Stopped-flow computer control

FIA is capable of very high precision, better than 1%, as a result of very controlled sample dispersion in a continuous flow stream. Kinetic FIA measurements can be made by precisely stopping (under computer control) the flow of the analyte product in the detector cell at a position corresponding to the peak or some point past it, depending on concentrations. Then the change of signal, for example, absorbance, is recorded for a period of time to obtain the rate. The slope of this recorded signal is plotted for different concentrations to prepare a calibration curve. 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

In order to use the stopped-flow technique, the reaction under study must have a convenient absorbance or fluorescence that can be measured spectrophotometri-cally. Another method, called rapid quench or quench-flow, operates for enzymatic systems having no component (reactant or product) that can be spectrally monitored in real time. The quench-flow is a very finely tuned, computer-controlled machine that is designed to mix enzyme and reactants very rapidly to start the enzymatic reaction, and then quench it after a defined time. The time course of the reaction can then be analyzed by electrophoretic methods. The reaction time currently ranges from about 5 ms to several seconds. 

This analyser is a computer-controlled automated batch analyser, using a stop-flow principle to analyse for pH, conductivity, turbidity and colour. TTie principle of analysis for each module is based on the recommended methods as detailed in the Examination of Waters and Associated Materials issued by the Standing Committee of Analysts of the Department of the Environment. The temperature of the sample hquid flow is measured in order that temperature-compensated results of pH and conductivity can be quoted. 

Computer control of pump, UV and NMR for stop-flow experiments 1 1 1... 

Figure 3.3-5. Stopped flow apparatus [19]. a, Syringe type pump b, thermostat c, mixing cell d, reaction cell e, stop syringe f, switch g, photo multiplier h, monochromatic filter i, lamp j, controller k, transducer 1, computer.

An automatic control system consists of various pressure, temperature, and flow sensors, valves and components controlled by a programmable logic controller (PLC), and an operator interface device. The operator interface control device can be as simple as a stop-go for totally dedicated systems, or a personal computer. A personal computer connected to the PLC to provide operator interface, data collection and analysis capability is recommended especially if the system parameter and/or the parts being processed will be changing. More importantly, a computer control system assures consistent and repeatable operation. 

Using a computer-controlled FIA system, certain technical arrangements can be implemented in order to raise the sensitivity of the assays. Stopped flow procedures where the sample is kept in contact with the immobilized enzyme preparation for a longer time than the passage takes is one such arrangement being used [37]. Reversal of flow rate in order to improve mixing in the enzyme... 

The basic level of control routinely used in the chemical process industries involves sequencing operations such as manipulating valves or starting/stopping pumps, instrument verification, data acquisition, on-line maintenance and fail-safe shut down procedures. The next level of computer control involves process control of parameters such as flow, pressure and temperature. RO/NF systems require both levels [43]. 

Figure 3 shows a schematic diagram of a stopped-flow spectrophotometer (or spectrofluorimeter) coupled to an automatic analyzer controlled by a computer that also handles data acquisition and processing. ... 

Hgure 2 Typical setup used for on-flow and direct stop-flow LC/NMR experiments. The control of the stop-flow valve is achieved by the computer of the LC/UV system which triggers both the valve of the HPLC pump and the NMR acquisition computer. A calibrated delay is used for parking the LC-peak of interest with precision at the center of the LC/NMR flow probe. 

The reflux scheme can be difficult to start up because initially there may not be enough light components accumulated in the reflux drum, and the column temperature may be too hot so, the controller may want more reflux flow than is available. This can pump the reflux drum level down until the distillate flow stops and then proceed to pump the reflux drum empty. However, this situation can be handled with computer control by using a low-level constraint control that will constrain the reflux flow rate to maintain a low-level constraint setpoint until the column temperature is low enough, so the temperature controller calls for less reflux. This reflux scheme is recommended when the reflux/distillate ratio is less than... 

Controlling a fluorimeter via computer software allows the stopped-flow technique to be implemented and making possible to obtain variable-angle spectra for chemometric processing exploiting a multivariate analysis technique. 

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