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.     

Hydrothermal preparation of hybrid carbon/silica monolithic capillary column for liquid chromatography

A simple, easy and economical approach for the preparation of a hybrid carbon/silica monolithic capillary column was described for the first time by using silica monolith as framework in combination with hydrothermal carbonization at 180°C. During the preparation process, formamide was introduced to the reaction solutions to reduce the dissolution rate of monolithic silica skeleton and its optimal concentration was 1.5 M. Fourier transform infrared spectrometry, scanning electron microscopy, energy dispersive X‐ray spectrometry, and inverse size exclusion chromatography were carried out to characterize the as‐prepared column. The results demonstrated that carbon spheres ranging from 150 to 1000 nm were successfully attached to the surface of silica skeleton. The prepared hybrid carbon/silica column had a permeability of 4.4 × 10^?14 m². Chromatographic performance of the column was evaluated by separation of various compounds including alkylbenzenes, nucleosides and bases, and aromatic acids. The column exhibited an efficiency of 75 000 plates/m for butylbenzene at the optimal linear velocity of 0.23 mm/s. The successful separation of these compounds and the study on mechanism indicated that the column can be applied in mixed‐mode chromatography.     

N-methylimidazolium-functionalized monolithic silica column for mixed-mode chromatography

The development of mixed-mode stationary phase to achieve multiple separation capabilities in one column is very important for high performance liquid chromatography. In this paper, a new specific stationary phase based on grafting N-methylimidazolium to a monolithic silica column was successfully prepared for performing capillary liquid chromatography. The characteristics of the column were evaluated by the separation of different types of compounds including inorganic anions, aromatic acids, nucleotides, polycyclic aromatic hydrocarbons, alkylbenzenes, and phenols. The mechanisms for the separation of these compounds were investigated and appeared to involve the mixed interactions including anion-exchange, hydrophilic, π-π, dipole-dipole, and hydrophobic interactions.     

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.     

Preparation and characterization of lauryl methacrylate-based monolithic microbore column for reversed-phase liquid chromatography

Poly(lauryl methacrylate-co-ethylene dimethacrylate) monoliths were in situ synthesized within the confines of a silicosteel tubing of 1.02 mm i.d. and 1/16" o.d. for microbore reversed-phase HPLC. In order to obtain practically useful monoliths with adequate column efficiency, low flow resistance, and good mechanical strength, some parameters such as total monomer concentration (%T), cross-linking degree (%C) and polymerization temperature were optimized. High-efficiency monoliths were successfully obtained by thermal polymerization of a monomer mixture (40%T, 10%C) with a binary porogenic solvent consisting of 1-propanol and 1,4-butandiol (7:4, v/v) at a high temperature of 90 °C. The morphology and porous structure of the resulting monoliths were assessed by scanning electron microscope (SEM) and inverse size exclusion chromatography (ISEC), while the column performance was evaluated through the separations of a series of alkylbenzenes in acetonitrile-water (50:50, v/v) eluent. At a normal flow rate of 50 μL/min (corresponding to 1.66 mm/s), the optimized monolithic columns typically exhibited theoretical plate numbers of 6000 plates/10 cm-long column for amylbenzene (k>40), and the pressure drop was always less than 1 MPa/10 cm. The monoliths, which were chemically anchored to the tube inner wall surface using a bifunctional silylation agent, exhibited adequate mechanical strength of up to 12-13 MPa, and were properly operated at 10 times higher flow rate than normal, reducing the separation time to one tenth. The lauryl methacrylate-based monolithic column was applied to a rapid and efficient separation of ten common proteins such as aprotinin, ribonuclease A, insulin, cytochrome c, trypsin, transferrin, conalbumin, myoglobin, β-amylase, and ovalbumin in the precipitation-redissolution mode. Using a linear CH(3)CN gradient elution at a flow rate of 500 μL/min (10-times higher flow rate), 10 proteins were baseline separated within 2 min.     

