Chemical compositional separation of styrene-methyl methacrylate copolymers using high-performance liquid chromatography with liquefied carbon dioxide as eluent

Chromatographic separation of styrene-methyl methacrylate (MMA) copolymers depending on the chemical composition was studied using liquefied carbon dioxide as an adsorption promoting solvent, and tetrahydrofuran, chloroform containing ethanol as a desorption promoting solvent in the mobile phase and the column packed non-bonded silica gel by a solvent gradient method. With the increase of MMA content, the elution was retarded indicating that the typical normal-phase type of adsorption occurred. The effects of type of desorption solvents, molecular mass of sample, and column temperature on the elution were investigated.     

Liquid Chromatographic Analysis of Styrene-Methacrylate and Styrene-Acrylate Copolymers

Abstract

Copolymers of styrene-methacrylate (methyl-, ethyl-, and n-butyl-) and styrene-acrylate (methyl-, ethyl-, and n-butyl-) were prepared by solution polymerization at a low degree of conversion. These copolymers were separated according to composition by liquid adsorption chromatography. Silica gel was the stationary phase and a mixture of chloroform and ethanol was the mobile phase. Ethanol content in the mobile phase affected the elution of the copolymers and methacrylate or acrylate rich copolymers required much ethanol to elute from a column. The retention of the copolymers was controlled by column temperature and the copolymers tended to retain in a column at higher column temperature. A linear gradient elution method way to increase ethanol in the mobile chase was effective to separate the copolymers in the order of increasing the methacrylate or acrylate content. Styrene rich copolymers eluted first from a column. Resolution between two adjacent peaks was improved with the increase in column temperature. In a mixture of copolymers of styrene-methacrylate or styrene-acrylate (methyl, ethyl, and n-butyl) having the same styrene content, the elution was in the order of n-butyl, ethyl, and methyl methacrylate or acrylate copolymers with styrene. In copolymrs having the same styrene content, a mixture of copolymrs having diffrent ester groups was separated and a mixtue of copolymrs having the same estr group (e.g., styrene-methyl mth-acrylate and styrene-methyl acrylate copolymers) was not separated.     

Effect of the surface coverage of endcapped C18-silica on the excess adsorption isotherms of commonly used organic solvents from water in reversed phase liquid chromatography

The excess adsorption isotherms of methanol, ethanol, 2-propanol, acetonitrile, and tetrahydrofuran from water were measured on five different silica-based packing materials by the minor disturbance method. These materials were prepared with the same lot of 5-microm particles (average pore size 90 A), all endcapped with trimethylchlorosilane (TMS), and bonded to octadecyl chains with different surface coverages (0, 0.42, 1.01, 2.03, and 3.15 micromol/m2). The relative adsorption of one eluent by respect to a second one informs on the heterogeneity of the material (alkyl-bonded and bare silica regions) and on the accessibility of the unreacted silanol groups to the mobile phase. It is shown that the total surface area of the adsorbent decreases with increasing degree of surface coverage with octadecyl chains and that the relative surface area of the regions occupied by accessible silanol groups to the regions occupied by alkyl-bonded groups decreases. For the five columns, an average of 10% of the adsorbent surface area is covered of bare silica accessible to the liquid phase, with a minimum of 5% with tetrahydrofuran and a maximum of 12% with ethanol or 2-propanol. Increasing the surface coverage by the C18 chains causes a significant increase of the attraction potential of the hydrophobic surface toward the organic solvent. This result is confirmed by the increase of the number of adsorbate monolayers with increasing bonding density of the octadecyl chains. This number is twice larger for the 315C18 column than for the C1 column.     

Silanol group effect in C18 bonded phases in HPLC

Summary If any residual (free) silanol groups remain at the surface of silica gel after bonding treatment, they may affect the retention of solutes since the dissociated groups (SiO^–) will attract cations. The silanol group effect on the retention of cationic solutes will increase with increasing pH of the mobile phase but the effect will decrease with increasing hydrophobic-ion concentration at the C₁₈ surface because such ions can mask the residual silanol groups. A method for the separation of metal complexes with 2-(5-bromo-2-pyridylazo)-diethylaminephenol (5-Br-PADAP) has been developed. The hydrophobic ion in the MeOH/H₂O mobile phase was tetrabutylammonium (TBA).     

