Selectivity comparisons of monolithic silica capillary columns modified with poly(octadecyl methacrylate) and octadecyl moieties for halogenated compounds in reversed-phase liquid chromatography

Stationary phase selectivities for halogenated compounds in reversed-phase HPLC were compared using C18 monolithic silica capillary columns modified with poly(octadecyl methacrylate) (ODM) and octadecyl moieties (ODS). The preferential retention of halogenated benzenes on ODM was observed in methanol/water and acetonitrile/water mobile phases. In selectivity comparison of selected analytes on ODM and ODS, greater selectivities for halogenated compounds were obtained with respect to alkylbenzenes on an ODM column, while similar selectivities were observed with a homologous series of alkylbenzenes on ODM and ODS columns. These data can be explained by greater dispersive interactions by more densely packed octadecyl groups on the ODM polymer coated column together with the contribution of carbonyl groups in ODM side chains. For the positional isomeric separation of dihalogenated benzenes (ortho-, meta-, para-), the ODM column also provided better separation of these isomers for the adjacently eluted isomers that cannot be completely separated on an ODS column in the same mobile phase. These results imply that the ODM column can be used as a better alternative to the ODS column for the separation of other halogenated compounds.     

Study of triacontyl-functionalized monolithic silica capillary column for reversed-phase capillary liquid chromatography

A novel stationary phase triacontyl-functionalized monolithic silica capillary column was successfully prepared for reversed-phase capillary liquid chromatography. The performance of the monolithic silica capillary column coated with triacontyl chain for the separation of alkylbenzenes, xylene isomers, polycyclic aromatic hydrocarbons, and mixture of α- and β-carotenes was studied, which was compared to that using the monolithic silica capillary column coated with octadecyl chain. The comparison results showed that triacontyl-functionalized monolithic silica capillary column would be a promising media to be used for the separation of isomeric solutes with long chain in reversed-phase capillary liquid chromatography.     

Polydimethylsiloxane-functionalized monolithic silica column for reversed-phase capillary liquid chromatography

A polydimethylsiloxane (PDMS)-modified monolithic silica column was prepared for performing reversed-phase capillary liquid chromatography. The prepared PDMS column has a permeability of 6.4×10(-14) m(2) with a plate height <9.2 μm. Alkylbenzenes and polycyclic aromatic hydrocarbons (PAHs) were well separated with the PDMS stationary phase, which exhibited similar selectivity and separation mechanism to that of octadecyl stationary phase. The hydrophobic interactions between the analytes and the PDMS stationary phase mainly play the roles for the separation of alkylbenzenes and PAHs. The characteristics of the PDMS column for the separation of alkylbenzenes and PAHs demonstrated that it would be a promising alternative to the octadecyl column.     

An attempt directed toward enhanced shape selectivity in reversed-phase liquid chromatography: preparation of the dodecylaminated beta-cyclodextrin-bonded phase.

With the aim of preparing a stationary phase with a high shape-recognition ability for liquid chromatography, a new bonded phase was synthesized by coupling multiply dodecylamino-substituted beta-cyclodextrin (beta-CD) to 3-glycidoxypropyl-derivatized silica gel. The stationary phase prepared in this way was expected to have increased shape selectivity compared with that of conventional reversed-phase materials, due to solute interactions with the alkyl chain piles built up on the beta-CDs bonded to silica. The separation characteristics of the bonded phase were investigated using polycyclic aromatic hydrocarbons (PAHs) with different molecular shapes and compared with those of monomeric ODS and native beta-cyclodextrin-bonded phases. The newly developed stationary phase was found to be highly selective for PAHs.     

Characterization of polymer monolithic stationary phases for capillary HPLC

Monolithic capillary columns have been prepared in fused‐silica capillaries by radical co‐polymerization of ethylene dimethacrylate and butyl methacrylate in the presence of porogen solvent mixtures containing various concentration ratios of 1‐propanol, 1,4‐butanediol, and water with azobisisobutyronitrile as the initiator of the polymerization reaction. The through pores in organic polymer monolithic columns can be characterized by “equivalent permeability particle size”, and the mesopores with stagnant mobile phase by “equivalent dispersion particle size”. Increasing the concentration of propanol in the polymerization mixture diminishes the pore volume and size in the monolithic media and improves the column efficiency, at a cost of decreasing permeability. Organic polymer monolithic capillary columns show similar retention behaviour to packed alkyl silica columns for compounds with different polarities characterized by interaction indices, I_x, but have different methylene selectivities. Higher concentrations of propanol in the polymerization mixture increase the lipophilic character of the monolithic stationary phases. Best efficiencies and separation selectivities were found for monolithic columns prepared using 62–64% propanol in the porogen solvent mixture. To allow accurate characterization of the properties of capillary monolithic columns, the experimental data should be corrected for extra‐column contributions.     

