Ultraviolet detectors selectivity

Hplc techniques are used to routinely separate and quantify less volatile compounds. The hplc columns used to affect this separation are selected based on the constituents of interest. They are typically reverse phase or anion exchange in nature. The constituents routinely assayed in this type of analysis are those high in molecular weight or low in volatility. Specific compounds of interest include wood sugars, vanillin, and tannin complexes. The most common types of hplc detectors employed in the analysis of distilled spirits are the refractive index detector and the ultraviolet detector. Additionally, the recent introduction of the photodiode array detector is making a significant impact in the analysis of distilled spirits. 

The fluorescence detector is available as either a filter fluorimeter or as a continuous wavelength fluorimeter. The filter fluorimeters are less expensive, but in most cases low wavelength excitation is not possible with these instruments, and this makes, e.g., the determination of indoles and catecholamines by their native fluorescence impossible. The selectivity of the fluorescence detector is much better than that of the ultraviolet detector, and for favorable compounds the sensitivity may also be better. 

Ultraviolet detectors. Ultraviolet detectors function by monitoring the light absorbed by the solute molecules from the incident beam. Ultraviolet detectors are the most commonly used type with LC systems, they are not appreciably flow or temperature sensitive, have a good dynamic linear range, but are, however, selective. The absorbance is proportional to concentration and obeys the Beer-Lambert Law which is defined as follows ... 

The fundamental properties of fluorescence make this a particularly attractive basis for an HPLC detection system [27], for whereas photometers depend upon the measurement of fairly small differences between the intensity of a full and slightly attenuated beam the measurement of fluorescence starts in principle from zero intensity. At sufficiently low values of concentration (<0.05 absorbance) then the intensity of fluorescence is directly proportional to concentration with a linear range of three to four decades. Consequently, fluorescence detectors are more selective and sensitive than ultraviolet detectors in LC by a factor of 10 giving noise equivalent sensitivities of better 1 ngmP. ... 

Photoionisation detector, PID component molecules in the detector are ionised by photons from a high energy ultraviolet source, selectivity can be achieved by using a different source, e.g. 10.2eV low energy krypton lamp for aromatics and alkenes, 11.7eV argon lamp for alkanes, halogenated compounds as well as aromatics. Sensitivity is similar to a FID, 10 to 10- 2gs-. ... 

Because of refractive-index effects, an unretained solvent used to dissolve the sample— if different from the chromatographic mobile phase—often deflects the base-line when passing through an ultraviolet detector cell. This indicates the void volume or the void time. Consider the chromatogram in Figure 21.19. (a) Determine the capacity factors for each nitroaniline isomer, (b) Determine the selectivity factor for the m- and p-substituted isomers relative to the o-nitro-aniline. 

The most common detectors in HPLC are ultraviolet, fluorescence, electrochemical detector and diffractometer. However, despite all improvements of these techniques it seems necessary to have a more selectivity and sensitivity detector for the purposes of the medical analysis. It should be therefore improvements to couple analytical techniques like infrared IR, MS, nuclear magnetic resonance (NMR), inductively coupled plasma-MS (ICP-MS) or biospecific detectors to the LC-system and many efforts have been made in this field. 

Select the detector. To acquire molecular weight distribution data, use a general detector such as a refractive index detector. To acquire structural or compositional information, employ a more selective detector such as an ultraviolet (UV) or infrared (IR) detector. Viscometric and light-scattering detectors facilitate more accurate molecular weight measurement when appropriate standards are not available. 

High-performance liquid chromatography (HPLC) with a micellar mobile phase or with a selective pre-column or reaction detection system has also been used to determine alkylenebis(dithiocarbamaes). ° Zineb and mancozeb residues in feed were determined by ion-pair HPLC with ultraviolet (UV) detection at 272 nm. These compounds were converted to water-soluble sodium salts with ethylenediaminetetra-acetic acid (EDTA) and sodium hydroxide. The extracts were ion-pair methylated with tetrabuthylammonium hydrogensulfate (ion-pair reagent) in a chloroform-hexane solvent mixture at pH 6.5-8.S. The use of an electrochemical detector has also been reported. ... 

These detectors are based on selective absorption of ultraviolet or visible light by the desired solute. A large number of single or multiple wavelength... 

The function of this subunit is to present so-called monochromatic radiation to the detector, i. e. to separate or disperse the radiation so that selected frequencies corresponding to particular energy transitions within the sample may be individually examined. For instruments designed to operate in the ultraviolet, visible and infrared regions of the spectrum, there are two approaches to this problem. 

A continuously monitoring detector of high sensitivity is required and those that measure absorption in the ultraviolet are probably the most popular. These may operate at fixed wavelengths selected by interference filters but the variable wavelength instruments with monochromators are more useful. Wavelengths in the range of 190-350 nm are frequently used and this obviously means that a mobile phase must not absorb at those wavelengths. 

Photoionization a gas chromatographic detection system that uti-hzes an ultraviolet lamp as an ionization source for analyte detection. It is usually used as a selective detector by changing the photon energy of the ionization source. 

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