HPLC separation of fullerenes on two charge-transfer stationary phases

Two charge-transfer stationary phases were prepared by immobilizing p-nitrobenzoic acid and naphthyl acetic acid onto silica. The nitrophenyl moiety and the naphthyl moiety were grafted to silica gel through the spacer of aminoalkyl silanes. The HPLC separation of C60, C70, and higher fullerenes on the new stationary phases was also studied. The influence of mobile phase and column temperature on the separation of C60 and C70 was examined, respectively. The retentions of C60 and C70 on the two stationary phases increased with decreasing toluene content in the mobile phase or with increasing column temperature. Higher fullerenes can be separated well using toluene as the mobile phase on the stationary phase of p-nitrobenzoic acid-bonded silica.     

Preparation of pyrenebutyric acid-modified magnesia-zirconia stationary phases using phosphonate as spacers and their application to the separation of fullerenes

A novel method was proposed for the preparation of pyrenebutyric acid-modified magnesia-zirconia stationary phases. Pyrenebutyric acid was grafted to magnesia-zirconia composites with different pore sizes via the sodium salt of cis-(3-methyloxiranyl)phosphonic acid (fosfomycin) as spacers. Aminated fosfomycin was first absorbed onto the surface of magnesia-zirconia composites during the preliminary step to provide amino and hydroxy reactive sites. And then the pyrenebutyric acid was covalently attached to the amine or hydroxyl groups via amide or ester bonds. The resulting stationary phases were characterized by elemental analysis, diffused reflectance FT-IR, nitrogen adsorption analysis and ¹³C solid state NMR spectra. The HPLC separation of fullerenes on the new stationary phases with different pore sizes was also investigated. The chromatographic performance showed a dependence on the pore size of the magnesia-zirconia matrix. Little retention of fullerenes was observed on the stationary phase with pore sizes about 4.5 nm. However, on the modified magnesia-zirconia with pore sizes about 10 nm, selectivity factors (α) for C₇₀/C₆₀ separation were determined to be 1.76, 2.29, 2.41, 3.10, with carbon disulfide, chlorobenzene, xylene and toluene as mobile phases, respectively. And the high solubility of fullerenes in these solvents dramatically increased the overall potential with regard to preparative fullerene purification. Among the reported stationary phases with pyrene ligands, the pyrenebutyric acid-modified magnesia-zirconia (PYB-F-(ZrO₂-MgO)) with larger pore sizes exhibited the best selectivity for fullerenes. The thermodynamic and kinetic behavior of fullerenes was also examined.     

芘基丁酸键合硅胶固定相对含π电子物质的高效液相色谱分离研究

通过芘基丁酸键合硅胶固定相对含π电子物质的分离研究, 考察了该固定相的色谱性能. 结果表明, 该固定相不仅具有疏水作用和电荷转移性能, 还具有很强的平面识别能力, 可很好地分离烷基苯、 稠环芳烃及取代硝基苯等各种含π电子物质， 且在pH 3.5～7.0 时化学稳定性能良好, 硅醇基效应得到了很好的抑制, 可有效地用于碱性化合物的分离分析.     

Fast separation and quantification of C60 and C70 fullerenes using thermostated micro thin-layer chromatography.

Thermostated micro planar chromatography was applied for systematic separation studies of C60 and C70 fullerenes using n-alkanes as mobile phases on TLC and HPTLC plates coated with polyamide, silica gel, aluminum oxide as well as two types of octadecylsilica (C18) sorbents. Retention data were collected at constant temperature at 20 degrees C (+/-0.05 degrees C) using an unsaturated chamber mode with an eluent, such as n-pentane, n-hexane and n-heptane. The separation results under both saturated and unsaturated chamber modes for selected mobile/stationary phases were also examined, and several parameters, including separation factor (alpha) and resolution (R(S)), were compared with data obtained with high-performance liquid chromatography conditions. Interestingly, C60/C70 fullerenes separation performed on HPTLC plates with a developing distance of 45 mm was better for those observed on a 25 cm length analytical HPLC column under similar conditions to that on carbon coverage of the stationary phase, n-hexane as the mobile phase and separation temperature (R(S) = 1.84 and 1.68 for HPTLC, and HPLC, respectively). Moreover the advantage of the planar chromatographic separation of fullerenes studied is a short elution time of less than 6 min. Furthermore, the reported separation protocol shows a capability for the evaluation of fullerenes quantity in commercial samples.     

