Daoqian Zhu

Gas chromatographic enantiomer separation on single and mixed cyclodextrin derivative chiral stationary phases

SummaryCapillary gas chromatographic enantiomer separation of some polar compounds, including α-phenylethylamine, styrene oxide, pyrethroid insecticides and other carboxylates, was investigated on modified cyclodextrin (CD) chiral stationary phases. The chiral stationary phases studied included permethylated β-CD (PMBCD), heptakis (2,6-di-O-butyl-3-O-butyryl)-β-CD (DBBBCD), heptakis (2,6-di-O-nonyl-3-O-trifluoroacetyl)-β-CD (DNTBCD), the mixture of PMBCD and DBBBCD, and the mixture of PMBCD and DNTBCD. On the mixed chiral stationary phases containing the mixtures of derivatized cyclodextrins, enantiomer separation was improved significantly for some compounds as compared to the single cyclodextrin derivative chiral stationary phases, and synergistic effects were observed for some compounds on the mixed cyclodextrin derivative chiral stationary phases.

Gas chromatographic enantiomer separation on long-chain alkylated β-cyclodextrin chiral stationary phases

Two new chiral stationary phases, heptakis(2,6-di-O-nonyl-3-O-trifluoroacetyl)-beta-cyclodextrin (DNTBCD) and heptakis(2,6-di-O-dodecyl-3-O-trifluoroacetyl)-beta-cyclodextrin (DDTBCD), were synthesized. The gas chromatographic enantiomeric separation of amines, alcohols, diols, carboxylic acids, amino acids, epoxides, halohydrocarbons and ketones were investigated on DNTBCD, DDTBCD and heptakis(2,w6-di-O-npentyl-3-O-trifluoroacetyl)-beta-cyclodextrin (DPTBCD) stationary phases. It is observed that DNTBCD exhibits the best chiral selectivity among three stationary phases for most racemates investigated. An enthalpy-entropy compensation effect exists within the a-phenylethylamine derivatives separated on DPTBCD and DNTBCD stationary phases

MOLECULE IMPRINTING CHIRAL STATIONARY PHASE

Molecule imprinting polymers (MIPs) with high chiral selectivity for N(alpha)-protected amino acids were synthesized in polar solvent using acrylamide (AM) or combined functional monomer methacrylic acid (MAA) + vinyl pyridine (VP). Factors that influence the chiral selectivity of MIPs and mechanisms of the chiral recognition process were investigated. It was found that the rigid structure and the polar functional group of the print molecule were very important for the chiral selectivity of MIPs. For MIPs made using a combined functional monomer, ionic and hydrophobic interaction may be responsible for the chiral recognition process in aqueous media. The number of binding sites on MIPs and dissociation constants for the complex of enantiomers and MIPs were determined by frontal chromatography analysis.

Investigation of the effect of acylation on the enantiomeric separation of amino acid isopropyl esters by gas chromatography

Abstract The effects of acylation on the enantiomeric separation of isopropyl esters of alanine, valine, allo-isoleucine, isoleucine, leucine and aspartic acid were investigated with a cross-linked polycyanoethyl vinyl siloxane- l -Val- tert -butylamide fused-silica capillary column. The gas chromatographic behaviour of N-formyl, N-acetyl, N-trifluoroacetyl and N-benzoyl derivatives of the amino acid isopropyl esters were studied. The chiral recognition mechanism of the solutes on diamide chiral stationary phases is discussed.

Design and use of a thermal conductivity detector at reduced pressure for temperature-programmed capillary gas chromatography

Abstract Based on a systematic study of the response characteristics of a thermal conductivity detector (TCD) at reduced pressure, a newly designed TCD was constructed and evaluated for use in temperature-programmed capillary gas chromatography. The effective cell volume is as low as 1.3 μl (physical volume 100 μl), and there is no dead corner along the sensing path. Capillary columns of I.D. ⩾0.25 mm can couple directly with the TCD without a make-up gas. The detector response time is less than 0.1 s, the linear dynamic range is about four orders of magnitude and the detection limit is about 10 ppm (v/v) for octadecane. In the split injection mode, a simple configuration of the capillary inlet system offers a constant carrier gas flow-rate through the column with temperature-programmed operation. Practical examples are given that demonstrate the applicability of this detector and the system.

Separation of the enantiomers of 2-phenylcyclopropanecarboxylate esters by capillary gas chromatography on derivatized cyclodextrin stationary phases

SummarySeparation of the enantiomers of 2-phenylcyclopropanecarboxylate esters has been investigated on derivatized cyclodextrin chiral stationary phases (CD CSPs) to enable direct determination of the enantiomeric purity of the products of enantioselective cyclopropanation. Four stereoisomers of these chiral compounds could be resolved to baseline on permethylated β-cyclodextrin CSP. Some unusual phenomena, iso-enthalpy retention behavior and entropically driven chiral separation, were observed for the enantioseparation of 2-phenylcyclo-propanecarboxylates on the CD CSPs. Thermodynamic parameters were evaluated and an enthalpy-entropy compensation effect was observed forn-alkyl esters of 2-phenylcyclopropanecarboxylate separated on CD CSPs.

Response characteristics and operating limits of thermal conductivity detectors at reduced pressure in capillary gas chromatography

Abstract The response characteristics of thermal conductivity detectors (TCDs) at reduced pressure, especially in the pressure range 0.2–25 Torr, was studied systematically. It was found that there existed an operational pressure limit (Pmin) below which the gas flow in the detector cell was no longer laminar and the detector response became pressure dependent. In addition, the noise level increased exponentially as the pressure decreased below Pmin. When the detector was operated at pressures above Pmin, both the responses and relative responses of the TCD were constant for a given concentration of a sample. Pmin as a function of temperature, cell structure and carrier gas properties was interpreted theoretically and verified experimentally. Using this technique, coupling of capillary columns directly with a conventional TCD without make-up gas was realized while maintaining the efficiency of the capillary columns and the concentration sensitivity of the detector.