Real-time reaction-monitoring method

FT-IR microspectroscopy is a new nondestructive, fast and rehable technique for solid-phase reaction monitoring. It is the most powerful of the currently available IR methods as it usually requires only a single bead for analysis, thus it is referred to as single bead FT-IR [166]. (See also Chapter 12 for further details). The high sensitivity of the FT-IR microscope is achieved thanks to the use of an expensive liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. Despite the high cost of the instrument, this technique should become more widely used in the future as it represents the most convenient real-time reaction monitoring tool in SPOS [166, 167]. 

The adsorption species was concluded to be GaCH3 under monolayer growth conditions, as detected by a real time optical monitoring method/" A possible reaction of GaCH3 with ASH3 was proposed, as shown in Equation (2). 

Dantan et al. (2001) developed a FIA system coupled with HPLC or CE for monitoring chemical and pharmaceutical production processes. In this system, a derivatization step, automated using a FIA-system (see Figure 5.8), was introduced to transform the activated ester involved in the reaction—suitable for the detection by HPLC but less so for amine quantification—to a nonreactive amide. The FIA-CE hyphenation method developed yielded good results appropriate for near real-time process monitoring. 

The performance of a biotreatment system ultimately depends on optimization of the activity of microbes and the ability to control the process parameters of the treatment system [157]. In this respect, the ability to monitor gene copy numbers and gene expression is highly useful for real time optimization of the efficiency of a biotreatment system. Advanced molecular techniques as well as low cost methods (e.g., antibody detection of enzymes based on color reaction strips fluorescence i.e., GFP marked organisms with UV light detection) can also be applied to monitor the microbial community structure, persistence of the added bacteria, and their interactions with indigenous populations. 

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. 

Direct monitoring of the catalysed reaction has most usually been carried out in real time by light absorption or fluorescent emission analysis and some initial progress has been made with light emission detection. The low quantity of abzyme usually available at the screening stage puts a premium on the sensitivity of such methods. However, some work has been carried out of necessity using indirect analysis, e.g. by hplc or nmr. 

In some manufacturing process analysis applications the analyte requires sample preparation (dilution, derivatization, etc.) to afford a suitable analytical method. Derivatization, emission enhancement, and other extrinsic fluorescent approaches described previously are examples of such methods. On-line methods, in particular those requiring chemical reaction, are often reserved for unique cases where other PAT techniques (e.g., UV-vis, NIR, etc.) are insufficient (e.g., very low concentrations) and real-time process control is imperative. That is, there are several complexities to address with these types of on-line solutions to realize a robust process analysis method such as post reaction cleanup, filtering of reaction byproducts, etc. Nevertheless, real-time sample preparation is achieved via an on-line sample conditioning system. These systems can also address harsh process stream conditions (flow, pressure, temperature, etc.) that are either not appropriate for the desired measurement accuracy or precision or the mechanical limitations of the inline insertion probe or flow cell. This section summarizes some of the common LIF monitoring applications across various sectors. 

The reaction between olefins and ozone produces light that can be measured and related to the concentration of the reactants. One of the preferred methods for measuring ambient ozone concentrations utilizes the chemiluminescence generated in the ozone-ethylene reaction for detection. Recently, Hills and Zimmerman (16) described the use of this detection principle for determining hydrocarbon concentrations. They utilized the chemiluminescence created when ozone reacts with isoprene for development of a continuous, fast-response isoprene analyzer. This real-time isoprene system is reported to be linear over three orders of magnitude and to have a detection limit of about 1 ppbv. Because the system doesn t include a preseparation of hydrocarbons, interferences from other olefins (ethylene, propylene, and so forth) could occur. Thus far the chemiluminescent detector has been used to monitor isoprene emissions under conditions in which the concentrations of olefins that could interfere are negligible compared to those of the biogenic hydrocarbon. 

The condensation of silanols in solution or with surfaces has not been as extensively studied and therefore is less well understood. The limitation until recently has been the lack of suitable analytical methods necessary to monitor in real time the many condensation products that form when di- or trifunctional silanols are used as substrates. With the advent of high-field wSi-NMR techniques, this limitation has been overcome and recent studies have provided insights into the effects of silanol structure, catalysts, solvent, pH, and temperature on the reaction rates and mechanisms. Analysis of the available data has indicated that the base catalyzed condensation of silanols proceeds by a rapid deprotonation of the silanol, followed by slow attack of the resulting silanolate on another silanol molecule. By analogy with the base catalyzed hydrolysis mechanism, this probably occurs by an SN2 -Si or SN2 -Si type mechanism with a pentavalent intermediate. The acid catalyzed condensation of silanols most likely proceeds by rapid protonation of the silanol followed by slow attack on a neutral molecule by an SN2-Si type mechanism. 

As classical analytical methods only give delayed results, no evolution of the global antioxidant capacity of these media with time can be examined. For the first time, owing to fast response of cyclic voltammetry, the presented direct electrochemical measurements give results in real time, thus allowing the monitoring of reaction kinetics. 

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