On-line preconcentration of trace carcinogenic polycyclic aromatic hydrocarbons (PAHs) in microcolumn liquid chromatography via large volume injection

The ability and efficiency of micro precolumns made of C30 particles, monolithic silica C18 stationary phase and quartz wool coated with C30, which act as novel solid phase absorbing materials, for the on-line enrichment of aqueous polycyclic aromatic hydrocarbons (PAHs) in microcolumn liquid chromatography (LC) was investigated. The enrichment unit was designed in such a way that micro precolumns were directly connected to a 6-port micro injection valve via fused-silica tubing (0.05 mm I.D.) in order to minimize band broadening of the samples, and the enrichment efficiency of the three materials was tested using 14 PAHs, which are selected by the US Environmental Protection Agency (US EPA), as the analytes. The separation of PAHs was evaluated by using laboratory-made C30 or ODS capillary columns and the results were compared. There were no significant differences showed from the separation of PAHs in terms of peak signal between the C30 and ODS capillary columns, but the C30 capillary column was chosen for the following experiment due to its ability to produce better repeatability than the ODS column. By using the three kinds of precolumn materials, results showed that the precolumn packed with C30 particles as well as the capillary monolithic C18 precolumns (0.1 or 0.2 mm I.D.) provided better recovery than those of the quartz wool's. As long as the recovery and separation of the PAHs were concerned, 0.1 mm I.D. monolithic C18 precolumn showed the best results and the R.S.D.s (N = 7) for the retention time, peak area and peak height were between 0.70-1.5, 2.3-5.8 and 2.4-6.6%, respectively. Large volume injection up to 0.5 mL, i.e. 2500-fold enrichment, was possible and no negative effect on the separation profile was found. The LOD (S/N = 3) were between 0.10 and 4.6 pg mL⁻¹, while the LOQ (S/N = 10) were in the range of 0.32-15 pg mL⁻¹, which showed that the system is comparable to many major analytical techniques and is sensitive enough for the trace analysis of PAHs in environmental samples. The system was then applied to the determination of trace PAHs present in soil sample which was randomly taken from a nearby highway.     

Development of a new C14 monolithic silica column containing embedded polar groups for pressurized capillary electrochromatography

Monolithic columns have been prepared with a novel bonded silica stationary phase, tetradecylamine bonded silica (TDAS), and used in pressurized capillary electrochromatography (pCEC). The monolithic silica column matrix was prepared by a sol-gel process and then chemically modified with the spacer (3-glycidoxypropyl)trimethoxysilane and tetradecylamine. The introduced embedded polar amine groups dominated the charge on the surface of the monolithic stationary phase and generated an EOF from cathode to anode under acidic conditions. The tetradecyl hydrophobic chains in TDAS provide chromatographic interactions. The chromatographic characteristics of the prepared monolithic column were studied. Some aromatic compounds including alkylbenzenes, aromatic hydrocarbons, phenols, and anilines were successfully separated on the TDAS monolithic column in pCEC mode. As expected, the TDAS monolithic stationary phases exhibit typical reversed-phase electrochromatographic behavior toward neutral solutes due to the introduced tetradecyl groups. Hydrophobic as well as electrophoretic migration processes within the monoliths were observed in the separation of basic anilines. Symmetrical peaks can be obtained for anilines because the embedded polar amine groups on the surface can effectively shield the adsorption of positively charged analytes onto the stationary phase.     

Thermo-responsive monolithic materials

One of the recent major improvements of HPLC was the introduction of monolithic silica columns, which have the advantage of faster separation and lower back pressure as compared to common silica beads. Here, we present an interesting alternative to such reversed-phase monolithic columns by a convenient coupling route of a thermo-responsive polymer to hydrophilic silica monoliths. Poly(N-isopropylacrylamide) (PNIPAM) was polymerized in solution via a reversible addition fragmentation chain transfer (RAFT) polymerization technique and coupled then in situ onto an amino-modified silica monolithic column. These columns were compared with RP-18 monolithic columns in the separation of steroids under isocratic condition in aqueous mobile phase. Separation is optimized just by changing the temperature, instead of changing the mobile phase composition.     

