Methods for studying reaction kinetics in gas chromatography, exemplified by using the 1-chloro-2,2-dimethylaziridine interconversion reaction

Exploring thermal isomerisation in gas chromatography analyses using natural pyrethrins: Comparison of comprehensive two-dimensional and one-dimensional gas chromatography

This study aims to assess and qualitatively compare the visual presentation of chromatographic data from the isomerisation of natural pyrethrins - a group of pesticides derived from Chrysanthemum flowers - using one-dimensional gas chromatography (1DGC) and comprehensive two-dimensional gas chromatography (GC×GC). Molecular structural changes, such as thermal isomerisation in this case, occur during gas chromatography injection and separation, to provide characteristic patterns which may not be routinely recognised on the 1D chromatogram. To demonstrate the influence of analytical method parameters on isomerisation processes, variations in oven temperature (isothermal vs. temperature programmed analysis), inlet mode (split vs. splitless), inlet temperature, and carrier gas flow rate were investigated. Increasing oven temperature was the most significant factor affecting isomerisation. Splitless injection mode and increasing inlet temperature promoted isopyrethrin formation, while the effect of inlet temperature appeared minimal with a split injection technique, most likely due to the short residence time in the inlet. Increased carrier gas flow rates in a temperature programmed analysis reduced retention time and minimised isomerisation. The unique presentation of isopyrethrin peaks on a GC×GC contour plot allows for facile recognition of isomerisation especially at low concentrations, simplifies chromatogram interpretation, and aids in analyte identification. It also confirms that the isomerisation process is irreversible since the pyrethrin I and II compounds are absent throughout the bridge formation. These benefits support the use of GC×GC over 1DGC to study isomerisation. Additionally, due to limited data in the literature, Kováts retention indices and linear retention indices of the natural pyrethrins, including isopyrethrins, were experimentally determined on four columns: DB-5 ms UI, Rxi-17Sil MS, SLB-IL60i, and SLB-IL111i.

Headspace study of chiral interconversion of N-acetyl-homocysteine thiolactones

The rate constants for (L)-N-acetyl homocysteine thiolactone enantiomerization have been obtained from batch-wise studies and by dynamic gas chromatography of racemic mixtures. Results from the batch-wise experiments show that the kinetics of racemization at 150 °C is the same for vials made of glass, silanized glass or Teflon-coated glass so that the vial surface exhibited no effect on the kinetics of racemization. From the temperature dependence of the rate constants the preexponential factor, activation energy, the activation Gibbs energy and activation entropy have been obtained from transition state theory. The catalytic effect of G-DP, G-BP and B-DP GC chiral stationary phases on racemization has been observed and quantified by the values of rate constants; B-DP exhibited the greatest activity. The Eyring activation parameters obtained from batch-wise experiment were compared with theoretical values acquired from quantum chemical modelling. Agreement between the experimental and calculated values of activation Gibbs energy, activation enthalpy and activation entropy is good. The dynamic gas chromatography of racemic mixture on chiral B-DP, G-DP and G-BP capillary columns indicate that the rate constants of forward and reverse reactions are different in chiral environments. The greatest accelerating effect in the process of enantiomerization has been identified for G-BP both in the batch-wise experiments and by the dynamic gas chromatography.

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.

High-Throughput and Direct Sample Screening Using a Laser Spray Ionization Miniature Mass Spectrometer

Over the past two decades, great efforts have been made toward mass spectrometer instrument miniaturization. With increasing analytical performances, miniature mass spectrometers are on the edge of being applied to more application scenarios. Besides sensitivity, mass resolution, and instrument portability, high-throughput and little or no sample preparation are also critical features in practical applications. In this study, we report the development of a miniature mass spectrometry (MS) system equipped with a 2D moving platform and a laser spray ionization (LSI) source. The method to make a patterned sample holder was also introduced and optimized for automatic high-throughput sample analyses. With the LSI source, analytes in complex matrix could be directly mass analyzed; in addition to the 2D moving platform, different samples could be analyzed in a high-throughput fashion. Results show that good linearity of quantitation could be achieved for multiple samples. Tens of nanograms of drugs, peptides, and vitamin B could be identified in diluted whole blood samples, and it takes 10 s on average to scan one sample.

