Back extraction technique

The back extraction technique is also applicable as a rapid "milking" method in which a daughter product i3 separated from the mother activity in the organic solvent layer by shaking with an appropriate aqueous washing solution. 

Chromium Removal System. Chlorate manufacturers must remove chromium from the chlorate solution as a result of environmental regulations. During crystallization of sodium chlorate, essentially all of the sodium dichromate is recycled back to the electrolyzer. Alternatively, hexavalent chromium, Cr, can be reduced and coprecipitated in an agitated reactor using a choice of reducing agents, eg, sodium sulfide, sulfite, thiosulfate, hydrosulfite, hydrazine, etc. The product is chromium(III) oxide [1333-82-0] (98—106). Ion exchange and solvent extraction techniques have also... 

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. 

When a 350 ml seawater sample was spiked with 54Mn and taken through the chelation, extraction, and back-extraction procedures, the observed recovery of the radio-tracer was 100.6%. Estimates of detection limits for manganese based on sets of both shipboard and shore laboratory separations are of the order of 0.1 nmol/1. The accuracy of the technique is demonstrated by data from the ICES fifth-round intercalibration exercise for trace metals in seawater [449 ]. 

The table also shows that a three-phase LLE (organic extraction followed by back-extraction into aqueous phase) yields lower recoveries and enrichment compared to three-phase LPME, as reflected in peak heights from the two techniques as shown in Figure 1.29. Furthermore, three-phase LLE is sensitive to the magnitude of Ka/org and LPME is not. 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

Unlike clenbuterol, salbutamol is a difficult compound to analyze due to its particular chemical attributes. It is a basic compound subjected to protein binding poor recoveries are obtained especially when protein precipitation techniques are used to prepare the extracts (145). In addition, salbutamol is charged at all pH values and does not readily lend itself to simple, specific back-extracting procedures. This severely restricts the options of sample cleanup. However, a Subtilisin protease digestion step followed by acid clarification and solid-phase extraction has been suggested (146) as an adequate extraction and cleanup procedure prior to the end-point determination of salbutamol by an enzyme immunoassay (139) based on the cross-reactivity of anticlenbuterol antibodies. 

The handling and disposal problems associated with the use of liquid solvent extractors have resulted in increased attention to the separation and preconcentration of organic compounds in water by collection in synthetic polymers followed by elution with an organic solvent (2). For example, selective collection and concentration of organic bases on methylacrylic ester resin from dilute water samples have been reported (3). Such collection techniques are especially well-suited to flow-injection measurement techniques. In this study, ionizable organic analytes such as salicylic acid and 8-hydroxyquinoline (oxine) were extracted into a polymer and then back extracted by an aqueous solution. Amperometric measurement using a flow-injection technique was employed to monitor the process. 

With this technique it is easy to repeat the operations of extraction, back-extraction and washing with proper solvents to improve the separations without great expenditure of time. 

APDC/DDDC into chloroform and back-extraction into 7.5 N HN03). Vertical profiles obtained by both techniques are shown in Figure 4. The profiles for cadmium and zinc show good agreement between the solvent extract and Chelex results, although the Chelex results are somewhat lower than those by solvent extraction concentration. The values for nickel were also lower when the Chelex technique was employed. The mean difference between the two techniques was 65 35 ng l-1. 

More recently, a highly selective, sensitive, and rapid HPLC method has been developed and validated to quantify tadalafil in human plasma [44]. The tadalafil and internal standard (loratadine, I.S.) were extracted by liquid-liquid extraction technique followed by an aqueous back-extraction allowing injection of an aqueous solvent in the HPLC system. The chromatographic separation was performed on a reverse phase BDS Hypersil C18 column (250 mm x 4.6 mm, 5 pm. Thermo Separation Co., USA) with a mobile phase of acetonitrile and aqueous solution containing 0.012 M... 

Lecithin has a long history of use in foods, dating back more than 60 years (9). The 1930s brought widespread use of commercial solvent extraction techniques for... 

Membrane-assisted solvent extraction processes have known an increasing number of applications in the last decades. The technique not only overcomes the limitations of conventional liquid extraction, such as flooding, intimate mixing, hmitations on phase flow rate variations, and requirement of density difference but also provides a large surface area of mass transfer per volume of contactor. Simultaneous extraction and stripping of the solute has been developed using two HE modules in series, one for the extraction and the other for the back-extraction processes. 

The challenges inherent in the analysis of gas bubbles trapped in the ice were reviewed by J. Chappellaz (84). The gas, in fact, should be extracted without losses or contamination and the minute amounts available require sophisticated analytical approaches. On the other hand, unique information can be gained in this way on the composition of the atmosphere as far back as 100,000 years ago. The extraction techniques employed are basically dry extraction, melt extraction and sublimation. Past changes in greenhouse gases can be reliably documented so that global biogeochemical cycles can be better understood. 

To be able to determine theophylline in the ng range, Arbin and Edlund improved their method by derivatizing theophylline and the internal standard theobromine using penta-fluorobenzyl bromide, in connection with an electron capture detector. To 100 ul samples of plasma containing 50-1000 ng theophylline, 100 yl of a solution of the internal standard were added. The extraction was performed by column extraction (cellulose) using dichloro-methane as solvent. After back extraction into an alkaline aqueous phase (NaOH), theophylline and theobromine were alkylated with pentafluorobenzyl bromide by an extractive alkylation technique. The derivatives were extracted with cyclohexane and, after concentration to 3 ml, 2 yl were injected for the gas chromatographic assay. Standard deviation of the method was 2.5 % at a concentration level of 200 ng per sample, and 8 % at 20 ng per sample. The sample amounts were 100 yl plasma and 250 mg rat brain tissue, and the sensitivity limit... 

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