in Figure 3, inlet 24 is preferably positioned closer to first regeneration electrode 22 than to second regeneration electrode 23 along a horizontal axis. Thus, the distance that the first chromatography effluent streams travels from inlet 24 to first regeneration electrode 22 is preferably shorter than the distance traveled by the second chromatography effluent stream from inlet 24 to second regeneration electrode 23. Most preferably, the horizontal distance X'' between the center of inlet 24 and the second regeneration electrode 23 is about .930 inches (2.36 cm) to about 1.205 inches (3.06 cm) and the horizontal distance X' between the center inlet 24 and the first regeneration electrode 22 is about 0.466 inches (1.18 cm) to about 0.741 inches (1.88 cm). Preferably, the distance Y between the center axis of inlet 24 and first outlet is about 0.232 inches (0.589 cm) to about 0.464 inches (1.17 cm). The distance Z between first outlet and second electrode is about 0.466 inches (1.18 cm) to about 0.741 inches (1.88 cm). The distance Z' between first outlet and first electrode 22 is preferably about 0.930 inches (2.36 cm) to about 1.025 inches (3.06 cm). Figures 4 and 4a further illustrate suppressor 15 of the system described with respect to Figure 1. The suppressor comprises end caps 302 and 310. The suppressor further comprises first regeneration electrode 304 and second regeneration electrode 308. Positioned within first regeneration electrode (not shown in Figure 4) and second regeneration electrodes are frits 313 and 311, respectively. Frits 313 and 311 are preferably constructed from porous, non-conductive, non-electroactive materials such as polyolefins, or PAT™ (PEEK alloyed with TEFLON), or surface-oxidized titanium. Or the frits may preferably be constructed from inert, electro-active materials such as platinum coated titanium. The suppressor also includes O-rings 305 and 310a for providing a fluid tight seal between suppressor housing 306 and regeneration electrodes 304 and 308. The suppressor 15 further comprises an inlet 307, a first outlet 309, a second outlet 323 and a third outlet 321. Preferably, it is desirable to flow the gas bubbles (oxygen and hydrogen gas) formed by the electrolysis away from the detector. Alternatively, it is desirable to remove the gas bubbles from the system prior to the detector. This is desirable because the gas bubbles could interfere with the detection of the analyte ions at the detector. The gas bubbles can be flowed away from, or removed prior to, the detector in a variety of ways. One method for removing gas bubbles prior to the detector was previously illustrated by using gas permeable tubing 17 prior to the detector. In any event, as those skilled in the art will appreciate, gas permeable tubing 17 may be used in any method of ion analysis where it is desired to remove gas bubbles prior to detecting sample ions at the detector. For example, in methods of electroelution chromatography and of generating a high priority eluant disclosed in U.S. Patent No. 5,259,405, oxygen and hydrogen gas by-products of the electrolysis of water may be removed prior to detection of the sample ions. Similarly, in methods of suppressed ion chromatography disclosed, for example, in this application and co-pending application Serial No. 09/075,652, gas permeable tubing may be placed between the suppressor and detector to remove the oxygen and hydrogen gas by-products from the electrolysis of water prior to the detector. By increasing the back pressure in the system using back pressure source 21a, gas bubbles can be "forced" out of the system through gas permeable tubing 17 prior to the detector 21. Of course, it is desirable to balance the back pressure generated by back pressure source 21a, otherwise sample analysis could be affected. Therefore, back pressure sources 21b and 21c are preferably provided to counter the back pressures generated by source 21a to permit efficient operation of the system. Back pressure sources 21a, 21b and 21c may preferably be constructed from an in-line filter comprising a porous frit of plastic or metal from about 2-10 microns. Instead of two sources 21b and 21c, one such source could be used where the fluid flow in tubing 17a and 17b is merged in a T-configuration into one such source (not shown). Alternatively, instead of back pressure sources 21b and 21c, the back pressures created by source 21a can be balanced by altering the length of tubing 17a and 17b. Increasing the tubing length increases the back pressure created by the suppressed chromatography effluent flowing therethrough. In another aspect , sensor electrodes may be placed in the suppressor 15 resulting in an integrated suppressor and detector. A system for continuous electrochemically suppressed ion chromatography using an integrated suppressor and detector is illustrated in Figure 2. In this embodiment, the