Comparison of the adsorption mechanisms of pyridine in hydrophilic interaction chromatography and in reversed-phase aqueous liquid chromatography

The adsorption isotherms of pyridine were measured by frontal analysis (FA) on a column packed with shell particles of neat porous silica (Halo), using water–acetonitrile mixtures as the mobile phase at 295 K. The isotherm data were measured for pyridine concentrations covering a dynamic range of four millions. The degree of heterogeneity of the surface was characterized by the adsorption energy distribution (AED) function calculated from the raw adsorption data, using the expectation-maximization (EM) procedure. The results showed that two different retention mechanisms dominate in Per aqueous liquid chromatography (PALC) at low acetonitrile concentrations and in hydrophilic interaction chromatography (HILIC) at high acetonitrile concentrations. In the PALC mode, the adsorption mechanism of pyridine on the silica surface is controlled by hydrophobic interactions that take place on very few and ultra-active adsorption sites, which might be pores on the irregular and rugose surface of the porous silica particles. The surface is seriously heterogeneous, with up to five distinct adsorption sites and five different energy peaks on the AED of the packing material. In contrast, in the HILIC mode, the adsorption behavior is quasi-homogeneous and pyridine retention is governed by its adsorption onto free silanol groups. For intermediate mobile phase compositions, the siloxane and the silanol groups are both significantly saturated with acetonitrile and water, respectively, causing a minimum of the retention factor of pyridine on the Halo column.     

Rheological Properties of Silica Suspensions in Aqueous Solutions of Block Copolymers and Their Water-Soluble Components

Dynamic moduli of fumed silica suspensions in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers and PEO homopolymers were measured as a function of surface coverage. Since the block copolymers and PEO are adsorbed on the silica surface through hydrogen bonding between the ether oxygen and the silanol group on the silica surface, the interaction between the silanol groups, which is dominant for the aggregation of silica particles, should be prohibited. Dynamic moduli in the silica suspensions were strongly related to the stability of the silica suspensions and the block copolymer, and the longest PEO portion was useful for stabilizing the silica particles. However, the PEO homopolymer did not support stability of the silica particles, suggesting that chain conformation of the PEO portion in the block copolymer is different from that for the PEO homopolymer. Copyright 2001 Academic Press.     

Preparation and characterization of mixed-mode monolithic silica column for capillary electrochromatography

A silica-based monolithic stationary phase with mixed-mode of reversed phase (RP) and weak anion-exchange (WAX) for capillary electrochromatography (CEC) has been prepared. The mixed-mode monolithic silica column was prepared using the sol–gel technique and followed by a post-modification with hexadecyltrimethoxysilane (HDTMS) and aminopropyltrimethoxysilane (APTMS). The amino groups on the surface of the stationary phase were used to generate a substantial anodic EOF as well as to provide electrostatic interaction sites for charged compounds at low pH. A cathodic EOF was observed at pH above 7.3 due to the full ionization of residual silanol groups and the suppression in the ionization of amino groups. A variety of analytes were used to evaluate the electrochromatographic characterization and column performance. The monolithic stationary phase exhibited RP chromatographic behavior toward neutral solutes. The model anionic solutes were separated by the mixed-mode mechanism, which comprised RP interaction, WAX, and electrophoresis. Symmetrical peaks can be obtained for basic solutes because positively charged amino groups can effectively minimize the adsorption of positively charged analytes to the stationary phase.     

Siloxane/silarylene copolymers as stationary phases for capillary gas chromatography part I: Evaluation of silanol terminated dimethyl substituted polymers

The thermal stability of silicones can be improved on replacement of certain of the oxygen atoms in the polymer backbone by phenyl groups. Such a polymer has been synthesized and evaluated for use as stationary phase in fused silica capillary gas chromatography; the polymer was dimethyl substituted and silanol terminated. A selectivity was provided by the phenyl groups in the backbone. For comparative purposes, a silanol-terminated dimethylpolysiloxane has also been evaluated. Both stationary phases gave columns of highest separation efficiency and the supporting fused silica surface was deactivated by the stationary phases on thermal treatment. Further, low column bleeding was observed at the maximum temperature tested, 370°C. The phenyl-containing phase could be immobilized to 60% by heat treatment, but the pure dimethylpolysiloxane was 10% immobilized. The influence on immobilization of factors such as nature of the supporting surface, stationary phase silanol content, reaction temperature and atmosphere in the column during reaction has been studied.     