Liquid crystalline polymers as stationary phases. IV. Chemical bonding and immobilization of the polymer on silica characterization by solid-state nuclear magnetic resonance spectroscopy

Chemical bonding reaction and immobilization through low energy radiation (heating) have been investigated to fix a side-chain liquid crystalline polymer (SC-LCP) on silica particles in order to use the resulting modified silica in normal-phase HPLC. Highly stable chromatographic stationary phases are observed under excellent polymer solvent flow conditions (THF) for both methods and better column efficiencies are also exhibited towards PAHs' separation compared to the classical coated stationary phase. The characterization of these new stationary phases and the rationale for improved column stability have been investigated by solid state 13C and 29Si CP/MAS NMR spectroscopy. It is clearly shown that the chemical bonding is achieved by the classical hydrosilylation reaction between PHMS chains and vinyl modified silica. The bonded polymer is likely a copolymer than a homopolymer. The immobilization of the SC-LCP by heating results in the breaking of Si-O-Si bonds of the polysiloxane chain after the attack of the silica surface silanols. Applications to fullerenes and carotenes separation of these bonded stationary phases are compared to the separation power of a classical monomeric C18 stationary phase in NP-HPLC as n-hexane-toluene or methyl-tertiobutyl ether-methanol mixtures.     

Fullerenes separation with monomeric type C30 stationary phase in high-performance liquid chromatography

The temperature effect on the separation of fullerenes in LC was examined using monomeric type C30, C18 and C8 alkyl bonded stationary phases. It appears that the C30 phase exhibits superior separation ability for fullerenes. It is observed that the maximum retention temperature of fullerenes on the C30 phase is around 20 degrees C. A strong correlation between the changes in NMR spectra and the retention behavior of the solutes was found. The interpretation of the retention behavior of fullerenes on the alkyl bonded stationary phases, including the behavior in subambient temperature, is discussed using the information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Retention of C60 and C70 fullerenes on reversed-phase high-performance liquid chromatographic stationary phases.

The separation of C60 and C70 fullerenes on four different polysiloxane stationary phases was examined. It was determined that polar solvents can be used as mobile phases effectively for the separation of fullerene molecules. Unlike previously published work, a polymeric octadecyl siloxane (ODS) stationary phase provided higher separation factors for C70/C60 than did monomeric ODS stationary phases or phenyl substituted stationary phases. For example, for a methanol-diethyl ether (50:50, v/v) mobile phase and C60, k' approximately 5.0 separation factors, alpha = 3.3, were achieved with polymeric ODS compared to alpha = 2.2, with a monomeric ODS stationary phase. A linear solvation energy relationship (LSER) was used to model the importance of solvent interactions and stationary phase interaction to solute retention.     

Nanoparticles as stationary and pseudo-station+ary phases in chromatographic and electrochromatographic separations

To improve selectivity, chemical stability, and separation efficiency of chromatography, many past papers reported on nanoparticles (NPs) being used as stationary phases in chromatography. This article covers applications of NPs, including carbon nanotubes, fullerenes, gold NPs, silica NPs, zirconia NPs, and titanium-oxide NPs, as stationary phases in gas chromatography, high-performance liquid chromatography, capillary electrophoresis and capillary electrochromatography.We discuss the advantages and the disadvantages of nanomaterials as stationary phases compared to other materials, including traditional stationary phases. We also discuss future possibilities for developing nanomaterial-based stationary phases.     

A humic acid stationary phase for the high performance liquid chromatography separation of buckminsterfullerenes: theoretical and practical aspects

The influence of the mobile phase composition and column temperature on the chromatographic separation of five buckminsterfullerenes (C60, C70, C76, C78, C84) on a stationary phase based on silica gel with chemically bonded humic acid (Bonded humic acid column (BHAC)) was studied. The retention behavior of the fullerenes was measured under isocratic conditions with different mobile phase compositions, ranging from 0.05-0.70 (v/v) of toluene in cyclohexane. The column temperature was analysed in the range 35-75 °C. The retention factors of the five fullerenes do not depend linearly on the toluene fraction but follow a quadratic relationship. The best chromatographic conditions for baseline separation of the five fullerenes were selected. The retention of the fullerenes on the HA stationary phase was strongly affected by temperature. Positive values of thermodynamic parameters (changes of enthalpy and entropy) were due to the abnormal solubility behaviour of fullerenes in toluene in the temperature range 35-75 °C. The information obtained in this work makes this BHAC very simple to prepare and low cost, useful for fullerene research applications.     