Determination of the purity of phenoxymethylpenicillin by micellar electrokinetic chromatography and reversed phase liquid chromatography on a monolithic silica column

A micellar electrokinetic chromatographic and a fast reversed‐phase liquid chromatographic method have been developed for determination of the purity of phenoxymethylpenicillin. The optimized running buffer composition was 40 mM phosphate–borate–125 mM SDS–3.5% (v/v) methanol. The HPLC method employed a monolithic silica C18 column and a mobile phase composed of phosphate buffer, pH 3.5, and ACN, the flow‐rate being 3.5 mL/min. Both methods were successfully validated. Linearity, intermediate precision, limits of quantitation, accuracy, and a good correlation of the HPLC and MEKC results were demonstrated. Both methods proved to be fast and reliable and sufficiently sensitive. A combination of the two methods can be very useful in impurity profiling.     

Electrically assisted capillary liquid chromatography using a silica monolithic column

A silica monolithic capillary column was linked to an open capillary of the same internal diameter via a Teflon sleeve to form a duplex column to investigate the combination of chromatography and electrophoresis in the mode of electrically assisted capillary liquid chromatography (eCLC). Using a commercial CE instrument with an 8.5 cm long, 100 μm i.d. reversed phase silica monolithic section and a window 1.5 cm beyond the end of this in a 21.5 cm open section, a minimum plate height of 9 μm was obtained in capillary liquid chromatography (CLC) mode at a low driving pressure of 50 psi. In eCLC mode, high speed and high resolution separations of acidic and basic compounds were achieved with selectivity tuning based on the flexible combination of pressure (0–100 psi) and voltage. Taking advantage of the excellent permeability of silica monolithic columns, use of a step flow gradient enabled elution of compounds with different charge state.     

Reduced-bore monolithic silica column modified with C8-TEOS for reversed-phase electrochromatography

Monolithic silica columns of 2.7 mm ID were prepared and derivatized with C8-TEOS and TEOS by on-column sol-gel reaction. These C8 large diameter monolithic silica columns gave 21 000 theoretical plates for aromatic hydrocarbons in 60% acetonitrile and 40% Tris-HCI buffer. The surface areas as well as the separation reproducibility were improved on coating by the sol-gel approach. Joule heating was greatly reduced by using monolithic columns to which fine quartz sand had been added during column preparation. Since this is a preliminary investigation on a monolithic column with such a large inner diameter, the separation efficiency was not so high as that presently achieved in normal capillary electrochromatography (CEC). However, use of the columns improved sample loadability and concentration detectability of electrochromatography, and semi-preparative separations could be performed.     

Preparation of chitosan functionalized monolithic silica column for hydrophilic interaction liquid chromatography

A novel cationic hydrophilic interaction monolithic stationary phase based on the chemical modification of carboxymethyl chitosan (CMCH) to the monolithic silica skeleton using carbodiimide as an activation reagent was prepared for performing capillary liquid chromatography. The amino and hydroxy moieties of CMCH functioned as both the ion-exchange sites and polar providers. The performance of the column was studied by the separation of polar acidic compounds. The chitosan functionalized monolithic silica column showed good selectivity for nucleosides, nucleotides, aromatic acids and aliphatic acids. The mechanism for the separation of these compounds was also studied. The results showed that these compounds were separated primarily based on the hydrophilic interaction mechanism.     

The effect of hydrothermal treatment on column performance for monolithic silica capillary columns

Monolithic silica capillary columns with i.d. 100 μm and monolithic silica rods were prepared with tetramethoxysilane (TMOS) or a mixture of TMOS and metyltrimethoxysilane (MTMS) using different hydrothermal treatments at T=80 °C or 120 °C. Nitrogen physisorption was applied for the pore characterization of the rods and inverse size exclusion chromatography (ISEC) for that of the capillary columns. Using nitrogen physisorption, it was shown change of pore size and surface area corresponds to that of hydrothermal treatment and silica precursor. The results from ISEC agreed well with those from nitrogen physisorption regarding the pore size distribution (PSD). In addition, the retention factors for hexylbenzene with the ODS-modified capillary columns in methanol/water=80/20 at T=30 °C could also support the results from nitrogen physisorption. Furthermore, column efficiency for the columns was evaluated with alkylbenzenes and three kinds of peptides, leucine-enkephalin, angiotensin II, and insulin. Column efficiency for alkylbenzenes was similar independently of the hydrothermal treatment at T=120 °C. Even for TMOS columns, there was no significant difference in column efficiency for the peptides despite the difference in hydrothermal treatment. In contrast, for hybrid columns, it was possible to confirm the effect on hydrothermal treatment at T=120 °C resulting in a different column efficiency, especially for insulin. This difference supports the results from both nitrogen physisorption and ISEC, showing the presence of more small pores of ca. 3-6 nm for a hybrid silica without hydrothermal treatment at T=120 °C. Consequently, the results suggest that hydrothermal treatment for a hybrid column with higher temperature or longer time is necessary, compared to that for a TMOS column, to provide higher column efficiency with increase in molecular size of solute.     

