Practical aspects of using methacrylate-ester-based monolithic columns in capillary electrochromatography (original) (raw)
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Microchemical Journal, 2013
Monolithic poly(octadecyl methacrylate-co-ethylene dimethacrylate) capillary columns for use in capillary electrochromatography (CEC) were developed and characterized by porosimetric measurements and scanning electron microscopy (SEM). The stationary phases were prepared using 2-acryloylamido-2-methylpropanesulfonic acid (AMPS) as the ionizable monomer and 2,2′-azobisisobutyronitrile (AIBN) as the initiator of thermal polymerization, which occurred at 60°C. The porogenic solvents used in this work were amyl alcohol and 1,4-butanediol, in the proportion 65:35 (v/v). The ratio between monomers and porogenic solvents was varied in a range of 60 to 80% (v/v) of porogenic agents. The porosimetry showed that the increase in content of porogenic solvents caused an increase in pore surface area, but a relationship with the separation efficiencies was not observed. SEM agreed with the conclusions made through porosimetry, where decreases in globule size could be noted with increases in porogenic solvents.
Journal of Chromatographic Science, 2012
Butyl methacrylate (BMA)-ethylene dimethacrylate (EDMA)-methacrylic acid (MAA) and BMA-EDMA-2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) monolithic columns were prepared by varying the percentage of ionic monomers for capillary electrochromatography. Monolithic columns with a higher content of ionic monomers provided better column efficiency, and the performance of BMA-EDMA-MAA monoliths was better than BMA-EDMA-AMPS. To characterize and optimize BMA-EDMA-MAA monoliths, the effects of the content of cross-linker and the total monomer in the polymerization mixture on column performance were also studied. Plate heights of 8.2 mm for the unretained solute (thiourea) and 12.6 mm for the retained solute (naphthalene) were achieved with a monolithic column using 2.5% MAA (Column I).
Methacrylate monolithic columns of 320 μm I.D. for capillary liquid chromatography
Journal of Chromatography A, 2002
Monolithic capillary columns (320 mm I.D.) were prepared for capillary liquid chromatography (CLC) by radical polymerization of butylmethacrylate (BMA) and ethylenedimethacrylate (EDMA) in the presence of a porogen solvent containing propan-1-ol, butane-1,4-diol and water. The influence of the contents of the porogen solvent and EDMA in the polymerization mixture on the monolith porosity and column efficiency was investigated. The composition of the polymerization mixture was optimized to attain a minimum HETP of the order of tens of mm for test compounds with various polarities. The separation performance and selectivity of the most efficient monolithic column prepared was characterized by van Deemter curves, peak asymmetry factors and Walters hydrophobicity and silanol indices. It was demonstrated that the 320-mm I.D. monolithic column exhibited CLC separation performance similar to that observed for 100-and 150-mm I.D. monolithic columns reported in the literature; moreover, the 320-mm I.D. column was easier to operate in CLC and exhibited a higher sample loadability.
Journal of Chromatography A, 2008
A monolithic stationary phase was prepared by in situ copolymerization of 2-hydroxyethyl methacrylate (HEMA), ethylene dimethacrylate (EDMA), and methacrylic acid (MAA), in a binary porogenic solvent consisting of toluene and 1-dodecanol. The resulting monolith was evaluated as a hydrophilic interactioncapillary electrochromatography (HI-CEC) stationary phase under the mode of pressurized capillary electrochromatography (pCEC). Effects of the buffer pH, salt concentration and the mobile phase composition on the electroosmotic flow (EOF) velocity and the retention factors of the compounds were investigated. The generation of cathodic EOF under a broad pH range was attributed to the presence of the carboxyl groups on the surface of the polar stationary phase. The carboxyl groups offered at the same time the possibility of weak electrostatic interaction with analytes. The separation mechanism of the monolithic column was discussed in detail. It was found that the separation mechanism of charged solutes could be attributed to a mixed mode of HI and weak electrostatic interaction, as well as the effect of electrophoresis, while the separation of neutral solutes was based on the hydrophilic interaction at high acetonitrile (ACN) content.
Sol−Gel Monolithic Columns with Reversed Electroosmotic Flow for Capillary Electrochromatography
Analytical Chemistry, 2000
Sol-gel chemistry was used to prepare porous monolithic columns for capillary electrochromatography. The developed sol-gel approach proved invaluable and generates monolithic columns in a simple and rapid manner. Practically any desired column length ranging from a few tens of centimeters to a few meters may be readily obtained. The incorporation of the sol-gel precursor, N-octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride, into the sol solution proved to be critical as this reagent possesses an octadecyl moiety that allows for chromatographic interactions of analytes with the monolithic stationary phase. Additionally, this reagent served to yield a positively charged surface, thereby providing the relatively strong reversed electroosmotic flow (EOF) in capillary electrochromatography. The enhanced permeability of the monolithic capillaries allowed for the use of such columns without the need for modifications to the commercial CE instrument. There was no need to pressurize both capillary ends during operation or to use high pressures for column rinsing. With the developed procedure, no bubble formation was detected during analysis with the monolithic capillaries when using electric field strengths of up to 300 V cm -1 . The EOF in the monolith columns was found to be dependent on the percentage of organic modifier present in the mobile phase. Separation efficiencies of up to 1.75 × 10 5 plates/m (87 300 plates/ column) were achieved on a 50 cm × 50 µm i.d. column using polycyclic aromatic hydrocarbons and aromatic aldehydes and ketones as test solutes.
