Process and Apparatus for the Quantitative Determination of Cations or Anions by Ion Chromatography

The invention concerns a procedure and an apparatus for the quantitative determination of anions or cations in a sample using ion chromatography, whereby a sample (containing the ions to be determined) is injected into and then carried by a buffer stream onto a chromatography column; the background ions in the eluate from the column are suppressed by an ion exchange mechanism and the determination is by conductivity measurements.

The invention begins with the current state of the technology as described, for example, in DE-OS No. 24 37 600. There the basic problem, that the eluant has such a high conductivity that measuring the conductivity of the ion species separated in the chromatography column is impossible, is solved by connecting to the latter a suppressor column of opposite ion exchange type as to the chromatography column. In the suppressor column the ion species passing the ions to be determined are exchanged for OH.sup.- or H.sup.+ ions. The disadvantage of this system is that the suppressor column can not be run continuously as the column must be periodically regenerated. In addition, the effect of the suppressor column varies with use and is dependent on chemical properties such as pH, diffusions, adsorption and exchange effects, all of which will reduce the reproducibility and exactness of the analyses.

The invention enables the suppressor reaction to be dramatically improved, so that the analysis can be carried out continuously under constant chemical conditions. To do this, the background ions from the chromatography column are suppressed by an ion exchange mechanism and the determination is by conductivity measurements.

To suppress the background ions the eluate is passed between two ion exchange membranes which are not permeable to ions, of opposite charge to those which are to be determined. Two electrodes are so arranged that an electric field can be set up across the direction of flow so that strongly dissociated electrolytes (in similar or quite different solutions) can be separated from each other between the electrodes and the ion exchange membrane. Under the influence of the electric field the weakly dissociated ions of low conductivity in the buffer are carried through, whilst the buffer ions of opposite charge are removed after passing through the membrane into the electrolytic solution.

The single FIGURE is a schematic exploded perspective view of the components of a cell according to the present invention.

The invention consists of a procedure and an apparatus as follows: storage for the buffers, a sample inject valve, a chromatography column filled with ion exchange resin, a second ion exchange device to suppress the background ions, and a conductivity cell. The second ion exchange device for suppressing background ions consists of a channel for the eluate which is at least partially partitioned by ion exchange membranes on the outside of which are electrolyte solutions and the adjacent spaces contain electrodes which can set up an electric field across the eluate channel.

Before going into the background of the various chemical methods to analyse anions and cations, we should look more closely at the cell (see diagram) to obtain a better understanding of the nature of the invention. The cell consists of two ion exchange membranes (10,12) which are fixed between two platinum electrodes (14,16) attached to the side walls of the cell (not shown in diagram). The eluate from the separation column flows through the channel marked 18 on the diagram. The platinum electrodes (14,16) are connected to a potential which can be set to between 50 and 550 Volts which leads to currents of between 10 mA and 1.2 A. In the main a current of 120 mA is obtained running at 200 Volts.

The distance between the electrodes (14,16) and the adjacent ion exchange membranes (10 or 12) is between ca. 4 mm and ca. 50 mm, often more than not 8 mm. The two membranes are usually between ca. 0.1 mm and ca. 3 mm apart. The two ion exchange membranes and the electrodes are pressure stable up to 30 bar (at the moment), so that they form in cross section three square channels, i.e. on either side of the eluate channel (18) there are side channels formed by the membrane and the anode electrode (14)--channel 20--and channel 22 between the cathode (16) and membrane (12).

The two side channels have electrolytic solutions pumped through them which act as electrode buffers. Both these buffers have to be pumped from different buffer reservoirs in quite separate circulation channels through the side channels. The chemical and physical composition of the two electrode buffers is sometimes identical but often different.

The chemical reactions in the cell are dependent on

the type and condition of the ion exchange membranes (10,12)

the nature and concentration of the electrode buffers and the eluate

The voltage across the electrodes and the resulting flow rates only influence the reaction time and not the reaction as such.

In practice a length of ca. 100-500 mm (normally 270 mm) for the membranes and a thickness of ca. 0.4-5 mm (normally 1 mm) has been found to be best. The materials to be used for the membrane and the solutions are determined by which type of ions, anions or cations, are to be measured. For the analysis of anions, cations exchange membranes are used (consisting of a vinylchloride or teflon matrix with SO.sub.3 groups on the surface) whose thickness varies from 0.09-0.2 mm (usually 0.12 mm) and which have an electrical resistance of ca. 2.0-5.9 ohm/cm.sup.2. As the electrode buffer the strongly dissociated mineral acids, mainly H.sub.2 SO.sub.4, HNO.sub.3 and H.sub.3 PO.sub.3 in concentrations from 0.001 to 1 mole/1 are used. These electrode buffer flow in the side channels (20,22) in the opposite direction to the eluate flow in the central channel (18).

