Technique: ion chromatography

Columns and mechanism

The IC separation process is based on ion exchange where the sample ions dynamically bind to groups of opposite charge at the accessible surface of the stationary phase in the column. The strength of this interaction is mediated by the composition of the eluent which also transport the sample through the system. Anion IC thus employs an anion exchange column, typically containing organic polymer particles carrying a stationary phase with positively charged groups.

Most frequently, the term ion chromatography (IC) refers to the separation of small ions using ion exchange, whereas the designation ion exchange chromatography (IEX) more often represents the separation of larger molecules (e.g., proteins and peptides). However, there are no fundamental differences in the underlying principles and mechanisms of IC and IEX, only in the application and terminology of them.

Mobile phase conditions

Typical eluents for suppressed anion IC are strongly basic aqueous solutions, most often containing mixtures of carbonate (CO₃²¯), and bicarbonate (HCO₃¯), or hydroxide (OH¯). Isocratic elution with carbonate-bicarbonate is the most popular approach since these eluents are easy to prepare and have some built-in buffer capacity. However, for gradient analysis of more complex samples hydroxide eluents are dominating, but these are sensitive to contamination and must be protected appropriately. Alternative eluents may be comprised of tetraborate (B₄O₇²¯) or different phenolate ions, and there are also examples of routine methods employing p-hydroxybenzoic acid at high pH as eluent. Acetonitrile and other organic solvents are sometimes added to IC eluents to reduce the impact of hydrophobic interactions with the core material carrying the ion exchange sites in the column, especially if the stationary phases are of older and more hydrophobic design.

Instrumentation

The instrument design in IC is rather similar to other modes of liquid chromatography, especially HPLC. This allows HPLC instruments to be converted to IC use, provided that the other modules can tolerate the mobile phase conditions. One modification that often is required though, is to exchange stainless steel tubing to polymeric poly(etheretherketone), PEEK, and passivating the remaining stainless steel surfaces. Detection of the separated ions in IC is typically carried out using techniques such as conductivity or ultraviolet (UV) light absorption spectrometry, and sometimes mass spectrometry (MS) or electrochemical oxidation-reduction reactions. When performing detection by suppressed conductivity or mass spectrometry, a suppressor device is placed after the column, in order to remove cationic counterions from the eluent and neutralize it. The suppressor can be regenerated intermittently, or continuously, and especially for strong eluents and gradient elution, the latter is typically preferred and often even necessary.

Learn more

To learn more, browse the application examples below or contact Diduco to discuss your specific analysis.