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Ion Chromatography – IC
The most important form of ion chromatography involves a combination of specific ion-exchange phases with conductivity detection. It is a sensitive technique, in some cases being able to detect ppb levels of ions.Suppressed or non-suppressed detection Eluents used in IC contain a relatively high level of salt ions and therefore exhibit high conductivity. This leads to a high background signal which could inhibit the detection of low level analytes. Suppression of eluent conductivity post column is necessary for efficient detection of sample ions, and is the most common method for anion analyses. Although isocratic elution is more commonly used, the use of suppressors enables gradient elution to be used for complex samples.
Silica and polymer based phases are available for anion and cation analyses. Silica based columns, although showing better efficiency, have a limited pH range. As a result, they are not compatible with anion IC methods requiring suppressed detection, due to the high pH of the eluents required. Polymer based materials are stable over a wider pH range and have higher capacities. Tables 1 and 2 show typical base materials and bonding for anion and cation chromatography phases respectively. The majority of phases for anion chromatography are bonded with quaternary ammonium groups, with a permanent cationic charge. For cation chromatography phases, sulphonate is the most common functionality.Columns optimised for non-suppressed IC generally have lower capacity than those for suppressed detection, in order to achieve a relatively low background conductivity. They are therefore not suitable for suppressed detection.
The choice of eluent for IC depends on whether the method is suppressed or non-suppressed. For suppressed anion IC, carbonate/bicarbonate or hydroxide are the most common eluents. Hydroxide eluents have the advantage of producing water only in the suppressor and thereby giving a very low background conductivity. However, some phases are not stable at the high pH (12) of this eluent. For non-suppressed anion IC analyses, typical eluents include p-hydroxybenzoic acid and phthalic acid. Typical eluents for suppressed and non-supressed cation IC include HCl, HNO3, tartaric acid and succinic acid.
High sensitivity ion analyses are important in a wide spectrum of industries including pharmaceutical, food, water,semiconductor etc. In addition to the common inorganic anions eg F-, Cl-, NO2-, NO3-, SO42-, acid salts can also be analysed by IC eg formate, acetate. Quantitative analysis of anions at the ppb level can be achieved. Cation chromatography is used for the separation and detection of Group 1 and 11 metal ions, in addition to some transition metal ions, ammonium ions and ethanolamines. Small ions are generally eluted before larger ions and monovalent ions before di- and trivalent ions.
Table 1. Anion Chromatography Phases
Table 2. Cation Chromatography Phases[table “26” not found /]
Process Scale Chromatography