Liquid-phase monitoring

Commercially available sensor systems that are based on the use of conducting electroactive polymers include the Aromascan from Aromascan, pic and the Electronic Nose of Neotronics. Scientific both for gas and vapor phase monitoring. The SmartSense biosensor assay system of Ohmicron Corporation, and the EPSIS and Bio-SenSys systems of ABTECH Scientificare for liquid-phase monitoring. Each of these systems is based on conductimetry, the measurement of a change in imped-... 

When running hydrolysis reactions in the liquid phase, it was observed that the sodium could be washed from the bed by flushing with water and monitoring the change in pH of the bed effluent. If this precaution was not observed, initial low yields of product were observed until sodium was entirely removed from the bed. 

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

Hitherto, there are primarily three ways in which an amperometric electrode can be used to simultaneously induce and monitor interfacial processes. These are illustrated schematically in Fig. 2 for the most general case where diffusion may occur in both of the liquid phases, which comprise the interface of interest. 

A recent stndy (13,27) describes the use of Co-Si-TUD-1 for the liquid-phase oxidation of cyclohexane. Several other metals were tested as well. TBHP (tert-butyl hydroperoxide) was used as an oxidant and the reactions were carried out at 70°C. Oxidation of cyclohexane was carried out using 20 ml of a mixture of cyclohexane, 35mol% TBHP and 1 g of chlorobenzene as internal standard, in combination with the catalyst (0.1 mmol of active metal pretreated overnight at 180°C). Identification of the products was carried out using GC-MS. The concentration of carboxylic side products was determined by GC analysis from separate samples after conversion into the respective methyl esters. Evolution and consumption of molecular oxygen was monitored volumetrically with an attached gas burette. All mass balances were 92% or better. 

In a closely related study, Tung and Sun discussed the microwave-assisted liquid-phase synthesis of chiral quinoxalines [80], Various L-a-amino acid methyl ester hydrochlorides were coupled to MeOPEG-bound ortho-fluoronitrobenzene by the aforementioned ipso-fluoro displacement method. Reduction under microwave irradiation resulted in spontaneous synchronous intramolecular cyclization to the corresponding l,2,3,4-tetrahydroquinoxalin-2-ones (Scheme 7.71). Retention of the chiral moiety could not be monitored during the reaction, but after release of the desired products it was found that about 10% of the product had undergone racemization. 

In addition, a novel fluorous support has been developed recently as an alternative to traditional polymer supports and applied successfully to oligosaccharide synthesis in combination with the trichloroacetimidate method [541]. Each intermediate in the fluorous oligosaccharide synthesis [542,543] could be obtained by simple fluorous-organic solvent extraction, and the reactions could be monitored by TLC, NMR and MS, in contrast to solid-phase reactions. Moreover, the new liquid-phase technique is anticipated to be easily applicable to the large-scale synthesis. 

If a solid is heated at a constant rate and its temperature monitored during the process, the melting curve as illustrated in Fig. 4.1 is obtained. Below the melting point, the added heat merely increases the temperature of the material in a manner defined by the heat capacity of the solid. At the melting point, all heat introduced into the system is used to convert the solid phase into the liquid phase, and therefore no increase in system temperature can take place as long as solid and liquid remain in equilibrium with each other. At the equilibrium condition, the system effectively exhibits an infinite heat capacity. Once all solid is converted to liquid, the temperature of the system again increases, but now in a manner determined by the heat capacity of the liquid phase. 

Figure 9.14. Precision and accuracy of the instrument at various concentrations of oxygen as compared to a standard oxygen analyzer (Servomex). The fiber optic sensor monitored the concentration of oxygen in saline solution (continuous stair) in equilibrium with various nitrogen-oxygen gas mixtures monitored by the gas analyzer (dotted staircase). The absolute concentration of oxygen in the gas phase is about 60 times larger than the corresponding equilibrium concentration in the liquid phase. The bath temperature was 37 C For the purpose of comparison both measurements have been scaled to percent oxygen. (From Ref. 21 with permission.)...

Schuster et al. reported work on monitoring a complex ace-tone-butanol-ethanol (ABE) fermentation system.22 They looked at the qualitative nature of the biomass as well as the solvents present in the liquid phase. A hierarchical cluster analysis was performed on samples from various times of the fermentation. The clusters were then classified using classical markers and analyses. The resultant table, combining qualitative interpretation and quantitative results, shows an interesting mosaic of the system over time. Total solvents, optical density, and butyric acid are given as numeric values in either absorbance units of g/1. 

A frequent complication in the use of an insoluble polymeric support lies in the on-bead characterization of intermediates. Although techniques such as MAS NMR, gel-phase NMR, and single bead IR have had a tremendous effect on the rapid characterization of solid-phase intermediates [27-30], the inherent heterogeneity of solid-phase systems precludes the use of many traditional analytical methods. Liquid-phase synthesis does not suffer from this drawback and permits product characterization on soluble polymer supports by routine analytical methods including UV/visible, IR, and NMR spectroscopies as well as high resolution mass spectrometry. Even traditional synthetic methods such as TLC may be used to monitor reactions without requiring preliminary cleavage from the polymer support [10, 18, 19]. Moreover, aliquots taken for characterization may be returned to the reaction flask upon recovery from these nondestructive... 

Inline UV-vis spectroscopy may certainly be used for monitoring the course of liquid-phase reactions. The concentrations of all UV-vis active components may be followed by a proper insertion probe. 

For liquid-phase reactions it is known that they may be enhanced by applying higher pressures if the activation volumes are strongly negative, for example, with Diels-Alder reactions. At pressures up to 600 bar (6 x 10 Pa), Benito-Lopez et al. used fiber-based on-line UV-vis spectroscopy to monitor the nucleophilic aromatic substitution of l-fluoro-4-nitrobenzene with a ten-fold excess of pyrrolidine. ... 

An early example of an MIP-QCM sensor was a glucose monitoring system by Malitesta et al. (1999). A glucose imprinted poly(o-phenylenediamine) polymer was electrosynthesized on the sensor surface. This QCM sensor showed selectivity for glucose over other compounds such as ascorbic acid, paracetamol, cysteine, and fructose at physiologically relevant millimolar concentrations. A unique QCM sensor for detection of yeast was reported by Dickert and coworkers (Dickert et al. 2001 Dickert and Hayden 2002). Yeast cells were imprinted in a sol-gel matrix on the surface of the transducer. The MIP-coated sensor was able to measure yeast cell concentrations in situ and in complex media. A QCM sensor coated with a thin permeable MIP film was developed for the determination of L-menthol in the liquid phase (Percival et al. 2001). The MIP-QCM sensor displayed good selectivity and good sensitivity with a detection limit of 200 ppb (Fig. 15.7). The sensor also displayed excellent enantioselectivity and was able to easily differentiate the l- and D-enantiomers of menthol. 

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