Continuous detection methods

Capillary electrophoresis with amperometric detection was also used for the determination of four electroactive preservatives (methylparaben, ethylparaben, propylparaben, and butylparaben) and, indirectly, two nonelectroactive preservatives (potassium sorbate and sodium lactate) in various types of foodstuffs [17]. Moreover, high-performance micellar electrokinetic capillary chromatography with amperometric detection (MECC-AD) has been also employed for the fast determination of melamine (2,4,6-triamino-s-triazine), which was occasionally used to increase the apparent protein content of milk products [18]. 

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Concerns over safe handling of radioactive materials and issues around the cost and disposal of low level radioactive waste has stimulated the development of nonradiometric products and technologies with the aim of replacing radioactive tracers in research and medical diagnosis (25). However, for many of the appHcations described, radioactive tracer technology is expected to continue to be widely used because of its sensitivity and specificity when compared with colorimetric, fluorescent, or chemiluminescent detection methods. 

SERS has also been applied as a sensitive, molecule-specific detection method in chromatography, e.g. thin layer, liquid, and gas chromatography. SERS-active colloids were deposited on the thin layer plates or mixed continuously with the liquid mobile phases. After adsorption of the analytes, characteristic spectra of the fractions were obtained and enabled unambiguous identification of very small amounts of substance. 

Continuous flow devices have undergone careful development, and mixing chambers are very efficient. Mixing is essentially complete in about 1 ms, and half-lives as short as 1 ms may be measured. An interesting advantage of the continuous flow method, less important now than earlier, is that the analytical method need not have a fast response, since the concentrations are at steady state. Of course, the slower the detection method, the greater the volumes of reactant solutions that will be consumed. In 1923 several liters of solution were required, but now reactions can be studied with 10-100 mL. 

Some Chemical Considerations Relevant to the Mouse Bioassay. Net toxicity, determined by mouse bioassay, has served as a traditional measure of toxin quantity and, despite the development of HPLC and other detection methods for the saxi-toxins, continues to be used. In this assay, as in most others, the molar specific potencies of the various saxitoxins differ, thus, net toxicity of a toxin sample with an undefined mixture of the saxitoxins can provide only a rough approximation of the net molar concentration. Still, to the extent that limits can be placed on variation in toxin composition, the mouse assay can in principle provide useful data on trends in net toxin concentration. However, the somewhat protean chemistry of the saxitoxins makes it difficult to define conditions under which the composition of a mixture of toxins will remain constant thus, attaining a reproducible level of mouse bioassay toxicity is difficult. It is therefore useful to review briefly some of the chemical factors that should be considered when employing the mouse bioassay for the saxitoxins or when interpreting results. Similar concepts will apply to other assays. 

Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

Various optical detection methods have been used to measure pH in vivo. Fluorescence ratio imaging microscopy using an inverted microscope was used to determine intracellular pH in tumor cells [5], NMR spectroscopy was used to continuously monitor temperature-induced pH changes in fish to study the role of intracellular pH in the maintenance of protein function [27], Additionally, NMR spectroscopy was used to map in-vivo extracellular pH in rat brain gliomas [3], Electron spin resonance (ESR), which is operated at a lower resonance, has been adapted for in-vivo pH measurements because it provides a sufficient RF penetration for deep body organs [28], The non-destructive determination of tissue pH using near-infrared diffuse reflectance spectroscopy (NIRS) has been employed for pH measurements in the muscle during... 

Workers at the Department of the Environment UK [174] have described continuous flow methods for the determination of total oxidised nitrogen and nitrite in seawater. limits of detection are 1.3 pg/1 (total oxidised nitrogen) and 0.26 pg/1 (nitrite). Within-batch standard deviations for total oxidised nitrogen range from 0.28 pg/1 to 17.5 pg/1 at the total oxidised nitrogen level, to 0.96 pg/1 at the 560 pg/1 total oxidised nitrogen level. Within-batch standard deviations for nitrite range from 0.056 pg/1 at the 3.5 pg/1 nitrite level to 0.042 pg/1 at the 70 pg/1 nitrite level. 

Continuous monitoring methods based on amperometric (Nikolic et al. 1992) or spectrophotometric (Kuban 1992 Ma and Liu 1992) techniques for the quantification of free cyanide are also available. Ion chromatography with amperometric determination provides good sensitivity (2 ppb) and selectivity for free cyanide and the weak complexes of cadmium and zinc (Rocklin and Johnson 1983). Postcolumn derivatization and fluorescence detection provides low detection limits as well (0.1 ppb) (Gamoh and Imamichi 1991). 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

Other anomalies were observed, since Beer s law was fulfilled at 220 nm and between 0 and 2 g/L and not at other wavelengths. Considering the possibility of unattended anomalies of the HEC after fractionation, we had to abandon this detection method. For this reason, for the continuous concentration determination, we used a flow-differential refractometer (type R403, Waters Associates, Milford, MA, USA) connected to an integrating recorder. Since there is no monochromator on the apparatus, it was not possible to use the refractive index increment value used for the light-scattering measurements. 

Arsenic Toxicosis. Urine arsenic is the best indicator of current or recent exposure. Atomic absorption spectrophotometry is preferred as the detection method. Hair or fingernail sampling may also be helpful. Use of blood is useful if analyzed soon after exposure or in cases of continuous chronic exposure. After acute exposure, chelation therapy is instituted utilizing either (1) Dimercaprol BAL (British Anti-Lewisite) and analogues ... 

Chemical interferences are dependent upon the detection method. This results from the fact that many of the detection methods are not molecule-specific. For example, many of the detection papers change color based on a physical property that many similar chemicals share. Thus, the topic of interferences will be discussed in a method-specific manner. This discussion has been limited to the sensor systems that are candidates for use as continuous real-time monitors. 

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