[Show abstract][Hide abstract] ABSTRACT: Monolithic silica capillary columns synthesized following a three-level design were evaluated for the electrochromatographic separation of acidic and neutral compounds. The influences of four factors in the sol-gel synthesis, i.e. the concentrations of tetramethylorthosilicate (TMOS) and PEG in the starting mixture, the gelation temperature and the silanization modifying time, on the electrochromatographic performance of the resulting C18-silica capillary monoliths were studied. The considered responses were retention factor, resolution, symmetry factor, column efficiency, electrokinetic porosity and the equivalent length of the monolith. The four factors were varied to change the pore structure and the surface coverage with octadecyl moieties, resulting in nine stationary phases. The retentive properties of the columns were initially characterized with alkylbenzenes. Next, the separation for acetylsalicylic acid (ASA) and its related compounds was optimized and used to evaluate the performance of the nine stationary phases considering six responses. A compromise between the different responses was found around higher concentrations of tetramethylorthosilicate and PEG with a lower gelation temperature and a modifying time of 2 h. Column efficiencies up to 96 000 plates/m and resolutions above 1.9 were obtained for the acetylsalicylic acid separation, with a sufficient EOF to yield rapid analysis, which showed improvements over the center-point stationary phase.
[Show abstract][Hide abstract] ABSTRACT: In the past few years, monolithic methacrylate-based columns have attracted some attention in separation science. The mobile-phase optimization on these columns for drug analysis has not yet been thoroughly examined. This paper evaluates the separation of acetylsalicylic acid and its impurities as a case study. First, the best pH was determined as 2.3. Methacrylate-based phases can be employed at such pH because they remain charged, necessary to generate electro-osmotic flow. Then, a suitable solvent strength was determined. Trifluoroacetic acid (0.1%) was added to the mobile phase to improve peak shapes. The optimal organic modifier composition was then determined, using isoeluotropic mobile phases, based on the theory of Snyder's solvent triangle. Quadratic models were built to predict the retention of the compounds at all mobile-phase compositions within the triangle. The predictions were tested and found appropriate. Eventually, a baseline separation of acetylsalicylic acid and its impurities was not obtained. However, it could be concluded that one can optimize the mobile phase on methacrylate-based monolithic columns in CEC using Snyder's solvent triangle approach.
[Show abstract][Hide abstract] ABSTRACT: A pressurized capillary electrochromatographic (pCEC) fingerprint of Ginkgo biloba leaf extract was developed on three different types of capillary columns. A commercial column packed with 3-microm particles and an in-house column packed with 5-microm particles were investigated for their performance. Additionally, a monolithic column was included in the fingerprint study as a potential alternative to the conventional packed columns. The effects of experimental parameters, such as the composition of the mobile phase, the concentration and pH of the buffer, and the applied voltage, were studied. Binary mobile phases consisting of acetonitrile and a 5 mM sodium dihydrogen phosphate electrolyte at pH 2.8 were used in gradient elution mode with an applied voltage of 5 kV. Under optimal gradient conditions, at least 45 peaks were observed within 60 min on the commercial packed column, whereas only about 20 peaks were separated on the methacrylate-based monolithic and the in-house packed columns. The commercial column thus clearly outperforms the two other. However, the properties of the monolithic stationary phase still might be adapted (i.e., by changing the polymerization-mixture composition, the porosity, and thus the selectivity of the phase might be changed), which could lead to an improved efficiency.
[Show abstract][Hide abstract] ABSTRACT: New stationary phases play an important role in the evolution of separation methods. The use of monolithic phases in capillary columns has become rather widespread. The ease of their preparation, their versatility, and the abundance of available chemistries increase their attractiveness, even dethroning the particulate phases. This review paper overviews the different types of monoliths used in capillary electrochromatography and pressurized capillary electrochromatography for the enantiomeric separation of chiral molecules and the analysis of pharmaceutically relevant molecules. The diverse methods of monolith preparation as well as the different types of used materials are discussed, as well as the ways they can be modified to fulfill given analysis needs (e.g., selectivity, efficiency of separation, speed of analysis, and economical interest). In addition, some of the advantages and drawbacks of the different monolithic materials are mentioned.
[Show abstract][Hide abstract] ABSTRACT: Microfluidic devices on chip-based systems are becoming increasingly popular, because of many advantages, like low mobile phase consumption and greatly reduced analysis times. The ultimate goal is to design all-including labon-a-chip tools that can be employed in the field and that integrate all elements necessary for an analysis, e.g. one or more separation channels, (multiple) detectors, mixing chambers and sample pretreatment areas. The chip format permits to make any channel pattern, rendering the applications of lab-on-a-chip unlimited. However, overcoming the limitations inherent with the detection systems remains a challenging problem. Whereas UV-detection is predominant in the larger systems, its sensitivity is inadequate due to the minute pathlengths employed in a chip. This review summarizes the short-comings of UV detection, and discusses some possible solutions. Besides, it presents an overview of the alternative detection methods that have been employed in pharmaceutical and chiral chip analysis, including their positive and negative aspects as well. Finally, the applications reported on the chips are discussed.