Investigation of π–π and ion–dipole interactions on 1-allyl-3-butylimidazolium ionic liquid-modified silica stationary phase in reversed-phase liquid chromatography

1-Allyl-3-butylimidazolium bromide ionic liquid [AyBIm]Br was prepared and used for the modification of mercaptopropyl-functionalized silica through surface radical chain-transfer addition. The obtained ionic liquid-modified silica (SiImBr) was characterized by elemental analysis, infrared spectroscopy, NMR spectroscopy, and thermogravimetric analysis. The selective retention behaviours of polycyclic aromatic hydrocarbons (PAHs) including some positional isomers were investigated using SiImBr as a stationary phase in reversed-phase liquid chromatography. The results showed that SiImBr presented multiple interactions including hydrophobic, π–π, and ion–dipole interactions during the separation of PAHs and dipolar compounds. However, it is proposed that π–π and ion–dipole interactions play important roles in the separation of PAHs and dipolar compounds. These results indicate that the ionic liquid-modified silica stationary phase is promising for future applications. A commercially available monomeric octadecylated silica (ODS) column and a custom-made poly(styrene)-grafted silica (Sil-St_n) column were used as references.     

Temperature optimization for the separation of PAHs on micropacked LC ODS columns

The effect of column temperature on the separation of the sixteen polycyclic aromatic hydrocarbons (PAHs) of mixture SRM 1647a of the US Environmental Protection Agency has been studied on different micropacked ODS columns. Isothermal temperature optimization was successfully used for complete separation of the PAHs on a polymeric ODS stationary phase, whereas temperature programmed conditions were selected for separation on a monomeric ODS stationary phase.     

Controlling retention, selectivity and magnitude of EOF by segmented monolithic columns consisting of octadecyl and naphthyl monolithic segments--applications to RP-CEC of both neutral and charged solutes

Monolithic capillaries made of two adjoining segments each filled with a different monolith were introduced for the control and manipulation of the electroosmotic flow (EOF), retention and selectivity in reversed phase-capillary electrochromatography (RP-CEC). These columns were called segmented monolithic columns (SMCs) where one segment was filled with a naphthyl methacrylate monolith (NMM) to provide hydrophobic and π-interactions, while the other segment was filled with an octadecyl acrylate monolith (ODM) to provide solely hydrophobic interaction. The ODM segment not only provided hydrophobic interactions but also functioned as the EOF accelerator segment. The average EOF of the SMC increased linearly with increasing the fractional length of the ODM segment. The neutral SMC provided a convenient way for tuning EOF, selectivity and retention in the absence of annoying electrostatic interactions and irreversible solute adsorption. The SMCs allowed the separation of a wide range of neutral solutes including polycyclic aromatic hydrocarbons (PAHs) that are difficult to separate using conventional alkyl-bonded stationary phases. In all cases, the k' of a given solute was a linear function of the fractional length of the ODM or NMM segment in the SMCs, thus facilitating the tailoring of a given SMC to solve a given separation problem. At some ODM fractional length, the fabricated SMC allowed the separation of charged solutes such as peptides and proteins that could not otherwise be achieved on a monolithic column made from NMM as an isotropic stationary phase due to the lower EOF exhibited by this monolith.     

Gas chromatographic analysis of high-molecular-mass polycyclic aromatic hydrocarbons II. Polycyclic aromatic hydrocarbons with relative molecular masses exceeding 328

Three columns were used for the gas chromatographic analysis of polycyclic aromatic hydrocarbons (PAHs) with relative molecular masses (M_r) up to 450. Two of the columns were commercially available, coated with a 50% methyltrifluoropropyl-substituted polysiloxane a 5% diphenyl-substituted methylpolysiloxane. The third column was laboratory made, coated with a biphenyl-substituted silarylene-siloxane copolymer. All three columns were utilized for the analysis of high-M_r PAHs as regards both thermal stability of the stationary phases, i.e., low bleeding rate, and chromatographic efficiency. The column coated with a trifluoropropyl-substituted stationary phase showed, however, a low separation efficiency, possibly owing to low solute stationary phase compatibility. The biphenyl-substituted stationary phase, on the other hand, showed a very high separation efficiency, but the retention of the PAHs was significantly higher on this column compared with the other two, leading to the demand for higher oven temperatures. Different retention mechanisms were observed on these columns, as shown by differences in the retention indices of the PAHs measured in a system using PAHs as retention index markers. A comparatively faster elution of non-planar PAHs was observed on the columns coated with the trifluoropropyl-substituted stationary phase and the biphenyl-substituted stationary phase compared with the column coated with the 5% diphenyl-substituted polymer. The usefulness of the columns for separations of high-M_r PAHs is demonstrated by gas chromatograms of carbon black extracts and a coal tar extract standard reference material.     

