Six Liquid Chromatography Detectors That Analytical Researchers Should Know

Pharmaceutical analysis is the application of analytical chemistry in pharmacy. It is a "method science" for the research and development of drug quality control. It mainly uses chemical, physical-chemical, or biochemical methods and techniques to study chemical structures that have been clarified.

UV-Vis Absorption Detector

Ultraviolet-visible detector (UVD) is one of the most widely used detectors in HPLC, and almost all liquid chromatographs are equipped with this detector. UVD is characterized by high sensitivity, wide linear range, low noise, and is suitable for gradient elution. The detection limit for strongly absorbing substances can reach 1ng, and the sample will not be destroyed after detection. It can be used for preparation and can be used in series with any detector. The working principle and structure of the UV-visible detector are similar to the general spectrophotometer, and it is actually a UV-visible photometer equipped with a mobile ground.

Ultraviolet absorption detector and Photodiode array detector (PDAD) are the two most commonly used ultraviolet-visible detectors.

Fluorescence detector

Fluorescence detector (FD) is a highly sensitive and selective detector that can detect compounds that produce fluorescence. Some non-fluorescent substances can be chemically derivatized to generate fluorescent derivatives, which can then be detected by fluorescence.

Its minimum detection concentration can reach 0.1ng/ml, which is suitable for trace analysis. Under normal circumstances, the sensitivity of fluorescence detectors is about 2 orders of magnitude higher than that of ultraviolet detectors, but its linear range is not as wide as that of ultraviolet detectors.

In recent years, laser-induced fluorescence detectors produced by using laser light as the light source of fluorescence detectors have greatly enhanced the signal-to-noise ratio of fluorescence detection, so they have high sensitivity and are widely used in trace and ultra-trace analysis.

Differential Refractive Index Detector

The differential refractive index detector (RID) is a concentration-type universal detector that responds to all solutes, and some components that cannot be detected by selective detectors, such as polymers, sugars, and fats, can be detected with a differential detector. Differential detectors are based on the continuous measurement of the change in refractive index between a sample flow path and a reference flow path to determine sample content.

When light passes from one medium to another, refraction occurs due to the difference in the refractive index of the two substances. As long as the refractive index of the sample component is different from that of the mobile phase, it can be detected. The greater the difference between the two, the higher the sensitivity. In a certain concentration range, the output of the detector is proportional to the solute concentration.

Electrochemical detector

Electrochemical detectors (ED) mainly include amperometric, polarographic, coulombic, potential, conductometric and other detectors, which are selective detectors and can detect compounds with electroactivity. At present, it has been widely used in the determination of various inorganic and organic anions and cations, metabolites of biological tissues and body fluids, food additives, environmental pollutants, biochemical products, pesticides and medicines. Among them, conductivity detectors are most widely used in ion chromatography.

Chemiluminescence detector

Chemiluminescence detector (CD) is a fast and sensitive new detector developed in recent years, known for its advantages of low price and wide linear range. The principle is that certain substances undergo chemical reactions at room temperature to generate reaction intermediates or reaction products in an excited state, and when they return to the ground state from the excited state, they emit photons.

Since the energy of the excited state of matter comes from chemical reactions, it is called chemiluminescence. When the separated components are eluted from the chromatographic column, they are immediately mixed with an appropriate chemiluminescent reagent to cause a chemical reaction that causes the luminescent substance to emit radiation, the intensity of which is proportional to the concentration of the substance.

This detector does not require a light source or a complicated optical system. As long as there is a constant flow pump, the chemiluminescent reagent is pumped into the mixer at a certain flow rate, so that it can be mixed with the column effluent quickly and evenly to produce chemiluminescence, and then use the photomultiplier tube to convert the optical signal into an electrical signal. The minimum detection amount of this detector can reach 10-12g.

Evaporative Light Scattering Detector

Evaporative Light-scattering Detector (ELSD) can detect compounds without chromophores, such as carbohydrates, lipids, polymers, underivatized fatty acids and amino acids, surfactants, drugs, and can detect unknown compounds in the absence of standards and unknown compound structural parameters.

The unique detection principle of the evaporative light scattering detector is that the column eluent is first atomized to form an aerosol, then the solvent is evaporated in a heated drift tube, and finally the remaining non-volatile solute particles are detected in the light scattering detection cell.