Effect of solvents on the selectivity of terbutylazine imprinted polymer sorbents used in solid-phase extraction

A solid-phase extraction sample preparation method using a molecularly imprinted polymer (MIP) selective for the triazine type pesticide terbutylazine has been developed. The method involves preconcentration from large volumes of water samples on a C18 disk coupled to selective clean-up on the MIP. The method has been optimised by studying the recovery and retention of terbutylazine and some other structurally related triazine derivates as a function of the selective washing solvent used. The effect of the water content of the selective washing solvent was also investigated on the recovery of the MIP. River water samples were analysed with the coupled technique, and efficient clean-up of the samples was observed.

Energetic heterogeneity of the surface of a molecularly imprinted polymer studied by high-performance liquid chromatography

The influence of thermal annealing on the surface homogeneity of a polymer imprinted against L-phenylalanine anilide (LPA) was examined using high-performance liquid chromatography (HPLC) for the measurement of the adsorption isotherms. The isotherms obtained for LPA and for its enantiomer, D-phenylalanine anilide (DPA) were fitted to the Freundlich (F) equation which accounts for the energetic heterogeneity of the surface with a separate parameter. Changes in the adsorptive properties of the polymer produced by thermal annealing were deduced by comparing the heterogeneity parameters given by the nonlinear regression. These changes were also illustrated by deriving the isosteric heats of adsorption as functions of the amounts adsorbed and by calculating the associated affinity distributions. This latter technique involves an application of the affinity spectrum (AS) combined with the F adsorption model. The plausibility and accuracy of the combination is discussed. It is shown that the derivation of the amplitudes of the affinity distributions from the F parameters is inaccurate, making difficult the proper estimate of the changes in the total population of adsorption sites. In contrast, the AS method gives correct estimates of the parameter that characterizes the slope of the affinity distributions. The results derived from the three sets of results (F model parameters, isosteric heats of adsorption, AS + F method) show consistently that annealing reduces the energetic heterogeneity of the polymer surface for both LPA and DPA. In practice, however, the improvement of the polymer performance in HPLC is relatively limited.

Determination of phenytoin in plasma by molecularly imprinted solid-phase extraction

A molecularly imprinted polymer (MIP) using phenytoin as template and methacrylamide as the functional monomer was prepared. The selectivity was measured by comparing capacity factors of phenytoin and other structurally related compounds. The polymer was evaluated as a selective sorbent in molecularly imprinted solid-phase extraction (MISPE). Several washing solvents were tested to study their ability to disrupt the non-specific interactions occurring between the sample and the polymer matrix and the role of water in the recognition process was also investigated. It was shown that the key step of successful sample extraction is the right choice of the washing solvent. Plasma samples spiked with phenytoin were analyzed by the MISPE methodology developed in this work. Method validation (intra- and inter-day precision, recovery, specificity) was carried out. The calibration curve showed good linearity in the 2.5-40 microg/ml range corresponding to therapeutically relevant plasma levels. The intra- and inter-day precision values were below the 15% limit established for bioanalytical methods. The results showed that the method could be successfully applied for the determination of phenytoin in plasma samples.

Influence of the pH on the behavior of an imprinted polymeric stationary phase--supporting evidence for a binding site model

The equilibrium isotherms of the two enantiomers of phenylalanine anilide (PA) were measured by conventional frontal analysis at three different pH on a thermally-treated imprinted stationary phase selective for the L enantiomer. The first of these pH (buffer pH=3.0, pH(app)=4.0) is well below the apparent pKa (6.4) of the two solutes, the second (buffer pH=5.8, pH(app)=7.0) slightly below this pKa, and the third (buffer pH=7.0, pH(app)=8.3) well above it. The experimental data were fitted to several isotherm models. The best estimates of the parameters of these models are reported and discussed. The corresponding isotherms are compared with the experimental ones. The contributions of the enantioselective and nonselective interactions could be separated. The results obtained show that the saturation capacity is always smaller for D-PA than for L-PA, the template. The analytical separation is best at pH=3.0 because of a good separation factor (2.82) and short retention times. A good compromise between the resolution and the saturation capacity is obtained at pH=5.8, for which the best preparative separation is found. Both analytical and preparative results are poor at pH=7.0 because the separation factor is low (1.32). At this pH, the isotherm remains nearly linear in the whole concentration range accessible to measurements. The number of nonselective sites increases with increasing mobile phase pH slightly faster than the number of selective sites, suggesting different pKa ranges for the two type of sites. Moreover, the binding energy and the homogeneity of the selective sites decreases with increasing pH. These results agree with a binding site model involving more than one carboxylic acid group, providing charge complementarity and hydrogen bond donors for binding of L-PA.

