Steven J. Gluck

Automated pKa determination at low solute concentrations by capillary electrophoresis

Abstract Capillary electrophoresis is investigated for the determination of thermodynamic pKa measurements at low solute concentrations, a current limitation of potentiometric titrations. It is not necessary to accurately know the concentration of a titrant or solute. The method relies on measuring the ionic mobility of the solute as a function of pH. Mobility and pH data are fitted to an equilibrium expression with a non-linear regression. The detection limit for benzoic acid is 2 μM. Equations are introduced to remove the need to measure buffer pH outside of the capillary and to handle potential discontinuities in solute mobility between different buffer solutions caused by changes in the shape of the solute molecule and the buffer viscosity.

Capillary zone electrophoresis for the determination of dissociation constants

Abstract Automated capillary electrophoresis is an effective method for the determination of pKa values. Advantages include having high sensitivity for poorly soluble solutes, not having to know the concentration of the solutes, and the simplicity of the method. The procedure, further investigated in this work, yielded determined pKa values to within 0.07 units of literature values from the IUPAC database for 18 solutes having pKa values of less than 9. The range evaluated was 2.43 to 9.99. Bases with pKa values above 9 had significant differences with the literature values. Wall adsorption and concentration effects were not potential contributors to these differences. Spectroscopic pKa determinations for three solutes showing large differences with the literature values agree with the capillary electrophoresis determinations.

Determination of acidity constants of monoprotic and diprotic acids by capillary electrophoresis

Abstract Capillary electrophoresis as a tool to determine thermodynamic p K a values of monoprotic and diprotic acids has been extended to the p K a range between 1.5 and 3.4. Previously developed procedures for determinations of p K a values in this range have been difficult to implement using conventional fused-silica capillaries. The improved procedure uses a dynamic coating of a positively charged polymer to impart a positive charge to the capillary thus making the electroosmotic flow stable and less dependent on pH than is generally found with uncoated fused-silica capillaries. The dynamic coating reverses the electroosmotic flow allowing the acids to elute before neutral solutes. The procedure was evaluated using five monoprotic and three diprotic acids. For multiprotic solutes, a general model was presented. The p K a values in all cases were easily determined by nonlinear regression with the appropriate equation. It was found that for diprotic cases, where the two p K a values differ by approximately one p K a unit, the determination of the p K a from the migration data is less precise. This is an inherent limitation of this method that is also shared with any other method for determining the p K a values for multi-protic solutes.

Indirect determination of octanol-water partition coefficients by microemulsion electrokinetic chromatography

Abstract Microemulsion electrokinetic chromatography (MEEKC) was assessed and developed as a screening tool for the indirect determination of octanol-water partition coefficients. The capacity factors from MEEKC were correlated to the octanol-water partition coefficients. The same microemulsion (50 m M SDS, 400 m M butanol and 32 m M heptane) was used at pH 1.19 and pH 12 allowing most compounds to be run in their neutral state. This procedure was evaluated using a set of 24 structurally diverse solutes and 4 homologs. It was found that MEEKC can determine a range of over 5 orders of magnitude in the log K ow covering from −1 to over 4. MEEKC provides all the advantages of an HPLC system to estimate lipophilicities including automation, small sample size, short run and analysis times and good reproducibility. However, MEEKC has neither the disadvantages of HPLC including pH limitations, column degradation and homologous series limitations nor the disadvantages of shake flask methods including large sample size, no automation and long turnaround.

Investigation of experimental approaches to the determination of pKa values by capillary electrophoresis

Abstract The calculation of p K a values from capillary electrophoresis data may be accomplished in several ways. Electrophoretic mobilities are fitted to a model which describes the dissociation versus the pH. For a linear model, a linear regression approach yields biased results. Weighted linear regression requires many replicates to determine weighting factors and is thus a time-consuming experiment. For an exponential model, non-linear regression of the electrophoretic mobilities in different, equally pH spaced buffers of the analyte gives the least biased determination. Another experimental approach to this determination is to use a permanently charged solute to correct for potential biases in the expected electrophoretic mobilities obtained between different buffers. In the buffer series chosen, there was no significant bias observed. Buffer pH may be determined by an in situ approach using an internal standard of known p K a . However, the precision obtained is much less than using a pH meter.

