Online optical method

TSS measurement can be carried out by gravimetry after separation by filtration or centrifugation, according to standard methods [5,24], For online measurement, these techniques cannot be applied without expensive devices. This is the reason why optical methods are often used, especially in industry. Among the different systems, turbidimetry,... 

The small inner diameter of the separation capillary used in CE implies a short optical pathway, and the consequent poor concentration sensitivity when conventional UV detectiOTi is used. To overcome this drawback several techniques have been developed some of them consist in application of general approaches that are not specifically addressed to CE analysis of alkaloids. One is the use of LIF detector for analysis of alkaloids with native fluorescence [68, 69] or after their off-line derivatization [64, 88, 112]. Sample pretreatment, a second major approach, is popularly employed in combination with sample extraction and can be conveniently applied in analysis of alkaloids because they can be easily retained in cationic-exchange sorbents in solid-phase extraction (SPE) mode [113, 114]. It may be interesting to focus on more specific aspects to detect very low levels of analytes using limited amoimts of samples to this regard chemiluminescence reactions and the use of online preconcentration methods will be considered. 

Additionally, online monitoring methods have been developed to adapt off-line characterization methods into in situ (i.e., in-reactor) probes for determination of kinetics and monomer conversion with optical methods such as mass spectroscopy (MS), ESR, FTIR, near IR, and Raman spectroscopy. However, frequently, due to high turbidity and viscosity of the polymer reaction milieu, the optical surfaces are easily fouled, leading to frequent sensor failure. Furthermore, data acquired with these probes are model dependent the empirical and inferential calibration schemes used can be expensive and time consuming to develop and can drift and become unreliable as reactor conditions change and as sensors become fouled. Another limiting feature of these methods is that they usually measure only one characteristic of the reaction, such as monomer conversion and are not directly sensitive to polymer molecular mass and intrinsic viscosity. More detailed discussion of these techniques can be found in Chapters 6-10 of this book. 

Preferential crystallisation is one option for optical resolution on a manufacmr-ing scale. Online polarimetry and refractometry have been used to d3mamically optimise the process for resolution of DL-threonine in aqueous solution by variation of process parameters such as degree of supersaturation, seed quantity, initial enantiomeric excess and scale [148]. The method is claimed to be suitable for control of quasi-continuous processes. 

Bruckner (2001) combined UV-vis with EPR spectroscopy, using online gas chromatography for product analysis. For many transition metal ions, EPR and optical spectra are complementary, in that some states are detectable or distinguishable with only one of the methods. The UV-vis facility was added to a previously described flow reactor system for EPR spectroscopy (Bruckner et al., 1996). A fiber optical probe (Avantes, AVS-PC-2000 plug-in spectrometer) was inserted directly into the reactor via a Teflon -sealed feedthrough and placed in the catalyst bed. UV-vis spectra were reported for temperatures up to 810 K. The design was later expanded to include a third method, Raman spectroscopy (Bruckner, 2005 Bruckner and Kondratenko, 2006). A hole in the... 

Each one of the previous issues can influence the performance of the processing technique and the resulting composite in a variety of ways. Insufficient cure may result in a low and a consequent creep under stress. Inadequate flow may result in high levels of porosity or large voids. Online sensor systems permit process control [19] and are a useful tool toward improved quality in composites manufacture. Dielectric techniques have achieved the greatest success as commercial in-process monitoring systems. Optical-fiber methods show promise for further development, most notably where distinct features in chemical spectra can be obtained. 

FT-NIR spectroscopy in combination with a fiber-optic probe was successfully used to monitor living isobutylene, ethylene oxide and butadiene polymerizations using specific monomer absorptions. In the case of EO a temperature dependent induction period was detected when 5ec-BuLi/ BuP4 were used as an initiating system. This demonstrates the usefulness of this technique because this phenomenon had not been observed so far by other methods. We have also successfully conducted experiments in controlled radical polymerization. Then we were able to monitor the RAFT polymerization of A -isopropylacrylamide (NIPAAm). Thus it can be expected that with the help of online NIR measurements detailed kinetic data of many polymerization systems will become available which will shed more light onto the reaction mechanisms. Consequently, FT-NIR appears to be a method, which can be applied universally to the kinetics of polymerization processes. 

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