The eluant chosen for the buffer system of the separation column depends on the ions to be separated. In addition, for cation exchange (with H.sup.+ ions) the eluant must produce either demineralized water or a weakly dissociated compound of low conductivity (e.g. H.sub.2 CO.sub.3) after the exchange. Some examples are given below.

The reaction in the cell for anion determination is shown in the diagram using a concrete example. Chloride and nitrate are to be separated on the ion exchange column using NaOH as eluant. The anions are present in the sample as potassium and lithium salts respectively. KC1 and LiNO.sub.3 will flow into the cell from the column but because of the background from the NaOH identification of the anions by conductivity measurements or similar methods is not possible.

The flow rate of the eluate through the cell is approximately 2 ml/min. i.e. with a cell volume of 200 .mu.l there will be an average dwell time of 0.1 min. In the side channels 0.15 molar H.sub.2 SO.sub.4 is flowing as electrode buffer at a rate of 0.1 ml/min. Under the influence of the potential of 300 V the hydronium ions built-up in the region of the anode will flow to the cathode. As the cation exchange membrane is permeable to cations but not to anions (transport No. 0.97) the SO.sub.4.sup.= ions cannot move from the anion region to the cation region. The ions in the eluate stream undergo a parallel process. There is no volume mixing as the membranes are also not permeable as far as the solutions are concerned. The anions in the eluate stream would move in channel 18 towards the anode but cannot penetrate the cation exchange membrane 10, so that they are carried quantitatively with the eluate into the detector cell. The cations in the eluate stream will move through the cation exchange membrane 12 to the cathode 16 (in channel 22) and will be replaced by hydronium ions. The velocity of the movement will be effected by the electrical potential and the resulting rate of the flow. In addition the overriding factor will be the osmotic pressure. This can be optimized by reducing the molar concentration of the electrode buffer in channel 22 and increasing the flow rate in this channel. A reduction of the H.sub.2 SO.sub.4 concentration to 5.times.10.sup.-3 mol/l with a flow rate of 0.5 ml/min produces no deterioration in the cell measurements but much increased base line stability in the conductivity detector.

The reactions can be summarized as follows:

(a) in the region of the anode

1. Dissociation of the H.sub.2 SO.sub.4

and also (see Bromsted--

2. The H.sup.+ ions (protons) and the H.sub.3 O.sup.+ (hydronium ions) will move through the cation exchange membrane into the eluate stream.

The sulphate ions move in the direction of the anode 14 and are washed out of the system.

(b) in the eluate channel 18

1. Dissociation of NaOH, KCl, LiNO.sub.3

2. A movement of the cations Na.sup.+, LI.sup.+ and K.sup.+ in the direction of the cathode 16 through the cation exchange membrane 12.

A movement of the anions in the direction of the anode 14 along the non-permeable cation exchange membrane in the direction of flow causes the anions to react with the abundance of H.sub.3 O.sup.+ ions present.

i.e. the eluant, NaOH, of high conductivity will react to form a compound of low conductivity (H.sub.2 O) and the samples (KCl, LiNO.sub.3) will react to form compounds (HCl, HNO.sub.3) of high conductivity.

(c) in the region of the cathode 22

1. Dissociation of H.sub.2 SO.sub.4 as above

2. A movement of hydronium ions and cations from the eluate towards the cathode 16 and hence out of the system. A movement of the sulphate ions in the direction of the anion as far as the cation exchange membrane and then along the membrane and out of the system.

For the determination of cations, anion exchange membranes are used. These consist of a matrix of polyvinylchloride or fluorcarbons (polytetrafluoroethylene and polyperfluoethylene-propylene) with ammonium--NH.sub.4.sup.+ ions on the surface. The membrane thickness is normally between 0.11 and 0.15 mm and its electrical resistance is between ca. 2.4 and 4.6 ohm/cm.sup.2. The electrode buffer is normally NaOH or similar bases in concentrations between 0.001 and 1 mole/l. The buffer flows through channels 20 and 22.

The eluant acids used, depend upon the ions to be analysed. The overriding consideration is that such acids (e.g. HCl or H.sub.2 SO.sub.4) must result in demineralized water or other weakly dissociated compounds after exchange of the anions with OH.sup.-.

In the determination of cations, the movements of the ions are analogous to the description above in that the OH.sup.- ions from the cathode region and the anions in the eluate stream move to the anode through the anion exchange membrane, whereas the cations are held in their channels by the membrane.

Finally a few examples of the method are given below:

As the above table demonstrates, the method described using the new cell with ion exchange membranes results in the strongly dissociated compounds of high conductivity used as eluants being turned into weakly dissociated compounds of low conductivity in all cases which will not affect the quantitative measurement, by conductivity, of the cations or anions in the sample.