No abstract

Inversion of the elution order of enantiomers caused by enthalpy-entropy compensation at the isoenantioselective temperature (Tiso) was experimentally observed by gas chromatography on the diamide-type chiral stationary phase (CSP), Chirasil-L-Val-C11, with N-ethoxycarbonyl propylamide (ECPA) derivatives of a number of alpha-amino acids. For the first time, a clear visual representation of the increase of the apparent enantioseparation factor alpha app from 1.00 to 1.08 as the temperature is raised from 120 to 170 degrees C is presented. The increase of alpha app is accompanied by a concomitant reduction of the retention factors of the enantiomers. The Tiso values were in the range from 110 to 130 degrees C depending on the nature of the alpha-amino acid. On the contrary, the Tiso values of the N(O)-trifluoroacetyl ethyl ester derivatives (TFA-Et) of the same alpha-amino acids were approximately 80 degrees higher than that of ECPA derivatives. The comprehensive thermodynamic investigation of the enantioseparation of ECPA and TFA-Et derivatives of valine and alanine using the retention increment method showed that the Delta L,D(DeltaH) difference between the diastereomeric selector-selectand associates was almost the same for ECPA and TFA-Et derivatives despite a much stronger bonded selector-selectand association taking place for the ECPA derivatives. On the other hand, the Delta L,D(DeltaS) values were found to be more negative in the case of ECPA derivatives, resulting in the unusually low values of Tiso. A temperature-dependent inversion of the elution order of enantiomers was also observed on the cyclodextrin-type CSP, Chirasil-Dex, with TFA-Et derivatives of several alpha-amino acids. The Tiso values were in the range from 20 to 170 degrees C depending on the nature of the alpha-amino acid. The results obtained demonstrate the necessity to conduct temperature-dependent studies in order to optimize the enantiomeric separation of single racemates isothermally or of mixtures of racemates in temperature-programmed runs using enantioselective GC. It is also shown that consideration of the elution order of enantiomers and the value of the apparent enantioseparation factor alpha app alone, without performing temperature-dependent measurements, can easily lead to wrong conclusions regarding the enantiorecognition mechanism.

The chromatographic enantioseparation of small unfunctionalized chiral alkanes C*HR(1)R(2)R(3) (R = alkyl) represents a challenge in separation science. Because of the lack of any functional groups, enantiorecognition in the presence of a chiral selector is solely based upon weak enantioselective Van der Waals forces. Racemic alkanes containing seven and eight carbon atoms, i.e. 3-methylhexane (C7), 2,3-dimethylpentane (C7), 3-methylheptane (C8), 3,4-dimethylhexane (C8), 2,4-dimethylhexane (C8), 2,3-dimethylhexane (C8), and 2,2,3-trimethylpentane (C8) have been gas chromatographically enantioseparated on different modified cyclodextrins. The substitution pattern and cavity size of the cyclodextrin selectors have a pronounced effect on the degree of enantiorecognition observed. Thermodynamic parameters of enantiorecognition between four chiral alkanes and octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin (Lipodex G) have been determined. The possible role of molecular inclusion is indicated by the complete loss of enantioselectivity when the cyclodextrins are replaced by the corresponding linear dextrins ("acyclodextrins"). The enantioseparations of all seven chiral C7-C8 alkanes, six of them simultaneously, has been achieved on mixed binary selector systems whereby two different modified cyclodextrins are present in one gas chromatographic column. The smallest chiral (nonisotopically labeled) allene, i.e., 2,3-pentadiene, has been resolved gas chromatographically on a cyclodextrin selector.

The identification of volatile cis/trans-stereoisomers was accomplished by employing a hyphenated GC-NMR system. The chromatographic and spectroscopic conditions were optimized with respect to the (1)H NMR detection. A special processing technique was developed to handle the recorded NMR spectra in the gas phase with very low sample amounts. The processed stopped-flow (1)H NMR spectra of the investigated chromatographic peaks unequivocally revealed the structure of the corresponding compounds.

Whereas the hyphenation of gas chromatography (GC) with mass spectrometry is of great importance, little is known about the coupling to nuclear magnetic resonance spectroscopy (NMR). The investigation of this technique is an attractive proposition because of the valuable information given by NMR on molecular structure. The experiments shown here are to our knowledge the first hyphenating capillary GC to microcoil NMR. In contrast to liquids, gases have rarely been investigated by NMR, mainly due to the experimental difficulties in handling gases and the low signal-to-noise-ratio (SNR) of the NMR signal obtained at atmospheric pressure. With advances in NMR sensitivity (higher magnetic fields and solenoidal microprobes), this limitation can be largely overcome. In this paper, we describe the use of a custom-built solenoidal NMR microprobe with an active volume of 2 microL for the NMR detection of several compounds at 400 MHz, first in a mixture, and then with full coupling to capillary GC to identify them separately. The injected amounts of each analyte in the hyphenated experiments are in the range of 15-50 micromol, resulting in reasonable SNR for sample masses of 1-2 microg.