Ba Benno Ingelse

Determination of polar organophosphorus pesticides in aqueous samples by direct injection using liquid chromatography-tandem mass spectrometry.

It was demonstrated that four out of six of the very polar organophosphorus pesticides (OPs), i.e. acephate, methamidophos, monocrotophos, omethoate, oxydemeton-methyl and vamidothion, could not be extracted from water using commonly available SPE cartridges. In addition, GC analysis on all six compounds was found to be troublesome due to their polar and thermolabile character. This initiated the development of an alternative highly sensitive and selective method for the determination of the above mentioned very polar OPs in water, based on LC-MS. Large volume (1 ml) water samples were directly injected onto an RP18 HPLC column with a polar endcapping. The latter was essential for obtaining retention and maintaining column performance under 100% aqueous conditions during the sampling. The compounds were ionized using atmospheric pressure chemical ionization and detected on a tandem mass spectrometer operated in multiple reaction-monitoring mode. The detection limits were in the range of 0.01-0.03 microg/l. Compared to conventional GC methods, the developed LC-MS procedure is very straightforward, fast and more reliable. This application demonstrates the applicability of LC-MS for analysis of polar OPs in surface, ground and drinking water, as a more favourable alternative to GC.

HPLC-FID with superheated water as the eluent: improved methods and instrumentation

In the current study, an HPLC-FID system using superheated water as the eluent is constructed and further improved. Signal stability during temperature programmed operation was improved by using separate thermostatting of the 50 μm capillary restrictor. Operating the restrictor at 75°C prevented the superheated water from sputtering in the restrictor. Conventional octadecyl-modified silica, thermostable polymeric reversed phase and carbon type HPLC columns were used. The feasibility of the system is demonstrated by the analysis of a variety of compounds such as alcohols and aldehydes. It is shown that increased temperatures decrease elution times and, in addition to this, frequently improve peak shapes and column efficiencies. The separation of lower aldehydes is performed at 175°C with a detection limit in the low μg/ml range. The possibility of temperature programming is demonstrated for the separation of a range of alcohols. Furthermore, it is shown that temperature programming can be used for sample enrichment. Large volume injection with low temperature enrichment resulted in good peak shapes and a significant improvement of the detection limit. In this way, detection limits of 0.2 μg/ml (ppm) could be obtained for the aldehydes studied. Finally, the possibilities of using mobile phase additives as, e.g., buffers were briefly examined.

Enantiomeric separation by capillary electrophoresis using a soluble neutral β-cyclodextrin polymer

Abstract Enantiomers of several racemic basic compounds of pharmaceutical interest and three tryptophan derivatives were investigated by capillary electrophoresis employing a soluble β-cyclodextrin polymer and native β-cyclodextrin. The effects of the concentration of the polymer added to the background electrolyte and of the pH of the buffer on the effective mobility and resolution of the studied compounds were examined. The β-cyclodextrin polymer showed a higher stereoselectivity than the parent β-cyclodextrin. Enantionselectivity and resolution were influenced by the concentration of the β-cyclodextrin polymer and the background electrolyte. A pH study, carried out in the pH range 2.7–6, showed that an increase in pH caused a general decrease in both resolution and selectivity. The best results for the enantiomeric separation of the basic compounds studied were obtained at pH 2.7.

Capillary electrophoretic enantioseparation of selegiline, methamphetamine and ephedrine using a neutral β-cyclodextrin epichlorhydrin polymer

This paper describes the development of a capillary zone electrophoretic method for chiral separation of three basic compounds of the selegiline synthetic pathway: ephedrine, methamphetamine and selegiline. The method developed allows one to separate the studied compounds in one run using a neutral beta-cyclodextrin epichlorhydrin polymer. The effect of various experimental parameters, such as chiral selector concentration, concentration and composition of background electrolyte, pH, temperature, and the addition of some organic solvents, on the resolution and migration time is discussed. For selegiline and methamphetamine, it is possible, under optimal conditions, to quantify less than 0.5% of the minor isomer in an excess of the major one.

Thermodynamics of chiral selectivity in capillary electrophoresis : separation of ibuprofen enantiomers with β-cyclodextrin

The effect of temperature on the electrophoretic chiral separation of ibuprofen with β-CD was investigated. Background electrolytes with sodium acetate or formate were chosen because of their constant pK within 0.03 units in the temperature range 25–50°C. Ibuprofen has a temperature independent pK value of 4.36, and a mobility of 23.3·10−9 m2/Vs at 25°C. The mobility has a temperature coefficient of 2.0%/°C. At that same temperature, formation constants K1 for the uncharged enantiomers are 9955 and 10294 M−1 respectively. The formation constant K2 for the charged form is 5256 M−1 for both isomers. For these chiral formation constants, ΔH values are around −50 kJ/mol, whereas ΔS values are around −90 J/mol/K.

