http://hdl.handle.net/1957/17746 Fri, 24 May 2013 07:48:31 GMT 2013-05-24T07:48:31Z http://hdl.handle.net/1957/38251 The distribution of organic bases in reverse phase liquid chromatography : a study of mechanisms Jayasinghe, Dudley Shelton The distribution of hydrophobic ionogenic organic compounds (HIOC's) between aqueous and nonaqueous phases has been investigated. The HIOC's are aniline and methyl substituted anilines. The aqueous phases are methanol-water mixed solvents with varying pH and ionic strength. The nonaqueous phases are silica and alkyl derivatized silica with ethyl- (C-2), octyl- (C-8), and octadecyl- (C-18) silanes. A gas-liquid equilibration method has been developed to determine the Henry's law constants and the activity coefficients of methyl anilines in methanol-water mixed solvents. The logarithm of the Henry's law constants vary quadratically with the volume fraction of methanol. The chromatographic study has been performed to investigate the effect of three mobile phase factors, fraction of methanol, pH, and ionic strength, on the retention of methyl anilines on different stationary phases. The logarithm of mass distribution ratios, log k's, of methyl anilines vary linearly with the volume fraction of methanol. The ratios of H / k' of methyl anilines increase with increase in methanol in the mobile phase, indicating an increase in the activity coefficients in the stationary phases. This observation is found to be true for different methyl anilines in different stationary phases. A partition model is presented to describe the effect of pH and ionic strength on the retention of HIOC's in the mobile phase. The variation in log k' among neutral and ionic methyl anilines can be explained by linear free energy relationships. Equations are established to predict retention of neutral or ionized methyl anilines from their octanol-water partition coefficients at any methanol content in the mobile phase. Acid dissociation constants of methyl anilines determined from the dependence of log k' on pH are in close agrement with those determined by potentiometric titrations. The effect of ionic strength on the retention of both neutral and ionic species of methyl anilines is negligible. That is, the contribution from electrostatic energy and salting out effect are not significant. The chromatographic study is extended to study the retention of methyl anilines on silica as a function of pH and methanol at a constant ionic strength. The dependence of retention on pH is qualitatively interpreted from an ion exchange reaction between the dissociated silanol groups in the surface and the ionized methyl anilines. The effect of methanol on the retention of methyl anilines and of pyridine on silica is described by a quadratic relationship between the log k' and volume fraction of methanol. Graduation date: 1990 Tue, 05 Sep 1989 00:00:00 GMT http://hdl.handle.net/1957/38251 1989-09-05T00:00:00Z http://hdl.handle.net/1957/38148 The characterization and simplex optimization of a variable-diameter, multielectrode, direct current plasma for atomic emission spectroscopy McGuire, Joseph Graduation date: 1990 Fri, 03 Mar 1989 00:00:00 GMT http://hdl.handle.net/1957/38148 1989-03-03T00:00:00Z http://hdl.handle.net/1957/38138 Development of automated flow injection apparatus and a novel flow cell for chemi- and bioluminescence determinations Gander, Stuart C. A practical and versatile instrument has been developed both to optimize the chemistry of bio- and chemiluminescence reactions and to measure ultra-trace quantities of associated analytes. The instrument consists of a pneumatic flow injection system linked to a detection cell and a signal processing and readout system. The disk-like cell is defined by a pair of transparent polycarbonate plates and an "0-ring" seal. The lower plate is fitted with a reflective surface. Relative to conventional cells, this design affords a high light collection efficiency due to the large volume element viewed by an end-on photomultiplier detector. Rapid mixing of reagents within the cell is brought about by injection through concentric ports of a commercial burner assembly at a point immediately below the detector. The signal processing and readout system is interfaced to an IBM compatible personal computer and appropriate software was written to automate the instrument and to acquire, store and manipulate luminescence data. With this instrumentation, the chemistry of marine bacteria biolumin-escence was optimized for the determination of cis-11-hexadecenal and, ostensibly, for both the quantification of aldehyde insect pheromones and potential use in the control of insect pests. With the optimized conditions, cis- 11-hexadecenal was determined to 7 fmol. This value is more than an order of magnitude lower than detection limits for aldehyde pheromones reported in the literature. In this research, the less ideal substrates undecanal and heptanal were determined to 570 fmol and 65 pmol, respectively. Marine bacteria bioluminescence was used to quantify several epoxide analytes derivatized to aldehydes. 1,2-epoxyhexadecane and 1,2- epoxytetradecane were determined to 55 and 51 fmol, respectively. 1,2- epoxyoctane and cis-7,8-epoxy-2-methyloctadecane were determined to 100 and 3 pmol, respectively. The latter compound is the sex pheromone of the gypsy moth (Lymantria dispar), a well-known and serious agricultural pest. Epoxides have not been quantified previously with either chemi- or bioluminescence. The instrument was modified for use with corrosive solutions and for possible interfacing with a high performance liquid chromatograph. Lophine chemiluminescence was optimized for the analysis of Cr(VI) samples. With the optimized conditions, aqueous solutions of Cr(VI) were determined to 50 μg /L. A plausible explanation is offered for the dependence of lophine chemiluminescence on the concentration of the chromium species. Graduation date: 1991 Mon, 19 Nov 1990 00:00:00 GMT http://hdl.handle.net/1957/38138 1990-11-19T00:00:00Z http://hdl.handle.net/1957/38089 Microcomputer-based fluorometric kinetic determination of ascorbic acid and flow injection analysis methods for interference correction and kinetic determinations based on the peak profile Chung, Hyung-Keun Graduation date: 1990 Wed, 13 Dec 1989 00:00:00 GMT http://hdl.handle.net/1957/38089 1989-12-13T00:00:00Z