Dual ion chromatograph using parallel columns for ionic analysis

Abstract

Integrated system for the concurrent quantitative analysis, using ion exchange resin, of chromatographically separable cations and anions contained in a single measured portion of aqueous sample solution includes an eluant water reservoir, a sample injection valve, a pump delivering water from the reservoir to the sample injection valve, and thence to a first stream splitter feeding two parallel ion exchange columns. One column is charged with anion exchange resin and the other with cation exchange resin, preferably with differing bed depths but in any event selected in nature and geometry to achieve differing elution times for each ion present and being determined, and ordinarily to achieve elution of all the ions of one sign before elution of the ions of the opposite sign, e.g., all the anions before the cations. The effluent from each column is joined through a second stream splitter and the resulting single stream is directed to a conductivity cell with associated readout means. The effluent from the conductivity cell may be discarded but is ordinarily passed through clean-up resin bed means and cycled back to the eluant water reservoir. Preferably means are provided for (1) substituting columns with different resins and bed depths for specific analyses, (2) occasionally flushing the sample solution delivery system with bactericide, or (3) running known standard solution.

Description

CROSS-REFERENCE TO RELATED APPLICATION

In a copending application of T. S. Stevens and W. H. Parth, Ser. No. 386,259, filed Aug. 6, 1973, there is described an integrated system for quantitative analysis, using ion exchange resins, of the number of equivalents of ionic species present by converting all the species to a given ion pair and measuring the concentration of such pair with a conductivity cell.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an integrated system for the quantitative analysis of an aqueous solution containing a limited number of separable ion species, the determination of all cations and anions being determined being carried out on a single injected sample portion and the anions and cations being determined concurrently.

2. Description of the Prior Art

There is a constant and ever increasing demand for a rapid, inexpensive method of analysis of large numbers of samples of aqueous solutions containing a relatively small number of species of ion pairs dissolved therein. There is also a great need for a simple automatable apparatus for carrying out such analyses as well as for analysis of process streams for a small number of specified ion pairs or for a given ion in the presence of a small number of ion pairs, such as is present, theoretically, with up to six cation species and up to six anion species represented in the solution, in readily detectable amounts.

The determinations of dissolved ions in such aqueous solutions have generally been carried out using old classical, slow, and reinatively costly methods. Even where ion exchange separations, chromatographically or by exchange, have been utilized, the fractions obtained have generally been analyzed by classical methods which usually require a different test and/or a different instrument for each species to be determined, particularly with respect to the anions to be determined. Therefore, a new approach with new apparatus appears to be needed.

SUMMARY OF THE INVENTION

Concurrent quantitative determination of chromatographically separable cations and anions in aqueous solution from a single measured portion of sample solution is readily carried out using a system which includes: first and second tubular columns each adapted to hold a charge of ion exchange resin; first and second stream splitters, a sample selection valve, a pump, a water reservoir and a conductivity cell having associated readout means; and liquid conduit means sequentially connecting, in series, the water reservoir, the pump, the sample selection valve and the first stream splitter, then in parallel, between the first stream splitter and the second stream splitter, the first and second tubular columns, and finally, in series, the second stream splitter and the conductivity cell. In using the system, one column is charged with an anion exchange resin and cations are chromatographed thereon while the other column is charged with a cation exchange resin and anions are chromatographed thereon, concurrently, that is, substantially simultaneously with the cation separation and from the same sample portion divided by the stream splitter. The column or bed geometry is selected to facilitate elution of the separated ions at discretely different times so that quantitative detection with a conductivity cell can be accomplished. Generally, a clean-up resin bed means is employed following the conductivity cell, and the eluant water is cycled back to the water reservoir in a closed loop system which is readily automated, in which case it is generally desirable to include means for running a standard solution periodically and also flushing the sample selection valve and associated parts occasionally with a bactericide solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The single FIGURE of the drawing is a schematic representation of chromatographic apparatus for the concurrent quantitative determination of chromatographically separable cations and anions in aqueous solution from a single measured portion of sample solution per concurrent determination, the apparatus including dual columns connected in parallel for the substantially simultaneous separation of anions on one column and cations on the other column, according to the invention.