Temperature effect in separation of fullerene by high-performance liquid chromatography

Summary The effect of column temperature, especially at low temperatures, on the separation of fullerenes on monomeric and polymeric octadecyl silica (ODS) bonded phases has been studied. Decreasing the column temperature induces an increase in selectivity. The best temperature for the separation of fullerenes was determined for both types of ODS phase with n-hexane eluent. The selectivity for higher fullerenes on monomeric phases becomes similar to that on polymeric phases to low temperature. It has been found that as the carbon content of monomeric phases is increased, the selectivity also becomes similar to polymeric phases.     

Use of nanoparticles in capillary and microchip electrochromatography

Applications of nanoparticles are of rising interest in separation science, due to their favorable surface-to-volume ratio as well as their applicability in miniaturization. A stationary phase with large surface area in combination with an electroosmotic flow-driven system has great potential in a highly efficient separation system. This review covers the use of various nanoparticles as stationary or pseudostationary phase in capillary and microchip electrochromatography. The use of nanoparticles in pseudostationary phase capillary electrochromatography and open-tubular capillary electrochromatography are thoroughly discussed. The stationary and pseudostationary phases that are described include polymer nanoparticles, gold nanoparticles, silica nanoparticles, fullerenes and carbon nanotubes.     

2,4,6-三硝基苯酚改性锆镁复合氧化物固定相分离富勒烯的色谱性能研究

制备了用于分离C60和C70的2,4,6-三硝基苯酚改性锆镁复合氧化物高效液相色谱固定相,并用元素分析仪、红外光谱仪和比表面积分析仪等对其进行了表征。考察了流动相中甲苯含量和柱温对C60和C70分离的影响。同时考察了以甲苯为流动相,在348 K的柱温下,该固定相分离富勒烯混合物(含3%高富勒烯)的情况。结果表明,2,4,6-三硝基苯酚改性锆镁复合氧化物固定相对富勒烯有较强的保留,并表现出较强的温度关系,提高柱温能提高固定相对富勒烯的分离能力,并具有用于富勒烯制备分离的潜力。     

New stationary phases for normal-phase HPLC based on silica gel modified with cobalt,nickel, zinc,and cadmium salts

Several new stationary phases for normal-phase high-performance liquid chromatography were synthesized by the modification of silica gel with cobalt, nickel, zinc, and cadmium salts and subsequent calcination at 700°C. These sorbents exhibit an increase in the retention of organic compounds of the basic character, an increase in the selectivity of separation, and an eightfold increase in efficiency in comparison with initial silica gel. The experimental results were explained on the basis of the interpretation of the IR spectra of pyridine adsorbed on the synthesized and initial silica gel. It was demonstrated that the surface of silica gel modified with cadmium ions exhibits the highest acidity. The advantage of the synthesized sorbents was demonstrated with the example of the separation of fullerenes containing diethylamino and hydroxy groups.__________Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 4, 2005, pp. 412–416.Original Russian Text Copyright © 2005 by Pirogov, Kuzavlev, Shpigun.     

Effect of temperature on the mechanism of retention of fullerenes in liquid chromatography using various alkyl bonded stationary phases

Summary The temperature-dependency of the separation of fullerenes in liquid chromatography (LC) has been examined using various alkyl bonded stationary phases. It has been found that a maximum retention temperature exists with long alkyl bonded stationary phases, whereas there is no similar effect with the newly synthesized alkyl bonded phases which have two phenyl groups at the base of the bonded phase. The interpretation of the retention behavior of fullerenes in the low temperature region on alkyl bonded stationary phases is discussed using information obtained by CP-MAS solid-state NMR spectroscopy and LC.     

Retention Behavior of Proteins on Glutamic Acid-Boned Silica Stationary Phase

A glutamic acid-bonded silica (Glu-silica) stationary phase with cation-exchange properties was synthesized using l-glutamic acid as ligand and silica gel as matrix. The effects of solution pH value, salt concentration and metal ion on the retention of proteins were examined. The standard protein mixture was separated with a prepared chromatographic column and an iminodiacetic acid column, and compared. The influence of the binding capacity of an immobilized metal ion on the complexation of metal chelate column was studied. The results indicate that the obtained column displays cation-exchange characteristic and better separation ability for proteins. As fixing metal ion on the Glu-silica column, retention of proteins on the column is a cooperative interaction of metal chelate and cation-exchange. The stationary phase shows the typical metal chelate properties with the increase of the sorption capacity of immobilized metal ion.     

