Back-extraction efficiency

Li and coworkers synthesized the novel IL l-butyl-3-trimethylsilylimid-azolium hexafluorophosphate and demonstrated its utility for liquid/ liquid extraction of inorganic mercury. Using o-carboxyphenyl diazoamino p-azobenzene as a chelator to form a stable neutral complex with the metal ion, the authors demonstrated selective extraction into the hydrophobic IL phase [19]. When sodium sulfide was added to the IL phase, the mercury ion was back-extracted into the aqueous layer, providing an avenue for recycling the IL. The authors report extraction and back-extraction efficiencies of 99.9 and 100.1%, respectively, for a 5.0 pg/L aqueous mercury standard. The mercury detection limit was 0.01 ng/mL in water and the method was successfully applied to detecting trace mercury in natural water samples. 

As organic and aqueous phases are macroscopically separated by the membrane, HFM offer several hydrodynamic advantages over other contactors, such as the absence of flooding and entrainment, or the reduction of feed consumption (160, 161). The flowsheets tested in HFM were similar to those developed for centrifugal contactor tests. Computer codes based on equilibrium (162) and kinetics data, diffusion coefficients (in both phases and in the membrane pores), and a hydrodynamic description of the module, were established to calculate transient and steady-state effluent concentrations. It was demonstrated that, by selecting appropriate flow rates (as mass transfer is mainly controlled by diffusion), very high DFs (DI A 11 = 20,000 and DFrm = 830) could be achieved. Am(III) and Cm(III) back-extraction efficiency was up to 99.87%. 

Carlson and Nagarajan have reported that the addition of alcohol improved the back extraction efficiency [10]. It is well known that alcohol is a representative cosurfactant so far. A long-chain alcohol stabilizes the reverse micelles, while a short-chain alcohol sometimes inhibits the formation of reverse micelles. In haemoglobin back extraction, the addition of isopropyl alcohol or ethanol is significant in facilitating haemoglobin release from DOLPA reverse micelles (Figure 14.6) [6]. 

The thermodynamic approach led to the first conclusion that the optimal working temperature should not exceed 700 °C. The two main reasons are a significant loss of back-extraction efficiency and an increase of AICI3 volatility. 

The weight evolution, the uranium content in the salt phase and the U back extraction efficiency are summarised in Table 6.3.3. The total loss of weight observed after each experiment (that includes both metal and salt phase) is partly due to AICI3 evaporation. It is particularly the case for run Nal where an important volatilisation of AICI3 was observed. The mechanical treatment of the salt after experiment may also explain a part of the total loss of weight. Table 6.3.3 shows that the amount of U in the salt phase, determined by UV-visible spectrophotometry is in perfect agreement with the loss of weight of the metal phase. 

Figure 6.3.2 Thermodynamic extrapolation of U back-extraction efficiency as a function of the AlCI /U ratio,...

Table 6.3.4 shows that the back-extraction efficiencies of the studied elements remained almost constant for the three experiments. This behaviour was expected since the experimental conditions of this step were the same for the three runs only the initial amount of An and Nd in the metallic phase slightly differed from one experiment to the other. Pu and Am are quantitatively back-extracted (>99%) in a single stage while the... 

Table 6.3.4 Back-extraction efficiency of the different studies on Am and Nd...

Several extraction techniques have also been described that use enzymatic or chemical reactions to improve extraction efficiency. A technique that has been used to increase the overall recovery of the marker residue is enzymatic hydrolysis to convert specific phase II metabolites (glucuronides or sulfates) back into the parent residue. Cooper etal used a glucuronidase to increase 10-fold the concentration of chloramphenicol residues in incurred tissue. As an example of a chemical reaction, Moghaddam et al. used Raney nickel to reduce thioether bonds between benomyl and polar cellular components, and as a result achieved a substantially improved recovery over conventional solvent extraction. In choosing to use either of these approaches, thorough characterization of the metabolism in the tissue sample must be available. 

There are two classes of parameters that influence the efficiency of back extraction first, the parameters that govern the forward extraction such as pH, salt type and concentration, surfactant type and concentration, and protein type and concentration and second, the pH, salt type and concentration of stripping solutions, and extraction temperature. 

Salt type and concentration For back-extraction, increases in pH are not enough to strip the protein out from reverse micelles this is also due to the size exclusion elfect resulting from a decrease in the reverse micelle size [31,32]. This means that high salt concentration and salts that form small reverse micelles favor back transfer. Most of the work reported in the literature used KCl solution, normally 1.0 mol dm KCl coupled with a pH around 7.5. Marcozzi et al. [23] also showed that the back transfer efficiency of a-chymotrypsin depends on the salt type and concentration used in the forward transfer. 