FURTHER DESCRIPTION OF THE INVENTION

An embodiment of the present chromatographic apparatus, or dual ion chromatograph, for the concurrent quantitative analysis of chromatographically separable cations and anions in a single sample portion, as shown in the drawing, includes a water reservoir 10, normally containing a substantial quantity of water 11 which is used as eluant, a sample selection valve 12 ordinarily in the form of a conventional sample injection valve, a pump 13 for conveying eluant water 11 to the sample selection valve 12, a first stream splitter 14 receiving eluant water and any sample admixed therewith in the sample selection valve 12, and dividing the stream and directing the divided parts to the first tubular column 15, which is charged with cation exchange resin when ready for use, and the second tubular column 16, which is charged with anion exchange resin when ready for use, a second stream splitter 17 which here serves as a stream combining means on receiving the effluent from both of the tubular columns 15, 16, a conductivity cell 18 with associated readout means 19, receiving the conjoined column effluents, clean-up resin bed means indicated generally by the numeral 20 and taking the form here of two columns 21, 22 in series and normally charged with a bed of cation exchange resin in the hydrogen form and a bed of anion exchange resin in the hydroxide form in the respective columns for deionizing the effluent water, and liquid conduit means 23 for connecting the above-described parts in the sequence described, as well as connecting the clean-up resin bed means 20 to the water reservoir 10.

The first tubular column 15, when ready for use, is charged with a cation exchange resin in the hydrogen form, while the second tubular column 16 is charged with an anion exchange resin in the hydroxide form. Anions must become separated chromatographically while traversing the first column 15, else a smaller sample or a deeper bed of resin must be employed or both, if success is to be achieved, that is, the ion species separated, so that the conductivity cell detects separated bands or groups of ions. Or, a particular ion exchange resin may be selected that holds the ions of interest in a differential manner. Similarly, cations must become separated chromatographically while traversing the second column 16 containing anion exchange resin.

In order to avoid the elution of cations and anions simultaneously, that is, from both columns at once, column 16 is selected from or made up as a longer column than column 15, or at least the charge of resin therein is deeper in column 16. Generally separate issuance of anions prior to the issuance of cations is assured upon making the resin bed or column 16 at least 25 percent greater in depth, and preferably at least 50 percent greater in depth, than the resin bed depth in column 15.

While the stream splitter 14 will ordinarily be one that divides the stream evenly between the two columns, by adjusting the split between the two columns, and therefore the amount of eluant water applied to the columns respectively, further adjustments are possible in the relative elution times from the two columns as will be appreciated by those skilled in the art.

The sample selection valve 12 is of the general type commonly used for chromatographic analyses and typically is provided with a measuring bore in the valve plug of known volume or a pair of ports to the valve body are connected by a tubing loop of known volume, and the valve is provided with bypass means for continuously directing eluant water through the valve to the stream splitter 14 and through the system while sample solution, or standard solution, as the case may be, flows continuously through the measuring bore or the tubing loop and discharges continuously to a waste stream.

In the case of individual samples under analysis a syringe or pipet may be used to fill the measuring loop, or the sample, as measured by the syringe, is simply injected through a septum into the eluant stream. In the case of a sample stream, a by-pass line, not shown, can be used to bring sample to the sample intake of the sample selection valve 12, and through the measuring section described above.

Wherein the measuring loop is used, the valve is manipulated to bring the sample-filled volume into series with the eluant stream of water constantly passing through a portion of the valve, and the selected sample portion of known volume is swept on into the system. If the apparatus is automated, a computer-controller, not shown, will cause the selection valve to be actuated to bring the sample measuring loop to be swept out by eluant water and will coordinate the readout of the conductivity cell therewith.