ELECTROPHORESIS, 2001
Separation of basic, acidic and neutral compounds by capillary electrochromatography using uncharged monolithic capillary columns modified with anionic and cationic surfactants A mode of capillary electrochromatography (CEC), based on the dynamical adsorption of surfactants on the uncharged monolithic stationary phases has been developed. The monolithic stationary phase, obtained by the in situ polymerization of butyl methacrylate with ethylene dimethacrylate, was dynamically modified with an ionic surfactant such as the long-chain quaternary ammonium salt of cetyltrimethylammonium bromide (CTAB) and long-chain sodium sulfate of sodium dodecyl sulfate (SDS). The ionic surfactant was adsorbed on the surface of polymeric monolith by hydrophobic interaction, and the ionic groups used to generate the electroosmotic flow (EOF). The electroosmotic mobility through these capillary columns increased with increasing the content of ionic surfactants in the mobile phase. In this way, the synthesis of the monolithic stationary phase with binary monomers can be controlled more easily than that with ternary monomers, one of which should be an ionic monomer to generate EOF. Furthermore, it is more convenient to change the direction and magnitude of EOF by changing the concentration of cationic or anionic surfactants in this system. An efficiency of monolithic capillary columns with more than 140 000 plates per meter for neutral compounds has been obtained, and the relative standard deviations observed for t 0 and retention factors of neutral solutes were about 0.22% and less than 0.56% for ten consecutive runs, respectively. Effects of mobile phase composition on the EOF of the column and the retention values of the neutral solutes were investigated. Simultaneous separation of basic, neutral and acidic compounds has been achieved.
Journal of Chromatography A, 2009
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Journal of Chromatography A, 1999
Capillary columns with monolithic stationary phase were prepared from silanized fused-silica capillaries of 75 mm I.D. by in situ copolymerization of divinylbenzene either with styrene or vinylbenzyl chloride in the presence of a suitable porogen. The porous monolithic support in this study was used either directly or upon functionalization of the surface to obtain a stationary phase that was appropriate for the separation of peptides by capillary electrochromatography (CEC). The main advantages of monolithic columns are as follows. They do not need retaining frits, they do not have charged particles that can get dislodged in high electric field, and they have relatively high permeability and stability. Whereas such columns are designed especially for CEC, they find application in micro high-performance liquid chromatography (m-HPLC) as well. Five different porogens were employed to prepare the monolithic columns that were examined for permeability and porosity. The flexibility of fused-silica capillaries was not adversely affected by the monolithic packing and the longevity of the columns was satisfactory. This may also be due to the polymerization technique, which resulted in a fluid-impervious outer layer of the monolith that precluded contact between the fused-silica surface and the liquid mobile phase. For the most promising columns, the conductivity ratios and the parameters of the simplified van Deemter equation, both in m-HPLC and CEC, were evaluated. It was found that the efficiency of the monolithic columns in CEC was significantly higher than in m-HPLC in the same way as observed with capillary columns having conventional particulate packing. This is attributed to the relaxation of band-broadening with electroosmotic flow (EOF) with respect to that with viscous flow. It follows then that the requirement of high packing uniformity to obtain high efficiency may also be relaxed in CEC. Angiotensin-type peptides were separated by CEC with columns packed with a monolithic stationary phase having fixed n-octyl chains and quaternary ammonium groups at the surface. Plate heights of about 8 mm were routinely obtained. The mechanism of the separation is based on the interplay between EOF, chromatographic retention and electrophoretic migration of the positively charged peptides. The results of the complex migration process, with highly nonlinear dependence of the migration times on the organic modifier and the salt concentration, cannot be interpreted within the framework of classical chromatography or electrophoresis.
Journal of Biochemical and Biophysical Methods, 2007
This paper describes the fabrication of long alkyl chain methacrylate monolithic materials for using as stationary phases in capillary liquid chromatography. Following deactivation of the capillary surface with 3-(trimethoxysilyl)propyl methacrylate (γ-MAPS), monoliths were formed by co-polymerisation of stearyl methacrylate (SMA) with ethylene glycol dimethacrylate (EDMA) in the presence of the initiator AIBN and a mixture of porogens including iso-amyl alcohol and 1,4-butanediol. The monoliths were prepared in 100 μm i.d. capillaries and the composition of the polymerisation mixtures were optimised in terms of the ratio of SMA/EDMA, the porogen composition and ratio of porogen to monomers. As the porogen weight fraction decreased, the microglobules became smaller and as expected, the total porosity decreased. In order to determine the usability of such materials, the column permeability K was measured by pumping water through the columns at different linear flow rates. Good results were obtained when these capillaries were used to separate mixtures of weak acids, neutral and basic compounds.
Journal of Chromatography A, 2005
A variety of porous polymer monoliths (PPMs) have been synthesized using the 'conduct-as-cast' format. The resulting polymers have been evaluated for use as separation media in capillary electrochromatography (CEC). The results have shown that substituting a small percentage of the standard polymer formulation with a more hydrophobic monomer produced columns with expected increases in retention for a neutral analyte series. However, substituting larger percentages of a more hydrophobic monomer resulted in columns that exhibited less retention. The unexpected behavior of these hydrophobic columns has been attributed to the non-uniform polymerization of the more hydrophobic monoliths. Van Deemter plots of polyaromatic hydrocarbons have been examined to further analyze the unexpected behavior of these columns. H min values ranged from 8.7 to 9.1 m for the columns evaluated. The effect of the percentage of organic modifier in the mobile phase on the separation has also been studied. The retention window decreases when altering the ACN concentration in the mobile phase from 50% to 80% (v/v).