Henri Snijders

Optimization of temperature-programmed gas chromatographic separations I. Prediction of retention times and peak widths from retention indices

A numerical method is described to predict retention times and peak widths of a mixture containing components with known identities in capillary gas chromatography. The procedure is based on extracting thermodynamic values (enthalpy and entropy terms) from Kovats retention indices. Next, a numerical procedure is developed that uses these data to calculate retention times and peak widths on any capillary column containing the same stationary phase but with a different phase ratio. The estimations are based on a sound theoretical basis. The predictions can be performed either in the isothermal or temperature-programmed (single- or multi-ramp) mode. In the temperature programs, which cover a broad temperature range, isothermal plateaus are allowed. Errors in the predictions of retention times are generally less than 4%. Prediction of peak widths under the same conditions can be performed with errors of about 10%. An attractive feature of the approach is, that once the thermodynamic values of the solutes of interest are known, future optimizations can be performed without the need to perform experimental input runs. This indicates that the concept can be used for complete off-line simulations and/or optimizations of gas chromatographic separations.

Determination of phenylurea herbicides in water samples using on-line sorptive preconcentration and high-performance liquid chromatography with UV or electrospray mass spectrometric detection

A recently developed method for the extraction of organic micropollutants from aqueous samples based on sorptive enrichment in columns packed with 100% polydimethylsiloxane (PDMS) particles was coupled on-line with HPLC analysis. The sorptive enrichment procedure originally developed for relatively nonpolar analytes was used to preconcentrate polar phenylurea herbicides from aqueous samples. PDMS extraction columns of 5, 10 and 25 cm were used to extract the herbicides from distilled, tap and river water samples. A model that allows prediction of retention and breakthrough volumes is presented. Despite the essentially apolar nature of the PDMS material, it is possible to concentrate sample volumes up to 10 ml on PDMS cartridges without losses of the most polar analyte under investigation, fenuron. For less polar analytes significantly larger sample volumes can be applied. Since standard UV detection does not provide adequate selectivity for river water samples, an electrospray (ES)-MS instrument was used to determine phenylurea herbicides in a water sample from the river Dommel. Methoxuron was present at a level of 80 ng/l. The detection limit of the current set-up, using 10 ml water samples and ES-MS detection is 10 ng/l in river water samples. Strategies for further improvement of the detection limits are identified.

Optimization of temperature-programmed gas chromatographic separations II. Off-line Simplex optimization and column selection

In this work a method is described which allows off-line optimization of temperature-programmed GC separations. The method is based on a new numerical method that allows the off-line prediction of retention times and peak widths of a mixture containing components with known identities in capillary GC. In the present work we apply this method for off-line optimization of single- and multi-ramp temperature-programmed GC separations. First, it will be shown how the numerical methods are incorporated in a Simplex optimization method. Next, it is described how the method can be used to determine the optimal temperature program for the separation of a mixture containing components of different functionalities. Finally, it is shown that the optimization strategy followed here allows selection of the capillary column most suited for the separation problem under study from a given set of capillary columns containing the same stationary phase and varying inner diameters and film thicknesses. The process can be performed without any experimental effort. The results indicate that fully off-line simulation and optimization of single- and multi-ramp temperature-programmed GC separations as well as column selection is possible.

Automated on-line ionic detergent removal from minute protein/peptide samples prior to liquid chromatography-electrospray mass spectrometry

An automated on-line ionic detergent removal pre-column system coupled to capillary liquid chromatography-electrospray mass spectrometry is described. The system involves two micro precolumns, composed of a specific ionic detergent trapping column and a preconcentration column, respectively, and a packed 300 microns I.D. analytical column. Sample loading to the micro precolumns and regeneration of the detergent trapping column were performed at a flow-rate of 50 microliters/min, while the flow-rate through the analytical column was set at 5.0 microliters/min. Ionic detergent-containing tryptic-digested protein samples were directly applied to the micro precolumns without sample pretreatment and were analysed by UV absorption detection and electrospray mass spectrometry. The presented system allows for the fully automated removal of SDS with virtually no loss in protein/peptides. Maximum SDS load and breakthrough have been determined. Excellent protein recovery and complete removal of SDS is found. The chromatographic separation after SDS removal was completely restored and equalled the reference chromatogram. Mass spectral data confirm these findings. Finally, this technique allows for SDS removal from minute protein samples without the need for any sample handling.

The Use of Non-Splitting Injection Techniques for Trace Analysis in Narrow-Bore Capillary Gas Chromatography

The use of hot splitless, cold splitless, and on-column injections for trace analysis in narrow-bore capillary GC is evaluated. Despite the low flow rates for the columns used, the required splitless times for splitless injections can be surprisingly short if liners with a small inside diameter are used. On-column injection can be applied by using an appropriate normal-bore precolumn coupled to the narrow-bore analytical column using a specially designed low dead volume column connector. The effects of the experimental conditions such as sample volume, injection temperature, and initial oven temperature on peak focusing and the discrimination and degradation behavior of the analytes are discussed. The possibilities to obtain sensitive and fast separations are illustrated by various applications.

Design and optimization of a novel type nitrogen-phosphorus detector for capillary gas chromatography

The design and optimization of a novel type alkali flame ionization detector (AFID) for capillary gas chromatography is presented. The design differs in that the alkali salt is continuously introduced, as a solution in water, into the detector by means of a liquid chromatographic syringe pump. This design permits continuous refreshment of the enhancement source and circumvents the need of source replacement, thereby eliminating detector response fatigue over time. Modification of an existing nitrogen-phosphorus detector (NPD) to allow direct introduction of the alkali salt into the detector will be described. Next, important operational parameters such as the location of introduction, type and concentration of the salt and the flow of the various detector gasses are optimized experimentally with regard to detector sensitivity, selectivity, minimum detectability and noise level. Moreover the linearity, reproducibility and stability of the detector are evaluated. It will be shown that the new detector design exhibits excellent response characteristics for phosphorus compounds which are comparable to or better than those observed with commercially available AFIDs or NPDs. Finally, the performance of the detector will be demonstrated by two environmental applications.