Chromatography is based on the principle of flow of two or more components in a mixture through the stationary phase at different speeds with the help of a mobile phase.
Definition of Chromatography
Chromatography, which is the combination of the Greek words chroma (color) and graphein (to write), was first developed in 1903 by the Russian botanist Michael Tsvett. Tsvett used this method to separate the colored components of plant pigments from each other. He named this separation method as chromatography because colored bands were formed in the column. Thereafter, in various multicomponent samples, chromatographic methods were started to be used for separation and purification of components.
Chromatographic methods are widely used in the last decades thanks to its simplicity, rapidity and effectivity. By using chromatographic methods, qualitative and quantitative analyses can be performed.
Chromatography is based on the principle of flow of two or more components in a mixture through the stationary phase at different speeds with the help of a mobile phase. By using chromatographic methods, it is possible to separate and purify the mixtures which are composed of chemically and physically similar components completely.
In chromatographic methods, the components in a mixture are separated from each other by passing the mobile phase through the stationary phase. During this separation, both phases interact with each other. Interactions of components with different physical and chemical properties with both phases are different. Thus, the components driven by the mobile phase leave the stationary phase at different times, as they are held on to the stationary phase differently. The least interacting component in the environment leaves first, and the most interacting component leaves last.
As a result of the difference in these movement speeds and retention times, the sample components are separated from each other. They are divided into different bands or regions that can be analyzed qualitatively and quantitatively.
Today, chromatographic methods are used to analyze the species that form a mixture (qualitative analysis) and determination of the quantities of the components that form a mixture (quantitative analysis). It has become one of the most used instrumental analysis methods.
Ion Exchange Chromatgraphy
Chromatographic methods can be categorized according to the mode of application, according to the mechanism of separation and according to their phase types. The analyses performed in this study are based on ion chromatography (IC).
Ion chromatography, which is also known as ion exchange chromatography, is a separation and determination method of ions by ion exchange resins. In order to separate anions, anion exchange resins are used and in order to separate cations, cation exchange resins are used. The resins are insoluble solids in the form of beads and they have porous structure which provides a large surface area. Ion exchange occurs by trapping and releasing the ions by the resin. These are divided into two groups as inorganic and organic.
The inorganic ones are clay and zeolites that have been used for about a century. Today, organic ion exchangers are used much more than inorganic ones. Ion chromatography is used in toxicology, forensic medicine, drinking water and wastewater analysis, air pollution analysis, content of ionic species in biological solutions, industrial waste analysis, food and beverage analysis and in the determination of anions, cations, organic acids, amines, amino acids, carbohydrates or nucleic acids in different samples.
All ion chromatography systems mainly consist of an eluent and a pump, injection part, a separation column in which ion separations take place and a detector. In some ion chromatography systems, between separation column and detector, there is a suppressor column which reduces the ground conductivity of the ions generated by the eluent and increases the conductivity of sample ions.
Ion exchange occurs between a charged analyte, a stationary phase and a mobile phase which has the opposite charge of the stationary phase. The charged regions of the stationary phase are the functional groups. Before the injection of the analytes to the system, eluent is given to the system to ensure electrical neutrality. When the sample is injected, analyte ions they are in a race with the eluent ions to interact with the functional groups of the stationary phase. This race continues between two phases throughout the chromatography column. The more an ion interacts with the stationary phase, the slower it leaves the column. Ion exchange is basically represented by the Figure 1.
Separation of analyte ions occurs according to the difference of their interest to the functional group of the stationary phase. The relative affinity of analytes to the stationary phase is defined as selectivity. Selectivity depends on various parameters such as the type of functional group of the stationary phase, the properties of the eluent ions, the concentration of the eluent ions, the solvents and temperature.
The amount of functional groups in the stationary phase is defined as capacity. The capacity is usually, the equivalent of 1 g of resin or the equivalent coefficient of a column. The higher the capacity leads the longer the retention of analyte ions in the column.
The structure of the ion exchangers is defined by the ionic functional groups at the surface and a support matrix. Commonly used functional groups in ion chromatography are sulfonic, carboxylic, phosphonic, phosphinic, arsenic, selenoic acids and phenols as cation exchangers and quaternary ammonium, tertiary, secondary and primary amines as anion exchangers.