The sample selection valve 12 should provide for measuring and injecting a predetermined sample size in the range of about 2 to about 1000 microliters. Typically chosen sample sizes adequate for detecting most ionic materials and small enough to avoid unnecessary exhaustion of the ion exchange resins used are in the range of about 5 to about 50 microliters.

The sample selection valve 12 is preferably either a rotary valve such as the Model R 6031 SVA-K valve supplied by Chromatronix, Inc., or a slide valve such as the Model CSVA-K valve from the same supplier, or the equivalent of either. When using remotely actuated valves in an automated system the valves must each be provided with appropriate actuators.

The columns used to house the ion exchange resins, viz, columns 15 and 16, are best selected from glass or metal columns now readily available commercially and having the proper fittings to be easily connected into a system. While larger columns may generally be used, if desired, such as those having 25 to 50 mm. inside diameter (ID), the smaller columns utilizing smaller resin beds better serve the purposes of obtaining rapid, sharp analytical separations and the preferred column sizes are in the range of about 1 to 10 mm. ID and from about 5 to 1000 cm. length, provided the two columns are relatively proportioned, one to the other as described above, but more generally selected sizes are 2.8 mm. or 9 mm. ID and the first column has a length of about 20 to 30 cm. and the second column is proportionately longer to provide for the requisite deeper resin bed needed, although both columns may be the same length and the second column filled with a greater depth of ion exchange resin, i.e., with a larger charge of resin.

The resin charged to the columns 15 and 16 should be high specific exchange capacity ion exchange resins. The ion exchange resins used for chromatographic separations herein gradually become exhausted according to their specific exchange capacities, amounts of resin used, and the number of equivalents of ion species in the samples exchanging with the ions at the active sites of the resins. Preferably, for the sake of economy of operator attention the number of equivalents in selected sizes of samples and the total exchange capacity of the ion exchange resins used permit the analysis of at least about 500 and more preferably 5000 or more samples before the charge of resin in a given column becomes exhausted.

The ion exchange resins usable in the present method and apparatus are typically polystyrene or modified polystyrene copolymers cross-linked, e.g., with divinylbenzene, and carrying nuclear groups, the latter providing active exchange sites. The cation exchange resins carry nuclear sulfonic acid or sulfonate groups along the polymer chain. The strong base anion exchange resins carry nuclear chloromethyl groups which have been quaternized.

For further information on ion exchange theory, processes and resins synthesis reference is made to the monograph "Dowex: Ion Exchange" 3rd Ed. 1964, published by The Dow Chemical Company, Midland, Mich., and the two-volume work "Ion Exchange" Ed. by Jacob A. Marinski and published by Marcel Dekker Inc., New York, 1966. Chapter 6, Vol. 2, of "Ion Exchange" is devoted to a description of synthesis of ion exchange resins of various types usable herein.

The dimensions of the clean up resin bed columns 21, 22 used to house the clean up resins beds are not critical as analytical separations are not carried out therein. Most any geometry suffices so long as the ion exchange resin placed therein has a total exchange capacity sufficient to deionize the eluant water effluent from the conductivity cell for a very large number of samples, preferably handling at least as many samples as the ion exchange resins in columns 15 and 16. Usually columns with the same bed volumes as that of column 15 suffices.

Instead of using two columns in series as shown in the drawing the clean up resin bed means 20 may take the form of a single column which will be charged with either a two layer bed or a mixed bed resin for deionization of the eluant water. In carrying out analyses according to the invention, both anions and cations in the effluent from the conductivity cell will need to be exchanged for hydroxide ions and hydrogen ions respectively in the clean up bed resins. The quality of the deionized water should be such as to give a very low base line reading, e.g., about 2 micromho/cm, when passed through the conductivity cell.

The conductivity cell employed is selected from most any of the conventional commercially available models regularly used in conductimetric detection chromatography.

An example of a suitable conductivity cell is Model MCC 75 available from Chromatromix, Inc. A suitable conductivity meter for use therewith is Model CM 1A from the same supplier. Most any meter selected is preferably modified, for the present purposes, to reduce zero suppression.

