Hexane-buffer system

Method. The stationary phase consists of the appropriate amount of picric acid dissolved in a citrate buffer system (Titrisol Merck). The pH of these solutions are checked and when necessary are adjusted to the desired pH with 5 N sodium hydroxide. The columns are packed with a slurry of silica gel by means of the equal-density procedure described earlier [54]. For other adsorbents, a new technique [55] is used for packing the slurry. After packing, twice the dead volume of chloroform is pumped through the column. The column is then heated at 180 °C for 2 h and flushed simultaneously with a gentle stream of nitrogen. The columns (usually 10 cm long) are treated with ca. 10 ml of the stationary phase at a flow-rate of 0.5 ml/min, and are then flushed with 20-40 ml of hexane. The column is equilibrated with chloroform at a flow-rate of 0.2 ml/min for various times. The samples are injected as ion pairs on to the column. Formation of the scopolamine and hyoscyamine ion pairs occurs in 5 ml of buffer solution (pH 5 or 6) to which a solution of picric acid is added containing 40 mg of picric acid in 5 ml of buffer. 

A more promising approach for the synthesis of hydrophobic substances with ADHs is published by Kruse et al. [159, 238], They use a continuously operating reactor where the enzyme containing water phase is separated from the hydrophobic substrate-containing organic phase by a membrane. The hydrophobic product is extracted continuously via a hydrophobic membrane into an hexane phase, whereas the coenzyme is regenerated in a separate cycle, that consists of a hydrophilic buffer system. This method decouples advantageously the residence time of the cofactor from the residence time of the substrate. Several hydrophobic alcohols were prepared in this way with (S)-ADH from Rhodococcus erythropolis (Table 16). 

A comparative study was made of asymmetric hydrolysis of styrene epoxide to (R)-l-phenyl-l,2-ethanediol by mung bean epoxide hydrolase in a biphasic system (n-hexane-buffer) containing hydrophilic ionic liquids (ILs). Compared to the biphasic system alone, the introduction of a small amount of hydrophilic ILs decreased the degree of non-en matic hydrolysis and increased the reaction rate by 22%. The ILs with a cation containing an alkanol group,... 

The synthesis of n-hexanal from 2-pentene in water using Rh(CO)2(acac) and sulfonated NAPHOS as catalyst precursors was investigated. It was found that by lowering the partial pressure of carbon monoxide in a buffered system at pH 7 and 8, significant increase in the aldehyde yield (up to 73%) and excellent regioselectivity (normal iso = 99 1) were observed. Similar results for the hydroformylation of 2-pen-tene were also obtained in the presence of tertiary amines such as triethylamine [110]. 

Table 9.6 Enzymatic reduction of ketones in a scC02/buffer or hexane/buffer biphasic system.

Figure 3.10 shows the use of sol-gel-encapsulated alcohol dehydrogenase (WHOA mutant of Thermoanaerohacter ethanolicus alcohol dehydrogenase, TeADH) for asymmetric reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol, a precursor for both bufeniode and labetalol (antihypertensive agents) [20], Immobilization made it possible to use the biocatalyst in hexane containing 2-propanol and increase the substrate concentrations to 140 mM (vs. 35 mM in an aqueous buffer system). It also allowed the biocatalyst to be reused... 

Carrier-free immobilized EHs were prepared using the cross-linked enzyme aggregate (CLEA) technology, as exemplified by the CLEA formation of a mixture of two EHs from mung beans. Compared to the free enzymes, the CLEAs exhibited significantly shorter reaction times and higher enantioconvergence for the hydrolysis of SO in a biphasic n-hexane buffer reaction system, which ensured low levels of nonenzymatic hydrolysis of the substrate [59]. 

Eluent components should be volatile. Solvents such as ethyl acetate, isopropyl ether, diethylketone, chloroform, dichloromethane, and toluene as modifiers and n-hexane as diluent are recommended for normal phase chromatography. For reversed-phase systems, methanol or acetonitrile are used as modifiers. Such components as acetic acid or buffers, as well as ion association reagents, should be avoided. 

Kennedy et al. developed a lasalocid immunoassay for application to residues in chicken meat and liver samples. The antibody was specific and did not cross-react with salinomycin, maduramicin, or monensin. Sample preparation consisted of homogenization in aqueous acetonitrile, removal of fat from an aliquot of the aqueous acetonitrile by hexane extraction, and evaporation of acetonitrile. The sample was then reconstituted with assay buffer. Liver required an additional solid phase extraction step. The LOQ was 0.02 xgkg for muscle and 0.15 agkg for liver. These workers were able to use the system to determine the half-life of lasalocid in the tissues. 

Additional modes of HPTC include normal phase, where the stationary phase is relatively polar and the mobile phase is relatively nonpolar. Silica, diol, cyano, or amino bonded phases are typically used as the stationary phase and hexane (weak solvent) in combination with ethyl acetate, propanol, or butanol (strong solvent) as the mobile phase. The retention and separation of solutes are achieved through adsorp-tion/desorption. Normal phase systems usually show better selectivity for positional isomers and can provide orthogonal selectivity compared with classical RPLC. Hydrophilic interaction chromatography (HILIC), first reported by Alpert in 1990, is potentially another viable approach for developing separations that are orthogonal to RPLC. In the HILIC mode, an aqueous-organic mobile phase is used with a polar stationary phase to provide normal phase retention behavior. Typical stationary phases include silica, diol, or amino phases. Diluted acid or a buffer usually is needed in the mobile phase to control the pH and ensure the reproducibility of retention times. The use of HILIC is currently limited to the separation of very polar small molecules. Examples of applications... 

