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HPLC Detectors

Liquid chromatography detectors are broadly categorized into three categories; non- specific (bulk property) detectors, partially specific (partial solute property) detectors and the totally specific detector (solute property detectors). Generally, sensitivity increases as the detector becomes more specific, specific detectors are the most sensitive.
There are four broad categories of detectors used in LC analysis. They include: refractive index detectors, UV detectors, fluorescence detectors and electrical conductivity detector.

Refractive Index Detector.

Refractive index is a bulk property of the column eluent. Detection depends on the ability of the solute to modify the overall refractive index of the mobile phase to provide a signal twice that of the noise. Bulk property detectors have limited sensitivity regardless of instrumental technique used. Many types of refractive index detectors have been developed and a number of optical systems deployed. These detectors include: Christiansen effect detector, interferometer detector, optilab interference refractometer detector, thermal lens detector, dielectric constant detector and others.

UV Detectors

Majority of all HPLC analysis are carried out using one of UV detectors. There are four types of UV detectors.

The UV Absorption Detectors

These detectors respond to substances that absorb light in the range 180 to 350 nm. Many substances absorb light in this wavelength range. UV absorption detectors include.

a)    The fixed wavelength UV detector

This is one of the mostly employed detectors in LC analyses. It is very sensitive, linear and relatively inexpensive. It is used to monitor separation of aromatic hydrocarbons by exclusion chromatography. Simple fixed wavelength detectors are used for preparative chromatography. Fixed wavelength detectors uses light of a single wavelength that is produced by a specific type of discharge lamp. The most popular discharge lamp is the low pressure mercury vapor lamp. It generates light at a wavelength of 254nm. Other lamps that are employed are the low-pressure cadmium lamp that generates light at 225nm and the zinc lamp that emits light intensely at 214 nm.

b)    Multi- Wave length UV Detector

This detector utilizes a light source that emits light over a wide range of wave lengths. It employs an optical system that aids in selection of a specific wavelength for detection purposes. The specific wavelength is usually chosen where the solute has an absorption maximum to allow maximum sensitivity. Alternatively, absorption spectra of an eluted substance is obtained and scanned over a range of wavelength for identification purposes. A multi-wavelength detector requires  broad emission light sources such as xenon and deuterium lamps. There are two basic multi-wavelength detectors, the diode array and the dispersion detector. An ideal multi –wavelength detector is a combination of both the diode array detector and the dispersion system. A common application of multi- wavelength choice is to increase the sensitivity of the detector by selecting a wavelength that is characteristically absorbed by the solute of interest. Alternatively, a wavelength can be selected that substances of little interest in the mixture do not absorb and, thus make the detector more specific to substance that absorb. This technique is used in separation of carboxylic acids monitored by absorption at 210 nm.

c)    Diode Array Detector

This detector utilizes a xenon or deuterium lamp that emits light over the UV region. Diode array detectors are used to verify the purity of a given substance, separation of aromatic hydrocarbons and in demonstrating peak purity.

The Fluorescence Detector

The fluorescence detector is one of the most sensitive detectors used in liquid chromatography. It is often applied in trace analysis. Although it is very sensitive, its response is only linear over a limited concentration range. Most of substances do not naturally fluoresce, which is a limitation of application of this type of detector. Derivatization of non- fluorescing substances must be carried out prior to analysis to render them detectable by fluoresce detector. There are several types of fluorescence detectors, the single wavelength excitation fluorescent detector, multi length fluorescence detector and the fluorescence spectrometer detector.

The Electrical Conductivity Detector

This detector is used in analysis of substances that ionize, thus it is largely used in the analysis of inorganic salts, acids and bases. It is also used in the analysis of some organic acids that are frequently required in biotechnology and environmental studies. It is also used in separation of alkali and alkaline earth cations in very low concentrations. The sensor is the simplest of all the detection systems, consisting only of two electrodes placed in a suitable flow cell.

The trident Multi functional detector

The popularity of fluorescence detector, UV detector and the electrical conductivity detector motivated Schmidt and Scott to develop a detector that detected substances by all the three methods at the same time in a single low volume cells. This detector allows the analyst a choice of the three most useful detector function in one system. It’s used in analysis of organic compounds.

Several manufactures that specialize in development of liquid chromatography detectors include:
The water Corporation, Varian Corporation, Agilent Corporation and Shimadzu Corporation. You can also find new and used laboratory equipment on www.oxfordlabs.co.uk


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