Instantaneous detection system

Separations in liquid chromatography occur in a dynamic manner and, therefore, require detection systems which work online and produce an Instantaneous record of the column events. A prototypic detector must have good sensitivity to deal with low concentrations of analytes typical of analytical liquid... 

Imagine that a reactor is operating in the steady state with a constant throughflow, Q, of 3.7 dm min. At the reactor inlet, 50 g of concentrated dye are instantaneously injected. The outlet concentration of this tracer is detected and recorded as a function of time. Steady-state calibrations of the dye-detecting system show that its sensitivity is essentially constant and equal to 3.88 mV (g dye dm over the range of dye concentrations encountered. The data in Table 1 is collected. 

A discussion of the various detectors employed in GC is covered in Chapter 5. Our purpose here is only to categorize the detection system according to whether they are an integral type system or a differential type system. This classification is an old one any detection system can be made integral or differential simply by a modification of the detector electronics. A more modern categorization would be instantaneous (differential) and... 

The use of chemiluminescence reactions for the detection of metal ions by liquid chromatography was recently reported [59,60]. The detectors made use of the chemiluminescence produced in the reaction between luminol and hydrogen peroxide which is catalyzed by transition metals. The column effluent was mixed with the reagents in order to yield the chemiluminescence. The reaction was fast and was carried out at room temperature. By varying the pH of the buffer, selectivity towards certain metals was also achieved. For example, at pH 10-11 nickel could be analyzed but lead and aluminium were inactive at pH 13-14, the converse was true [59]. Aminco-Bowman has marketed a liquid chromatographic system in which amino acids and amines are analyzed by means of the fluorescence produced on reaction with the reagent fluorescamine. Fluorescamine does not fluoresce, but it does react with primary amino groups to produce fluorescent derivatives. The reaction is instantaneous and may be carried out at room temperature, usually at pH 9. This detection system promises to be far more sensitive than the ninhydrin detection system and is much more easily adapted to HPLC. 

To overcome the disadvantages of these previously described systems, the HELM tracker system allows continuous measurement of the current flow in every cathode in an electrolytic tankhouse. The system allows for instantaneous detection of the magnitude and specific location of electrode current distribution anomalies such as short circuits and poor contacts, which is displayed to the operators through the systems HMI. The system requires no electrical or physical contact with the cathodes, and can be easily retrofitted into an existing plant, without interfering with the operation. 

If the leak detection system is not actuated and the leak, smaller than the detection threshold, is instantaneously built up into a larger one as a result of an annular closed crack formation, the tube ends may move apart, but the safeguard housing prevents the separation of the ends of broken pipeline. The leak detection system detects the leak as soon as the detection threshold is reached, and the reactor is safely shut down and cooled down. 

Such relays are normally instantaneous, highly sensitive and operate at low spill cuiTents. Since they detect the residual current of the system, the current may contain third-harmonic components (Section 23.6) and operate the highly sensitive relay in a healthy condition. To avoid operation of the relay under such conditions, it is a normal practice to supply the relay coil with a tuned filter, i.e. a series L-C circuit to filter out the third-harmonic components. The capacitance of the filter circuit may also tame a steep rising TRV (Section 17.10.3) during a momentary transient condition and protect the relay. 

A practical method of predicting the molecular behavior within the flow system involves the RTD. A common experiment to test nonuniformities is the stimulus response experiment. A typical stimulus is a step-change in the concentration of some tracer material. The step-response is an instantaneous jump of a concentration to some new value, which is then maintained for an indefinite period. The tracer should be detectable and must not change or decompose as it passes through the mixer. Studies have shown that the flow characteristics of static mixers approach those of an ideal plug flow system. Figures 8-41 and 8-42, respectively, indicate the exit residence time distributions of the Kenics static mixer in comparison with other flow systems. 

While in the frequency domain all the spectroscopic information regarding vibrational frequencies and relaxation processes is obtained from the positions and widths of the Raman resonances, in the time domain this information is obtained from coherent oscillations and the decay of the time-dependent CARS signal, respectively. In principle, time- and frequency-domain experiments are related to each other by Fourier transform and carry the same information. However, in contrast to the driven motion of molecular vibrations in frequency-multiplexed CARS detection, time-resolved CARS allows recording the Raman free induction decay (RFID) with the decay time T2, i.e., the free evolution of the molecular system is observed. While the non-resonant contribution dephases instantaneously, the resonant contribution of RFID decays within hundreds of femtoseconds in the condensed phase. Time-resolved CARS with femtosecond excitation, therefore, allows the separation of nonresonant and vibrationally resonant signals [151]. 

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