Current Pharmaceutical Analysis 05/2009; 5(2):101-111. DOI:10.2174/157341209788172951 · 0.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Pressure-assisted CEC (pCEC) can either be performed on a CE instrument by adding pressure at the column inlet, or by applying voltage on a capillary liquid chromatography system. This study investigates the pressure's added value in pCEC using an LC instrument as well as the influence of the polymerization-mixture composition on monolithic columns in such experimental circumstances. Two factors of the polymerization mixture, which is used to prepare the monolithic capillary columns, were varied according to an experimental design approach: the pore-forming solvent/total monomer ratio and the pore-forming solvents composition. Initially, the effect of the resulting stationary phase on the elution behavior of mainly pharmaceutical compounds was studied. Four responses were used to evaluate the effects on the chromatography: retention time, retention factor, peak asymmetry and number of theoretical plates. After processing the results, the stationary phase composition with the best chromatographic behavior was determined and tested. The advantageous properties of this stationary phase compared with the design center-point column were demonstrated. Secondly, the results of these pCEC experiments were compared with those generated in an identical experimental setup previously performed using CEC. Chromatographic conditions were chosen so that the center-point column showed similar retention in CEC and pCEC. The expected advantage (faster analysis) and drawback (decreased efficiency) of pCEC in the analysis of pharmaceuticals was evaluated. Analysis time and efficiency were both found to depend greatly on the porosity of the column. The conclusion of this comparison is that pCEC did not have a significant added value to CEC. However, this was mainly due to the instrument's limitation of the pressure-driven flow over the column. A clear benefit of the pCEC setup was apparatus-related, i.e. the presence of a loop injection system on the pCEC instrument, which avoids the injection problems that were occasionally observed in CEC.
[Show abstract][Hide abstract] ABSTRACT: Methacrylate-based monolithic stationary phases were evaluated for the analysis of drug molecules using capillary electrochromatography (CEC) as separation technique. The effect of the polymerization-mixture composition on the retention behavior of a small test set of mainly drug molecules was studied. Two factors were varied in a central-composite design-based approach: the ratio between the pore-forming solvents and the monomers on one hand, and the ratio within the pore-forming solvents on the other hand, resulting in nine different stationary phases. The central point of the design was chosen at 70% (m/m) pore-forming solvents (PFS) of which 30% (m/m) is 1,4-butanediol, i.e. 21% of the total polymerization mixture. Experiments were conducted using both a basic (pH 11.5) and an acidic (pH 3) mobile phase. Retention times, retention factors, peak asymmetry and number of theoretical plates are the responses used to evaluate the performance of the resulting monoliths. The best compromise between the different responses was found around 67% PFS and 18% 1,4-butanediol (relative to the total mass), i.e. rather close to the center point. At these conditions, retention times were generally below 15min and retention factors below 5. Asymmetry values between close to 1 were found, and theoretical plate numbers up to 10,900, which were improvements compared to the central point of the design.
Journal of Pharmaceutical and Biomedical Analysis 09/2008; 48(2):264-77. DOI:10.1016/j.jpba.2007.11.040 · 2.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Polymeric methacrylate-based monoliths are evaluated in capillary electrochromatography (CEC) and pressurized capillary electrochromatography (p-CEC) for their potential in pharmaceutical analysis. Using a given polymerization mixture as a basis for the monolith synthesis, different mobile phase pH at constant organic modifier concentrations are tested in both CEC and p-CEC. The test set consists of basic, acidic, amphoteric, and neutral compounds, which are mainly pharmaceuticals. Because of the mainly hydrophobic character of the stationary phase, the interactions are largest when the compounds appear in an uncharged state, but some ion-exchange phenomena with negatively charged compounds can also be observed. In CEC, acidic substances are most retained at low pH. For amphoteric and neutral compounds, no preference regarding analyzing pH can be derived from these experiments. For basics, a high pH is chosen, but a reduced solvent strength is needed to enhance the retention of these compounds. The retention mechanism in p-CEC can also be assigned to both hydrophobic and ionic interactions. For acidic, amphoteric, and neutral compounds, acceptable retention is seen. For the basic compounds, the retention with a mobile phase containing 50% organic modifier is low, as in CEC. However, when the organic modifier content in the mobile phase is decreased, retention increases and the selectivity of the stationary phase is more pronounced. This mode of operation presents a possibility for separating some test mixtures, thus some potential for pharmaceutical analysis is seen. More efforts are needed to obtain higher efficiencies and better peak shapes, which might be solved by a further optimization of both the stationary phase synthesis and the mobile phase composition.
[Show abstract][Hide abstract] ABSTRACT: The effect of five factors on the capillary electrochromatographic enantioseparation of acidic compounds was studied using an experimental design. The studied factors were pH, acetonitrile content in the mobile phase, temperature, buffer concentration, and applied voltage. These experiments allowed defining a generic separation strategy applicable on acidic compounds with chemical and structural diversity. The starting screening conditions consist of a 45 mM ammonium formate electrolyte at pH 2.9 mixed with 65% acetonitrile, an applied voltage of 15 kV, and a temperature of 25 degrees C. The screening phase occasionally can be followed by an optimization procedure. Evaluation of the proposed strategy pointed out that it allows achieving baseline resolution within a relatively short time when a beginning of separation is obtained at the starting conditions. This strategy revealed enantioselectivity for 11 compounds out of 15, of which 10 could be baseline-separated after the proposed optimization steps.