Ion Chromatography in Practice

Jul 07, 2014

Ion chromatography (IC) is often used for the chromatographic separation of inorganic anions or cations in waters, low-molecular-weight organic acids in beverages, drugs in blood plasma, preservatives or vitamins in food, modified sugars in high-throughput screening of new bioactive formulations, ionic chelates in production samples, amino acids in biospecimens, and certain organometallic compounds in metallurgical products. In a laboratory that already uses high performance liquid chromatography (HPLC) or ultrahigh-pressure liquid chromatography (UHPLC), will the implementation of ion chromatography be expensive in terms of instrumentation or resources? The Column spoke to Ade Kujore of Cecil Instruments Limited to find out.

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Q: How does ion chromatography differ from high performance liquid chromatography (HPLC) or ultrahigh-pressure liquid chromatography (UHPLC)?

A. Ion chromatography (IC) is any method for the chromatographic separation of ionic or ionizable species in solution. Many different ions can be analyzed within single elutions. The actual analytical methods and instrumentation are similar to those of high performance liquid chromatography (HPLC) and ultrahigh-pressure liquid chromatography (UHPLC). The main forms of IC are based on one or more of the following interactions:

  • Ion-exchange chromatography: This is where ionic interactions occur between analyte ions and polar functional groups in the stationary phase. The stationary phase is a chemically modified silica or styrene-divinylbenzene copolymer resin, onto which ionic side groups are introduced. Examples of the ionic side groups include: Quaternary ammonium groups (strong anion exchanger); tertiary amine group (weak anion exchanger); sulphonic acid (strong cation exchanger); and carboxylic acid (weak cation exchanger). Ion-exchange chromatography is currently the most widely used of all forms of ion chromatography interactions. Increased selectivity in ion-exchange separations are achieved by manipulating the type of counter-ion, ionic strength, and pH of the mobile phase. Further to this, more than one mobile phase can be used to create a gradient, to progressively elute different analytes through the column.
  • Ion-exclusion chromatography: This is mainly used for the separation of weak acids such as carboxylic acids, phenols or amino acids, or for weak bases.
  • Ion-pair chromatography: Used to separate ions on a reversed-phase column, detection is normally performed using a UV–vis detector. An ion-pair reagent is added to alter the separation and resolution of ions. The separation of complex mixtures of very polar with ionic molecules, such as halides and imidazolium and pyridinium derivatives, or a mixture of artificial sweeteners, is achieved.