purifying organic compounds and combinatorial libraries in VS .NET

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purifying organic compounds and combinatorial libraries
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loaded column, and the recovery of the product decreased from 80 85% to 75% with the elevated temperature. As in reverse-phase preparative HPLC, the fast gradient mode can also be applied in the normal-phase preparative HPLC. With use of a short column such 125 25 mm or 125 50 mm and a high ow rate, the separation of eight organic molecules can be achieved in 12 minutes.21
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LLE AND SLE
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Liquid liquid extraction (LLE) is based on a simple principle that a compound will be partitioned between two immiscible solvents with concentration at a distribution ratio proportional to its solubility in each of the solvents. LLE is a common method of working up organic reaction mixtures. A conventional LLE application is to separate compounds between water and an organic solvent such as diethyl ether, ethyl acetate, or methylene chloride. Acidic or basic buffers are often used to control the distribution ratio of a certain substance. However, conventional LLE requires precise removal of the aqueous layer, which is not amenable to large number of samples. To solve this problem, solid supported liquid-liquid extraction (SLE) was developed.26 Instead of using separation funnels, the reaction mixture is loaded on a cartridge packed with diatomaceous earth, which is pretreated with an aqueous buffer and contains an aqueous layer. A water-immiscible solvent, usually methylene chloride or ethyl acetate, is then applied to elute the products off the cartridge, leaving more water-soluble impurities on the column. Like conventional LLE, SLE is also based on partitioning of compounds between two liquid phases. Hydrophilic amines (c log P < 3.1) were removed with an acid buffer of 1N HCl, but most hydrophobic amines (C log P > 3.1) were retained (Table 10.1). All acids were removed with a basic buffer of 1N NaOH. Since all the acids used in this study were hydrophilic (c log P < 3.1), it remains unclear how hydrophobic organic acid would respond to basic SLE. One would expect a c log P threshold for acidic compounds as well. One distinctive advantage for SLE over the more traditional LLE is its ease of automation and parallel processing. Diatomaceous earth can be packed into a 96-deep-well lter plate.The plate is frozen to prevent leaking during the transfer. The eluent was directly collected to a 96-well microtiter plate. With the help of robotic liquid handlers, the SLE process can be automated with a throughput of four plates per hour.27 Since the aqueous phase is immobilized on the cartridge, any water-immiscible organic solvent can
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solid-phase extraction
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be used regardless of its density. The elution is usually driven by gravity, while a slight negative pressure can facilitate this process. The introduction of a third phase uorous phase is an effective way to purify compounds with a uorous tag.28 This method can remove both hydrophilic and organic impurities from the reaction mixture. Studies have shown that only the desired product is taken by the uorous solvent while impurities remain in organic and water phases. The uorous tag can be removed later by a simple reaction such as desilylation. The purity of all nal products is higher than 95%. LLE and SLE also suffer from some limitations. There are often hydrophobic by-products, such as that from an incomplete removal of a protecting group, in combinatorial samples. These impurities will not be removed by SLE. This will affect the product purity. On the other hand, hydrophilic samples with low log Ps may get lost during the process.
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SOLID-PHASE EXTRACTION
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Solid-phase extraction (SPE) is the method of sample preparation that concentrates and puri es analytes from solution by sorption onto a disposable solid-phase cartridge, followed by elution of the analyte with an appropriate solvent. The SPE technique was developed in the mid-1970s as an alternative means of liquid-liquid extraction29 but become particularly attractive for its automation, parallel puri cation, and pre-concentration. Since 1995, SPE has been applied in various elds, environmental, food sciences, biomedical analyses, pharmaceutical analyses, and organic synthesis.30 34 There are a numbers of publications and reviews on the subjects of development of new solid-phase supporting materials,26,35 instrumentation and device,37 techniques,38 40 and theoretical aspect.41 In general, the procedures of SPE consists of the following four steps:
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Conditioning the sorbent by passing through the column with a small volume of the appropriate solvent to solvate the functional groups on the surface of the sorbents.The sorbent can also be cleaned at this point to remove any impurities present in the sorbent. The liquid sample is applied to the column with the aid of a gentle vacuum. The interested analyte and some interfering impurities will retain in the sorbent. In this retention step the analyte is concentrated on the sorbent. Rinse the column with some mixed solvents to remove the interfering impurities, and let the interested analyte retain on the sorbent.
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purifying organic compounds and combinatorial libraries Elute the analyte completely from the sorbent by an appropriate solvent.
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Depending on the mechanism of the interaction between the analyte and the sorbent, the SPE can be classi ed into three modes: reversed-phase SPE, normal-phase SPE, and ion-exchanged SPE. Like liquid chromatography, the sorbents used in the reverse-phase SPE are more hydrophobic, more hydrophilic in the normal-phase SPE, and ionic in ion-exchange SPE. Unlike HPLC, where the analyte is eluted continuously with mobile phase, and collected when the detected signal appears, the analytes collected in SPE process have no monitoring signal. Therefore, no matter what kind of mechanism, the retention of the interested analytes on the sorbents has to be very speci c and selective. A limitation of SPE in high-throughput puri cation of combinatorial libraries is the carryover of impurities with similar chemical properties. The selection of solid sorbents and the elution solvents will largely determine the recovery and purity of the desired products. The effects of sorbent properties and the elution solvents on the extraction ef ciency for different classes of molecules with different modes of SPE will be discussed in the sections below. 10.5.1. Reverse-Phase SPE
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The reverse-phase SPE involves the partitioning of organic solutes from a polar mobile phase, such as water, into a nonpolar solid phase, such as the C-18 or C-8 sorbent. The interaction between solute and sorbent involves van de Waals and dispersion forces.The speci city of the extraction depends on the difference in chemical potential or the solubility of the solutes between the two phases. The most common reverse-phase sorbent used in SPE is C-18 silica with various particle sizes (40 70 mm) and pore sizes (55 125 ). Other reversephase sorbents include C-8, C-4, C-2, cyclohexyl, and phenyl-bonded silica, as well as polymeric sorbents such as polystyrene-divinylbenzene (PSDVB) and graphitized carbon. For organic samples, there is a good correlation between the retention on C-18 silica sorbent and the octanol-water partition coef cient (log P) of the analytes. The more hydrophobic the analytes are, the higher the retention factor and the extraction recovery. For nonpolar or moderately polar analytes with log P values higher than 3, the extraction recovery can reach >95%.40 The effects of various solid supports on the extraction recovery for a set of polar carbamates have been studied, and the results are shown in Table 10.3. It is apparent that the extraction ef ciency is lower by using C-18/OH
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