The definition and principle of ion chromatography

Ion chromatography is a type of high-performance liquid chromatography. It is a liquid chromatography method for the analysis of anions and cations. This method has the advantages of good selectivity, sensitivity, rapidity, and convenience, and can measure multiple components simultaneously.

In general, ion chromatography can be divided into three types: ion exchange chromatography, ion exclusion chromatography, ion pair chromatography.

Ion exchange chromatography

Ion exchange chromatography is based on the principle of different interactions between ions and is mainly used for the separation of organic and inorganic anions and cations.

Ion exclusion chromatography

Ion-exclusion chromatography is based on Donnan's queuing repulsion, which is the separation of non-ionic interactions between a solute and a stationary phase. It is mainly used for the separation of organic weak acids and organic acids. It can also be used for the separation of alcohols, aldehydes, amino acids and sugars.

Ion pair chromatography

The separation mechanism of ion-pair chromatography is that the selectivity of adsorption and separation is mainly determined by the mobile phase.

This method is mainly used for the separation of surface active anions and cations and metal complexes.

Here we use ion exchange chromatography as an example to briefly introduce the principle of ion chromatography.

At a given acidity, there is an interaction between the sample ion and the stationary phase group. The size of this effect is different for different sample ions. Therefore, during the passage of the mobile phase through the column, the retention time of the strong ions of the sample is longer than that of the weaker ions. After a period of time, the separation of the sample can be achieved.

Taking the separation of anions as an example to explain the separation process of ion chromatography.

In the column, numerous ion exchangers are filled as the stationary phase for ion separation, and many cations are adsorbed on the stationary phase.

The mobile phase filled with the column is a solution of some kind of salt. When no sample enters, the anions in the mobile phase and the cations of the stationary phase are balanced.

There are two types of anions to be separated in the sample. The larger the volume of A, the greater the positive charge of the stationary phase, and the smaller the volume of B.

After the sample enters the column, the anions A, B, and the anion of the mobile phase advance together, and the three ions continuously alternately occupy the position where the positive phase attracts the positive ion; the anion A and the positive charge have a greater force and thus move slowly. But B moves faster, thus achieving separation.

Finally, because the number of anions in the mobile phase has an absolute advantage, the anions A and B of the sample flow out of the column, and the sample ions flow out of the column at different times to detect the sample ions that flow out of the column at different times. The type and content of the points.

The typical structure of an ion chromatograph consists of an infusion pump, an injection valve, a column, a suppression column, a detector, and a data processing system.

Infusion Pump

The double-headed reciprocating pump is a very common infusion pump. It is driven by the motor to rotate the cam. The two plunger rods reciprocate and suck and discharge the mobile phase. There is a time difference between the movements of the two plunger rods, which exactly compensates for the pulse of the mobile phase output, and thus the flow rate is quite stable.

Injection valve

The commonly used sample injection method is the six-way valve injection. This method has a large variable injection range, high pressure resistance, and is easy to automate.

Column

The main component of the separation system is the column, which is the place where the stationary phase is stored during chromatographic separation. The column packing of ion chromatography is a hotspot in the research of ion chromatography, and it is the main driving force for the development of ion chromatography. It has developed rapidly.

Ion reed detectors fall into two categories, electrochemical detectors and optical receptivity detectors. Electrochemical detectors include conductance, DC amperes, pulsed amperes, and integrated amperometric detectors. Optical detectors include ultraviolet, or visible, and fluorescent. Detector.

Among them, the conductivity detector is the most important detector for ion chromatography and is briefly described below.

All ionic compounds (organic ions, inorganic ions, strong acids and strong bases) as well as aqueous solutions of dissociable compounds (weak acids and weak bases) are able to conduct electricity. The conductivity detector is based on quantitative changes in the conductivity of the mobile phase of the ion chromatogram.

The structure of the conductance detector is relatively simple and the detection cell is in the middle of the two electrodes. When a voltage is applied to the electrode, the ions in the solution in the plug cell will generate motion. By measuring the current generated by the movement, the concentration of ions in the solution can be known.

If the mobile phase is highly conductive and the conductivity of the sample is low, then the conductivity detector will not be able to effectively detect the concentration of the sample ions.

Therefore, an inhibitor column is added between the column and the conductivity detector, which can change the conductivity of the mobile phase and the sample, so that the sample ions can be sensitively detected.

Role and development

After years of development, ion chromatography has played an important role in various fields of production and life.

Environmental Analysis

The most important application of ion chromatography in the initial stage of its production is the analysis of environmental samples. The main application of the ion chromatography is the qualitative and quantitative analysis of various anions and cations in environmental samples.

As a fast, accurate and effective analytical method, ion chromatography is widely used in the analysis of trace impurities in high-purity water and high-purity reagents in the microelectronics and power industry.

Food and beverage analysis

Compared with traditional analytical methods, the outstanding advantages of ion chromatography are simultaneous analysis of multiple components and simple sample handling, which makes it a better method for the analysis of anions, cations, organic acids, amines and sugars in foods and beverages.

Joint technology

Ion chromatography is a trend in the development of ion chromatography. The development of the combined use technology has greatly improved the application range and detection sensitivity of ion chromatography technology. About ion chromatography - atomic absorption (absorption) spectroscopy, ion chromatography - inductively coupled plasma, ion chromatography - mass spectrometry There have been many reports of joint use.