Wojciech Prus

Adsorption/partition model of liquid chromatography for chemically bonded stationary phases of the aliphatic cyano, reversed-phase C8 and reversed-phase C18 types.

A novel approach was introduced to modeling solute retention in the liquid chromatography systems, employing silica-based aliphatic chemically bonded stationary phases of the cyano, reversed-phase C8 and reversed-phase C18 types, and the mixed binary eluents most frequently used in the reversed-phase and normal-phase chromatography modes (i.e. using the methanol-water and the 2-propanol-n-hexane liquid mixtures, respectively). This approach takes notice of the mixed (adsorption/partition) mechanism of solute retention, in which both, the adsorptive and the dispersive forces contribute to the overall energetics of this process. Performance of our new model was compared with that of the widely recognized and on a routine basis applied Schoenmakers approach, and it was found out that both models perform with a practically equal and outstanding accuracy.

Monolithic molecularly imprinted polymeric capillary columns for isolation of aflatoxins.

Monolithic molecularly imprinted polymers extraction columns have been prepared in fused-silica capillaries by UV or thermal polymerization in a two-step process. First, a poly-(trimethylolpropane trimethacrylate) (polyTRIM) core monolith was synthesized either by UV or thermal polymerization. Then it was grafted with the mixture of methacrylic acid (MAA) as a functional monomer, ethylene dimethacrylate (EDMA) as a cross-linking agent, 5,7-dimethoxycoumarin (DMC) as an aflatoxin-mimicking template, toluene as a porogen solvent and 2,2-azobis-(2-methylpropionitrile) (AIBN) as an initiator of the polymerization reaction. Different thermal condition of the photografting and different concentrations of the grafting mixture were tested during polymerization. The extraction capillary columns were evaluated in the terms of their hydrodynamic and chromatographic properties. Retention coefficients for aflatoxin B1 and DMC were used for assessment of the selectivity and imprinting factor. The obtained results indicate that the temperature of photografting and concentration of the grafting mixture are key parameters that determine the quality of the prepared MIPs. From the MIP columns characterized by the highest permeability the column of the highest imprinting factor was applied for isolation of aflatoxins B1, B2, G1 and G2 from the model aqueous sample followed by on-line chromatographic separation. The process was performed using a micro-MISPE-microLC-LIF system of a novel design, which allowed for detection of the eluates from the sample preparation part as well as from the chromatographic separation.

Geochemical markers of soil anthropogenic contaminants in polar scientific stations nearby (Antarctica, King George Island).

The organic contamination of Antarctic soils and terrestrial sediments from nearby of five polar scientific stations on King George Island (Antarctica) was investigated. Gas chromatography-mass spectrometry (GC-MS) was applied to find composition of dichloromethane extracts of soil and terrestrial sediments. The presence of geochemical markers, such as n-alkanes, steranes, pentacyclic triterpenoids, and alkyl PAHs, their distribution types, and values of their ratios indicates the predominating source of organic fossil fuels and products of their refining rather than from the natural Antarctic environment. Fossil fuel-originated compounds well survived in conditions of Antarctic climate over long times thus enabling to characterize geochemical features of source fossil fuel identified as petroleum expelled from kerogen II of algal/bacterial origins deposited in sub-oxic conditions and being in the middle of catagenesis. Both microbial activity and water leaching play an important role in degradation of terrestrial oil spills in the Antarctica climate, and petroleum alteration occurs lowly over long periods of time. Synthetic anthropogenic compounds found in terrestrial Antarctica sediments included diisopropylnaphthalenes, products of their sulfonates degradation in paper combustion, and organophosporus compounds used as retardants and plasticizers.

THE ROLE OF LATERAL ANALYTE–ANALYTE INTERACTIONS IN THE PROCESS OF TLC BAND FORMATION. II. DICARBOXYLIC ACIDS AS THE TEST ANALYTES

ABSTRACT A second attempt has been made to investigate intermolecular hydrogen-bonding interactions among adsorbed analyte molecules (so-called lateral interactions), and their impact on the retention process. In this work, three compounds from a homologous series of dicarboxylic acids were selected as the test analytes. To this effect, a novel model was investigated. The quantitative validity of this novel model was checked experimentally with three dicarboxylic acids (succinic, adipic, and suberic) as test solutes. TLC was performed with cellulose powder and 1,4-dioxane, respectively, as stationary and mobile phases. The results obtained, fully confirmed the practical utility of our approach. The traditional definition of the R F coefficient was re-examined and its irrelevance to analytes participating in lateral interactions was demonstrated, as was the use of densitograms (rather than flat overall pictures of TLC chromatograms).

Molecular modelling of a template substitute and monomers used in molecular imprinting for aflatoxin B1 micro-HPLC analysis

The preparation of molecularly imprinted polymers (MIPs) involves the polymerisation of functional monomers in the presence of template molecules. 5,7-Dimethoxycoumarin (DMC) was found to be a structural analogue for aflatoxin B1 (AB1) and serves as its substitute in a grafting solution for the MIP synthesis. It was found that both methacrylic acid and allylamine are functional monomers which could provide a similar binding towards AB1 and DMC.