suppressor described with reference to figures 1, 3, 4, and 4a may be adapted by placing sensor electrodes in the second chromatography effluent flow path. The sensor electrodes are connected to a recording device and the separated analyte ions are detected while in the second chromatography effluent flow path within the suppressor. Another adaptation of an integrated suppressor and detector is illustrated in Figure 5. The chromatography effluent is preferably introduced into the integrated suppressor and detector 416 at inlet 417. Upon entering the integrated suppressor and detector, the chromatography effluent is split into two flow paths; namely a first chromatography effluent flow stream and a second chromatography effluent flow stream much like previously described. The first chromatography effluent flow stream is flowed towards a first regeneration electrode 422 and the second chromatography effluent flow stream is flowed to the second regeneration electrode 424. The first and second regeneration electrodes are preferably flow-through electrodes as previously described. As those skilled in the art will appreciate, by configuring the chromatography effluent flow paths and the regeneration electrodes in this manner, the oxygen and hydrogen gas bubbles formed by the electrolysis of water are flowed away from the sensor electrodes 426 and 428, and, therefore, will not interfere with the detection of the analyte ions at the sensor electrodes 426 and 428. In ion analysis, the integrated suppressor and detector works as follows. The chromatography effluent comprising aqueous sodium hydroxide and separated analyte anions is flowed from the chromatography column to the integrated suppressor and detector 416. The chromatography effluent is introduced into the suppressor and detector at inlet 417, where the flow path of the chromatography effluent is split. A portion of the chromatography effluent - the first chromatography effluent flow stream - is flowed to the first regeneration electrode 422 and a second portion of the chromatography effluent - the second chromatography effluent flow stream - is flowed to the second regeneration electrode 424. The flow of hydronium ions from the second regeneration electrode 424 to the first regeneration electrode 422 causes the sodium ions and sample countercations to migrate towards the first regeneration electrode 422 (the cathode) and away from the sensor electrodes 426 and 428, which are positioned in the second chromatography effluent flow path. Additionally, the sodium ions, the sample countercations, and the hydronium ions combine with the hydroxide ions generated at the first regeneration electrode 422 to form an aqueous sodium hydroxide and sample countercation hydroxide solution that may be reused as the mobile phase. Thus, because sodium ions migrate towards the first regeneration electrode 422 and away from the sensor electrodes 426 and 428, the hydronium ion concentration in the area around the sensor electrodes 426 and 428 far exceeds the sodium ion concentration. The analyte anions combine with hydronium ions to form the relatively more conductive acid of the analyte ion in the areas around the sensor electrodes which increases the sensitivity of the analyte ions to detection in the area around the sensor electrodes. After detection, the acid of the analyte ions is flowed through second regeneration electrode 424 and out of the integrated suppressor and detector 416. Moreover, the electrolysis of water provides a continuous supply of hydronium ions at regeneration electrode 424 that are flowed across the stationary phase 420 to first regeneration electrode 422. The source of the water for the electrolysis may be from the aqueous chromatography effluent or from a separate aqueous regenerant source. The previously described embodiments offer certain advantages. For example, gas bubbles formed by the electrolysis of water are flowed away from the sensor electrodes, which reduces the extent to which these bubbles interfere with the detection of the analyte ions. Additionally, the analyte ions do not have to flow through, or be in contact with, regeneration electrodes before detection by the sensor electrodes. This reduces the possibility that analyte ions will be chemically altered by contact with the regeneration electrodes. The concentration of unwanted counterions of the analyte ions in the area of the sensor electrodes is reduced which increases sensitivity of the system. On this point, it has unexpectedly been discovered that the above-described T-cell embodiment produces greater sensitivity over conventional suppressor systems. Without being restricted to theory; it is presently believed that this increased sensitivity is due to the preferential migration of incoming analyte ions toward the oppositely charged regeneration electrode which concentrates the analyte ions