Design of experiments for zeroth and first-order reaction rates

This work presents optimum designs for reaction rates experiments. In these experiments, time at which observations are to be made and temperatures at which reactions are to be run need to be designed. Observations are performed along time under isothermal conditions. Each experiment needs a fixed temperature and so the reaction can be measured at the designed times. For these observations under isothermal conditions over the same reaction a correlation structure has been considered. D-optimum designs are the aim of our work for zeroth and first-order reaction rates. Temperatures for the isothermal experiments and observation times, to obtain the most accurate estimates of the unknown parameters, are provided in these designs. D-optimum designs for a single observation in each isothermal experiment or for several correlated observations have been obtained. Robustness of the optimum designs for ranges of the correlation parameter and comparisons of the information gathered by different designs are also shown.

Stereodynamics of small 1,2-dialkyldiaziridines

Diaziridines are very interesting representatives of organic compounds containing stereogenic nitrogen atoms. In particular, 1,2-dialkyldiaziridines show extraordinarily high stereointegrity. The lone electron pairs of the nitrogen atoms are in trans configuration, avoiding a four-electron repulsive interaction. Furthermore, the trans configuration of the substituents at the nitrogen atoms is energetically favored because of reduced steric interactions. Therefore only two stereoisomers (enantiomers) are observed. At elevated temperatures the enantiomers are interconverting because of the limited stereointegrity of the chirotopic nitrogen atoms. The enantiomerization rate constants and the activation parameters of interconversion are of great interest. Here, we investigated the stereodynamics of a set of small 1,2-dialkyldiaziridines bearing short substituents (Me, Et, iPr, tBu), using enantioselective dynamic gas chromatography (DGC). Separation of enantiomers of all compounds, including the highly volatile 1,2-dimethyldiaziridine, was achieved using heptakis(2,3-di-O-ethyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin in 50% PS086 (w/w) as chiral stationary phase in fused silica capillaries with a length of up to 50 m. Measurements at variable temperatures were performed and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers at room temperature for 1-(tert-butyl)-2-ethyldiaziridine, ΔG(╪)(298K) = 123.8 kJ mol(-1) (ΔH(╪) = 115.5±2.9 kJ mol(-1), ΔS(╪) = -28±1 J mol(-1) K(-1)), and 1-ethyl-2-isopropyldiaziridine, ΔG(╪)(298K) = 124.2 kJ mol(-1) (ΔH(╪) = 113.1±2.4 kJ mol(-1), ΔS(╪) = -37±2 J mol(-1) K(-1)), were determined, representing some of the highest values observed for nitrogen inversion in diaziridines.

Stereodynamics of tetramezine

The antidepressant drug tetramezine [1,2-bis-(3,3-dimethyldiaziridin-1-yl)ethane] consists of two bridged diaziridine moieties with four stereogenic nitrogen centers, which are stereolabile and, therefore, are prone to interconversion. The adjacent substituents at the nitrogen atoms of the diaziridines moieties exist only in an antiperiplanar conformation, which results in a coupled interconversion. Therefore, three stereoisomers exist (meso form and two enantiomeric forms), which epimerize when the diaziridine moieties are regarded as stereogenic units due to the coupled interconversion. Here, we have investigated the epimerization between the meso and enantiomeric forms by dynamic gas chromatography. Temperature-dependent measurements were performed, and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers of the epimerization were found to be ΔG(≠) = 100.7 kJ mol(-1) at 25°C and ΔG(≠) = 104.5 kJ mol(-1) at 37°C, respectively. The activation enthalpy and entropy were determined to be ΔH(≠) = 70.3 ± 0.4 kJ mol(-1) and ΔS(≠) = -102 ± 2 J mol(-1) K(-1) .