Spectrophotometric determination of ionization constants by capillary zone electrophoresis

Abstract One of the historical means of determining the ionization constant of a solute relies on the ionized and neutral states of a molecule having different absorption coefficients at an analytical wavelength. An ionic distribution curve can be constructed by directly determining the ratio of neutral species to ionized species in a series of buffer solutions of differing pH. This method is complementary to a recently developed procedure which utilizes electrophoretic mobilities to determine ionization constants. The same data system used to generate mobilities at different pH values also includes information on peak areas, which correlate with the absorbance of a species. Thus, absorbances regressed against pH yields ionization constants which can corroborate those derived from electrophoretic migration.

Development of a phase system for intermediate polarity compounds in centrifugal partition chromatography

Abstract A new phase system and an approach to developing phase systems relying on composition, physical data and an empirical polarity index have been proposed for general use in centrifugal partition chromatography. The system developed possesses a comparably low viscosity necessary for improved efficiency at a composition where compounds of intermediate polarities have a high solubility in both the upper and lower layers. This system should be useful for traditional isocratic use and in normal-phase ascending gradient separations, because the composition and polarity changes of the lower phase were small relative to the changes in the composition and polarity of the upper phase.

Evaluation of NOx Flue Gas Analyzers for Accuracy and Their Applicability for Low-Concentration Measurements

Abstract The requirements of the Texas State Implementation Plan of the U.S. Clean Air Act for the Houston-Galveston Ozone Nonattainment Area stipulate large reductions in oxides of nitrogen (NOx ) emissions. A large number of sources at Dow Chemical Co. sites within the nonattainment area may require the addition of continuous emission monitoring systems (CEMS) for online analysis of NOx , CO, and O2 . At the outset of this work, it was not known whether the analyzers could accurately measure NOx as low as 2 ppm. Therefore, NOx CEMS analyzers from five different companies were evaluated for their ability to reliably measure NOx in the 2–20 ppm range. Testing was performed with a laboratory apparatus that accurately simulated different mixtures of flue gas and, on a limited basis, simulated a dual-train sampling system on a gas turbine. The results indicate that this method is a reasonable approach for analyzer testing and reveal important technical performance aspects for accurate NOx measurements. Several commercial analyzers, if installed in a CEMS application with sampling conditioning components similar to those used in this study, can meet the U.S. Environmental Protection Agency’s measurement data quality requirements for accuracy.

Concentration and determination of the propylene glycol butyl ether esters of 2,4-dichlorophenoxyacetic acid (PGBE 2,4-D) in air

A method has been developed and validated for the determination of airborne propylene glycol butyl ether ester and the dipropylene glycol butyl ether ester of 2,4-dichlorophenoxyacetic acid. The method is designed to collect samples in the breathing zone of workers for 4 hours at 150 mL/min. using a tube containing 750 mg of Amberlite XAD-2 resin and a portable pump. Recoveries of 85.2% and 81.4% were obtained for the two esters over a range of 0.67 and 13.3 mg/m3 by desorption wih hexane. The extracts were analyzed by electron capture gas chromatography using a highly efficient and inert bonded Carbowax 20-M column packing. The recovery was not affected by 80% relative humidity or refrigerated storage for two weeks. Some loss in recovery was observed for the 0.67 mg/m3 level when samples were stored at room temperature for two weeks. The overall precision of the method for the two esters is +/- 17% and +/- 16% at the 95% confidence level.

Improved methodology for field validation of industrial hygiene monitoring methods

During the development and validation of industrial hygiene monitoring methods in the laboratory, various parameters of collection, storage, and analysis are studied. It is not feasible, however, to study all the environmental variables in the laboratory, so field validation is necessary to detect problems which are specific to the actual work site. A statistical procedure to evaluate the field validation experiment is described together with the methodology necessary to perform these experiments and the equipment to facilitate these experiments. These procedures are demonstrated in an experiment measuring acrylonitrile in the workplace.