1" sensitivity selective detection

In addition to TMS ethers, dimethylsilyl ethers [36] were studied for the analysis of alcohols and phenols. They were prepared by reaction with reagents analogous to those used for TMS ethers [dimethylmonochlorosilane and tetramethyldisilazane in pyridine (1 3 9)] and they provide shorter retention times in comparison with TMS ethers on Apiezon L. Grant [37] converted phenols into bromomethyldimethylsilyl ethers. In addition to a higher sensitivity, selective detection, permitting, e.g., phenolic substances to be determined in tars without any preliminary separation, was even achieved by using an ECD. 

Sensitive, selective detection of biochemically active compounds can be achieved by employing antigen-antibody, enzyme-substrate, and other receptor-protein pairs, several of which have been utilized in the development of piezoelectric immunoassay devices. The potential analytical uses of these materials has been reviewed, particularly with respect to the development of biochemical sensors [221-224], The receptor protein (e.g., enzyme, antibody) can be immobilized directly on the sensor surface, or it can be suspended in a suitable film or membrane. An example of the sensitivity and response range that can be... 

CW AIR MONITORING EQUIPMENT COMBINED WITH APPROPRIATE AIR SAMPLING DEVICES, INCLUDING ENHANCEMENT OF SENSITIVITY, SELECTIVITY, DETECTION AND... 

Sensitive, selective detection techniques are of crucial importance for capillary electrophoresis (CE), microfluidic chips, and other microfluidic systems. Electrochemical detectors for microfluidic systems have attracted considerable interest, with features that include high sensitivity, inherent miniaturization of both detection and control instrumentation, low cost, low power demand, and high compatibility with microfabrication technology. The electrochemical detectors commonly used can be classified into three general modes conductimetry, potentiome-try, and amperometry. 

The pump-probe concept can be extended, of course, to other methods for detection. Zewail and co-workers [16,18, 19 and 2Q, 93] have used the probe pulse to drive population from a reactive state to a state that emits fluorescence [94, 95, 96, 97 and 98] or photodissociates, the latter situation allowing the use of mass spectrometry as a sensitive and selective detection method [99, 100]. 

Recently, the state-selective detection of reaction products tluough infrared absorption on vibrational transitions has been achieved and applied to the study of HF products from the F + H2 reaction by Nesbitt and co-workers (Chapman et al [7]). The relatively low sensitivity for direct absorption has been circumvented by the use of a multi-pass absorption arrangement with a narrow-band tunable infrared laser and dual beam differential detection of the incident and transmission beams on matched detectors. A particular advantage of probing the products tluough absorption is that the absolute concentration of the product molecules in a given vibration-rotation state can be detenuined. 

Designing an experimental procedure involves selecting an appropriate method of analysis based on established criteria, such as accuracy, precision, sensitivity, and detection limit the urgency with which results are needed the cost of a single analysis the number of samples to be analyzed and the amount of sample available for... 

Rapid, simple, quaUtative methods suitable for determining the presence of benzene in the workplace or surroundings have been utilized since the 1930s. Many early tests offered methods for detection of aromatics but were not specific for benzene. A straightforward test allowing selective detection of benzene involves nitration of a sample to y -dinitrobenzene and reaction of the resultant ether extract with an ethanoHc solution of sodium hydroxide and methyl ethyl ketone (2-butanone), followed by the addition of acetic acid to eliminate interferences from toluene and xylenes. Benzene imparts a persistent red color to the solution (87). The method is claimed to be sensitive to concentrations as low as 0.27 ppm benzene from 10 mL air samples. 

Just like the physical and microchemical methods of detection, the indirect, biological-physiological detection procedures are very selective when apphed to thin-layer chromatography. Here it is not chemical functional groups or particular physical properties that are selectively detected but effects on highly sensitive biodetectors . The following detection techniques have been employed ... 

The advantages of controlled-potential techniques include high sensitivity, selectivity towards electroactive species, a wide linear range, portable and low-cost instrumentation, speciation capability, and a wide range of electrodes that allow assays of unusual environments. Several properties of these techniques are summarized in Table 1-1. Extremely low (nanomolar) detection limits can be achieved with very small sample volumes (5-20 pi), thus allowing the determination of analyte amounts of 10 13 to 10 15 mol on a routine basis. Improved selectivity may be achieved via the coupling of controlled-potential schemes with chromatographic or optical procedures. 

The first part of the book consists of a detailed treatment of the fundamentals of thin-layer chromatography, and of measurement techniques and apparatus for the qualitative and quantitative evaluation of thin-layer chromatograms. In situ prechromatographic derivatization techniques used to improve the selectivity of the separation, to increase the sensitivity of detection, and to enhance the precision of the subsequent quantitative analysis are summarized in numerous tables. 

Analysis of methyl parathion in sediments, soils, foods, and plant and animal tissues poses problems with extraction from the sample matrix, cleanup of samples, and selective detection. Sediments and soils have been analyzed primarily by GC/ECD or GC/FPD. Food, plant, and animal tissues have been analyzed primarily by GC/thermionic detector or GC/FPD, the recommended methods of the Association of Official Analytical Chemists (AOAC). Various extraction and cleanup methods (AOAC 1984 Belisle and Swineford 1988 Capriel et al. 1986 Kadoum 1968) and separation and detection techniques (Alak and Vo-Dinh 1987 Betowski and Jones 1988 Clark et al. 1985 Gillespie and Walters 1986 Koen and Huber 1970 Stan 1989 Stan and Mrowetz 1983 Udaya and Nanda 1981) have been used in an attempt to simplify sample preparation and improve sensitivity, reliability, and selectivity. A detection limit in the low-ppb range and recoveries of 100% were achieved in soil and plant and animal tissue by Kadoum (1968). GC/ECD analysis following extraction, cleanup, and partitioning with a hexane-acetonitrile system was used. 

Isotopic-labeled tracers behave like the components in the fluid of interest. For example, tritium water behaves like water. If less similar chemicals are used as tracers, selective adsorption, chemical reaction, and liquid-liquid distribution must be considered. The tracer must be chosen so that the analytic method is sufficiently sensitive to detect the tracer in the desired amounts. 

A further extension of the DFG S19 method was achieved when polar analytes and those unsuitable for GC were determined by LC/MS or more preferably by liquid chromatography/tandem mass spectrometry (LC/MS/MS). Triple-quadrupole MS/MS and ion trap MS" have become more affordable and acceptable in the recent past. These techniques provide multiple analyte methods by employing modes such as time segments, scan events or multiple injections. By improving the selectivity and sensitivity of detection after HPLC separation, the DFG S19 extraction and cleanup scheme can be applied to polar or high molecular weight analytes, and cleanup steps such as Si02 fractionation or even GPC become unnecessary. 

As compared to HPLC, cSFC shows higher efficiency, universal and selective detection, minimal derivatisation for separation and the ability to separate thermally labile organic compounds. Often, cSFC analyses are also considerably faster. This arises because higher mobile phase diffusion coefficients translate directly into higher optimum velocities. However, sensitivity, detection dynamic range and sample capacity... 

HPLC (in both NP and RP modes) is quite suitable for speciation by coupling to FAAS, ETAAS, ICP-MS and MIP-MS [571,572]. Coupling of plasma source mass spectrometry with chromatographic techniques offers selective detection with excellent sensitivity. For HPLC-ICP-MS detection limits are in the sub-ng to pg range [36]. Metal ion determination and speciation by LC have been reviewed [573,574] with particular regard to ion chromatography [575]. 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

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