Recent developments of monolithic and open‐tubular capillary electrochromatography (2017–2019)

Capillary electrochromatography, which combined the high selectivity of high‐performance liquid chromatography and the high separation efficiency of capillary electrophoresis, is an attractive separation tool. In this review, the developments on monolithic and open tubular capillary electrochromatography during 2017 to August 2019 are summarized. Considering the development of novel stationary phases is the most active research field in capillary electrochromatography, monolithic capillary electrochromatography is classified according to the polymer‐based and hybrid monolithic columns, while open‐tubular capillary electrochromatography is categorized by cyclodextrin, silica, polymer, nanomaterials, microporous materials, and biomaterials‐based open tubular columns.     

有机-无机杂化硅胶基质整体柱的制备及其电色谱性能

采用溶胶-凝胶法,以四乙氧基硅烷和苯基三乙氧基硅烷作为反应单体,通过酸碱两步催化在毛细管中进行原位缩聚反应,制备了新型有机-无机杂化硅胶基质毛细管整体柱,制备过程简单。整体柱基质中均匀分布的苯基基团可直接用于反相毛细管电色谱的分离,因而不需要对基质再进行衍生化。优化了整体柱的制备条件,采用扫描电镜和压汞法对整体柱的微观结构和孔径分布进行了表征。分别考察了溶胶-凝胶初始反应液中水的用量对柱床结构的影响和两种单体的配比对材料孔径分布的影响。研究了稠环芳烃类化合物在整体柱上的保留行为,用所制备的整体柱分离了7种苯酚类化合物,平均柱效达100000塔板/m。     

Short communication performance of octadecylsilylated monolithic silica capillary columns of 530 microm inner diameter in HPLC

Monolithic silica capillary columns were successfully prepared in a fused silica capillary of 530 microm inner diameter and evaluated in HPLC after octadecylsilylation (ODS). Their efficiency and permeability were compared with those of columns pakked with 5-microm and 3-microm ODS-silica particles. The monolithic silica columns having different domain sizes (combined size of through-pore and skeleton) showed 2.5-4.0-times higher permeability (K= 5.2-8.4 x 10(-14) m2) than capillary columns packed with 3-mm particles, while giving similar column efficiency. The monolithic silica capillary columns gave a plate height of about 11-13 microm, or 11 200-13 400 theoretical plates/150 mm column length, in 80% methanol at a linear mobile phase velocity of 1.0 mm/s. The monolithic column having a smaller domain size showed higher column efficiency and higher pressure drop, although the monolithic column with a larger domain size showed better overall column performance, or smaller separation impedance (E value). The larger-diameter (530 microm id) monolithic silica capillary column afforded a good peak shape in gradient elution of proteins at a flow rate of up to 100 microL/min and an injection volume of up to 10 microL.     

Comprehensive two‐dimensional monolithic liquid chromatography of polar compounds

Two‐dimensional liquid chromatography largely increases the number of separated compounds in a single run, theoretically up to the product of the peaks separated in each dimension on the columns with different selectivities. On‐line coupling of a reversed‐phase column with an aqueous normal‐phase (hydrophilic interaction liquid chromatography) column yields orthogonal systems with high peak capacities. Fast on‐line two‐dimensional liquid chromatography needs a capillary or micro‐bore column providing low‐volume effluent fractions transferred to a short efficient second‐dimension column for separation at a high mobile phase flow rate. We prepared polymethacrylate zwitterionic monolithic micro‐columns in fused silica capillaries with structurally different dimethacrylate cross‐linkers. The columns provide dual retention mechanism (hydrophilic interaction and reversed‐phase). Setting the mobile phase composition allows adjusting the separation selectivity for various polar substance classes. Coupling on‐line an organic polymer monolithic capillary column in the first dimension with a short silica‐based monolithic column in the second dimension provides two‐dimensional liquid chromatography systems with high peak capacities. The silica monolithic C₁₈ columns provide higher separation efficiency than the particle‐packed columns at the flow rates as high as 5 mL/min used in the second dimension. Decreasing the diameter of the silica monolithic columns allows using a higher flow rate at the maximum operation pressure and lower fraction volumes transferred from the first, hydrophilic interaction dimension, into the second, reversed‐phase mode, avoiding the mobile phase compatibility issues, improving the resolution, increasing the peak capacity, and the peak production rate.     

