Ultraviolet-visible absorption detector

Ultraviolet/visible absorption detection is the most common technique found in HPCE. Several types of absorption detectors are available on commercial instrumentation, including the following ... 

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. 

Once the transient species has been formed, it has to be monitored by some form of kinetic spectroscopy, typically with ultraviolet-visible absorption or emission, infrared (time-resolved infrared or TRIR) (74), or resonance Raman (time-resolved resonance Raman or TR3) (80) methods of detection. The transient is usually tracked by a probe beam at a single characteristic frequency, thereby giving direct access to the kinetic dimension. Spectra can then be built up point by point, if necessary, with an appropriate change of probe frequency for each point, although improvements in the sensitivity of multichannel detectors may be expected to lead increasingly to the replacement of the laborious point-by-point method by full two-dimensional methods of spectroscopic assay (that is, with both spectral and kinetic dimensions). 

Atomic-absorption and emission lines occur in the ultraviolet and visible regions, so a monochromator for either technique should be a general-purpose ultraviolet-visible instrument. Detectors are generally photomultiplier tubes. The most commonly used photomultiplier is the RCA 1P28 tube or the equivalent with an... 

To obtain a high degree of precision in the measurement, the chromatographic peaks should be as close as possible to Gaussian and be separated right to their base. It should also be checked that no other product has the same retention time as either of the enantiomers. In effect, even a small quantity of an impurity with a strong ultraviolet-visible absorption (in the case that such a detector is used) can have a significant effect on the measurement. 

Thus while one never sees commercial FT spectrometers for ultraviolet-visible (UV-VIS) absorption measurements (because photomultiplier tubes are much quieter detectors than are microphones), FT-VIS/PA spectrometers have been built that permit speedier acquisition of high S/N photoacoustic spectra (6-7). 

For ultraviolet and visible spectroscopic detectors, a standard solution of a compound whose molar absorption constant is known must be prepared, and placed in the flow cell. The absorbance obtained is then compared with the value measured by a standard spectrophotometer. 

CE is based on the use of narrow-bore capillaries with internal diameters typically betwen 20 and 150 pm. Because most commercial instruments equipped with ultraviolet/visible (UV-Vis) absorption detectors use a segment of the same capillary as the detection cell, the path length in CE is much less compared to those in HPLC or spectrometry. Therefore, the most commonly used CE detectors... 

In liquid chromatographic analysis of nitrofuran antibacterials, the most popular detector is the ultraviolet visible (UV-vis) spectrophotometer. Nitrofurans exhibit strong absorption at wavelengths around 365 nm and are, therefore, ideal for direct determination (Table 29.5). Detection wavelengths of 275 nm (56, 57) and 400 nm (175) have also been suggested. Electrochemical detection is also frequently applied in liquid chromatographic methods for the determination of various nitrofuran antibacterials in edible animal products (172, 173, 179). 

Ultraviolet-visible (UV-Vis) spectrophotometric detectors are used to monitor chromatographic separations. However, this type of detection offers very little specificity. Element specific detectors are much more useful and important. Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectroscopy (ICPAES) and inductively coupled plasma-mass spectrometry (ICP-MS) are often used in current studies. The highest sensitivity is achieved by graphite furnace-AAS and ICP-MS. The former is used off-line while the latter is coupled to the chromatographic column and is used on-line . 

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... 

Analysis is an integral part of research, clinical, and industrial laboratory methodology. The determination of the components of a substance or the sample in question can be qualitative, quantitative, or both. Techniques that are available to the analyst for such determinations are abundant. In absorption spectroscopy, the molecular absorption properties of the analyte are measured with laboratory instruments that function as detectors. Those that provide absorbance readings over the ultraviolet-visible (UV-vis) light spectrum are commonly used in high-performance liquid chromatography (HPLC). The above method is sufficiently sensitive for quantitative analysis and it has a broader application than other modes of detection. 

Two general type of detection devices are available. These are bulk property detectors and solute property detectors. The bulk property detector measures a change in some overall physical property in the mobile phase as it emerges from the column. Two typical examples are measurement of refractive index and conductance. The solute property detector is sensitive to changes in a physical property of the solute as it emerges from the column in the mobile phase a typical example is the measurement of ultraviolet and / or visible absorption. In general,... 

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. 

UV detector HPLC detector based on an ultraviolet-visible spectrophotometer using microsample flow cells (10 pi). Response to components depends on their absorption spectrum and absorptivity coefficient as defined by the Beer-Lambert law. Detector wavelength is set to give maximum sensitivity ideally at Amax however an optimum wavelength, Aopt at which all the components have a satisfactory absorbance, may be used. 

The measurement of absorption of ultraviolet-visible radiation is of a relative nature. One must continually compare the absorption of the sample with that of an analytical reference or blank to insure the reliability of the measurement. The rate at which the sample and reference are compared depends on the design of the instrument. In single-beam instruments there is only one light beam or optical path from the source through to the detector. This usually means that one must remove the sample from the light beam and replace it with the reference after each reading. Thus, there is usually an interval of several seconds between measurements. 

The continuous detectors most often used in liquid chromatography are based on ultraviolet or visible absorption, fluorescence, and differential refractometry. 

Table 21.1 classifies popular LC detectors according to several criteria for purposes of comparison. At the present time, LC detectors are generally less sensitive than GC detectors, which can detect picograms of material under good conditions. Most LC detectors provide only limited structural information. However, spectrophotometers fitted with micro flow-cells can be used to obtain a stop-flow ultraviolet-or visible-absorption spectrum of an LC peak trapped in the flow cell. On-line coupling of liquid chromatographs with mass or infrared spectrometers offers sophisticated, but indeed expensive, detection/identification methods. Such systems have been described in the literature, but are quite limited by the solvents that can be used in the chromatography step. 

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