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Ion Chromatography is closely related to high performance liquid chromatography (HPLC). It is an effective method for separation and determination of ions, based on the use of ion exchange resins.
There are several ways of detection of analytes.
In conductivity detection, the conductivity of ion analyte is measured as it passes through a conductivity cell. Both, the eluent and sample are ionic species and the background conductivity is too high to detect ions with a good sensitivity. To get it,it is necessary to eliminate chemically the background conductivity of the eluent, in a post column reaction. These columns are known as contuctivity suppressors, and can be chemical or electrochemical. This mode of detection is the most common in ion chromatography.
In Amperometric detection the generated electric current is measured when the ion analyte is oxidized or reduced in an amperometric cell composed by three electrodes: working electrode, reference electrode and auxiliary electrode. Oxidation or reduction of ions depends on the application of a potential between the working electrode and the reference electrode. The signal measured is the intensity of the current generated between the working electrode and the auxiliary electrode. Within this detection mode highlights the Pulsed Amperometry Detection(PAD), widely used for the detection of carbohydrates. This mode of detection is highly selective since only oxidizable or reducible ions to potential applied may be measured.
UV-VIS spectrophotometric detection is based on the absorption of electromagnetic energy by certain groups, called chromophores, present in the molecules or ions. The wavelength and energy absorbed depend on the chemical bonds within the molecule or ion, and are characteristic thereof. In ion chromatography direct photometric detection, only ions presenting chromophore groups may absorb some of the light that reaches the cell, causing a decrease in the intensity of the light beam, that is directly proportional to the amount of ion eluted. In ion chromatography, UV-VIS detection is not very significant, since the inorganic anions and cations and aliphatic organic molecules lack chromophores, therefore is only used to supplement the conductivity detection. One of the most common applications of this mode of detection is the determination of nitrite and ammonium in seawater. The UV-VIS detection of these minority ions (chromophores) at sea, can be achieved by removing the interference of the major ions in these samples: sodium and chloride (not chromophores).
In ion chromatography, spectrophotometric detection UV-VIS usually requires a previous derivatization step, in which cations/anions are transformed in chromophore species. An examples of this technique, the determination of chromium (VI) by ion chromatography and UV-VIS detection.
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