Modelling of solute retention in the reversed-phase high-performance liquid chromatography system with the chemically bonded 3-cyanopropyl stationary phase

We investigated the applicability of three models of solute retention (devised by Snyder et al., Schoenmakers et al., and Kowalska) for reversed-phase high-performance liquid chromatography (RP-HPLC) for description of the chromatographic behaviour of 25 test solutes in the RP-HPLC systems with the 3-cyanopropyl stationary phase and the methanol+water eluent. The employed test solutes considerably differed with respect to molecular structure, as they belonged to chemical classes of (i) benzene and alkylbenzenes, (ii) hydroxyaromatics, and (iii) crown ethers. In the case of the chemically bonded stationary phases, the 3-cyanopropyl phase included, one encounters the mixed mechanism of solute retention with the contributory effects of partition and adsorption. From our investigations, it comes out that the retention model proposed by Schoenmakers et al. (i.e., model II) is the best suited for description of the chromatographic behaviour of solutes on such packings, while the remaining two models (i.e., models I and III) perform less accurately. An additional investigation was performed with aid of the relationship between the retention parameters (i.e., log k) of the employed test solutes and the logarithms of their partition coefficients (log P) in the octanol/water system (as calculated from the hydrophobic fragmental constants after Rekker), depending on the quantitative composition of the binary eluent. The greater is the deviation of the log k vs. log P relationship from linearity (as monitored by the decreasing values of the respective linear correlation coefficients, r), the less pronounced becomes the contribution of partition and hence the importance of adsorption for the overall retention increases. Thus, the log k vs. log P study was performed in order to follow the partition↔adsorption balance with the changing volume proportions of methanol in the mixed eluent, and ultimately to link an extent of contamination of the nominal reversed-phase partition mechanism by adsorption with the predictive power of models I–III. Finally, an effort was undertaken aimed at demonstration of a considerably enhanced performance of model III, when applied to the experimental conditions that corresponds better with the theoretical assumptions of this approach.

Thermal aromatization of selected TLC-type aliphatic chemically bonded stationary phases, as monitored by use of HPLC-DAD

In our earlier papers we provided indirect experimental evidence strongly in support of our long-established hypothesis that aliphatic ligands on chemically bonded TLC-type stationary phases (i.e. RP-18, RP-8, CN, and Diol) are partially aromatized relatively easily at considerably elevated temperatures (i.e. much higher than 100°C). The first indication of such a possibility was apparent from Raman spectra obtained from these phases by use of a high-power laser-light source emitting in the NIR range. Irradiation of samples of the adsorbents with the NIR light caused their evident heating and the consequent appearance of a predominant Raman band in the so-called aromatic region of the spectra. Later this unexpected, and in some ways disturbing, phenomenon was utilized positively in a new method for Raman spectroscopic evaluation of the density of coverage of the silica matrix with octadecyl ligands. In this paper we report results from an experiment designed to provide more convincing and direct evidence in support of our hypothesis about this partial aromatization. To collect such evidence we had to re-design the procedure used to heat the samples of stationary phase, and to find the most efficient means of extracting and separating the aromatization products. It became obvious that to obtain readily measurable quantities of such products the samples should be heated in a closed system (e.g. in a sealed glass ampoule or in the stainless steel tube which forms part of the apparatus for accelerated solvent extraction, ASE) which prevents the compounds from escaping from the reaction system at high temperatures. We then used two advanced methods of solvent extraction - ultrasonic extraction and ASE - to separate the expected aromatization products from the solid matrix of the adsorbents investigated. Finally, analysis of the extracts obtained was performed by HPLC with diode-array detection. Our results furnish convincing evidence of an efficient aromatization process occurring equally with TLC-type octadecyl and octyl ligands heated at 170°C.

OPTIMIZATION OF THE MOBILE PHASE COMPOSITION IN NP-TLC WITH THE CHEMICALLY BONDED 3-CYANOPROPYL STATIONARY PHASE

We investigated the dependence between retention of five different test solutes and the mobile phase composition in thin-layer chromatographic systems with the chemically bonded 3-cyanopropyl stationary phase and binary 2-propanol - n-hexane eluents. Then, we calculated regression parameters for the selected three models of solute retention (applicable to the adsorption liquid chromatography mode), first upon all 36 experimental data points and, then, upon a restricted number of four experimental data points. Finally, the relevant conclusions were drawn regarding efficiency of the retention models considered in predicting solute retention (and consequently, in optimization of separation selectivity) based upon the results of an absolutely minimal introductory experiment.

GC-MS study of the products cleaved from RP-18 chemically bonded phases for TLC during thermal modification

The growing popularity of liquid chromatographic techniques in analysis has recently led to attempts to standardize the adsorbents used, currently an important theoretical and practical issue. To a large extent this tendency results from the desire to obtain the most reliable separation results and to establish repeatable analytical procedures. Practical standardization of chromatographic adsorbents (especially for reversed-phase liquid chromatography) has so far seemed very difficult, however, or even barely possible. The main difficulties are the complex nature of commercial phases (and particularly mixed inorganic–organic chemically bonded adsorbents) and the synthetic procedures used, which depend on a variety of different physical and physicochemical conditions. In these circumstances a search for novel methods of the assessment and characterization of the adsorbents used in liquid chromatography (TLC included) seems fully justified. This has led to attempts to expand the arsenal of tools already suitable for gathering information about the structural features of adsorbents that determine the mechanism of retention.

Formation of Aromatics in Thermally Induced Reactions of Chemically Bonded RP-C18 Stationary Phase

In continuation of the research on the thermally induced chemical transformation of the silica-based chemically bonded stationary phases (C18), the oxidative cleavage of the silicon-carbon bonds with hydrogen peroxide and potassium fluoride was utilized, followed by the gas chromatography coupled with mass spectrometry (GC-MS) study of the resulting products. These investigations allowed determination of the probable structures of certain thermal modification products as the various different alkyl derivatives of the phenylsilane ligands. Apart from aromatic compounds, the products with unsaturated bonds and carbonyl functionalities were found in the analyzed extracts. The analysis of the GC-MS chromatograms reveals that under the applied working conditions, the investigated process runs with relatively low yields.