Liquid detectors

The liquid detector model simulates the behaviour of detection papers, which are in operational use by the Dutch and many other Defence forces. It simulates whether or not a paper will show a visible coloration, depending on deposition density and droplet sizes. 

Pumps handling high vapor pressure hydrocarbon liquids (detector sited close to the pump seals). 

Paper, Chemical Agent Liquid Detectors, 3-WAY C2 chemical agent detector kit CUB 800... 

Overflow detection may take several forms. It may be automatic, using suitably located gas/ liquid detectors to operate valves or pumps, or it may be manual, relying on operator response to various forms of detection (including alarms raised by suitable instrumentation, visual indications such as direct observation or via CCTV, or smell). The details of overflow detection measures will be site-specific, and a number of factors need to be taken into consideration. 

Flammable liquid detectors should be located anywhere a mixture could exceed 25% of the lower flammable limit. 

Small amounts of organic compounds of low ionization potential (see Section 4.2) can be dissolved in liquefied rare gases. The solubility increases from argon to xenon. The scintillation light leads to single photon ionization from which electron/ion pairs originate. The electrons are detected in the usual manner with a charge-sensitive amplifier (see Section 2.10). This way the sensitivity and resolution of rare gas liquid detectors can be improved. Another method uses the scintillation photons to eject electrons from a photocathode (Aprile et al., 1994). 

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

The most widely used experimental method for determining surface excess quantities at the liquid-vapor interface makes use of radioactive tracers. The solute to be studied is labeled with a radioisotope that emits weak beta radiation, such as H, C, or One places a detector close to the surface of the solution and measures the intensity of beta radiation. Since the penetration range of such beta emitters is small (a ut 30 mg/cm for C, with most of the adsorption occurring in the first two-tenths of the range), the measured radioactivity corresponds to the surface region plus only a thin layer of solution (about 0.06 mm for C and even less for H). 

Fig. XVII-5. Schematic detector response in a determination of nitrogen adsorption and desorption. A flow of He and N2 is passed through the sample until the detector reading is constant the sample is then cooled in a liquid nitrogen bath. For desorption, the bath is removed. (From Ref. 28. Reprinted with permission from John Wiley Sons, copyright 1995.)...

Dorn H C 1984 H NMR—a new detector for liquid chromatography Anal. Chem. 56 747A-58A... 

Figure Bl.27.8. Schematic view of Picker s flow microcalorimeter. A, reference liquid B, liquid under study P, constant flow circulating pump and 2, Zener diodes acting as heaters T and T2, thennistors acting as temperature sensing devices F, feedback control N, null detector R, recorder Q, themiostat. In the above A is the reference liquid and C2is the reference cell. When B circulates in cell C this cell is the working cell. (Reproduced by pemiission from Picker P, Leduc P-A, Philip P R and Desnoyers J E 1971 J. Chem. Thermo. B41.)...

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

Graham, R. C. Robertson, J. K. Analysis of Trihalomethanes in Soft Drinks, /. Chem. Educ. 1988, 65, 735-737. Trihalomethanes are extracted from soft drinks using a liquid-liquid extraction with pentane. Samples are analyzed using a packed column containing 20% OV-101 on 80/100 mesh Gaschrom Q equipped with an electron capture detector. 

The principle of headspace sampling is introduced in this experiment using a mixture of methanol, chloroform, 1,2-dichloroethane, 1,1,1-trichloroethane, benzene, toluene, and p-xylene. Directions are given for evaluating the distribution coefficient for the partitioning of a volatile species between the liquid and vapor phase and for its quantitative analysis in the liquid phase. Both packed (OV-101) and capillary (5% phenyl silicone) columns were used. The GG is equipped with a flame ionization detector. 

The second set of experiments describes the application of high-performance liquid chromatography. These experiments encompass a variety of different types of samples and a variety of common detectors. 

Flow injection analysis (FIA) was developed in the mid-1970s as a highly efficient technique for the automated analyses of samples. °> Unlike the centrifugal analyzer described earlier in this chapter, in which samples are simultaneously analyzed in batches of limited size, FIA allows for the rapid, sequential analysis of an unlimited number of samples. FIA is one member of a class of techniques called continuous-flow analyzers, in which samples are introduced sequentially at regular intervals into a liquid carrier stream that transports the samples to the detector. ... 

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