In addition, the apparatus may include provision (not shown) for bringing sample solution from a process stream to the sample selection valve 12, and filtering the solution, if desirable.

To assure accurate results by making corrections from time to time for such changes as instrument component drift, it is highly desirable and usually considered essential to supply a known or standard solution to the analysis apparatus on an intermittent basis and mutually exclusively to the supplying of sample solution. Accordingly, standard solution stored in a reservoir 24 is supplied as by gravity through a three-port selector valve 25 which, when appropriately set, delivers standard solution to the sample injection valve 12 with the aid of pump 26, if necessary, while stopping the flow of sample solution for the duration of the running of the standard solution to the sample injection valve 12.

To prevent the growth of bacteria, algae or anaerobic organisms in the system, it may be found desirable, especially in the sampling and analysis of some solutions using automated apparatus, to provide means for periodically flushing the three-port valve 25, the pump 26 and parts of the sample injection valve 12 and the liquid conduit means interconnecting the same with a bactericide solution. Solutions such as aqueous sulfuric acid, or inhibited hydrochloric acid, having a concentration of e.g., 4 normal, or an aqueous solution of most any of the organic chemical bactericidal, algaecidal or slimicidal compounds used in keeping cooling towers, sampling lines, and the like free from organisms, may be used. Such bactericide solution is stored in a container 27 which serves as a reservoir and, on appropriately setting the selector valve 25, the bactericide solution flows to the pump 26 and into the sample selection valve 12, at times when sample and also standard solutions are mutually exclusively shut off, relative to the bactericide solution. Since the bactericide would be deleterious to the resin beds it is excluded therefrom by closing off access to the columns at the sample injection valve 12; all of the bactericide solution exits through a waste port in the sample injection valve 12.

While the columns 15, 16 and the resin beds therein may be readily replaced when the resin is exhausted, it may also be desirable to backwash and regenerate each bed in place, utilizing multi-port valves 28 and 29 in working with column 15, and multi-port valves 30 and 31 in connection with column 16.

Candidate sample solutions analyzable by the present method and apparatus therefor are aqueous solutions, as indicated herein above, containing ion species that are resolvable on an ion exchange resin column serving as a chromatographic column, a cation exchange resin column for the separation of anions and an anion exchange resin column for the separation of cations, and the column geometries varying sufficiently that the individual ions elute at different times, usually all of the anions first. It is not essential that all of the ions of a given balance sign elute first so long as the peaks in the output of the conductivity cell are identifiable.

Resolvability of all of the ions present is most likely to be achieved easily if the number of cations and the number of anions in each is no more than six and even more so if each is no more than three. Expecially likely to be resolvable is a solution containing the conjugate bases of a weak and a strong acid and the conjugate acids of a weak and a strong base, or either alone.

Traces of other ions may be present providing they elute at different times than the ions under determination or providing they do not significantly alter the conductivity cell output at peaks for the ions under determination. The output is not altered significantly if it does not change the readout of apparent ion concentration by an increment greater than the limits of accuracy required of the analyst. Typically, this limit might be 1 to 5 percent of the actual ion concentration.

The following example serves to illustrate and not to limit the scope of the invention.

EXAMPLE

Using substantially the apparatus represented schematically in the drawing and having a cation exchange resin column 9 mm ID and 250 mm in length and charged with a commercial cation exchange resin, Dowex 50W X16, 200-400 mesh (37-74 microns) in the hydrogen form and an anion exchange resin column 9 mm ID and 500 mm in length and charged with a commercial anion exchange resin, Dowex 1 X8, 200-400 mesh (37-74 microns), in the hydroxide form, a solution containing sodium chloride, sodium carbonate, and ammonium hydroxide that was too concentrated to analyze without dilution was diluted 1:25 with deionized water and 40 microliters of the diluted sample was introduced into the system through the sample selection valve. With eluant water flowing at the rate of 345 ml per hour the readout of the conductivity cell on a meter with stripchart recorder appeared as well separated peaks at 1.5, 3.0, 4.5, and 7.0 minutes for, respectively, chloride ion, carbonate ion, sodium ion, and ammonium ion. Identification was confirmed by running known standardized mixtures from which it was also possible to compute the sample composition, from conductivity cell response, to be, by weight, 13.0 percent sodium chloride, 12.5 percent sodium carbonate, and 13.0 percent ammonia. Molar concentrations are as follows:

