Multiple extractions, efficiency

The method procedures are very time-efficient by always using a well defined portion of the extracts, thus avoiding multiple extractions, time-consuming rinses and overloading of the chromatographic cleanup systems with co-extracted sample matrix. 

The extraction efficiencies using a blender and a shaker were compared and both methods gave similar results. A corn sample treated with radiolabeled carfentrazone-ethyl and collected from a metabolism study was used for comparison. Multiple samples can be extracted simultaneously if extraction is performed by shaking. In addition, since the extraction procedures in the residue study closely followed the extraction scheme in the metabolism study, the resulting extraction efficiencies from both studies were almost identical. 

Solvent Extraction Aqueous sample is partitioned with an immiscible organic solvent. Extraction efficiency depends on the affinity of the solute for the organic solvent. All sample types Samples with a high affinity for water are not extracted. Extractions can be performed by a simple single equilibration or by multiple equilibrations with fresh solvent. Solvent impurities concentrated along with sample. 

The need to use multiple extraction to achieve efficient extraction required the development of new types of continuously working extractors, especially mixer-settlers and pulsed columns, which were suitable for remotely controlled operations. These new extractors could be built for continuous flow and in multiple stages, allowing very efficient isolation of substances in high yield. A good example is the production of rare earth elements in >99.999% purity in ton amounts by mixer-settler batteries containing hundreds of stages. These topics will be further developed in Chapters 6 and 7. 

Extraction of 25 different binary mixtures of racemic acids (2-(4-isobutylphenyl)-propionic acid (1), and cis- and trans-chrysanthemic (2)), and various chiral bases with supercritical carbon dioxide permitted the conclusion that molecular chiral differentiation in a supercritical fluid is more efficient than in conventional solvents. In the majority of cases, however, complete separation could not be achieved. In five cases, remarkable partial resolutions were realized (30-75% ee) and resolution was possible on a preparative scale. The pair ds-chrysanthemic acid and (S)-(-i-)-2-(benzylamino)-1-butanol (3) was studied in detail. Pressure, temperature, and time, as well as the molar ratio of base and acid, had a marked influence on the quantity and quality of the products. Increasing pressure or decreasing temperature resulted in higher ee values. (-)-cw-Chrysanthemic acid in 99% ee was obtained from the raffinate in a single extraction step. Multiple extractions produced the (-i-)-cA-acid in 90% ee (see fig. 6.3) (Simandi et al., 1997). 

The cleanup procedure used was discussed in the section entitled Resin Elution Concentration. The extraction procedure was not the optimum procedure that would result from the observations. The data indicated that a base extraction at pH >12 and a base-to-solvent ratio of 15 1 with multiple extraction steps were needed to maximize removal of the matrix interferences. However, the extraction efficiency must be balanced against minimal sample destruction and operational ease. 

Figure 3.9. In the oven cavity is a carousel (turntable or rotor) that can hold multiple extraction vessels. The carousel rotates 360° during extraction so that multiple samples can be processed simultaneously. The vessels and the caps are constructed of chemically inert and microwave transparent materials such as TFM (tetrafluoromethoxyl polymer) or polyetherimide. The inner liners and cover are made of Teflon PFA (perfluoroalkoxy). The vessels can hold at least 200 psi of pressure. Under elevated pressures, the temperature in the vessel is higher than the solvent s boiling point (see Table 3.11), and this enhances extraction efficiency. However, the high pressure and temperature may pose safety hazards. Moreover, the vessels need to be cooled down and depressurized after extraction.

There are many factors involved in optimizing static headspace extraction for extraction efficiency, sensitivity, quantitation, and reproducibility. These include vial and sample volume, temperature, pressure, and the form of the matrix itself, as described above. The appropriate choice of physical conditions may be both analyte and matrix dependent, and when there are multiple analytes, compromises may be necessary. 

The mixtures of racemic acids and chiral bases were extracted with supercritical carbon dioxide. The extracts were collected as separate samples successively in time. Each of them was examined separately. The results of ibuprofen extraction are presented in Figure 3a. The extracts contained the (+)-enantiomer in 20-40 % optical purity while the raffinate was rich in (-)-enantiomer (enantiomeric excess= 44 7 %). Further purification of the extracts can be carried out by multiple extraction (Figure 3b). We have succeed to separate the cis-chrysanthemic acid mixture in a single extraction step with excellent enantiomeric excess for the (-)-enantiomer (greater than 95 %). The enantiomeric excess of the (+)-enantiomer was 90 % after three subsequent extraction. The resolution of ibuprofen was less efficient The... 

