Retention phase trapping

Lopez, R, Batlle, R., Neiin, C., Cacho, J., and Ferreira, V. (2007). Use of new generation poly(styrene-divinylbenzene) resins for gas-phase trapping-thermal desorption - Application to the retention of seven volatile organic compounds. J. Chromatogr. A., 1139, 36 4. 

Control of surfactant retention in the reservoir is one of the most important factors in determining the success or failure of a surfactant flooding project. In a typical surfactant flood, chemical cost is usually half or more of the total project cost. Based on the mechanisms, surfactant retention can be broken down into precipitation, adsorption, and phase trapping. However, it is difficult to separate the surfactant losses from different mechanisms. Therefore, we usually report the total surfactant loss as surfactant retention without clearly specifying the losses from different mechanisms. 

Experiments by Glover et al. (1979) for a type n(-i-) system showed that much of the surfactant retention could be caused by phase trapping. They also showed that much of this retained surfactant could be remobilized with a low-salinity drive. This view was supported by Hirasaki (1981). He pointed out that in a type II(-i-) environment, in the presence of dispersion, not only did the peak surfactant concentration decrease, but the location lagged behind with increased dispersion. These two factors resulted in a lower oil recovery and delay in oil production. 

SEE with solid-phase trapping has been used for the extraction of DBDE and PBBs together with PCBs and chlorinated benzenes from sediment samples, with CO2 as the supercritical fluid. Before the extraction, the sediment sample was mixed with copper powder and sodium sulphate. Total extraction time was 60 min (20 min static, 40 min dynamic). The extraction temperature was 120°C and the pressure 374 bar. Use of CO2 with modifiers (diethylamine, methanol, and acetone) gave only slightly better extraction yields than CO2 alone. Compared with Soxhlet extraction, SEE gave similar yields, but the extracts were much cleaner and it was not necessary to clean the extracts before GC-ECD analysis. In GC a two-channel system was used for verification of the identification. The on-column injector was connected to a retention gap, which was split with a t-piece to two columns (60 m X 0.25 mm i.d.) with different stationary phases (DB 5 and DB 17). The method was proven to be quantitative and sensitive, with LODs ranging from 0.01 to... 

Normally, retention of surfaetants, which involves adsorption, precipitation, and phase trapping, has been regarded as one of the main factors for the unfavorable economics in chemical flooding. Adsorption at the solid-hquid interface should be at minimum and be the only retention mechanism for a properly designed surfactant system for a Type II( —) phase behavior. Commercial products of actual surfactants are poly-disperse in the PO and EO groups, and mixtures of them are potential flooding chemicals. 

A variation on the method of thermal modulation is the use of a length of eapillary eolumn eoated with a thiek film of stationary phase. At ambient oven temperatures, this results in a retention of semivolatile analytes, whieh may be subsequently released to the seeondary eolumn onee the trap is heated. The rapid eyeling time possible with this methodology has resulted in its eommon applieation as the intermediate trap in eomprehensive GC. 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

Thermolysis of 16e,f in either solution or gas phase (150-350 °C) gave deuteriated ethylenes (i.e. 40e from 16e and 41f from 16f) with about 95% retention of stereochemis-try ". Similarly, pyrolysis of the stereoisomeric 2,3-diphenylthiirane oxides 16g,h proceeded smoothly to yield stilbenes and sulfur monoxide in more than 70% yield . The extrusion of SO from the trans-isomer proceeds almost stereospecifically, while that from the cis-isomer occurs with complete loss of stereochemistry. This indicates the intervention of a stepwise mechanism, and not a symmetry-allowed nonlinear chelatropic reaction . Based on the fact that all attempts to trap the intermediate with 1,3-dipolarophiles were in vain, whereas a 1 1 adduct was obtained in good yield (about 60%) with the carbon radical scavenger di-p-anisyl thioketone, a mechanistic scheme as depicted in equation 10 has been proposed . Although the radical intermediates are capable of internal rotation about the carbon-carbon bond, for the 2,3-diphenyl case (i.e. 16g,h), the rotation would be... 

Contaminants may be adsorbed on the solid phase or on suspended particles in the liquid phase. Environmental factors, such as temperature, pH, and water content in the subsurface prior to contamination, also affect the nature of contaminant adsorption. Other physical processes of retention include precipitation, deposition, and trapping. Under natural conditions, pollutants often consist of more than a single contaminant, comprising a mixture of organic and inorganic toxic compounds. Each of these compounds can react differently with the existing minerals and chemicals in the subsurface. 

Hydrogenolysis of vinyl sulfones.2 Vinyl sulfones are reduced by Na2S204 and NaHC03 in aqueous ethanol with retention of configuration. The reaction is facilitated by use of a phase-transfer catalyst such as Adogen. The reduction involves addition of HSOz to form a sulfone sulfinate salt which can be trapped by CH3I. The overall reaction thus involves a (3-syn-addition followed by an anti-elimination. This hydrolysis was used in a synthesis of several insect pheromones, such as (Z)-8-dodecenyl-l-acetate (1). 

Apart from the above-discussed parameters for HPLC optimization of chiral resolution on antibiotic CSPs, some other HPLC conditions may be controlled to improve chiral resolution on these CSPs. The effect of the concentrations of antibiotics (on stationary phase) on enantioresolution varied depending on the type of racemates. The effect of the concentrations of teicoplanin has been studied on the retention (k), enantioselectivity (a), resolution (Rs), and theoretical plate number (N) for five racemates [21]. An increase in the concentration of teicoplanin resulted in an increase of a and Rs values. The most surprising fact is that the theoretical plate number (N) increases with the increase in the concentration of teicoplanin. It may be the result of the resistance of mass transfer resulting from analyte interaction with free silanol and/or the linkage chains (antibiotics linked with silica gel). This would tend to trap an analyte between the silica surface and the bulky chiral selector adhered to it. This is somewhat... 

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