Ultraviolet-visible detector

The ultraviolet-visible, UV-Vis, detector is considered the workhorse of detectors for HPLC systems. Since UV principles are well-described in several undergraduate textbooks, only a limited description of the principles involved in UV-absorbance will be given in here. 

As the sample enters the flow cell, the sample concentration is related to the fraction of hght transmitted through the cell by Beer s law (equation (15))  

I = transmitted light intensity lo = incident light intensity T = transmittance 

In commercial fixed wavelength UV detectors, Xmax is often set at 254 nm. This specific wavelength will provide a wide range of applications, giving strong absorption bands for biologically important compounds such as amino acids, proteins, enzymes and nucleic acid constituents. 

On the other hand, the more sophisticated variable wavelength UV-Vis detector wiU allow the detection of carbohydrates in the low wavelength (190-225 nm) region. Carbohydrates such as fructose, glucose, sucrose, etc. which are transparent at 254 or 280 nm, required either RI detection or post column derivatisation before the advent of the variable wavelength detector. 

A is the absorbance of a species of concentration C, and with an absorptivity s, in a cell of length b. Concentration is usually in molar concentration and the path length is measured in cm. The term (molar) absorptivity has units that are the inverse of the C and e units. This leaves A dimensionless it is usually described in terms of absorbance units. A detector set to a certain sensitivity, for example, 0.16, is said to beat 0.16 Absorbance Units Full Scale sensitivity (0.16 AUFS sensitivity). 

The Lambert-Beer equation is useful for choosing conditions for the separation and detection of ions. The eluent ions should have a low absorptivity and the sample ions should have reasonably high absorptivity. In a special case of indirect detection (discussed later), this should be reversed. In this case, the eluent has an absorption signal and the sample is detected by a decrease of the background signal. 

It is important to note that when discussing the properties of the eluent and sample ions, it makes a difference whether one is separating anions or cations. For example, if a separation of anions is being discussed, then the absorptivities of the eluent anion and sample anions are considered. But if a low background signal is needed, then of course the cation that is counter to the eluent anions must have a low absorptivity as well. 

Some absorbance data are given in this chapter and in some of the applications described in this book. If an ion absorptivity for a particular wavelength is unknown, it can be measured with a spectrophotometer and ion solution of known concentration. 

The discussion of UV-VIS detectors for use in ion chromatography is divided into two parts (a) the direct monitoring of column effluents and (b) post-column derivati-zation with subsequent spectrophotometric measurement. 

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Table 20-2 Minimum detectable signal (photons/s/detector element) of ultraviolet/visible detectors...

Four detectors have found widespread application. These are the ultraviolet-visible detector, the fluorescoice detector, the refiactive index d ector, and the electrochemical detector. Only the refractive index detector can be considered as a imiversal detector as virtually all compounds cause a (diange in refiactive index whoi solved in [Pg.202]

Ultraviolet spectrophotometers cont.), single-beam, 225 standardisation, 226 Ultraviolet spectrophotometry, 221-232 absorption cells, 226 colorimetry, 228 derivative, 230 difference method, 229 dual-wavelength, 229 identification by, 231 influence of pH, 224 influence of solvent, 224 laws of absorption, 222 quantitative applications, 227 stray-light effects, 224 Ultraviolet-visible detector, 202 multiwavelength, 211 Unicontin, 1011 Unidiarea, 474 Unidone, 356 Uniflu, 557, 893 Unilobin, 709 Unimycin, 846 Uniphyllin, 1011 Uniprofen, 677 Unisom, 576... 

A typical pSFC instrument, at first glance, is designed like an HPLC system. The major differences are encountered at the pump, the column oven, and downstream of the column. pSFC is best carried out using pumps in a flow-control mode. A regulator mounted downstream of the column and ultraviolet-visible detector (UV) controls the pressure drop in the chro-... 

Photodiode array (PDA) or variable ultraviolet/ visible detector... 

Ultramicroelectrode Synonymous with microelectrode. Ultraviolet/visible detector, HPLC Detector for high-performance liquid chromatography that uses ultraviolet/visible absorption to monitor eluted species as they exit a chromatographic column. 

Detection systems for GC are chosen for their sensitivity and selectivity for a particular class of VOCs. Detectors for GC include FID, the BCD, the photoionization detector (PID), the pulsed discharge detector (PDD), and the reduction gas detector (RGD). A variety of mass spectrometers can also be interfaced with a GC for confirmation of molecular structure and quantitation. Singlewavelength ultraviolet-visible detectors (190 to 600 nm) and diode array detectors are used to detect carbonyls as their 2,4-dinitrophenylhydrazone derivatives. The absorption maxima for aliphatic carbonyls, aromatic carbonyls, and dicarbonyls are near 360 nm, 385 to 390 nm, and 415 to 430 nm, respectively. Formic, acetic, and pyruvic acid are detected by ion conductivity. 

Diode array detector HPLC multichannel ultraviolet-visible detector consisting of a linear array of photodiodes onto which the full spectrum falls, i.e. 200 400 nm ultraviolet, 400-800 nm visible regions. Thus a complete spectrum can be obtained in less than 0.1 s, also several wavelengths can be monitored simultaneously to achieve maximum sensitivity for each component. 

In this regard, various analytical methods have been developed and their potential for the VOO authentication has been evaluated. Thus, spectroscopic tools, such as FT-near-infrared spectroscopy, FT-Raman spectroscopy, fluorescence and ultraviolet-visible detectors, and chromatographic techniques have been widely used in the field of VOO authentication [43-48],... 

PBLG molecular weights were verified by gel permeation chromatography (GPC). Instrumentation (Waters Associates, Inc.) included a dual pump Multi-Solvent Delivery system (Waters 600/600E), 712 WISP Autosampler, a Waters 486 tunable ultraviolet/visible detector, and a Waters 410 differential refractometer detector. Data collection and analysis are facilitated by a Maxima 825 software program. 

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