Ultraviolet absorption detectors

An ultraviolet absorption detector was used In tandem with a differential refractometer detector to obtain chemical composition data (3, 5 6). 

HPLC has been used, with an ultraviolet absorption detector set for 254 nm, for the determination of aromatic hydrocarbons and with a flow calorimeter for the detection of all hydrocarbons. Increased sensitivity and decreased interference can be achieved with the ultraviolet absorption detector by measuring absorption at two wavelengths and using the ratios of the absorption at those wavelengths [28]. 

Many ultraviolet absorption detectors use a low pressure mercury lamp as a source the cells are about 1 cm in path length and have volumes of 8 to 30 pL. Most are double beam. Normally, they have absorbance ranges from 0.001 to 3 absorbance units that corresponds to a minimum sample requirement of about 5 x lO " g/mL for a favorable compound. Figure 19-35 is a diagram of one such detector. 

Fluorescence detectors can give improved selectivity over ultraviolet absorption detectors because fewer compounds fluoresce than absorb (Chapter 16). Sensitivities at least as good as and perhaps better than the UV detector are achieved, depending on the geometry of the excitation source-detector arrangement, the intensity of the source, and the quantum efficiency of the fluorophore. The amper-ometric detector (see Chapter 15) is useful for detecting electroactive substances and has found considerable use in biological applications, for example, in the HPLC separation and detection of trace quantities of catecholamines from the brain. 

A series of random, block and graft copolymers of VC with styrene (S), butadiene (B), MMA, VAc, and VDC was characterized with the help of three consecutive detectors a differential refractometer (RI), an ultraviolet absorption detector (UV) and an automatic viscometer [33]. Figure 5.2 summarizes the relation between solution viscosity and molecular mass for these copolymers. 

Figure 3.36. Size exclusion chromatography system with an on-line viscometer and a lightscattering detector allows to create the Mark-Houwink-Sakurada plot without fractionating the polymer somewhere else. The concentration is detected by a refractive index detector or an ultraviolet absorption detector.

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

Many kinds of detectors have been designed, ranging from the widely used, cheap but robust flame ionization (GC) or ultraviolet absorption type (LC) to the much more exciting and informative, if much more expensive, mass spectrometer. 

Kim et al. [161] used particle beam mass spectrometry and ultraviolet absorption spectrometry as detectors in a method for the determination of down to lppm of chlorinated phenoxy and ester herbicides in soil. 

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... 

The photodiode array detector (PDAD) measures absorption of light waves by a sample. This is considered the most powerful of the ultraviolet spectrophotometric detectors. The optical system focuses light from a deuterium source through the sample flow cell onto several photodiodes. These act as capacitators by holding a fixed amount of charge. When light strikes the photodiodes, they discharge a certain amount of current. 

GC = gas chromatography EC = electrochemical (detector ECD = electron capture (detector HCD = Hall conductivity detector HFBA = heptafluorobutyric anhydride HPLC = high performance liquid chromatography NCI-MS = mass spectrometry in the negative chemical ionization mode NPD = nitrogen-phosphorus detector ppb = parts per billion UV = ultraviolet absorption SPE = solid phase extraction wt wt = weight weight... 

Since amphenicols exhibit strong ultraviolet absorption, they are ideal for direct determination by liquid chromatography, without any need for derivatization (Fig. 29.2.1). Their detection wavelengths have been set at 224 nm for thiamphenicol, 220 or 225 nm for florfenicol, and 270-290 nm for chloramphenicol (Table 29.2). Use of photodiode array detectors has been suggested for tentative confirmation of the identity of chloramphenicol residues analyzed by liquid chromatography (26, 37, 38, 40, 53, 58, 59, 61, 63, 66, 67). 

In liquid chromatographic analysis of macrolides and lincosamides, most popular is the ultraviolet detector (Table 29.4). Tylosin, tilmicosin, spiramycin, sedecamycin, and josamycin exhibit relatively strong ultraviolet absorption, but erythromycin, lincomycin, pirlimycin, and oleandomycin show extremely weak absorption in the ultraviolet region. Hence, detection at 200-210 nm has been reported for the determination of lincomycin (146). However, a combination of poor sensitivity and interference from coextractives necessitated extensive cleanup and concentration of the extract. Precolumn derivatization of pirlimycin with 9-fluorenylmethyl chloroformate has also been described to impart a chromophore for ultraviolet detection at 264 nm (140). 

In liquid chromatographic analysis of quinolone antibacterials, most popular is the fluorometric detector due to the inherent fluorescence of these drugs and the advantages in terms of selectivity and sensitivity that this detector offers (Table 29.6). Fluorometric detection after postcolumn derivatization with sulfuric acid has also been reported (203). However, quinolones exhibit also remarkable ultraviolet absorption and are therefore ideal for direct determination without derivatization. Detection can be performed in the wavelength range of 254-295 nm. 

Refractive index detectors are useless in gradient elution because it is impossible to match exactly the sample and the reference while the solvent composition is changing. Refractive index detectors are sensitive to changes in pressure and temperature (—0.01 °C). Because of their low sensitivity, refractive index detectors are not useful for trace analysis. They also have a small linear range, spanning only a factor of 500 in solute concentration. The primary appeal of this detector is its nearly universal response to all solutes, including those that have little ultraviolet absorption. 

To be effective, the detector must be capable of responding to concentration changes in all of the compounds of interest, with sensitivity sufficient to measure the component present in the smallest concentration. There are a variety of HPLC detectors. Not all detectors will see every component separated by the column. The most commonly used detector is the variable ultraviolet (UV) absorption detector, which seems to have the best combination of compound detectability and sensitivity. Generally, the more sensitive the detector, the more specific it is and the more compounds it will miss. Detectors can be used in series to gain more information while maintaining sensitivity for detection of minor components. 

For several years LC detectors were limited to refractive index and ultraviolet absorption systems. Recently introduced systems include the electrochemical detector and a moving belt interface allowing for chemical ionization-mass spectrometric detection. Both of these techniques provide a degree of selectivity not previously available. 

The most common LC detectors are based on ultraviolet absorption (UV), refractive index (RI) changes, or molecular fluorescence emission. All three are very common phenomena, and the detectors used in LC are, with few exceptions, modifications of existing technology and not based on new principles. This was not the case with GC detectors, many of which were invented specifically for GC. In fact, many of the GC detectors, such as the FID, electron capture, and TID, have been adapted to LC as well. 

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... 

Although obviously limited to compounds having ultraviolet absorption, the UV detector described appears particularly applicable for quan-... 

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