Counterion extraction Due to the relative slowness of back extraction based on the methods above, the back-extraction of proteins encapsulated in AOT reverse micelles was evaluated by adding a counterionic surfactant, either TOMAC or DTAB, to the reverse micelles [33]. This novel backward transfer method gave higher backward extraction yields compared to the conventional method. The back-extraction process with TOMAC was found to be 100 times faster than back-extraction with the conventional method, and as much as three times faster than forward extraction. The 1 1 complexes of AOT and TOMAC in the solvent phase could be efficiently removed using adsorption onto montmorillonite so that the organic solvent could be reused. 

The ion -exchanger materials allow flow rates of 10 ml/min with minimum back pressure even at these flow rates, 95-100% of the transition metals are retained on the preconcentration column. Throughput is maximized without detriment to the extraction efficiency. 

Transfer of solubihzed proteins from the reverse micellar phase back to an aqueous phase constitutes back extraction. A successful RME should include both forward and back extraction processes in their optimized conditions. In contrast to the extensive studies investigating the forward extraction process, back extraction has been addressed to a much lesser extent. Most of the earlier studies tacitly assume that conditions, which normally prevent protein uptake in the forward transfer, would promote their release in the back transfer. That is to select a pH and salt condition that had minimal forward transfer efficiency. This... 

Efficient extraction of proteins has been reported with reverse micellar liquid membrane systems, where the pores of the membrane are filled with the reverse micellar phase and the enzyme is extracted from the aqueous phase on one side of membrane while the back extraction into a second aqueous phase takes place at the other side. By this, both the forward and back extractions can be performed using one membrane module [132,208]. Armstrong and Li [209] confirmed the general trends observed in phase transfer using a glass diffusion cell with a reverse micellar liquid membrane. Electrostatic interactions and surfactant concentration affected the protein transfer into the organic membrane and... 

AOT/octane Recombinant cytochrome bj Forward and back extractions were performed and the efficiency of back extraction was found to decrease at low temperature and high salt concentration [289]... 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

Using the described extraction system, we developed methods of amino acid recovery from pharmaceutical samples and fermentation broth. Amino acids were extracted efficiently from the diluted solution of fermentation broth into [C4Cilm][PFg] in the presence of DC18C6 and may be well back-extracted by the alkaline aqueous solution (pH > 9). These methods served as a basis for the corresponding analytical procedures. 

C4CiIm][PPg], a widespread IL, was recently used for fhe direcf exfraction of double-stranded DNA (dsDNA) [30]. The authors demonstrated that DNA may be extracted with high efficiency, >95%, while profeins and mefal-loproteins do nof inferfere extraction. The back-extraction of DNA info fhe aqueous phase wifh fhe efficiency of 30% was performed in fhe presence of phosphafe-cifrafe buffer solution (pH 4). 

Liquid-liquid partitioning is intended either to extract the drugs from an organic solvent into an aqueous solution or to wash out interfering substances from organic or aqueous solutions. In general, quinolones are extracted from chloroform or ethyl acetate sample extracts into alkaline buffers, to then be back-extracted into chloroform or ethyl acetate at acidic conditions (191, 195, 196, 200). Occasionally, sodium chloride may be added to the sample extracts in order to increase the extraction efficiency of ethyl acetate or chloroform (193-196, 200). To remove lipids, sample extracts are often also partitioned with u-hexane (183-186, 193-196, 202, 204), or diethyl ether (189, 190, 201). 

Liquid-liquid partitioning cleanup is sometimes directed solely to the removal of fat from the sample extract. This has been accomplished through use of either hexane for the removal of fat from acetonitrile (451, 434) or aqueous methanol sample extracts (452), or isooctane from acetonitrile sample extracts (454). When hexane was used for the initial extraction of 30 anabolics from fatty tissue samples, partitioning against methanol-acetate buffer, pH 5.2 proved to be an efficient procedure for fat removal because the analytes were preferentially partitioned into the aqueous buffer layer (433) further extract purification could be readily accomplished by back extraction of the analytes into dichloromethane. 

Selectivity. Several additional analytes were examined. Extraction efficiencies from 1 mM solutions for a 2-min extraction time were as follows glucose, 2 salicylic acid, 34 oxine, 75 phenol, 22 and p-cresol, 79. 8-Hydroxyquinoline and p-cresol are significantly more extractable than salicylic acid. 8-Hydroxyquinoline can be back extracted either by basic solution (pH >12) or acidic solution (pH <3) because of its amphiprotic behavior (pKi = 5.0 pK2 = 9.90). This experiment was performed by using either 0.1 M NaOH or 0.01 M HC1 with essentially the same results. The extraction step should be performed at pH 7-10. 

In the supercritical phase, both temperature and pressure play a significant role in determining the extraction efficiency. After the short-lived retrograde solubility effect subsides at about 55-60°C, a transition of the system back to the mass transfer controlled situation will take place where increasing temperature will, once again, bring about a surge in the extraction efficiency. In fact, for the supercritical phase,... 

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