Extraction solvents, screening

Timothy C. Frank, Ph.D. Research Scientist and Sr. Technical Leader, The Dow Chemical Company Member, American Institute of Chemical Engineers (Section Editor, Introduction and Overview, Thermodynamic Basis for Liquid-Liquid Extraction, Solvent Screening Methods, Liquid-Liquid Diversion Fundamentals, Process Fundamentals and Basic Calculation Methods, Dual-Solvent Fractional Extraction, Extractor Selection, Packed Columns, Agitated Extraction Columns, Mixer-Settler Equipment, Centrifugal Extractors, Process Control Considerations, Liquid-Liquid Phase Separation Equipment, Emerging Developments)... 

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

The collection procedure itself is straightforward. After cataloguing and identification, 1-2 kg of the plant material is dried, or stored in alcohol and brought back to the lab. The plant material is crushed and extracted with various solvents (most plant-derived bioactive molecules are low molecular mass substances, soluble in organic solvents of varying polarity). After removal of the solvent, the extracts are screened for desirable biological activities (e.g. inhibition of microbial growth, selective toxicity towards various human cancer cell lines, etc.). 

Separation processes (both liquid-liquid and gas-liquid) are a key element in many industrial processes. For this application, solvent molecules are built from UNIFAC submolecular groups, and the relevant properties of the new molecules such as distribution coefficients and selectivities are estimated. Strategies for the design of solvents for separation processes were initially formulated and later extended to better model the processes of solvent synthesis, solvent evaluation, and solvent screening. A method for solvent design for liquid-liquid extraction has been developed. 

In some applications like newborn screening and filter paper blood spots, the internal standard that is labeled cannot be mixed with blood. It can only be present in the extraction solvents. Therefore, only the extracted metabolites can be quantitatively measured. I have denoted a term called pseudo-isotope dilution to account for the differences between traditional isotope dilution and the technique commonly used in newborn screening by MS/MS. A special analysis is capable using this technique, however, in terms of an extraction efficiency experiment. With isotope-labeled standards you can perform an experiment whereby a traditional isotope-dilution technique (internal standard added to liquid blood and spotted) is compared to pseudo-isotope dilution techniques (internal standard is added to the extraction matrix). The ratio of the results of these two analysis (pseudo/traditional) is the extraction efficiency. 

Sparingly volatile organic compounds, for which the most suitable method is pH-dependent extraction with organic solvents (liquid-liquid extraction, LEE) or the solid phase extraction (SPE) screening technique... 

Generate a list of candidate solvents based on chemical knowledge and ejq)erience. Consider solvents similar to those used in analogous applications. Use one or more of the methods described in Solvent Screening Methods to identify additional candidates. Include consideration of solvent blends and extractants. 

T xtractive and azeotropic distillation in different types of chemical industry has become more important as more separations of close-boiling mixtures and azeotropic ones are encountered. Extractive distillation is used more because it is generally less expensive, simpler, and can use more solvents than azeotropic distillation. Solvent selection for azeotropic distillation has recently been discussed by Berg (I). Therefore, solvent screening for extractive distillation is discussed here. 

The approach adopted to obtain an exploitable pure plant constituent involves interdisciplinary work in botany, pharmacognosy, pharmacology, chemistry, toxicology. The plant material is extracted by solvents of increasing polarity, the extracts are screened with different bioassays and submitted to fractionation with chromatographic techniques. This process is repeated until the isolation of a pure active constituent which is finally identified by spectroscopic methods (bioactivity-guided isolation)(Fig. 1). 

Based on these electrochemical studies we developed a method for the quantitation of ajmalicine and catharanthine in cell cultures. These alkaloids were extracted from freeze-dried cells and purified by the solid-phase procedure described by Morris et al. (1985), except that ethanol was used as the extracting solvent instead of methanol. A dual-electrode coulometric cell was used in the screen mode. The potential of the first electode was set at +0.2 V (vs. Pd), which was at the base of catharanthine s voltammogram. The alkaloids were detected by the second electrode at +0.8 V, as this offered the best S/N ratio. Higher potentials led to lower S/N ratio, since the background current and noise started to increase exponentially above +0.85 V, due to the oxidation of water. The mobile phase was purified by a guard cell between the pump and injector. The guard cell operated at +0.8V. 

A network of 107 electrodes covering two-thirds of an acre was established. To treat beneath a warehouse, 85 of those electrodes were constructed directly through the floor of the building. Electrically conductive from 11-21 ft bg, the electrodes actively heated the depth interval from 5-24 ft bg. Once subsurface temperatures reach boiling, steam laden wifli chlorinated solvents was collected by a network of 37 soil vapor extraction wells screened to 5 ft bg. 

The mycotoxins are extracted with a solvent that dissolves as much of the mycotoxin and as little of the other components of the test sample as possible. Numerous methods have been devised to determine the presence of the various mycotoxins. Some of these have been studied collaboratively for accuracy and precision. Other methods have been designed to screen for more than one mycotoxin simultaneously. Acetonitrile, chloroform, acetone, ethyl acetate, and methanol, or mixtures of these solvents with water are used as the extraction solvents. An acid is sometimes added to extract compounds containing carboxylic acids or acidic hydroxyl groups. In general, the more polar the mycotoxin, the more polar the extraction solvent should be. 

---