in the area of the sensor electrodes, provided of course the sensor electrodes are positioned near the oppositely charged regeneration electrode as illustrated in Figure 5. With further reference to Figure 5, the horizontal distance A between first regeneration electrode and inlet 417 is preferably about 0.406 inches (1.03 cm) to about 0.509 inches (1.93 cm). The horizontal distance B between inlet 417 and sensor electrodes 426 and 428 is preferably about .447 inches (1.13 cm) to about 0.522 inches (1.32 cm). The horizontal distance C between sensor electrodes 426 and 428 and second regeneration electrode 424 is preferably about 0.391 inches (0.99 cm) to about 0.915 inches (2.32 cm). Figure 6 is an exploded view of an integrated suppressor and detector 500. First and second end caps 502 and 504 are provided. Positioned within first end cap 502 and a first female cell 506 is first regeneration electrode 507. Positioned within second end cap 504 and a male cell 508 is second regeneration electrode 509. The regeneration electrodes are preferably as described above. Also included are first and second sensor electrodes 521 and 513, respectively. The sensor electrodes are preferably made of inert, conductive materials, such as platinum, gold, or platinum or gold plated stainless steel or titanium. The electrodes must allow liquid flow from the suppressor inlet to regenerate electrode 509, and must therefore either allow flow around or through them. O-rings 506a and 509a are provided to provide a fluid tight seal between first regeneration electrode 507 and female cell 506 and second regeneration electrode 509 and male cell 508, respectively. Spacer 517 and seal gaskets 519 and 515 are positioned between sensor electrodes 513 and 521. The spacer functions to reproducibly set the distance between sensor electrodes 513 and 521 and the gaskets are provided for a fluid tight seal. Seal gaskets 523 and 511 are further provided to give a fluid tight seal between female cell 506 and sensor electrode 521 and male cell 508 and sensor electrode 513, respectively. An adapter 508a is provided for receiving chromatography effluent at inlet 508b. Preferably, end caps 502 and 504, female cell 506, male cell 508 and adapter 508a are constructed from an electrically non-conductive material, such as PEEK, polyolefin, acrylic, plysulfone, or glass. Figure 7 illustrates yet another aspect using a suppressor without porous electrodes. In this embodiment, the suppressor 600 comprises first and second regeneration electrodes 602 and 604, respectively. The first regeneration electrode 602 is the anode where hydronium ions are generated by the electrolysis of water. Hydroxide ions are generated at the cathode, second regeneration electrode 604. The suppressor further comprises first stationary phase 608 and second stationary phase 610 separated by flow restrictor 120. For anion analysis, the first and second stationary phase is cation exchange packing material as previously described. The chromatography column effluent is flowed to the suppressor 115 at first inlet 116. Power is applied during the sample run thereby creating an electrical potential across the first and second stationary phase. Using anion analysis in an aqueous sodium hydroxide mobile phase, for example, the chromatography effluent is flowed through the suppressor as indicated by the arrows where suppression occurs as previously described. Sodium ions are driven from the chromatography effluent by the combined action of the hydronium ion flow from anode 602 to cathode 604 and by the attraction of the negative charge at cathode 604. The sample anions are converted to their highly conductive acids by combining with the hydronium ions. The suppressed sample ions are then flowed from first outlet 116a through tubing 118 to the detector (D) where the sample ions are detected. The tubing 118 is preferably gas permeable as previously described. Thus, the gas generated from the electrolysis may be removed prior to the detector through the gas permeable tubing 118 as previously described. The detector effluent may then be flowed back through the suppressor at second inlet 117 and out second outlet 117a and then to waste. Figure 8 discloses yet another embodiment where the same suppressor 215 is configured for use in both cation analysis and anion analysis. In this embodiment, the suppressor comprises a first stationary phase 216 comprising cation exchange resin and a second stationary phase 217 comprising anion exchange resin. Preferably, the first and second stationary phase meet at the longitudinal central axis of inlet 220. Chromatography column effluent is flowed from the chromatography column to the suppressor 215 through inlet 220. Depending on whether the sample run comprises anion or cation analyte ions, a detector will be positioned downstream of either first outlet 222 or second outlet 