Liquid Chromatograph

A liquid chromatograph is an instrument that separates a mixture first and then analyzes and identifies it based on the difference in distribution ratios of the mixture between a liquid-solid phase or two immiscible liquids.

I. Basic Composition

High-pressure infusion pump: It draws the mobile phase from the reservoir and delivers it to the chromatographic system at a stable flow rate and pressure. The general pressure is 1.47×10? - 4.4×10? Pa, and the flow rate is adjustable and stable.

Sampling system: A diaphragm injection sampler or a high-pressure injection valve is usually used to complete the sampling operation, and the injection volume is constant to ensure the repeatability of the analyzed sample.

Chromatographic column: It is the core component of the liquid chromatograph, generally with a length of 10-50 cm and an inner diameter of 2-5 mm. It is made of high-quality stainless steel, thick-walled glass tubes, titanium alloy and other materials. The column is filled with a stationary phase with a particle size of 5-10 μm, which is composed of a matrix and a stationary liquid and has good selectivity for substances with different structures.

Detector: It is used to detect the sample components flowing out of the chromatographic column, convert the sample concentration into electrical signals and transmit them to a recorder or data processing system. Common detectors include ultraviolet detectors, fluorescence detectors, differential refractive index detectors, evaporative light scattering detectors, etc.

Data processing system: It collects, processes, stores and analyzes the signals output by the detector, can draw chromatograms, and calculate parameters such as peak area, peak height, and retention time, which are used for qualitative and quantitative analysis.

II. Working Principle

The mobile phase in the reservoir is pumped into the system by the high-pressure pump, and the sample solution enters the mobile phase through the sampler and is loaded into the chromatographic column by the mobile phase. Since each component in the sample solution has different distribution coefficients in the stationary phase and the mobile phase, when they move relatively in the two phases, after repeated adsorption-desorption distribution processes, the components have a large difference in moving speed and are separated into individual components, which flow out of the column in sequence. When passing through the detector, the sample concentration is converted into an electrical signal and transmitted to the recorder, and the data is printed in the form of a chromatogram.

III. Instrument Classification

Classified by stationary phase type: In liquid-liquid chromatography (LLC), the stationary phase is a liquid and the mobile phase is also a liquid, and separation is performed based on the difference in distribution coefficients of different components in two immiscible liquids; In liquid-solid chromatography (LSC), the stationary phase is a solid adsorbent and the mobile phase is a liquid, and separation is achieved based on the difference in adsorption capacity of sample components on the surface of the solid adsorbent.

Classified by separation mechanism: Adsorption column chromatography uses the difference in adsorption capacity of the adsorbent for different components for separation; Partition column chromatography separates components according to the difference in distribution coefficients between the stationary phase and the mobile phase; Ion exchange column chromatography is based on the exchange between ion exchange resins and ions in the sample for separation; Gel column chromatography uses the molecular sieve effect of gels to separate components according to their molecular sizes.

Classified by instrument performance and application range: It can be divided into analytical liquid chromatographs and preparative liquid chromatographs. The analytical type is mainly used for qualitative and quantitative analysis of each component in the sample; The preparative type is used for separating and purifying large quantities of target compounds.

IV. Application Fields

Biomedical field: It is used for the analysis of drugs and their metabolites in biological samples, the separation and identification of proteins and peptides, the analysis of nucleic acids, etc. For example, it studies the absorption, distribution, metabolism and excretion processes of drugs in the body, and the screening of biomarkers in disease diagnosis.

Food field: It can detect nutrients in food, such as vitamins, amino acids, carbohydrates, etc., and can also be used to detect harmful substances such as pesticide residues, veterinary drug residues, and food additives in food to ensure food safety.

Environmental field: It is used to analyze pollutants in environmental samples, such as heavy metal ions, polycyclic aromatic hydrocarbons, pesticide residues in water, and volatile organic compounds in the atmosphere, providing technical support for environmental monitoring and pollution control.

Petrochemical field: It conducts component analysis of petroleum and its products, such as the composition analysis of petroleum fractions, the determination of molecular weight and molecular weight distribution of polymers, etc., guiding the petrochemical production process and product quality control.