Ergot alkaloids as chiral selectors in capillary electrophoresis determination of the separation mechanism

Recently ergot alkaloids were introduced as novel chiral selectors in capillary electrophoresis. In the present study, stereoselectivities of several ergot alkaloids, added to the background electrolyte (BGE), towards some racemic hydroxy organic acids are compared. The 1-allyl derivative of (5R,8S,10R)-terguride (allyl-TER) proved to be the best chiral selector for these analytes. Only the capillary was filled with BGE containing the chiral selector. The in- and outlet vial did not contain any ergot alkaloid. The effects of pH, and MeOH added to the BGE were investigated. Low pH proved to have an adverse effect on enantioseparation. Good separation for the enantiomers of some α-hydroxy acids was obtained at pH 4.2, and 25 mM allyl-TER. The addition of 50% MeOH to the BGE altered stereoselectivity and increased the solubility of the chiral selector. Using a BGE containing 50% MeOH, and 62.5 mM allyl-TERat pH∗ 5.5, the optical isomers of all test compounds, including tropic acid and other organic acids, were baseline resolved.

Capillary electrophoretic separation of herbicidal enantiomers applying ergot alkaloids

The capillary electrophoretic separation of some herbicidal enantiomers is shown applying 1-allylterguride as chiral selector. Baseline separation is shown for the enantiomers of fluazifop, halossifop and fenoxaprop, whereas the optical isomers of flamprop could be partially resolved. Separation times are short compared to similar analyses, applying HPLC and a terguride chiral stationary phase. The degree of dissociation of the acidic analytes, as well as the amount of methanol present in the background electrolyte, are shown to have a major influence on enantioresolution, as expected form earlier studies.

Training software for chiral separations in capillary electrophoresis

A previously published steady-state simulation program for CE was extended with a sub-menu for chiral interaction. The interaction was modelled with a hypothetical (neutral) selector with properties similar to cyclodextrins. A three-type chiral interaction model was implemented in such a way that it was valid for both anionic and cationic analytes. User-defined chiral variables in the model were: the concentration of this chiral selector, the complex formation constants (K) of both non-ionic and ionic analyte species, the relative K differences between the two optical isomers (also for both non-ionic and ionic species) and the mobility of the analyte–selector complex, relative to that of the fully dissociated free analyte. In addition the following non-chiral variables are added: the dependence of the bulk viscosity on the chiral selector concentration and the dependence of the mobilities on the viscosity. Chiral components in the data base are simulated as mixtures of optical isomers with variable enantiomeric concentration ratios. These chiral parameters, together with the original features of the program, provide a very flexible training tool for chiral separations in capillary electrophoresis. For example, determination of the optimum chiral selector concentration can be done in seconds, using the simulation software. Although the software is intended mainly for training purposes, in cases where sufficient details of existing chiral selector–analyte combinations are available, the program is also suitable for method development of chiral separations in CE. The performance of the program is illustrated with simulations, using chiral parameters obtained from literature (ibuprofen, fenoprofen, homatropine) and from our own experiments (m-fenfluramine). Simulation results were compared with experimental electropherograms.

Influence of the nature of the buffer on chiral separation in capillary electrophoresis

The capillary electrophoretic separation of the enantiomers of a number of anionic sulfonamides was studied. Enantioselectivity of a range of native and modified cyclodextrins was tested. Also, from the experimental results, equilibrium constants and complex mobilities in different electrolyte systems were determined. In the present study it was found that the nature of the co-migrating buffer anion may significantly influence the magnitude of equilibrium constants, depending on the type of modification of a specific cyclodextrin. Consequently, this may also strongly influence the optimum cyclodextrin concentration for a particular separation. The results of the separation of sulfonamide enantiomers with cyclodextrins do not agree with the theoretical model suggested by Wren and Rowe, concerning the existence of a maximum in the mobility difference between two optical isomers.

Reversed determination of the formation constants of 1-allyl terguride with mandelic acid optical isomers using capillary electrophoresis

A method is presented for the accurate determination of equilibrium constants of complex formation between mandelic acid enantiomers and 1-allyl terguride. In earlier studies this ergot alkaloid has proven to be a potential chiral selector for racemic acidic compounds. In these studies, the formation constant was determined by measuring the effective mobility of some racemic acidic analytes at varying concentrations of ergot alkaloid. For these experiments, the assumption was made that the complex mobility was zero. In order to validate these data, the experimental set-up was reversed. In this new set-up, the cationic chiral selector is injected as sample, while the background electrolyte (BGE) contained either of the two optically pure mandelic acid enantiomers, in varying concentrations. For accurate determination of the effective mobility, tetrabutylammonium was used as a mobility reference and the ionic strength of the BGE was kept constant. By performing these experiments at two different pH values, it was possible to determine complex-formation constants, and chiral selectivity towards both the dissociated and the non-dissociated mandelic acid enantiomers. Results show that only the dissociated acid interacts selectively with the ergot alkaloid confirming our earlier results. In our earlier experiments we made the assumption that the non-dissociated acid does not interact with the ergot alkaloid. The experimental data obtained by the current method however, show that, although this interaction is not enantioselective, it cannot be neglected. Optically pure mandelic acid proved to be a suitable chiral selector for the separation of terguride enantiomers.