Analysis of pyridylaminated oligosaccharides using liquid chromatography–mass spectrometry with a monolithic capillary column

We examined the utility of a monolithic capillary column in the analysis of pyridylaminated oligosaccharides. Fluorescence detection and mass spectrometry were used to monitor a series of oligosaccharides. Although the total-ion chromatogram appeared similar to that obtained with fluorescence detection, the sensitivity of this technique was limited, especially in the case of smaller oligosaccharides. This limitation was overcome by applying selected ion current monitoring. Further, the capillary column also exhibited good reproducibility. We showed that the retention times obtained by using the monolithic capillary column could be converted into the standard data to enable comparison of the experimental data with the existing data. Furthermore, our studies revealed an important difference in the separation profile, i.e., the monolithic capillary column could resolve smaller oligosaccharides to a greater extent.     

Characterization of plant oils on a monolithic silica column by high-performance liquid chromatography-atmospheric pressure chemical lonization-mass spectrometry

Summary Five plant oils (peanut, pumpkin seed, sesame seed, soybean, and wheat germ) have been analyzed by high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (HPLC-APCI-MS). Gradient elution was performed with acetone-acetonitrile mobile phases on a short monolithic silica column (SilicaROD, RP-18e, 50 mm×4.6 mm). Identification of plant oil triacylglycerols (TAG) was based on the pseudomolecular ion [M+H]⁺ and the diacylglycerol [M−RCO₂]⁺ fragments. Positional isomers of triacylglycerols were identified from the relative intensities of the [M-RCO₂]⁺ fragments. Principal-component analysis, used to find similarities and differences between the different oils, indicated that the different plant oils could be clearly differentiated according to their triacylglycerol composition. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001     

Polydopamine‐functionalized monolithic silica column for mixed‐mode chromatography†

Dopamine is easy to self‐polymerize under alkaline conditions and the resultant polydopamine is easy to adhere to the surface of many organic and inorganic materials. Based on the characteristics of dopamine, in this paper, a new polydopamine functionalized monolithic silica column was successfully prepared for performing mixed‐mode chromatography. The performance of the column was evaluated by the separation of different types of samples including alkylbenzenes, polycyclic aromatic hydrocarbons, aromatic acids, phenols, and bases. The mechanism for the separation of these compounds was studied and appeared to involve the mixed interactions containing π?π, hydrophobic, electrostatic, and hydrophilic interactions.     

Sol-gel monolithic anion-exchange column for capillary electrochromatography

A single-step approach has been used to prepare a monolithic electrochromatographic column by sol-gel processing of an organofunctional silicon alkoxide precursor that contains a propyl-N,N,N-trimethylammonium group. We have found that the time of adding the porogen, poly(ethylene glycol), during the sol-gel reaction affected the separation performance. Since the surface charge of this material is switchable in sign upon manipulation of solution pH, the direction and magnitude of the electroosmotic flow (EOF) can be controlled by adjusting the pH of the running electrolyte. By controlling the direction of the EOF from cathode to anode, inorganic anions can be separated in a short time. Because of the quaternary ammonium functional group, the resulting material is anion exchangeable. Interestingly, the anion-exchange selectivity of inorganic anions on this column changes with solution pH or applied voltage. The column shows excellent run to run reproducibility (R.S.D. < 0.4%), good day to day reproducibility (R.S.D. < 4%), and reasonable column to column reproducibility (R.S.D. < 9%).     