Effect of coverage density and structure of chemically bonded silica stationary phases on the separation of compounds with various properties

The chemical character, geometry, and architecture of chemically formed surface layers determine interactions between stationary phase, analyte, and mobile phase, and therefore the retention mechanisms (partitioning, adsorption, ion exchange, steric exclusion) of separated analytes. These interactions also depend on the structure and chemical character of the solutes and the composition of the mobile phase. High-molecular-weight fullerenes (C60 and C70) and water-soluble selenium-containing peptides (833 and 2607 Da) were used for the evaluation of laboratory-prepared octadecyl stationary phases with high and low coverage density before and after end-capping. The aim of this work was to study differences in surface coverage density and homogeneity and conformational changes of chemically bonded moieties and the influence of these parameters on the separation of mixtures of selenopeptides and fullerenes with significantly different molecular masses. A topographical model of the chemically modified stationary surface is presented.     

碳笼化合物的高效液相色谱研究

采用化学键合固定相（ＤＮＡＰ－ＳＰ）及二元混合流动相（５０％苯－正已烷），对碳笼化合物Ｃ６０与Ｃ７０的高效液相色谱（ＨＰＬＣ）分析及分离进行了研究。比较了多种流动相对Ｃ６０、Ｃ７０分离效果的影响，苯／正已烷混合体系是最好的流动相。     

“Sock-Ball” Chromatographic Separation of High Molecular Weight Fullerenes with Sock-Shaped Stationary Phase

A method for “Sock-Bail” chromatographic separation of high molecular weight fullerenes is described. A prepared sock-shaped stationary phase (Sock-SAF-phase) was used for HPLC separation of several polycyclic aromatic hydrocarbons (PAHs) and fullerenes. Fullerenes, as ball-shaped molecules, are much more strongly retained than PAHs on this stationary phase and have the eluted order C₅₀ < C₇₀ < C₇₆ < C₇₈ < C₈₄ in the mobile phase of n-hexane/dichloromethane (100/0 ～ 80/20). In contrast, chromatography on the corresponding unmodified silica phase or SC-3OH-phase (an intermediate phase of Sock-SAF-phase) gave no separation of fullerenes. This fact indicated that the separation of fullerenes on Sock-SAF-phase was related to the selective interaction with the sock moiety.     

In-situ carbonizing of coal pitch on the surface of silica sphere as quasi-graphitized carbon stationary phase for liquid chromatography

A novel chromatography stationary phase with a quasi-graphitized carbon modified shell has been developed. Coal pitch was directly carbonized on the surface of porous silica with in-situ carbonization. The carbonized coal pitch coating exhibits some degree of graphitization with a 78 nm-thick layer on the surface of silica and a 0.5 nm-thick layer on the inner surface of the mesopores. Based on the special structure of the graphitized carbon coating, the novel stationary phase can provide multiple interactions such as hydrophobic interaction, π-π interaction and dipole-dipole interaction. The novel composite material exhibited unique separation selectivity and excellent separation efficiency for polar compounds, including imidazoles, nucleosides and pesticides. Besides, the packed column also exhibited great repeatability with the RSDs of the retention time of nucleosides between 0.07%-0.50% (n = 5). Finally, satisfied result was achieved in the separation of fullerenes on the new column, suggesting the great potential in the industrial-scale purification of fullerenes.     

Polar-copolymerized approach based on horizontal polymerization on silica surface for preparation of polar-modified stationary phases

A new approach for preparation of polar-modified reversed-phase liquid chromatography stationary phases was developed by using horizontal polymerization technique on silica surface, which was defined as “polar-copolymerized” approach. Based on this new approach, a representative polar-copolymerized stationary phase composed of mixed n-octadecyl and chloropropyl (C18–C3Cl) was synthesized. The resulting stationary phase named C18HCE was characterized with elemental analysis and solid phase ¹³C and ²⁹Si NMR, which proved the chemistry of polar-copolymerized stationary phases. Chromatographic evaluation and application of the C18HCE were also investigated. The results of preliminary chromatographic evaluation demonstrated that the C18HCE stationary phase exhibited 100% aqueous mobile phase compatibility, low silanol activity. In addition, the application results demonstrated that the C18HCE had superior separation performance in alkaloids separation at acidic conditions compared to some commercial stationary phases.