Simulation of Elution Profiles for Two-Dimensional Dynamic Gas Chromatographic Experiments

Gas chromatography-Fourier transform infrared spectroscopy reveals dynamic molecular interconversion of oximes

This study reports gas chromatography (GC) combined with Fourier transform infrared (FTIR) spectroscopy to investigate the elution profiles of individual oxime isomers undergoing characteristic interconversion (dynamic chromatography) in GC. The use of a light-pipe FTIR interface enables on-line acquisition of FTIR spectra, which in turn render unambiguous identification of the individual molecules. Here, acetaldehyde oxime and propionaldehyde oxime were chosen for comparison of elution behaviour under varying temperature and carrier flow velocities. The choice of selective responses (wavenumber selectivity), which were relatively stronger for each isomer, enabled display and retracing of the individual isomer over the chromatographic time scale and thus provided characteristic single isomer profiles. Chemometric data analysis using the multivariate curve resolution technique further confirmed this isomer elution profile. Simulation of the spectrum for each isomer allowed comparison with instrument-generated FTIR spectra to confirm the elution order of E and Z isomers. The effect of changing chromatographic parameters (temperature, flow) on interconversion rates and/or extents were studied and the corresponding change in FTIR spectrum intensity was noted. The GC-FID data acquired concurrently with GC-FTIR analyses ratified isomerisation chromatographic profiles.

Gas chromatographic high-throughput screening techniques in catalysis

Discovering highly efficient catalysts is of great scientific and economical interest. Advances in high-throughput assays in combination with sophisticated analytical techniques have increased the rapidity with which catalysts can be identified and optimized. Understanding how kinetics in the mechanism of catalysis is controlled by structural parameters is essential for a directed design of catalysts. To identify such rate-controlling elementary steps and to develop and refine models, comprehensive experimental kinetic data of a broad variety of substrates are necessary. In the present article concepts of high-throughput screening techniques in catalysis using gas chromatography are reviewed in a survey covering the period from 1998 to 2007. To cover also the origins of concepts and groundbreaking experiments in this research area milestones going back to 1950 are also reviewed. The first part of the review will focus on off-line gas chromatographic analysis, the second part on on-line gas chromatographic analysis covering sequential, parallelized and high-throughput multiplexing gas chromatography. The third part presents recent advances in the integration of chemical transformation and analysis in gas chromatography. The present review article describes the state-of-the-art, scope and limitations, and applications of these different high-throughput screening approaches.

Interconverting Conformations of Variants of the Human Amyloidogenic Protein β2-Microglobulin Quantitatively Characterized by Dynamic Capillary Electrophoresis and Computer Simulation

Capillary electrophoretic separation profiles of cleaved variants of beta2-microglobulin (beta2m) reflect the conformational equilibria existing in solutions of these proteins. The characterization of these equilibria is of interest since beta2m is responsible for amyloid formation in dialysis-related amyloidosis and thus is able to attain alternative conformations that lead to irreversible aggregation and precipitation. In this study, we quantitate the increased conformational instability of cleaved beta2m by extracting rate constants and activation energies by simulating the experimental data using a unified theory for dynamic chromatography and dynamic electrophoresis. The results are correlated with the outcome of independent experiments based on mass spectrometric measurement of H/D exchange. This study illustrates that dynamic capillary electrophoresis is suitable for the investigation of the interconversion of protein conformations of amyloidogenic molecules and is not only restricted to ideal model compounds.

Evaluation and Prediction of Stereoisomerizations in Comprehensive Two-Dimensional Chromatography

Conformational and configurational changes such as isomerizations, epimerizations, diastereomerizations and, enantiomerizations are important for the investigation of a large variety of processes ranging from protein folding to the stereostability of drugs. Under optimized conditions, these processes lead to an elution profile characterized by a plateau formation between the two interconverting species in chromatographic separations in a certain temperature range. By temperature-dependent measurements and subsequent computer simulation of the experimental chromatograms, the forward and backward rate constants k1 and k(-1), the Gibb's energy DeltaG++, activation enthalpy DeltaH++, and entropy DeltaS++ can be obtained. Due to its high efficiency two-dimensional chromatography is able to resolve the time-dependent distribution of the two species in the second dimension, thereby yielding the precise ratio of stereoisomers. An algorithm for the simulation and evaluation of two-dimensional chromatographic experiments has been developed, based on the theoretical plate model, which allows the determination of rate constants and barriers of isomerization, epimerization, and enantiomerization processes from two-dimensional chromatographic experiments. In the present article a detailed description of the extended theoretical plate model required for the simulation, the methods available, and examples for the evaluation of complex experimental data and the prediction of the separation conditions to observe isomerization, epimerizations, and enantiomerizations in two-dimensional chromatography are given.

Molecular interconversion behaviour in comprehensive two-dimensional gas chromatography

Comprehensive two-dimensional gas chromatography (GC x GC) is shown to provide information on dynamic molecular behaviour (interconversion), with the interconversion process occurring on both columns in the coupled-column experiment. The experiment requires suitable adjustment of both experimental conditions and relative dimensions of each of the columns. In this case, a longer column than normally employed in GC x GC allows sufficient retention duration on the second column, which permits the typical plateau-shape recognised for the interconversion process to be observed. The extent of interconversion depends on prevailing temperature, retention time, and the phase type. Polyethylene glycol-based phases were found to result in high interconversion kinetics, although terephthalic acid-terminated polyethylene glycol had a lesser extent of interconversion. Much less interconversion was seen for phenyl-methylpolysiloxane and cyclodextrin phases. This suggests that for the oximes, interconversion largely occurs in the stationary phase. Examples of different extents of interconversion in both dimensions are shown, including peak coalescence on the first column with little interconversion on the second column.