Off-line monitoring

Early investigations using API techniques were usually performed offline [20, 21]. A sequence of events in off-line study of a reaction in solution by mass spectrometry could be first to investigate the specific reaction conditions by mixing the reagents for the detection of the different intermediates, and then to determine the solution 

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What are the advantages and possible drawbacks of on-line GC monitoring compared to off-line monitoring Discuss the differences required in the equipment used in the two situations. 

Linking TLC with a tandem instrument differs from combining GC or LC with an appropriate spectrometer. Hyphenation of planar chromatographic techniques represents a niche application compared to HPLC-based methods. Due to the nature of the development process in TLC, the combination is often considered as an off-line in situ procedure rather than a truly hyphenated system. True in-line TLC tandem systems are not actually possible, as the TLC separation must be developed before the spots can be monitored. It follows that all TLC tandem instruments operate as either fraction collectors or off-line monitoring devices. Various elaborate plate extraction procedures have been developed. In all cases, TLC serves as a cleanup method. 

Rapid equivalent methods - Generally based on a principle different from that of the corresponding laboratory method, alternative or surrogate systems are used more and more often for on-line and off-line monitoring. For example, the spectrophotometric methods or biosensors proposed for the measurement of organic compounds or electrochemical techniques for metals must be considered as alternative methods. 

Tkac et al. [16] Ethanol Off-line monitoring of S. cerevisiae batch fermentation Alcohol oxidase (AOx) in Gluconobacter oxydans cells/covered with a cellulose acetate (CA) membrane Glassy carbon electrode/ +300mV vs. SCE Ferricyanide... 

Turbidity spectra and size exclusion chromatography (SEC) are two techniques which could be used for the on- and off-line monitoring of polymer particle size during the polymerization. An experimental evaluation of the two techniques is reported herein and it is shown that they satisfactorily follow the change of the particle size during the reaction. Information from SEC analysis is exploited to experimentally evaluate the turbidity spectra technique and investigate its potential to be used as an on-line measurement of particle size for latex reactor control. 

On-line and Off-line Monitoring of the Production of Cephalosporin C by Acremonium chrysogenum... 

The electrochemical detection utilized the re-oxidation of hexacyano-ferrate(II) on a platinum electrode. For pyruvate determination this assay was extended to a 3-enzyme system by the addition of glutamate p5u-uvate transaminase, which produces alanine from pyruvate. All enz5unes were used in solution in a reaction chamber of approximately 2 pi directly in front of the electrode. The cofactor NAD" " was coupled to dextran with a molecular weight of 40,000 to avoid its replacement for each assay. As the sensor responded to L-alanine and pyruvate again a differential measurement was required when a sample contained both compounds. It was applied to off-line monitoring of a cultivation of S. cerevisiae and data showed good correlation to the photometric assays. 

Off-line monitoring using traditional tests for physical and chemical properties as well as less or non-traditional testing such as NIR, To repeat an earlier comment. 

Seidel G, Tollnick C, Beyer M, Schugerl K. On-line and off-line monitoring of the production of cephalosporin C by Acremonium chrysogenum. Adv Biochem Eng Biotechnol 66 115-132, 2000. 

MALDI Off-line monitoring of reactions in segmented flow Hatakeyama et al. [42]... 

Nitrogen is the main impurity element in Uthium with respect to the rate of corrosive attack. Off-line monitoring techniques based on the Kjeldahl method are routinely used to determine nitrogen concentration. 

Smiechowski and Lvovich monitored the levels of acidity and basicity in industrial lubricant. The sensor was based on electrochemical impedance methodology. An iridium oxide potentiometric sensor was developed in both a conventional and MEMS configuration. Tests of the sensors in diesel lubricant showed good correlation between TAN, TEN, and the voltage output of each sensor [7]. Widera et al. used a potentiometric iridium oxide electrode as an indicating electrode with a silver/silver chloride reference electrode for the off-line monitoring of fuel acidity. The data showed that the iridium oxide sensor responds to compounds present in fuel that have acid-base character and it is possible to determine the acidity of different fuels and discriminate between unstressed and thermally stressed fuels. Experimental results indicated the ability to correlate the response of the iridium oxide sensor with the total acid numbers of different fuels [20]. 

Such off-line monitoring is common and has been reported in laboratory settings. Gueta-Neyroud et al. [38] confirmed in their study that the Al-OEt species characterized by the reaction of ethanol and methyl aluminoxane (MAO) were not the active species in the polymerization process, as a blank polymerization of propylene was performed with a 1 200 ethanokMAO solution in toluene. The reaction yielded no polymer, thus indicating that the polymerization proceeded via titanium species. ESR technique can be implemented at pilot plant level if the parahydrogen-induced polarization becomes a reality. 

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