ION CHROMATOGRAPHY. ELECTROLYTE ADDITIONAlexander M.Nemirovsky (firstname.lastname@example.org)
This paper discusses the effect of adding electrolyte to the eluent. In the book “Ion Exchange and Ion Chromatography” by A.M. Dolgonosov, M.M. Senyavin, I.N. Voloshchik (Moscow. Nauka. 1993.) there is a case described when in ion-exchange separation of anions the addition of electrolyte caused an increase of the hydrogen carbonate if carbonate eluent was used.
To explain the authors’ position, let us consider the anion separation scheme. Anion separation is carried out at anion-exchange resin, with solutions of weak acid salts such as carbonates, borates etc. being used as eluents. After a usual separation column a suppression cation-exchange one is placed. In the latter, the salts of the separated anions transform into acids, and the concentrations of the acids are registered by a conductivity sensor.
Hence, the authors explained the effect of the hydrogen carbonate peak increase with the fact that the carbonic acid produced after the suppression column has a lower conductivity than other acids. Adding an electrolyte to the eluent increases the overall electric conductivity of the solution which causes the considerable growth of the hydrogen carbonate peak. Such an explanation seemed unconvincing to me, which led to additional investigation of this process.
Experiments were carried out with a HIKS-1(made in Russia) ion-exchange reactor. A carbonate solution with a sodium nitrate additive was used as eluent. The solution was prepared the following way. Equal amounts of anhydrous carbonate and sodium bicarbonate were weighed. When the salts dissolved, sodium nitrate was added to the eluent. As a result, the solution contained 0.005M of carbonates and 0.001M of sodium nitrate. The suppression column contained resin KU-2-8 (made in Russia). Experiments were carried out on a chromatograph with a conductivity sensor.
In addition to the expected effect of increasing the hydrogen carbonate peak by a factor of 4 to 5, a curious fact has been noticed which was not noticed by the authors of the book mentioned above. It turned out that adding electrolyte made another peak appear. The position of the peak is determined by the electrolyte anion, i.e. if the electrolyte contains a nitrate then the peak appears in the position where the nitrate is usually located. But this is not an ordinary peak but a trough. Also, the following rule holds: the absolute value of this unusual peak grows with the contents of HCO3- growing from sample to sample.
The figure shows the results of chromatograph calibration by hydrogen carbonate. The straight line N1 illustrates the change of the hydrogen carbonate peaks and the straight line N1 illustrates that of the unusual nitrate peaks. Negative peak heights mean that there are troughs instead of peaks on the chromatogram.
Basing on these results, it is possible to conclude that carbonates can be detected not only by the hydrogen carbonate peak but also by the nitrate peak.
Let us try to interpret the observed effects.
Let us consider the behavior of the sample at the moment when it gets on the ion-exchange reactor. As the ion-exchange process bases on the fact that the ions of the analyzed compound displace those of hydrogen carbonate, it is natural to assume that excess HCO3- in the sample will displace nitrate ions from the surface of resin. Therefore the contents of nitrate ions in the mobile phase will be greater than normal, leading to an increase in the eluent conductivity. This process will be permanently observed throughout the migration of the excess hydrogen carbonate peak along the column. Finally, at the column exit an overall signal of HCO3- and the excess nitrate that has displaced hydrogen carbonate will be registered.
Further evolving this idea we can explain the appearance of the nitrate peak. Since HCO3 continuously displaces nitrate from ion-exchange resin, it impedes the formation of the nitrate peak. The lack of hydrogen carbonate will cause intensified absorption of nitrate by resin resulting in formation of the nitrate peak. Thus, the hypothesis stated above fully describes the effect of electrolyte addition on the chromatographic process.
In conclusion it should be mentioned that, besides the experiments with the sodium nitrate addition, experiments with sodium phosphate and potassium chloride were carried out. Their results were similar to those obtained by adding sodium nitrate to the eluent.