Diluted Sample Original Sample moles/liter moles/liter ______________________________________ Chloride 0.0089 0.222 Carbonate 0.0047 0.118 Sodium 0.0183 0.458 Ammonium 0.0148 0.371 ______________________________________

Claims

We claim:

1. Chromatographic apparatus for the concurrent quantitative determination of chromatographically separable cations and anions in aqueous solution from a single measured portion of sample solution which comprises:

first and second tubular columns each adapted to hold a charge of ion exchange resin;

first and second stream splitters, a sample selection valve, a pump, a water reservoir and a conductivity cell; and

liquid conduit means sequentially connecting, in series, the water reservoir, the pump, the same selection valve and the first stream splitter, then, in parallel, between the first stream splitter and the second stream splitter, the first and second tubular columns, and finally, in series, the second stream splitter and the conductivity cell.

2. The apparatus as in claim 1 wherein the second tubular column is at least 25 percent longer than the first tubular column.

3. The apparatus as in claim 1 wherein the second tubular column is at least 50 percent longer than the first tubular column.

4. The apparatus as in claim 1 wherein the first tubular column contains a cation exchange resin and the second tubular column contains an anion exchange resin, the cation exchange resin being in the hydrogen form and the anion exchange resin being in the hydroxide form.

5. The apparatus as in claim 1, including, in addition, at least one additional tubular column and additional liquid conduit means connecting in series the conductivity cell, the at least one additional tubular column and the water reservoir thereby providing a closed loop.

6. The apparatus as in claim 5 wherein the at least one additional tubular column is charged with a double bed of ion exchange resin combination adapted to substantially demineralize aqueous solution effluent from the conductivity cell passed therethrough the double bed.

7. The method of chromatographically quantitatively determining concurrently cationic species as well as anionic species in a given portion of aqueous sample solution with a single detector, the cations and anions being chromatographically separable from ion species of like charge, which comprises:

introducing and blending a known quantity of said aqueous sample solution into a flowing stream of water;

dividing the blend of sample and water stream into a first and second stream;

directing the first stream through a first tubular column containing a charge of anion exchange resin in the hydroxide form and chromatographically separating said cationic species therein and eluting the separated cationic species therefrom;

simultaneously directing the second stream through a second tubular column containing a charge of cation exchange resin in the hydrogen form and chromatographically separating said anionic species therein and eluting the separated anionic species therefrom; and

conjoining the effluent streams exiting from the first and second tubular columns and passing the conjoined streams through a conductivity cell capable of detecting each of the chromatographically separated cationic species and anionic species, the relative depths of the anion and cation exchange resin beds and the relative flow rates of the first and second streams being cooperatively preselected to avoid interference between ionic species being detected by the conductivity cell.

8. The method as in claim 7 wherein the number of each of the cationic species and anionic species present, respectively does not exceed six.

9. The method as in claim 7 wherein the number of each of the cationic species and anionic species present, respectively, does not exceed three.

10. The method as in claim 7 wherein the ion exchange resin bed depths and the relative flow rates are so selected that the each of the anionic species present elute from the ion exchange resin in the second tubular column and reach the conductivity cell before any of the cationic species elute from the ion exchange resin in the first tubular column and reach the conductivity cell.

11. The method as in claim 7 wherein the ion exchange resin bed in the first tubular column has a depth at least 1.25 as great as the ion exchange resin bed in the second tubular column.

12. The method as in claim 7 wherein the ion exchange resin bed in the first tubular column has a depth at least 1.5 as great as the ion exchange resin bed in the second tubular column.