A higher methanol content in the extraction mixture enhances the effectiveness of extraction of organic arsenic derivatives without altering the effectiveness of release of As(III) and As(V) salts, and arsenosugars [89]. The duration of the process can be reduced by ultrasound treatment or pressurized extraction [90]. Multiple extraction does not improve the efficiency of the process the quantity of arsenic remaining in the solid material depends solely on the volume of solvent retained in it (usually in the first cycle) and not on the number of repetitions of the leaching process [91]. 

In both organic and analytical chemistry laboratories, it is a common procedure to extract a compound from one solvent to another. It is also a common knowledge that it is more efficient to use small volumes and multiple extractions. This is shown by Eqs. (1) (given previously) and (25)-(27), assuming the two solvents are completely immiscible (e.g., H2O-CCI4). 

Liquid-liquid extraction (LLE) uses two immiscible liquids as the two phases. The sample is dissolved in one of the liquids (refinate) which comes in contact with the other liquid (extractant) into a separatory funnel, under agitation, to increase the contact area among the phases. Some mixing time is usually necessary for efficient phase exchange. Multiple extractions are also mandatory if quantitative extraction is desired. Sample transfer can become a problem, especially if phase emulsions are produced. 

An equation can be derived to calculate the increase in efficiency of multiple extractions versus one single extraction ... 

When there is no alternative to employing multiple extractions, chemical engineering approaches may afford more efficient operations in a pilot plant or manufacturing facility [10]. Many of these extractors are countercurrent devices, such as the Karr reciprocating plate tower, in which the light phase is continually fed into the bottom of the extractor while the heavy phase is fed into the top of the extractor. 

Figure 30-2 shows that the improved efficiency of multiple extractions falls off rapidly as a total fixed volume is subdivided into smaller and smaller portions. Clearly, little is to be gained by dividing the extracting solvent into more than five or six portions. 

The discussion on remaining sources of uncertainty which have arisen recently are the addition of an uncertainty component to the final certified value due to the precision of the recovery factor used to correct for extraction efficiency and purification [9] and somewhat the correction to dry mass. This debate is far from closed. It may be considered that the uncertainty of the recovery factor is reflected in the variability of the pretreatment procedure of the certification measurements. This is true if this uncertainty of the recovery factor as determined by e.g. spiking or multiple extraction procedures shows reproducibility close to the certification measurements. If not, as discussed above, the... 

Most of the derivatives shown in Figure 8 are solvent extractable at low pH, and thus one of the classical methods used for antibiotic purification becomes accessible to cephalosporin C. To be commercially feasible, solvents should be selective and only slightly miscible with water. Extraction efficiency should be sufficiently high that multiple extractions are not required, and ideally should be efficient at low ratios so as to effect a concentration of the desired component. Emulsions and insoluble solids are anathema to extraction. Using these criteria, extraction of most of the cephalosporin C derivatives at low pH are far from ideal since mostly non-selective solvents (such as n-butanol and ethyl acetate) usually work best several extractions seem to be required, and derivatized cephalosporin C broth upon acidification will frequently result in emulsion formation. However, some derivatives behave better than... 

Hale and Aneiro (1997) reviewed recent progress made in improving analytical techniques for determining components of creosote in environmental media. The multiple extraction and purification steps required prior to chromatographic analysis is problematic in that compounds may be lost through volatilization or transformed through photodegradation. More efficient extraction procedures include supercritical fluid extraction, accelerated solvent, and microwave, and solid-phase extraction. Newer... 

Most work in microchannel extraction focuses on improving the extraction efficiency or the phase separation or system development through adding multiple imit operations on a single chip or by scaling up. Because laminar flow exists in the microchannel devices, the intimate mixing of turbulent flow in traditional contactors is not present. Most studies have shown that the dissimilar phases flow parallel to each other with movement of solute molecules caused by molecular diffusion only. Thus, extraction is governed by contact time between phases [202]. 

Figure 1.15. Effect of multiple-stage arrangement on extraction efficiency.

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