Fluorescence, types sensitized

X-ray detectors also come in several varieties (1) single-photon counters which yield accurate results but require up to several weeks to acquire the 10,000 -100,000 (lO lO ) reflections necessary to compile a complete data set for a protein crystal (2) image plates that operate much like photographic film but are 10 times more sensitive (3) area detectors, electronic devices that detect X-ray photons on a two-dimensional surface. Both fluorescent-type detectors, image plates and fast area detectors, are more sensitive at the shorter wavelengths of X-ray radiation from synchrotron sources. 

The mechanism proposed for P-type sensitized delayed fluorescence requires that part of the energy from two separately absorbed quanta be transferred to the same molecule. Thus, by choosing as donor a compound whose lowest excited singlet state (D ) lies below that of the acceptor (A ), but whose triplet state ( iD) lies above that (3A) of the... 

Condition 1. vs in (47) and vs in (48) should differ by significant amounts so that the experimental distinction between sensitized fluorescence and sensitized phosphorescence in the laboratory is feasible. Color filters with photo and photomultiplier tubes are most convenient, but the results are subject to serious error unless care is used. Spec-trophotometric methods also may be difficult if scattered light for one type of radiation may be confused with the other. ... 

A listing of the most common types of HPLC detector is given in Table 6.1, along with some of their properties. By far the most widely used HPLC detector is the LTV absorbance detector. This is due to a combination of factors firstly, although sensitivity is not good compared to other detector types (e.g. fluorescence, electrochemical), sensitivity is adequate for the majority of HPLC analyses secondly, a great many compounds are detectable by UV absorbance measurements thirdly, simplicity of construction and economies of scale mean that UV absorbance detectors... 

Detection is the greatest problem associated with HPLC. If highly sensitive detectors such as the fluorescence type cannot be used, then the detection limit is much higher than in gas chromatography with a flame ionization detector. Derivatization reactions provide a useful way of increasing the detection limit in liquid chromatography. ... 

Detection is the greatest problem associated with HPLC. If highly sensitive detectors such as the fluorescence type cannot be used, then the detection limit... 

Arsenic is both toxic and cai cinogenic element. It is necessary to have a fast, reliable and accurate method for determination of ai senic in water. The hydride-generation atomic fluorescence spectrometry (HG AFS) is one of the simple and sensitive techniques for the determination of this element in various types of waters. 

Mercury generally is found in low and trace concentrations. So there is need to determine Hg in ranges corresponding to various types of water samples. Detection levels of Hg can be improved by the use of vapour generation technique. This technique allows to sepai ate the analyte from the sample matrix and so to overcome the matrix interference. The fluorescence technique, with its high sensitivity and linearity, in combination with vapour generation, provides for a possibility to detect Hg in parts per trillion per liter regions. 

Electrodriven separation techniques are destined to be included in many future multidimensional systems, as CE is increasingly accepted in the analytical laboratory. The combination of LC and CE should become easier as vendors work towards providing enhanced microscale pumps, injectors, and detectors (18). Detection is often a problem in capillary techniques due to the short path length that is inherent in the capillary. The work by Jorgenson s group mainly involved fluorescence detection to overcome this limit in the sensitivity of detection, although UV-VIS would be less restrictive in the types of analytes detected. Increasingly sensitive detectors of many types will make the use of all kinds of capillary electrophoretic techniques more popular. 

A sensitive determination of alkanesulfonates combines RP-HPLC with an on-line derivatization procedure using fluorescent ion pairs followed by an online sandwich-type phase separation with chloroform as the solvent. The ion pairs are detected by fluorescence. With l-cyano-[2-(2-trimethylammonio)-ethyl]benz(/)isoindole as a fluorescent cationic dye a quantification limit for anionic surfactants including alkanesulfonates of less than 1 pg/L per compound for a 2.5-L water sample is established [30,31]. 

The photo cell senses light of all wavelengths that is generated by fluorescence but the wavelength of the excitation light can only be changed by use of an alternative lamp. This simple type of fluorescence detector was the first to be developed, is relatively inexpensive, and for certain compounds can be extremely sensitive. Typical specifications for a fluorescence detector are as follows ... 

In this chapter, we present the theory and results of measurements on humic acid fractions using fluorescence techniques. The fluorescence techniques are attractive for this application because of the natural fluorescence of humic materials, the hi sensitivity of fluorescence detection, and the ability to directly observe the morphology of the molecule in aqueous solutions without the need for drying or applying harsh chemical conditions. Several interesting types of information are obtained from fluorescence measurements ... 

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. 

The most common final separation techniques used for agrochemicals are GC and LC. A variety of detection methods are used for GC such as electron capture detection (BCD), nitrogen-phosphorus detection (NPD), flame photometric detection (FPD) and mass spectrometry (MS). For LC, typical detection methods are ultraviolet (UV) detection, fluorescence detection or, increasingly, different types of MS. The excellent selectivity and sensitivity of LC/MS/MS instruments results in simplified analytical methodology (e.g., less cleanup, smaller sample weight and smaller aliquots of the extract). As a result, this state-of-the-art technique is becoming the detection method of choice in many residue analytical laboratories. 

The output signal of concentration-sensitive detectors is proportional to the concentration or weight of polymer in the column eluent. Examples of this type include the differential refractometer and the ultraviolet-visible spectrophotometer. Infrared and fluorescence detectors are used infrequently. None of the detectors described above is truly universal i.e., the response of these detectors varies with the chemical species, and, in case of the DRI, response is also a function of the chromatographic eluent.156 Recently, an... 

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