Extraction considerations

Although the imidazolidinone catalysts used within these transformations are simple, cheap, readily accessible and in some cases recyclable using acid/base extraction, considerable efforts have been made to examine alternative methods to separate and recycle the catalyst with good success. Examination of the structure of imidazolidinone 22 shows two convenient points for the introduction of a polymer or fluorous support, R and R, both of which have been examined (Fig. 4). Curran has shown that identical reactivity, diastereoselectivity and enantioselectivity can be obtained using a fluorous tag (23) [53]. The catalyst can easily be recovered and recycled using F-SPE with excellent yield, purity and levels of activity. Polymer- (24) and silica-supported (25) imidazolidinones reported by Pihko [54] (R substitution)... 

I have been able to extract considerable quantities of bromine from the mother liquors of the soda kelp which serves for the extraction of the iodine. Finally, it has seemed to me that the product of the evaporation of a mineral water from the eastern Pyrenees, which was strongly saline, became yellow in contact with chlorine. If the bromine actually existed in a water of this kind, one might hope to encounter it in the salt springs properly so called, and especially in the mother liquor of the rock salt. I have lacked material to verify it All this makes it very likely that bromine will be found in a large number of marine productions or those of submarine origin (28). 

In SLM extraction, the most widely applied type of three-phase membrane extraction, the membrane consists of an organic solvent, which is held by capillary forces in the pores of a hydrophobic porous membrane supporting the membrane liquid. Such membrane support can be either flat porous PTFE or polypropylene membrane sheet or porous polypropylene hollow fibers. Typical solvents are long-chain hydrocarbons like n-undecane or kerosene and more polar compounds like dihexyl ether, dioctyl phosphate, and others. Various additives can increase the efficiency of extraction considerably. The stability of the membrane depends on the solubility and volatility of the organic liquids, and it is generally possible to obtain membrane preparations that are stable up to several weeks. 

Unfortunately, the dose-survival times for the DSP toxins in the mouse assay fluctuate considerably and fatty acids interfere with the assay, giving false-positive results consequently, a suckling mouse assay that has been developed and used for control of DSP measures fluid accumulation after injection of the shellfish extract. Considerable effort has been applied recently to development of chemical assays to replace these bioassays. As a result a good high performance liquid chromatography (HPLC) procedure has been developed to identify individual PSP toxins (detection limit for saxitoxin = 20 fg per 100 g of meats 0.2 ppm), an excellent HPLC procedure (detection limit for okadaic acid = 400 ngg 0.4 ppm), a commercially available immunoassay (detection limit for okadaic acid=lfg per 100 g of meats 0.01 ppm) for DSP, and a totally satisfactory HPLC procedure for ASP (detection limit for domoic acid = 750 ngg 0.75 ppm). 

A number of authors have advised the use of continuous extraction. For the extraction of the aqueous phase when obtaining the acid-ether and alkaline-chloroform fractions this is of doubtful value, as, although it may extract marginally more of the drug, it is certain to extract considerably more impurities. Mention should be made, however, of a scheme for the continous extraction of minced tissues with ethanol described by Curry and Phang (323) this has proved most useful in extracting difficult compounds. 

Experimental data of coal pyrolysis in a sand fluidized-bed reactor indicate that it is possible to extract considerable amounts of hydrocarbons from caking coal by a rapid heating and subsequent cracking in the vapor phase. This is done by flowing pulverized coal into a fluidized... 

Recently, Shahidi et al. [31] found that total antioxidant activity of hazelnut extracts ranged from 29 to 148 xmol TE/g of ethanolic extract, with lowest activity in hazelnut kernel aud highest in hazelnut tree leaf (Table 13.1). The total antioxidaut activity of hazelnut by-product extracts were approximately 4-to5-fold higher than that of hazelnut kernel at the same extract concentration. At a given concentration, hazelnut by-product extracts would serve as a more effective antioxidant than hazelnut kernel extract. Consideration of defatted hazelnut (on an extract basis) and hazelnut (as is weight basis) make the quantitative comparison between the studies of Wu et al. [4] and Shahidi et al. [31] impossible. 

Immiscible Solvents. As in the case of cocurrent extraction, considerable simplification results if we may consider the liquids A and B substantially insoluble over the range of C concentrations experienced in the extraction (2, 5). The same notation will be used as in the previous case,... 

Despite these mechanistic intricacies and uncertainties, it is possible to extract considerable information from the electrochemical behavior. When two separate redox couples are observed, it is possible to determine the ratio KJKi, the factor of electrochemically-enhanced cation binding that occurs upon electrochemical switching. The enhancements observed always follow the trend Li >Na >K. This result is expected in view of coulombic considerations. Factors as high as 10 were observed for Li cation and lower values, typically 10 were observed for K [7]. Although these enhancement factors exhibit a clear selectivity, the overall binding... 

Table 5.1 gives the formulation of a polyethylene and a polystyrene used in some Uvitex OB extractability studies. In addition to the ultraviolet stabiliser, liquids obtained in extractability tests carried out on these plastics would contain various other substances, some of which are ultraviolet absorbers and which may be present in the extractant at higher concentrations than Uvitex OB. In this case the polymer also contained Santonox R, Wingstay T, phenolic antioxidant and styrene monomer. The presence of such ultraviolet absorbing substances in the extractant will interfere in the determination of Uvitex OB at 378 nm. In applying spectroscopic methods of analysis to extractants, consideration must always be given to the possibility of interference by any polymer additives present other than that which one is required to determine. 

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