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Problem Set V – High-Performance Liquid Chromatography 1. In reversed phase chromatography, the solutes are nonpolar and more soluble in a nonpolar mobile phase. In normal phase chromatography, the solutes are polar and more soluble in a polar stationary phase. 2. (a) small particles give increased resistance to flow. High pressure is required to obtain a usable flow rate. (b) A bonded stationary phase is covalently attached to the support. 3. Efficiency increases because solute equilibrates between phases more rapidly if the thickness of both phases are smaller. This effect decreases the C term in the van Deemter equation. Also, migration paths between small particles are more uniform, decreasing the multiple flow path (A) term. 4. 0.136 min and 0.304 min 5. Silica dissolves above pH 8 and the siloxane bond to the stationary phase hydrolyzes below pH 2.
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Unformatted text preview: 6. (a) Your sketch should be similar to B on page 21 of your study guide. (b) Tailing of amines might be eliminated by adding 30 mm triethylamine to the mobile phase. Tailing of acidic compounds might be eliminated by adding 30 mM ammonium acetate. For unknown mixtures, 30 mM triethylammonium acetate is useful. If tailing persists, 10 mM dimethyloctylamine or dimethyloctylammonium acetate might be effective. Tailing could also be caused by a clogged frit which you might be able to clean by washing with reversed flow. 7. (a) Since nonpolar compounds should become more soluble in the mobile phase, the retention time will be shorter in 90% methanol. (b) At pH 3, the predominant forms are neutral RCO 2 H and cationic RNH 3 + . The amine will be eluted first, since RNH 3 + is insoluble in the nonpolar stationary phase....
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This note was uploaded on 05/09/2008 for the course PHARM.CHEM 306 taught by Professor Hu during the Spring '08 term at Rutgers.

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