224. Alternatively, detectors may be placed downstream of both outlets 222 and 224. In cation analysis, a portion of the chromatography effluent will flow from the inlet 220 to first outlet 222. Conversely, in anion analysis, a portion of chromatography effluent will flow from inlet 220 to second outlet 224. The same suppressor, therefore, may be used for both cation and anion analysis. In operation, power is continuously applied thereby creating an electrical potential across the first and second stationary phases during the sample run. Water is supplied to the system, either from the chromatography effluent or from a separate water reservoir, and electrolysis occurs at the first electrode 240 and the second electrode 242. In this embodiment, the first electrode 240 is the anode and the second electrode 242 is the cathode. Hydronium ions are generated at the anode 240 and flowed from the anode towards cathode 242. Hydroxide ions are generated at cathode 242 and are flowed from the cathode towards anode 240. Thus, in anion analysis, the chromatography effluent is flowed from inlet 220 through first stationary phase 216 where the mobile is suppressed and the sample anions are converted to their conductive acids by ion exchange with hydronium ions. Sodium ions from the mobile phase flow away from first stationary phase 216 and into second stationary phase 217. The sodium ions then exit the suppressor 215 as sodium hydroxide at outlet 222. The suppressed mobile phase and sample anions exit suppressor 215 at outlet 224 and are flowed to the detector where the sample anions are detected. Conversely, the electrolyte of the mobile phase (sodium) migrates to second stationary phase 217 and exits at outlet 222 with the hydroxide ions generated by the electrolysis of water. The stream exiting at outlet 222 may be treated and re-used as the mobile phase in a subsequent sample run or flowed to waste. As discussed previously, the hydrogen gas and oxygen gas by-products from the electrolysis of water are preferably removed prior to detection of the sample ions at the detectors. A preferred way for doing this is through the use of gas permeable tubing such as TEFLON AF tubing. Applicants have discovered another advantage to placing degassing tubing or other means for removing gas prior to the detection of the sample ions at the detector. During suppression of carbonate/bicarbonate mobile phases dissolved carbonic acid is produced. The dissolved carbonic acid is relatively conductive, as compared to water, and thus creates a "background noise" which interferes with detection of the sample ions. Moreover, in gradient elution ion chromatography using carbonate/bicarbonate mobile phases, the background signal caused by the dissolved carbonic acid in the suppressed mobile phase fluctuates causing baseline drift that makes sample ion detection very difficult. Also, when using carbonate/bicarbonate mobile phases a water dip is seen at the beginning of the chromatograph because the water carrying sample ions has a lower conductivity than the suppressed carbonate/bicarbonate mobile phase. This water dip interferes with the detection of early eluting peaks such as fluoride. These problems associated with carbonate/bicarbonate mobile phases may be substantially reduced or eliminated by removing carbon dioxide gas from the suppressed sodium carbonate/bicarbonate mobile phase prior to detecting the sample ions. The dissolved carbonic acid from the suppression of the carbonate/bicarbonate mobile phase exists according to the following equilibrium: H + + HCO - 3  H 2 O + CO 2 (g) This equilibrium favors carbonic acid (HCO 3 - ). By removing the carbon dioxide gas, the equilibrium is moved to the right thereby removing dissolved carbonic acid. It has been discovered by removing sufficient amounts of carbon dioxide gas the levels of dissolved carbonic acid may be reduced so as to substantially eliminate the aforediscussed problems. It should be understood that the above method of removing carbonic acid is applicable to all methods of suppressed ion chromatography using an aqueous carbonate/bicarbonate mobile phase. Example 1 In this example, a chromatogram was generated using a suppressor illustrated in Figure 3 and the system of Figure 1 where, instead of back pressure sources 21b and 21c, long length tubing was connected to second and third outlets, 28 and 30, respectively, of the suppressor 15. The following equipment and parameters were used. Analytical Column ALLTECH ALLSEP column (Methacrylate-based anion exchanger with quaternary amine functionalities), 100 x 4.6 mm; 7 µm particle size Column Temp Ambient Eluant 0.85mM NaHCO 3 /0.90 mM Na 2 CO 3 Flow rate 1.0 ml/min. Detector Suppressed Conductivity Suppressor (see Figure 3) Bed length = 35.5 mm Distance X' = 11.85 mm Distance Y = 11.8 mm Distance Z = 11.85 mm Distance X''= 23.6 mm Distance Z ' = 23.6 