Evaluation of methods aimed at complete removal of template from molecularly imprinted polymers

Polymers imprinted with clenbuterol were used to study the influence of various post-polymerization treatments [e.g., thermal annealing, microwave assisted extraction (MAE), Soxhlet extraction and supercritical fluid template desorption] on the bleeding of residual template. The aim of the study was to reduce the bleeding to levels that would allow the use of the materials as affinity phases for extraction of clenbuterol from bovine urine at concentrations below 1 ng ml-1. After treatment, the clenbuterol imprinted polymers were packed into solid-phase extraction columns and the bleeding was estimated by quantifying the amount of template released in 10 ml of methanol-acetic acid (9 + 1 v/v). This was followed by an assessment of selectivity and recovery in comparison with non-treated material. The lowest bleeding level was found after MAE using 100% trifluoroacetic acid for 3 x 20 min at 100 degrees C. The collected eluate contained in this case 3 ng ml-1 of clenbuterol. The same material was subsequently used for the extraction of clenbuterol from spiked bovine urine. The resulting selectivity and recovery were lower compared with those obtained using the untreated material. A milder but still efficient method to reduce the bleeding level was found to be MAE with formic acid. In this case a bleeding level of 14 ng ml-1 was found after only a 1 h extraction time. In a second model system, using a polymer imprinted with L-phenylalanine anilide, the bleeding was reduced to a similar level by extensive on-line washing in good swelling solvents containing acid or base additives and after thermal annealing of the polymers in the dry state.

Evaluation of a multidimensional solid-phase extraction platform for highly selective on-line cleanup and high-throughput LC-MS analysis of triazines in river water samples using molecularly imprinted polymers

A novel highly selective sample cleanup procedure based on the use of molecularly imprinted polymers (MIPs) as solid-phase extraction materials has been evaluated with respect to its applicability and routine use in environmental analysis. The method comprises the combination of a restricted access material (RAM) and a MIP allowing a selective sample preparation to be achieved in the online mode. This combination is called the size-selective sample separation and solvent switch (six-SPE). The RAM column combines size exclusion and adsorption chromatography, reducing the concentration of matrix molecules by a cutoff of 15 kDa. The MIP column selectively retains the triazine analytes whereas the residual matrix is not retained and separated completely. Thus, the automated RAM-MIP is capable of excluding all matrix and nontarget compounds. The cleaned and enriched extract is subsequently eluted to an HPLC column and analyzed by LC-MS. A complete on-line analysis cycle including multidimensional solid-phase extraction, separation, and detection takes less than 15 min. Terbuthylazine, atrazine, propazine, simazine, ametryn, prometryn, irgarol, and also the metabolites deethylatrazine and deisopropylatrazine can be determined without any matrix interferences, e.g., by humic acids. The whole setup is fully automated and may be continuously operated. Nonspecific interactions with the polymer are below 1% in all cases. The accuracy of the LC-MIP-LC-MS system was controlled using a certified reference material (Aquacheck). The applicability of the method to the cleanup of real samples was demonstrated by injection of contaminated river water samples. The stability of different polymers was tested by consecutive injections, and it was shown that the performance of the materials did not vary even after more than 300 enrichment and desorption cycles.

Target analogue imprinted polymers with affinity for folic acid and related compounds

Two approaches to synthesize molecularly imprinted polymers with affinity for folic acid and other substituted pteridines have been compared. In the first approach, the folic acid analogue methotrexate was used as template and functional monomers capable of generating selective binding sites were searched in a miniaturized screening system based on binding assessment in the batch mode. Highest selectivity was seen using 2-vinylpyridine as functional monomer, which was confirmed in the chromatographic mode for a batch synthesized on a gram scale. However, the retentivity and selectivity of this phase were insufficient for anticipated applications. In a second approach, using methacrylic acid as the functional monomer, organic soluble inhibitors for the enzyme dihydrofolate reductase were used to develop sites complementary toward the pteridine substructure. This resulted in materials showing enhanced selectivity for substituted pteridines when evaluated by HPLC. Thus, methotrexate and leucovorine were selectively retained in mobile phases of either low or high aqueous content, thus showing the typical bimodal retention behavior of previously reported MIPs. In organic mobile-phase systems, the inhibitor used as template had an influence on the retentivity and selectivity of the MIP. The polymer imprinted with trimethoprim retained all folic acid analogues strongly and showed the highest selectivity among the MIPs in an organic mobile-phase system. This was supported by Scatchard analysis resulting in biphasic plots and a quantitative yield of high-energy binding sites. All templates were shown to associate strongly with MAA in CDCl(3), the strength of association correlating roughly with the template basicity and the selectivity observed in chromatography. Nonparallel complexation-induced shifts indicated formation of 1:2 template monomer complexes at concentrations corresponding to those of the prepolymerization solutions.