Optimization of binary porogen solvent composition for preparation of butyl methacrylate monoliths in capillary liquid chromatography

Butyl methacrylate monolithic columns in 320 microm i.d. fused silica capillaries for reversed-phase capillary liquid chromatography were prepared by radical polymerization initiated thermally with azobisisobutyronitrile (AIBN). Polymerization mixture contained butyl methacrylate (BMA) as the function monomer and ethylene dimethacrylate (EDMA) as the crosslinking agent with 1,4-butanediol and 1-propanol as a binary porogen solvent. Ratio of 1,4-butanediol to 1-propanol in the porogen solvent was optimized regarding the monolithic column efficiency and performance. Total porosity, column permeability, separation impedance, Walters hydrophobicity index, retention factors, peak asymmetry factors, height equivalents to a theoretical plate and peak resolutions were used for characterization of the prepared monolithic columns. The polymerization mixture consisting of 17.8% of BMA, 21.8% of EDMA, 18.0% of 1,4-butanediol, 42.0% of 1-propanol and 0.4% AIBN generated monolithic columns of the best performance having a sufficient permeability and the lowest separation impedance. It was also demonstrated that monolithic columns of this composition exhibited good preparation reproducibility and an excellent pressure resistance when applied in capillary liquid chromatography.     

Semi‐online nanoflow liquid chromatography/matrix‐assisted laser desorption ionization mass spectrometry of synthetic polymers using an octadecylsilyl‐modified monolithic silica capillary column

We have designed a semi‐online liquid chromatography/matrix‐assisted laser desorption/ionization mass spectrometry (LC/MALDI‐MS) system to introduce eluent from a octadecylsilyl (ODS) group modified monolithic silica capillary chromatographic column directly onto a sample plate for MALDI‐MS analysis. Our novel semi‐online system is useful for rapidly and sensitively examining the performance of a monolithic capillary column. An additional advantage is the small elution volume of a monolithic capillary column, which allows delicate eluents, such as 1,1,1,3,3,3,‐hexafluoroisopropyl alcohol (HFIP), to be used to achieve cost‐effective analysis. Using the semi‐online LC/MALDI‐MS system, chromatographic separation of polymers by the monolithic column with different eluents was studied. Separation of poly(methyl methacrylate) and Nylon 6/6 showed that the column functioned via size‐exclusion separation when tetrahydrofuran or HFIP eluent was used. On the other hand, the separation behavior of Nylon 11 indicated a reversed‐phase mode owing to the interaction of the polymer with the modified ODS group in the column. Using tetrahydrofuran/methanol (1:1, v/v) as the eluent, the LC/MALDI‐MS spectra of poly(lactic acid), which contains both linear and cyclic polymer structures, showed that the column could separate the hydrophobic cyclic polymer and elute it out relatively slowly. The monolithic column functions basically via size‐exclusion separation; the reversed‐phase separation by interaction with the ODS functions may have less influence on column separation. The semi‐online monolithic capillary LC/MALDI‐MS method we have developed should provide a means of effectively analyzing synthetic polymers. Copyright © 2010 John Wiley & Sons, Ltd.     

Rapid and high-resolution analysis of geometric polyprenol homologues by connected octadecylsilylated monolithic silica columns in high-performance liquid chromatography

A Chromolith Performance octadecylsilyl (ODS) monolithic silica column (Merck) was compared with a conventional microparticulate ODS-bonded silica column in the high-performance liquid chromatography separation of natural polyprenols. A system comprising two connected monolithic columns afforded an equivalent separation at half the analysis time of the conventional method. Furthermore, ten connected columns achieved a tremendously high-resolution separation, in which the complicated series of homologous polyprenols with geometric isomerism were fully separated.     

The Development of Reversed‐Phase Capillary Electrochromatography for the Separation of Steroids

This paper describes the preparation and optimization of packed capillary columns for reversed‐phase separation of steroids with CEC. The fabrication of on‐column frits is considered to be the most important step for obtaining a reproducible packed column for CEC separation. Porous silicate frits were generated in a fused‐silica capillary by heating the silica gel/sodium hydroxide solutions electrically. The optimized conditions involve silica gel (10.8%), sodium hydroxide (5.8%), and heating time (5 sec) with heating voltage (5V) for obtaining a 100‐μ end‐frit that can withstand pressure over 6000 psi. A HPLC pump was utilized to pack the 5‐μm ODS particle slurry into the capillary column. The ODS packed capillaries were then utilized for the separation of four anabolic cholesterols with a capillary electrophoresis system without pressurization of the column. The reproducibility of the packed columns was evaluated by measuring the relative standard deviations of four steroids. The relative standard deviations of migration time for column‐to‐column, day‐to‐day, and run‐to‐run are less than 7%, 2%, and 1% for four steroids, respectively.