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Optical Absorbance Detection

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. [Pg.781]

Absorbance detectors are also commonly used in combination with postcolumn reactors. Here, most issues of detector linearity and detection limit have to do with optimization of the performance of the reactor. In a typical application, organophosphorus compounds with weak optical absorbances have been separated, photolyzed to orthophosphate, and reacted with molybdic acid, with measurement being performed by optical absorbance.58... [Pg.18]

Detection UV absorbance detection is typically used for capillary electrophoresis. However, the short optical pathlength of the capillary results in poor detection limits... [Pg.350]

Laser flash photolysis techniques offer the possibility of examining in detail the transient processes responsible for the photostabilizing effect discussed above. The triplet lifetimes are frequently too short, even for this technique however, they can still be estimated using as a probe the quenching by 1-methyl-naphthalene, which leads to the formation of its easily detectable triplet. The optical absorbance due to the 1-methylnaphthalene triplet (Aft) produced as a result of energy transfer is related to the Stern-Volmer slope by equation 5, where N stands for... [Pg.22]

Expansion of Module 3 to include rules for selection of detectors and detector parameters. The rules will handle optical absorbance and fluorescence (including pre- and post-column deri-vatization) and electrochemical detection. [Pg.293]

Spectrophotometry (or colorimetry) has been used to measure chlorine dioxide in water using indicators that change colors when oxidized by chlorine dioxide. Spectrophotometric analyzers determine the concentration of chlorine dioxide by measuring the optical absorbance of the indicator in the sample solution. The absorbance is proportional to the concentration of the chlorine dioxide in water. Indicators used for this technique include jV,jV-diethyl-p-phenylenediamine, chlorophenol red, and methylene blue (APHA 1998 Fletcher and Hemming 1985 Quentel et al. 1994 Sweetin et al. 1996). For example, chlorophenol red selectively reacts with chlorine dioxide at pH 7 with a detection limit of 0.12 mg/L. The interferences from chlorine may be reduced by the addition of oxalic acid, sodium cyclamate, or thioacetamide (Sweetin et al. 1996). [Pg.117]

Although this section provides a brief description of most commonly nsed detectors for HPLC, most of the focus is on a few detection modes. Optical absorbance detectors remain the most widely nsed for HPLC, and are discnssed in some detail. We also focns on flnorescence, condnctivity, and electrochemical detection, as these methods were not widely nsed for HPLC in the past, bnt are especially well suited to micro- and nano-flow instrnments becanse of their high sensitivity in small sample volumes. Mass spectrometry has also come into wide and rontine nse in the last decade, but as it is the subject of another chapter, it will not be fnrther discnssed here. Miniaturization has been particularly important for capillary and chip-based electrophoresis, which often employs sub-nanoliter detection volnmes [36,37]. [Pg.211]

A deeper channel (e.g., 100 pm) can be used to enhance absorbance detection because of the longer optical path length. However, this is feasible only when non-aqueous CE, which produces low electrical current, is used [622]. An increased path length (720 pm) needed for optical absorbance detection was also achieved by constructing a 3D fluid path [709]. [Pg.201]

In another approach, optical absorbance detection was achieved in an injection-molded PMMA flow cell consisting of 1800 pillars (25 pm high) that were used as diffraction elements. The LOD of Nile blue A perchlorate was determined to be 1.2 pM [713],... [Pg.202]

Optical absorbance detection was also achieved using a CCD-based spectrophotometer. Two food dyes (FD C Blue 1 dye and FD C Red 3 dye) were first separated by open-tubular liquid chromatography and then detected on a PDMS chip. The LODs for the blue and red dyes were determined to be 80 pM and 200 pM, respectively [569]. [Pg.202]

When compounds are not optically active and are not easily derivatized, indirect detection is sometimes the best alternative. In indirect detection, an absorbing or fluorescing probe is added to the buffer. Displacement of the probe by the analyte produces a decrease in signal (10,11). Because of its universality and simplicity, indirect UV absorbance detection has been the predominant detection scheme for phos-phonic acid analyses by CE (12 17). [Pg.395]

In summary, the simple but versatile stopped-flow technique, coupled with optical detection (typically absorbance, fluorescence, or CD), has long been the primary source of kinetic insight into fast reactions. A major strength of stopped-flow methods is that they can be readily combined with a wide range of detection methods. As the need to understand molecular mechanisms increases, stopped-flow methods will continue to play an important role. [Pg.6327]

