Photoconductive Detection

The most common approaches to sulfonylurea determinations involve high-performance liquid chromatography (HPLC). The earliest reported methods utilized normal-phase liquid chromatography (LC) with photoconductivity detection this type of detector demonstrated undesirably long equilibration times and is no longer... 

Water samples of 500-1000 mL are extracted and purified simultaneously through an SPE cartridge such as Carbograph-1, Cig and RP-18, usually followed either by HPLC with ultraviolet (UV) or photoconductivity detection or by GC/ECD. The acidic-type diphenyl ether herbicides are derivatized with diazomethane and various kinds of chloroformates and determined by GC and HPLC. 

SM Walters. Preliminary evaluation of high-performance liquid chromatography with photoconductivity detection for the determination of selected pesticides as potential food contaminants. J Chromatogr 259 227-242, 1983. 

Diuron Crops Methanol extraction, methylene dichloride partitioning Silica gel HPLC with photoconductivity detection - [72]... 

The shallow hydrogenlc donors, on the other hand, have small binding energies and also have small central-cell corrections. This makes the resolution of different donors resulting from different chemical impurities difficult to achieve. The early experiments from which different chemical donors were identified employed hlgh-resolutlon Fourier-transform infrared magnetospectroscopy (FTIR) which used the modulated photoconductivity detection technique to monitor the 1S-2P transition In a fixed magnetic field. 

The FTIR experiment can be either a transmission experiment or a photoconductive experiment. Photoconductive detection is much more sensitive than the transmission experiment while still retaining the high resolution feature of the technique. It was shown by Stillman ( ) that photoconductive detection allows one to observe transitions between bound impurity states. The excited carrier can absorb a phonon and make a transition to a conduction band state. 

H2S04=sulfuric acid IDMS=isotope dilution mass spectrometry MS=mass spectrometry PD=photoconductivity detection PDMD=pendant mercury drop electrode TEA=thermal energy analyzer TSP=thermospray UV=ultraviolet detection... 

Both GC and HPLC have been used to separate MBOCA and its metabolites from urine. Most recently, HPLC has become the method of choice to selectively detect MBOCA and its metabolites in urine. The most sensitive and specific detection methods for HPLC are ED (Ichikawa et al. 1990 NIOSH 1986b Okayama et al. 1988 Trippel-Schulte et al. 1986 Vantulder et al. 1981) and photoconductivity detection (PCD) (Ducos et al. 1985). Of these, ED has been the most frequently used detection method. Ultraviolet detection (UV) has also been paired with HPLC (McKerrell et al. 1987 Angerer and Schaller 1985 Trippel-Schulte et al. 1986) but is less sensitive and less selective than either ED or PCD (Trippel-Schulte et al. 1986). 

A number of very useful and practical element selective detectors are covered, as these have already been interfaced with both HPLC and/or FIA for trace metal analysis and spe-ciation. Some approaches to metal speciation discussed here include HPLC-inductively coupled plasma emission, HPLC-direct current plasma emission, and HPLC-microwave induced plasma emission spectroscopy. Most of the remaining detection devices and approaches covered utilize light as part of the overall detection process. Usually, a distinct derivative of the starting analyte is generated, and that new derivative is then detected in a variety of ways. These include HPLC-photoionization detection, HPLC-photoelectro-chemical detection, HPLC-photoconductivity detection, and HPLC-photolysis-electrochemical detection. Mechanisms, instrumentation, details of interfacing with HPLC, detector operations, as well as specific applications for each HPLC-detector case are presented and discussed. Finally, some suggestions are provided for possible future developments and advances in detection methods and instrumentation for both HPLC and FIA. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

Measurements of photoconductivity and of the Hall potential [367] are accurate and unambiguous methods of detecting electronic conduction in ionic solids. Kabanov [351] emphasizes, however, that the absence of such effects is not conclusive proof to the contrary. From measurements of thermal potential [368], it is possible to detect solid-solution formation, to distinguish between electronic and positive hole conductivity in semi-conductors and between interstitial and vacancy conductivity in ionic conductors. 

Figure 3.13 The spectral dependence of the specific detectivity for several photoconduction detectors. The values corresponding to a typical thermopile and to a typical piroelectric detector are also shown.

Poly Chlorinated Biphenyls. The photoconductivity detector provides good responses for polychlorinated biphenyls separated by GPC. The normal matrix components are detected by RI and UV detectors while the polychlorinated species show high responses in the electrochemical detector (Figure 8). ... 

Nitrogen-phosphorus detection PC = Photoconductivity TEA = Thermal energy analyzer UV = Ultraviolet detector... 

Detection of the Specific Immune Complex Using the sNPS Photoconductivity... 

The experimental data presented show that sNPS can be used as transducers, which are stable for a long time after the construction of an immune biosensor. The specific immune complex formed on the sNPS surface may be registered by measuring its photoluminescence or photoconductivity. Such immune biosensors can be applied for control of T2 mycotoxin. The biosensors developed are sensitive and simple and allows for rapid analysis and analysis in field conditions. This approach may be applied for detection of any biochemical substances which can form an immune complex. Further investigations should be directed towards studying the mechanism of the biochemical signal detection by the sNPS and characterization of all the steps of analysis. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

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