Gradient chromatography

The crude residue was purified by flash chromatography (gradient elution with 10-30 % EtOAc/hexanes). 179 mg (98 % yield). 

New concepts presented in this edition include monolithic columns, bonded stationary phases, micro-HPLC, two-dimensional comprehensive liquid chromatography, gradient elution mode, and capillary electromigration techniques. The book also discusses LC-MS interfaces, nonlinear chromatography, displacement chromatography of peptides and proteins, field-flow fractionation, retention models for ions, and polymer HPLC. 

A sample of povidone or copovidone is analyzed after dissolution in a water/ methanol mixture using HPLC reverse phase chromatography (gradient run). The polymeric material is kept away from the chromatographic system using a precolumn and switching technique. 

AMD is the only method with which reproducible gradient elution on silica gel is possible. In column chromatography, gradient elution can in practice only be used in the reverse phase silica gel would be irreversibly changed, which, with a technique involving frequent re-use of the stationary phase, is not acceptable. This aspect is not relevant in TLC. (Note that the TLC plate is a disposable item ) [159]. 

Comprehensive multidimensional liquid chromatography is a relatively new development and has yet to develop a diverse application base. For the time being applications are dominated by the separation of proteins and synthetic polymers. For proteins the first dimension separations are usually based on ion exchange and the second dimension separations on reversed-phase liquid chromatography. Gradient elution was often used for both separation modes with a separation time less than 2 minutes for the second dimension separation and from 30 minutes to several hours for the first dimension separation. Current trends include the use of non-porous particles and perfusive stationary phases for the second dimension separation to reduce the total separation time and wider internal diameter columns in place of packed capillary columns to simplify interface construction and instrument operation and to allow the loading of larger sample sizes. 

The rapid separations offered by capillary electrophoresis have made it amenable as a detector in hyphenated techniques. For LC-CE, the total analysis time is usually governed by the LC separation, which generally takes minutes. However, capillary electrophoresis detection adds more peak capacity because of a second and orthogonal dimension for separation, and shorter separation conditions for LC can often be tolerated. For example, a 2.5 min reversed-phase liquid chromatography gradient was used in conjunction with 2.5 s CE separations for the detection of a tryptic digest of cytochrome c. ... 

To a solution of 10 mg 1,4-dione (0.021 mmol) in 0.5 mL methanol were added 23 fiL benzylamine (0.21 mmol), 12 ixL acetic acid (0.21 mmol), and 30 mg 4A molecular sieves activated by flame drying under vacuum. The reaction mixture was stirred at 70°C for 1 h. Upon completion, the reaction mixture was diluted with 50 mL EtOAc and washed with 1 M HCl (2 X 20 mL). The organic layer was then dried over MgS04, and the solvents were removed in vacuo. The crude material was purified by flash chromatography (gradient elution hexanes/EtOAc, 19 1-3 1, v/v) to afford 10 mg 5-benzoyl-l,4-dibenzyl-2-propyl-3b,4,5,6,6a,7-hexahydro-lH-l,5-diaza-cyclopenta[a]pentalene-4-carboxylic acid methyl ester, in a yield of 90%. 

C. Use of Automated Column Chromatography, Gradient Elution and Radioactivity Flow Cell... 

NP hydrophilic interaction chromatography gradients—concentration(s) of water (or of water and a more polar organic solvent) in a less polar solvent increase(s) aqueous mobile phases, polar adsorbent, or bonded-phase columns... 

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