Reinvestigating the Preferential Enrichment of DL-Arginine Fumarate: New Thoughts on the Mechanism of This Far from Equilibrium Crystallization Phenomenon.

Preferential enrichment (PE) is a crystallization process, starting from either a racemic of slightly enantio-enriched solution (ca. +5%) that results in a high enantiomeric excess in the liquid phase (>+90%ee) and a slight opposite excess in the deposited crystals (−2 to −5%ee). The mechanism(s) of this symmetry-breaking phenomenon is (are) still a matter of debate since it eludes rationalization by phase diagram formalism. In this publication, we thoroughly reinvestigate the PE phenomenon of arginine fumarate by using a new approach: the process is monitored by introducing isotopically labeled arginine enantiomers into the crystallization medium to better understand the mass exchanges during crystallization. These experiments are supported by chiral HPLC-MS/MS. This study permits re-evaluating the criteria that were thought mandatory to perform PE. In particular, we show that PE occurs by a continuous exchange between the solution and the crystals and does not require the occurrence of a solvent-mediated solid−solid phase transition.

Development of two-dimensional gas chromatography (GC×GC) coupled with Orbitrap-technology-based mass spectrometry: Interest in the identification of biofuel composition.

Comprehensive gas chromatography (GC) has emerged in recent years as the technique of choice for the analysis of volatile and semivolatile compounds in complex matrices. Coupling it with high-resolution mass spectrometry (MS) makes a powerful tool for identification and quantification of organic compounds. The results obtained in this study showed a significant improvement by using GC×GC-EI-MS in comparison with GC-EI-MS; the separation of chromatogram peaks was highly improved, which facilitated detection and identification. However, the limitation of Orbitrap mass analyzer compared with time-of-flight analyzer is the data acquisition rate; the frequency average was about 25 Hz at a mass resolving power of 15.000, which is barely sufficient for the proper reconstruction of the narrowest chromatographic peaks. On the other hand, the different spectra obtained in this study showed an average mass accuracy of about 1 ppm. Within this average mass accuracy, some reasonable elemental compositions can be proposed and combined with characteristic fragment ions, and the molecules can be identified with precision. At a mass resolving power of 7.500, the scan rate reaches 43 Hz and the GC×GC-MS peaks can be represented by more than 10 data points, which should be sufficient for quantification. The GC×GC-MS was also applied to analyze a cellulose bio-oil sample. Following this, a highly resolved chromatogram was obtained, allowing EI mass spectra containing molecular and fragment ions of many distinct molecules present in the sample to be identified.

Retention modeling and retention time prediction in gas chromatography and flow-modulation comprehensive two-dimensional gas chromatography: The contribution of pressure on solute partition.

This study aims at modelling and predicting solute retention in capillary Gas Chromatography (GC) and Flow Modulation comprehensive GC (FM-GCxGC). A new thermodynamic model, taking into account the effects of temperature and pressure, is proposed to describe the variation of the equilibrium partition constant of a solute during its elution. This retention model was challenged with the classical one, and both were applied to: (i) stationary phase film thickness indirect estimation; (ii) retention time (RT) prediction of a set of 11 model polycyclic aromatic hydrocarbons (PAHs) on the SLB-IL60 and DB-35MS columns, in temperature-programmed mode. Film thickness determination led to values about 2 times lower than those indicated by column nominal dimensions, whatever the employed model. Prediction of retention times in GC led to 0.84 and 0.26% mean errors using the classical and the extended models, respectively. Prediction in GCxGC gave 5.5 and 0.44% mean errors in 1st dimension RTs, and 7.3 and 2.2% mean errors in 2nd dimension RTs, using the classical and the extended models, respectively.

The incorporation of calix[6]arene and cyclodextrin derivatives into sol-gels for the preparation of stationary phases for gas chromatography.

New polyethylene-glycol-based sol-gels containing cyclodextrin or calix[6]arene derivatives have been synthesized. An original method for sol-gel preparation and capillary column coating, which consumes smaller quantities of selectors and allows for control of their amounts in the stationary phase, is reported herein. The new stationary phases exhibited excellent column efficiencies over a large range of temperatures and thermal stability up to 280°C. The cyclodextrin derivative generally showed the best separation factors for aromatic positional isomers. The calix[6]arene derivative exhibited the best selectivity for the polychlorobiphenyl congeners and some polycyclic aromatic hydrocarbon isomers. The relationship between the structure and the chromatographic properties of the selectors is discussed. The tert-butyl groups on the upper rim of the calix[6]arene were found to possibly play an important role in the recognition of solutes. The incorporation of the cyclodextrin derivative into the sol-gel matrix did not affect its enantioselective recognition capabilities.