Methods for improving ion chromatographic resolution

Ion chromatography is a type of high performance liquid chromatography. It is a liquid chromatography method for the analysis of anions and cations. The separation mechanism of ion chromatography is mainly ion exchange. There are three separation methods. They are high-performance ion exchange chromatography and ion exclusion chromatography. And ion pair chromatography.

1. Diluted Samples For complex samples, if the affinity of the tested ions for the resin varies considerably, several injections must be made and eluted with different concentrations or strengths of eluent or gradient. For ions with a large difference in affinity for the stationary phase, the easiest way to increase the degree of separation is to dilute the sample or perform sample preparation. For example, the separation of SO42- and Cl- in salt water. If the sample is injected directly, the chromatographic peak is very broad and the tailing indicates that the injection volume has exceeded the capacity of the separation column. Under the commonly used chromatographic conditions for anion analysis, the elution of Cl- continues after 30 minutes. In this case, no more injections can be made until the stable baseline is restored. A good separation between Cl- and traces of SO42- can be obtained if the sample is diluted 10 times before injection. The recommended maximum injection volume for anion analysis is generally 30% of the column capacity, beyond which large flat peaks or shoulder peaks will appear.

II. Changing Separation and Detection Methods If the affinity of the ion to be measured is similar or the same for the stationary phase, the effect of sample dilution is often unsatisfactory. In this case, in addition to selecting the appropriate mobile phase, consideration should also be given to selecting the appropriate separation method and detection method. For example, NO3- and ClO3-, due to their similar charge numbers and ionic radii, are co-rinsed on an anion exchange column. However, the hydrophobicity of ClO3- is greater than that of NO3-, which makes it easy to separate on ion-pair columns. Also, if the retention times of NO2- and Cl- are similar on the anion exchange separation column, the concentration of Cl- in common samples is much larger than that of NO2-, making the separation more difficult, but NO2- has strong UV absorption, and Cl- is very Weak, so should use UV as a detector to determine NO2-, with conductivity detection of Cl-, or the two detectors in series, at the same time detection of Cl-and NO2-. For the analysis of organic acids in high concentrations of strong acids, if ion exclusion is used, strong acids are not retained and are excluded in the dead volume and will not interfere with the separation of organic acids.

III. Sample Preparation For the determination of trace ions in high-concentration matrices, such as the determination of anions in seawater, the best method is to properly pre-treat the sample. The pretreatment method for removing excess Cl- is as follows: the sample is passed through the Ag pretreatment column to remove Cl-, or AgNO3 is added to the sample before the sample is precipitated to Cl-; valve switching technology may also be used, which is to make the sample weakly retained Components and more than 90% of Cl-into the waste liquid, only about 10% of the Cl- and the retention time greater than the Cl- component into the separation column for separation. For samples containing large organic molecules, the organics should be removed prior to injection. The simpler method is to remove the organic matrix with a RP or P column or on-line valve from Onion's pre-treatment column Dionex.

IV. Choosing the Right Eluent Ion chromatographic separation is based on the competition between the rinsing ions and the sample ions for the effective exchange capacity of the resin. In order to obtain effective competition, the sample ions and the elution ions should have similar affinities. The following is an example of the general principle of selecting an eluent. In the case of CO32--HCO3-as eluent, the ions eluting prior to Cl- are weakly retained ions, including monovalent inorganic anions, short carbon chain monocarboxylic acids, and some weakly dissociated components such as F-, formic acid Acetic acid, AsO2-, CN-, and S2-, etc. For the separation of acetic acid, formic acid and F-, Cl-, etc., weaker leaching ions should be used. The commonly used weak leaching ions are HCO3-, OH-, and B4O72-. Since HCO3- and OH- are easily absorbed in the air and CO2 is converted into CO32- in alkaline solution, the CO32- has a greater leaching strength than HCO3- and OH-, which is disadvantageous to the separation of the weakly retained ions. B4O72- is also a weakly eluted ion, but the solution is stable and is the recommended eluent to separate weakly retained ions. Medium-strength carbonate eluents have low elution efficiency for high-affinity components. There are two kinds of ions that have strong affinity for ion exchange resins. One is a large number of ions such as PO43-, AsO43-, and polyphosphates, and the other is a large ion radius and strong hydrophobicity, such as I. -, SCN-, S2O32-, benzoic acid and citric acid. For the former, increase the concentration of the eluent or choose a strong leaching ion. In the latter case, the recommended method is to add organic modifiers (such as methanol, acetonitrile, and para-cyanophenol, etc.) or hydrophilic columns to the eluent. The role of the organic improver is to reduce the sample ions and ions. The non-ion exchange effect between the exchange resins takes up the hydrophobic position of the resin, reducing the adsorption of hydrophobic ions on the resin, thereby shortening the retention time, reducing the peak tailing, and increasing the measurement sensitivity.

In ion chromatography, the selectivity can be improved by the addition of different eluent additives. This eluent additive affects only the interaction between the resin and the measured ions without affecting ion exchange. For ions with strong affinity to the resin, such as some polarizable ions, I- and ClO4-, and hydrophobic ions, benzoic acid and triethylamine, etc., an appropriate amount of polar organic solvent is added to the eluent. Such as methanol or acetonitrile can shorten the retention time of these components and improve the asymmetry of the peak shape. In order to reduce the non-ion exchange effect between the sample ions and the resin and reduce the adsorption of hydrophobic ions by the resin, in the anion analysis, p-cyanophenol can be added to the eluent. When trace amounts of I- and SCN- were measured in 1% NaCl, the addition of p-cyanophenol occupied the adsorption sites of the resin for I- and SCN-, thereby reducing peak tailing and increasing the sensitivity of the assay. In IC, one-price leaching ions elute a valence-tested ion, and divalent leaching ions elute a divalent analyte ion. The change of eluent concentration has greater influence on the retention time of divalent and multivalent ions to be measured. Price to be tested. If the retention time of polyvalent ions is too long, increasing the concentration of the eluent is the preferred method.