十八烷基键合硅胶整体柱的制备及其色谱性能评价

选用十八烷基二甲基氯硅烷作为硅烷化试剂,制备十八烷基反相键合硅胶整体柱,并用元素分析进行了表征。以苯、甲苯、联苯、萘、菲混合物作为测试溶质,在以甲醇和水为二元流动相的反相色谱条件下评价了该键合整体柱的色谱性能,考察了该整体柱适用的pH范围,以及柱压降、柱效与流速的关系。结果表明,该硅胶整体柱键合效果良好,具有较好的反相色谱性能,且在pH 2~8之间稳定性好,柱压降、柱效受流速影响较小,对5种心血管系统用药可以达到快速、有效的分离。     

A novel monolithic column for capillary electrochromatographic separation of oligopeptides

A monolithic column was prepared using l-phenylalanine as template and a covalent approach through the formation of Schiff base with o-phthalaldehyde (OPA). OPA, allylmercaptan, l-phenylalanine, and triethylamine were stirred at first, then methacrylic acid, 2-vinylpyridine, ethyleneglycol dimethacrylate, α,α-azobisisobutyronitrile, and 1-propanol were added to the reaction mixture. The resulting material was introduced into a capillary column. Following thermal polymerization, the template was then extracted with a mixture of HCl and methanol. The column was employed for the capillary electrochromatographic separation of oligopeptides. A capillary column of 75 (50) cm × 75 μm ID with a mobile phase of phosphate buffer (pH 7.0, 40 mM)/methanol (5%, v/v), an applied voltage of +15 kV, and detection at 214 nm, could baseline separate angiotensin I, angiotensin II, [Sar¹, Thr⁸] angiotensin, oxytocin, vasopressin, tocinoic acid, β-casomorphin bovine, β-casomorphin human, and FMRF amide within 20 min. The separation behavior of the templated polymer was also compared with that of the non-templated polymer. As a result, it can be concluded that the electrochromatographic separation of this set of peptides was mediated by a combination of electrophoretic migration and chromatographic retention involving hydrophobic, hydrogen bonding, electrostatic as well as the Schiff base formation with OPA in the cavity of the templated polymer.     

Evaluation of a microHPLC system dedictaed to packed capillary column liquid chromatography

Miniaturization and optimization of the solvent delivery system, mixing device, and detection system for gradient elution at few μl/min is the most important objective of instrumental development in microHPLC using packed capillary columns. Instrumental solutions and evaluation of the performence of a dedicated system for automatic gradient elution with packed capillary columns are reported. Retention time precision shown buy the system results in an RSD of 0.20–0.52% for a PAH model mixture eluted under gradient conditions at few μl/min. Compositional accuracy of gradient profiles is also demonstrated.     

Rapid determination of bromide in seawater samples by capillary ion chromatography using monolithic silica columns modified with cetyltrimethylammonium ion

Monolithic silica capillary columns dynamically modified with quaternary ammonium ions were evaluated for the determination of bromide in seawater samples. A quaternary ammonium ion such as cetyltrimethylammonium ion was dynamically introduced onto monolithic silica surfaces. The first layer of the modifier was introduced by electrostatic interaction, whereas the second layer was introduced by hydrophobic interaction. The latter layer worked as the anion-exchange sites. The modified monolithic silica capillary columns could be used for rapid separation of inorganic anions. Separation of authentic mixture of five anions was achieved within a few minutes. The addition of small amount of the modifier in the eluent improved the repeatability of the retention time. Seawater samples could be directly injected onto the prepared capillary columns, and bromide could be determined to be 63 mg/L.     

Polyhedral oligomeric silsesquioxanes as functional monomer to prepare hybrid monolithic columns for capillary electrochromatography and capillary liquid chromatography

A simple approach to fabricate hybrid monolithic column within the confines of fused-silica capillaries (75 μm i.d.) was introduced. A polyhedral oligomeric silsesquioxanes (POSS) reagent containing a methacrylate group was selected as functional monomer, and copolymerized with bisphenol A dimethacrylate (BPADMA) or ethylene dimethacrylate (EDMA) in the presence of porogenic solvents via thermally initiated free radical polymerization. After optimization of the preparation conditions, two POSS-containing hybrid monoliths were successfully prepared and exhibited good permeability and stability. By comparison of the separation efficiencies of the resulting poly(POSS-co-BPADMA) and poly(POSS-co-EDMA) monoliths in capillary electrochromatography (CEC) and capillary liquid chromatography (cLC), it was indicated the former has better column efficiencies for alkylbenzenes, phenols, anilines and PAHs in CEC and cLC than the latter. Particularly, the hybrid poly(POSS-co-BPADMA) monolith is more suitable for separation of PAHs due to π–π interaction between the analytes and aromatic rings in the surface of monolithic stationary phase.