Imprinted chiral stationary phases in high-performance liquid chromatography

Polymers imprinted with chiral templates offer a new generation of tailor-made chiral stationary phases (CSPs) with predictable selectivities. This review summarizes the present state of the art of molecular imprinting to generate tailor-made CSPs and provides an overview of the main factors involved in the manufacturing process that are crucial to the chromatographic performance of the phases.

Application of molecularly imprinted polymers in supercritical fluid chromatography

Molecularly imprinted polymers (MIPs), for the templates free base racemic propranolol and the L-enantiomer of phenylalanine anilide (L-PA), were investigated as stationary phases in supercritical fluid chromatography (SFC). Large retention differences were observed on the propranolol MIP for both the template molecule and the structural analogue metoprolol compared to that observed on the corresponding blank polymer. Mobile phase composition and solute concentration were found to affect this retention behaviour. The phenylalanine anilide MIP (L-PA MIP) was found to be enantioselective in SFC with stronger retention observed for the template enantiomer. Throughout the study, characteristic imprinting peak shapes for the stronger retained template molecule were observed for both MIPs examined. After a number of days under supercritical fluid conditions, the performance of the photochemically initiated L-PA MIP was found to significantly deteriorate whereas the thermally initiated propranolol MIP revealed only small changes in its separation performance after a long term of operation. The separation behaviour of these two MIPs in SFC was compared with results obtained on the same columns in high-performance liquid chromatography (HPLC) both before and after their application in SFC.

Application of imprinted synthetic polymers in binding assay development

The first part of the review describes a method for the synthesis of molecularly imprinted polymers for use in binding assays. The method considers the many factors involved that affect the recognition properties of the materials and describes an approach to screening and optimization of these factors. The second part describes the development of binding assays using such polymers. This includes the use of different labels, the effect of solvent and buffer, the scale of the assay (amount of solid polymer), and how these influence the quality of the assay in terms of sensitivity, selectivity, and speed of analysis.

Enantioselective ester hydrolysis catalyzed by imprinted polymers

Highly cross-linked network polymers prepared by molecular imprinting catalyzed enantioselectively the hydrolysis of N-tert-butoxycarbonyl phenylalanine-p-nitrophenyl ester (BOCPheONP). The templates were designed to allow incorporation of the key catalytic elements, found in the proteolytic enzyme chymotrypsin, into the polymer active sites. Three model systems were evaluated. These were constructed from a chiral phosphonate analogue of phenylalanine (series A, C) or L-phenylalanine (series B) attached by a labile ester linkage to an imidazole-containing vinyl monomer. Free radical copolymerization of the template with methacrylic acid (MAA) and ethylene glycol dimethacrylate (EDMA) gave a highly cross-linked network polymer. The templates could be liberated from the polymers by hydrolysis, giving catalytically active sites envisaged to contain an enantioselective binding site, a site complementary to a transition state like structure (series A, C), and a hydroxyl, imidazole, and carboxylic acid group at hydrogen bond distance. As predicted, the enantiomer of BOCPheONP complementary to the configuration of the template was preferentially hydrolyzed with D-selectivity for the series A polymers (kD/kL = 1.9) and L-selectivity for the series B polymers (kL/kD = 1.2). The maximum rate enhancement, when compared with a control polymer, prepared using a benzoyl-substituted imidazole monomer as template, was 2.5, and comparing with the imidazole monomer in solution, a maximum rate enhancement of 10 was observed. The catalytic activity was higher for polymers subjected to the nucleophilic treatment. This was explained by a higher site density and flexibility of the polymer matrix caused by this treatment. In a comparison of template rebinding to polymers imprinted with a template containing either a carboxylate (planar ground state structure) or a phosphonate (tetrahedral transition state like structure) functionality, it was observed that imprinted polymers are able to discriminate between a transition state like and a ground state structure for transesterification. However the influence of transition state stabilization on the observed rate enhancements remains obscure. Only at acidic pH's was catalysis observed, whereas at basic pH's the polymers inhibit the reaction. At a later stage, the catalytic activity of the polymers for nonactivated D- and L-phenylalanine ethyl esters was investigated. A rate enhancement of up to 3 was observed when compared to the blank. Most important, however, the polymers imprinted with a D template preferentially hydrolyzed the D-ethyl ester and exhibited saturation kinetics.