It was necessary to add over 10% buffer for the transesteiification of phosphatidyl choline by native lipase (5). Hydrolysis occurred as a side reaction in the hydrophobic solvent-water system. Tlie transesterification of phosphatidyl choline and eicosapentaenoic acid (EPA) was carried out in water-saturated n-hexane using palmitic acid-modified lipase. Table II shows the transesterification of phosphatidyl choline and EPA. Modified lipase made it possible for the transesterification of phospholipids in organic solvents. 

Addition of water always enhanced the efficiency of immobilized laccase. The optimal amounts of water were surprisingly low rates of syringaldazine oxidation comparable to those observed in buffer were observed in 65% aqueous acetonitrile, 50% aqueous acetone, 50% aqueous dioxane, and others. Immobilized laccase is surprisingly stable when stored in organic solvents. Stored in n-hexane at 30° C, it was stable for more than eight days with less than 10% loss of initial activity, whereas at the same conditions the system lost about 90% of its activity when stored in buffer. The addition of water to the pure solvents had a detrimental effect on stability. 

Biotin and biotin analogs Infant formula Protein precipitation using concentrated hydrochloric acid neutralization with 6 M NaOH lipid extraction with n-hexane Precolumn Microsorb C18 (15 X 4.6 mm, 5 jam Rainin). Analytical Microsorb C18 (250 X 4.6 mm, 5 /zm Rainin). Isocratic 100 mM phosphate buffer, pH 7.0 + methanol (80 + 20, v/v). 0.4 ml/min. Postcolumn reaction system UV absorbance at 220 nm followed by streptavidin-fluorescein isothiocyanate (2.0 mg/L) knitted open tubular reaction system (10.0 m x 0.5-mm ID) at a flow rate External standardization. 184 Linear range = 0.08-1.00 fjM biotin. LoD = 0.02 /zM or 97 pg biotin at SNR = 3. Repeatability CV 3.5% for biotin in infant formula. 

The pharmacokinetics of FLU and its metabolite 7-OH FLU in sheep tissue was studied using the HPLC method with fluorescence detection (207). Tissue samples were extracted with ethyl acetate. After drying, phosphate buffer (pH 7.8) and hexane were added, and aqueous (lower) phase was injected into an HPLC system. Extraction recovery was 75% for FLU and 60% for 7-OH FLU. The limit of detection was 1 and 4 ytrg/kg for both compounds. The elimination of OXO in eggs (albumen, yolk) was described using an HPLC assay with fluorimetric detection. The limits of quantitation were 5 tg/kg in albumen and yolk. Of the overall oxolinic acid detected in eggs, 95% was concentrated in the albumen. Detectable residues persisted for 9 and 7 days, respectively, in albumen and yolk after the treatment was discontinued (208). Albumen sample was homogenized with water and hydrochloric acid and extracted with ethyl acetate. The supernatant was evaporated and the rest dissolved in mobile phase. Extraction recovery was 65.2%, RSD of 5.3, for a concentration of 10 ig/kg. 

Method. The coating of the stationary phase on the support material is carried out in situ as described above in order to achieve a loading of 27% w/w. The stationary phase consists of 0.1 M tetrabutylammonium hydrogen sulfate in borate buffer (pH 9.2). The mobile phase is butanol-hexane (1 3). The HPLC separation of 12 sulfa drugs with the system described is shown in Fig.4.24. The distribution ratios of some drugs are listed in Table 4.8. The limits of detection range from ca. 10 to 100 ng per injection. 

To 2.0 ml of Amberlite XAD 7 was added 2.0 ml of a 0.05 M phosphoric acid buffer (pH 7.0) having dissolved therein 20 mg of lipoprotein lipase, and the system was allowed to stand at room temperature for 18 hours to thereby adsorb the enzyme onto the resin. The resin was filtered. A solution of 250 mg of 3,5-dinitrobenzoyl derivative of ()-3-acetoxymethyl-7,8-difluoro-2,3-dihydro-4H-[l,4]benzoxazine as a substrate in 25 ml of a mixed solvent of benzene and n-hexane (4 1 by volume) was added to the resin, followed by... 

The oxidation catalyst is believed to be ruthenium tetraoxide based on work by Engle,149 who showed that alkenes could be cleaved with stoichiometric amounts of ruthenium tetraoxide. Suitable solvents for the Ru/peracid systems are water and hexane, the alkene (if liquid) and aromatic compounds. Complex-ing solvents like dimethylformamide, acetonitrile and ethers, and the addition of nitrogen-complexing agents decrease the catalytic system s activity. It has also been found that the system has to be carefully buffered otherwise the yield of the resulting carboxylic acid drops drastically.150 The influence of various ruthenium compounds has also been studied, and generally most simple and complex ruthenium salts are active. The two exceptions are Ru-red and Ru-metal, which are both inferior to the others. Ruthenium to olefin molar ratios as low as 1/20000 will afford excellent cleavage yields (> 70%). vic-Diols are also... 

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