UV detector multi wavelength

A good compromise between a single wavelength UV detector and a DAD system is the use of a multi-wavelength UV detector, which monitors up to four different wavelengths. 

Figure 3.19 Schematic diagram of a multi-wavelength UV detector.

Detectors that are used for preparative work must be able to function linearly at very high solute concentrations. The refractive index detector is probably the most useful of the anal5 cal LC detectors for preparative work, but even at its lowest sensitivity setting, it may still be too sensitive. The multi wavelength UV detector can sometimes be used providing its conduits can tolerate the high flow rates. 

There are two types of UV detector the fixed wavelength detector and the multi-wavelength detector. A diagram of a fixed wavelength UV detector is shown in figure 5. 

Fixed and variable wavelength UV detectors. This is probably the HPLC detector found in most labs. Configurations vary from fixed wavelength (commonly 254 nm) to multi-wavelength and stop run scan-... 

A diagram of the multi-wavelength dispersive UV detector is shown in figure 5. 

The Multi-Wavelength Dispersive UV Detector Courtesy of the Perkin Elmer Corporation... 

Multi-wavelength detectors permit UV or UV-visible wavelength ran s to be scanned rapidly and therefore simultaneous plots at several wavelengths can be obtained and spectra of the analyte can be generated at very short time intervals during an entire chromatographic run. These detectors require a microcomputer for their operation and this allows all... 

The UV detector is the most popular and generally useful LC detector and this is particularly true if multi-wavelength technology is included in the genera of UV detectors. The UV detector has the best combination of... 

Some UV-VIS detectors, the so-called multi-wavelength detectors MWDs). are capable of monitoring several different wavelengths simultaneously, thereby enabling the sensitive detection of various analytes that differ in their adsorption maximum. 

Almost all quantitative LC analyses are carried out using UV, BCD or F detection also MS, CLND, ELSD and SCD play a role. The UV detector is probably the detector of choice for quantitative analysis as it combines the essential features of wide linear dynamic range with fairly high sensitivity. Most antioxidant stabilisers, whether phosphites, hindered phenols, etc., exhibit UV absorptivity. Simultaneous multi-wavelength quantitation at unlimited different... 

The spectrometer is sometimes used alone as a concentration detector but it is more commonly used as a secondary detector for composition analysis, especially with copolymers [50-52]. Obviously, solutes must absorb at a particular wavelength to which the spectrometer has to be set. The solvent must be transparent at this wavelength, which may cause problems of selection with regard to polymer solubility. An ultraviolet (UV) spectrometer is the most commonly used, and it is very concentration-sensitive when the sample has a good absorption coefficient. Classical UV detectors operate at 254 nm but many multi-wavelength spectrometers are available. Other spectrometers like infrared... 

The easiest hyphenated system consists of an LC instrument with a multi-wavelength (e.g. diode-array) UV detector. Such a system is excellent for characterizing copolymers consisting of two or more types of monomeric units, all of which exhibit (different) UV activity. Unfortunately, this is hardly ever the case. A combination of a UV detector and a refractive-index (RI) detector connected in series does in principle provide sufficient information for copolymers (two different monomeric units). However, the interdetector volumes and band broadening are a complicating factor, as are the different background and blank signals (solvent peaks) provided by the two instruments. 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

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