mm Electrodes Ti frits, 40 µ porosity and coated with Pt. Constant Current 75 mA with corresponding voltage 18V. Tubing exiting at third outlet 30 was 76 inches in length with 0.063" OD and 0.007" ID. Tubing exiting at second outlet 28 was 50 inches in length with 0.063" OD and 0.007" ID. At first outlet 25, and 10 µ pt frit was provided and tubing to the detector was 0.031" ID and 0.250" OD. The chromatogram of Fig. 9 was obtained. Example 2 Similar equipment and parameters as Example 1, except backflow sources (see Fig. 1, reference numerals 21b and 21c were used in the tubing connected to the second outlet 23 and third outlet 30 (see Fig. 3) of the suppressor 15. The backflow sources were placed 5 inches from the anode and cathode. The tubing had 0.040" ID. The backpressure sources were in-line 10 m micron filters, PEEK alloy TEFLON available from ALLTECH ASSOCIATES, Deerfield, IL as part no. 68250. An additional 20 inches of tubing was placed on the downstream side of the back pressure sources. Also, in this example, a constant current of 100mA was applied creating a corresponding voltage of 24V. The chromatogram of Figure 10 was obtained. Example 3 Samples were run to illustrate the advantage of removing carbon dioxide prior to detection of the analyte ions where an aqueous sodium carbonate/bicarbonate mobile phase is used. The following run conditions were used: Chromatography Column ALLSEP Anion A-2 (methacrylate-based anion exchanger with quaternary amine functionalities), 100 mL x 4.6 mm internal diameter, 7 µm particle size Column Temp Ambient (i.e., 23-25° C) Eluant 2.8 mM sodium bicarbonate, 2.2 mM sodium carbonate in water Flow rate 1.0 mL/min. Detector Suppressed Conductivity Sample 10 ppm each of nitrite, nitrate and sulfate in water; eluting in that order Suppressor dimensions and electrodes (see Figure 3) Same as Example 1 Current 100 mA Figures 11 and 12 are chromatograms that were generated from a system set-up with a length of TEFLON AF (gas permeable) tubing placed between the suppressor and the detector. Oxygen gas and hydrogen gas by-products from the electrolytes of water that may be present in the flow stream to the detector and carbon dioxide gas from the suppression of the sodium carbonate/bicarbonate mobile phase are removed through the TEFLON AF tubing. Figures 13 and 14 are chromatograms that were generated from the same system set-up, except TEFLON AF tubing was not used. Instead, a length of non-gas permeable tubing was placed between the suppressor and the detector. As can be ascertained from comparing the chromatograms, the chromatograms generated by the system with the gas permeable tubing are noticeably improved over the chromatograms from the system without the gas permeable tubing. The system with the gas permeable tubing has a background conductance of less than 8 uS whereas without the gas permeable tubing the background conductance is about 21 uS. The drop in background conductance is due to the removal of carbon dioxide. Also, the "water dip" is substantially reduced in the system with the gas permeable tubing. Finally, although the sample concentration and injection volumes were the same, the analyte peaks in the system with the gas permeable tubing were about 10% greater because the analyte peaks are detected against a lower background signal.

Claims: A method of suppressed ion chromatography wherein gas is removed prior to detection of analyte ions thereby reducing the amount of dissolved carbonic acid in a suppressed mobile phase, the method comprising: (a) chromatographically separating analyte ions in an aqueous mobile phase comprising a cation carbonate/bicarbonate; (b) suppressing the mobile phase thereby forming dissolved carbonic acid and carbon dioxide gas in equilibrium; (c) reducing the amount of carbonic acid in the mobile phase by removing carbon dioxide gas; and (d) detecting the separated analyte ions after step (c). The method of claim 1 wherein the cation comprises sodium ions and the sodium ions are suppressed in step (b) by ion exchange with hydronium ions. The method of claim 1 wherein the carbon dioxide gas is removed through gas permeable tubing. The method of claim 2 wherein the hydronium ions are generated by the electrolysis of water. The method of claim 4 wherein the electrolysis of water generates by-products comprising oxygen gas and hydrogen gas and comprising the additional step of removing the oxygen gas and hydrogen gas by-products prior to detecting the separated analyte ions.

Inventors: ANDERSON JAMES M JR, BENEDICT BART C, GURNER YURI, PHAM HUNG ANTHONY, SAARI-NORDHAUS RAAIDAH, SIMS CARL W, ANDERSON, JAMES M. JR., BENEDICT, BART C., GURNER, YURI, PHAM, HUNG ANTHONY, SAARI-NORDHAUS, RAAIDAH, SIMS, CARL W.
Applicants: ALLTECH ASSOCIATES INC, ALLTECH ASSOCIATES, INC.
Application year: 2000
Main classification: G01N30
IPC codes: B01D15, G01N30, G01N1