To be detected optically, species must absorb light. The basis for optical detection is the law of Lambert-Beer ... [Pg.101]

As described above, recent advances in accelerator technology have enabled the production of very short electron pulses for the study of radiation-induced reaction kinetics. Typically, digitizer-based optical absorbance or conductivity methods are used to follow reactions by pulse radiolysis (Chap. 4). However, the time resolution afforded by picosecond accelerators exceeds the capability of real-time detection systems based on photodetectors (photomultiplier tubes, photodiodes, biplanar phototubes, etc.) and high-bandwidth oscilloscopes (Fig. 8). Faster experiments use streak cameras or various methods that use optical delay to encode high temporal resolution, taking advantage of the picosecond-synchronized laser beams that are available in photocathode accelerator installations. [Pg.137]

In pulse radiolysis experiments with optical detection, the measured quantity is optical absorbance, so any measured absorbance A can be quantified as a value of Ge with Eq. 67 ... [Pg.614]

Good summaries of accepted experimental techniques can be found in the references that are cited for individual radionuclides in the sections below. Nitsche (1991) provides a useful general summary of the principles and techniques of solubility studies. A large number of techniques have been used to characterize the aqueous speciation of radionuclides. These include poten-tiometric, optical absorbance, and vibrational spectroscopy. Silva and Nitsche (1995) summarize the use of conventional optical absorption and laser-based photothermal spectroscopy for detection and characterization of solution species and provide an extensive citation list. A recent review of the uses of Raman and infrared spectroscopy to distinguish various uranyl hydroxy complexes is given by Runde et al. (2002b). [Pg.4757]

Similar to bacterial RC there is spectral and ESR evidence that a pheophytin a molecule operates as an intermediary electron acceptor in PSII-RC. Optical absorbance changes, with a spectrum similar to that of a pheophytin a anion radical could be detected in PSII-enriched particles illuminated at low redox potentials (— 0.65 V) [57,77]. The appearance of the Ph signal could be correlated to a decrease in the extent of the rise in fluorescence of PSII of chlorophyll a observed upon illumination [78]. This apparent discrepancy (reduction of an electron acceptor is expected to cause an increase of fluorescence) is now explained by the fact that the fluorescence increase is in reality a delayed fluorescence emitted by the return to the ground state of P -682 regenerated by electron transfer from the pheophytin anion [79]. The lifetime, of this transient fluorescence rise is 2-4 ns, and that of electron transfer from Ph to P -6%2 = 4 ns, when PSII particles are poised at —0.45 V [73]. This transient fluorescence increase is, however, almost totally suppressed when A,j,(Ph) is prereduced chemically before illumination. Using this experimental criterium the midpoint potential of the Ph /Ph couple has been estimated to be -0.61 V [73,80]. [Pg.112]

Protein Sequencing Using Microreactors and Capillary Electrophoresis with Thermo-optical Absorbance Detection... [Pg.3]

A miniaturized protein and peptide microsequencer consisting of either a fused silica capillary reactor or a microreactor made of Teflon is described. The performance of the miniaturized sequencer was evaluated by sequencing 33 and 27 picomoles of myoglobin that were covalently attached to Sequelon-DITC. The products generated by the sequencer were analyzed using capillary electrophoresis with thermo-optical absorbance detection. This CE system provides reproducible migration time (< 0.4% of RSD) and detection limits of less than 4 fmol. [Pg.3]

R vanderVos, PJ van Leeuwen, P Braun and AJ Hoff (1992) Analysis of the optical absorbance spectra ofD1-D2-cytochrome b559 complexes by absorbance-detected magnetic resonance. Structural properties of 6 80. Biochim Biophys Acta 1140 184-198... [Pg.288]

Optical Absorbance Detection 131 Table 6.13. Anions that can be detected by direct U V (see also Chapter 6). [Pg.131]


See other pages where Optical Absorbance Detection is mentioned: [Pg.331]    [Pg.447]    [Pg.214]    [Pg.354]    [Pg.171]    [Pg.238]    [Pg.202]    [Pg.64]    [Pg.226]    [Pg.705]    [Pg.579]    [Pg.310]    [Pg.505]    [Pg.155]    [Pg.4]    [Pg.3377]    [Pg.226]    [Pg.77]    [Pg.698]    [Pg.520]    [Pg.77]    [Pg.127]    [Pg.127]    [Pg.129]   
See also in sourсe #XX -- [ Pg.163 ]




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