Examination of the surface heterogeneity of reversed-phase packing materials with solvent adsorption

The adsorption isotherms of acetonitrile, ethanol, 2-propanol, and THF were measured using frontal analysis on six columns packed with octadecyl RP stationary phase. The effect of the bonding density of the end-capped octadecyl bonded phase on the adsorption properties was measured. Adsorption isotherm data were collected from aqueous solutions of the four organic modifiers. The isotherm model for solvent adsorption was selected using two independent parameter estimation methods, the regression analysis and adsorption energy distribution. The fitted isotherm parameters were tested by modeling of overloaded elution bands with the aid of the equilibrium-dispersive model of chromatography. The surface heterogeneity estimations and the effect of the silanol groups on the adsorption of solvents were based on those data.     

Ion-chromatographic behavior of alkali metal cations and ammonium ion on zirconium-adsorbing silica gel

The preparation and evaluation of zirconium-adsorbing silica gel (Zr-Silica) as an ion-exchange stationary phase in ion chromatography for inorganic anions and cations was carried out. The Zr-Silica was prepared by the reaction of silanol groups on the surface of the silica gel with zirconium butoxide (Zr(OCH2CH2CH2CH3)4) in ethanol. The ion-exchange characteristics of the Zr-Silica were evaluated using 10 mM tartaric acid at pH 2.5 as eluent. The Zr-Silica was found to act as a cation-exchanger under the eluent conditions. The retention behavior of alkali and alkaline earth metal cations was then investigated. The Zr-Silica column was proved to be suitable for the simultaneous separation of alkali metal cations and ammonium ion. Excellent separation of the cations on a 15 cm Zr-Silica column was achieved in 25 min using 10 mM tartaric acid as eluent.     

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.     

Analysis of the band profiles of the enantiomers of phenylglycine in liquid chromatography on bonded teicoplanin columns using the stochastic theory of chromatography.

The retention behaviour of the enantiomers of underivatized phenylglycine was studied on a Chirobiotic T column packed with amphoteric glycopeptide teicoplanin covalently bonded to the surface of silica gel. The retention and the selectivity of separation of the enantiomers increase with rising concentration of ethanol or of methanol in aqueous-organic mobile phases. The band profiles of the less retained L-phenylglycine are symmetrical, but the band profiles of the more strongly retained D-phenylglycine are tailing in all mobile phases tested. The band broadening does not diminish even at very low concentrations of phenylglycine, so that it cannot be attributed to possible column overload. The analysis of the band profile using the stochastic theory of chromatography suggests that the broadening can be attributed to at least two additional chiral centres of adsorption in the stationary phase contributing to the retention of the more strongly retained enantiomer in addition to the adsorption of the less retained one. This behaviour can be explained by the complex structure of the teicoplanin chiral stationary phase.     

Tetrazole-Functionalized Silica for Hydrophilic Interaction Chromatography of Polar Solutes

In this work, tetrazole-functionalized stationary phase was prepared with nitrile-modified silica by an ammonium-catalyzed (3 + 2) azide-nitrile cycloaddition reaction. The prepared stationary phase was used for separation of nucleobases and nucleosides by hydrophilic interaction chromatography (HILIC) mode. A typical HILIC mechanism was observed at higher content of acetonitrile (>85%, v/v) in the mobile phase. The retention mechanism of the column was investigated by the models used for describing partitioning and surface adsorption through adjustment ratio of water in the mobile phase, and by the influence of salt concentration, buffer pH, and temperature on the retention of solutes. The results illustrated that the surface adsorption through hydrogen bonding dominated the retention behavior of nucleobases/nucleosides under HILIC mode. From the separation ability, the tetrazole-functionalized stationary phase could become a valuable alternative for the separation of the compounds concerned.     

Preparation and application of a chiral stationary phase based on (+)-(1 8-crown-6)-2,3,11,12-tetracarboxylic acid without extra free aminopropyl groups on silica surface

A crown ether-based chiral stationary phase (CSP) without extra aminopropyl groups on the surface of silica gel was newly prepared by bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid to silica gel. The new CSP was applied to the resolution of various racemic alpha-amino acids, amines, and amino alcohols. The chiral recognition efficiency of the new CSP was generally superior to that of the original CSP containing unreacted residual aminopropyl groups on the surface of silica gel in terms of the separation (alpha) and the resolution factors (Rs). The retention behaviors of analytes on the new CSP with the variation of the content of organic and acidic modifiers in aqueous mobile phase were consistent with those on the original CSP in the resolution of alpha-amino acids, but somewhat different in the resolution of racemic amines and amino alcohols from those on the original CSP and the difference was rationalized by the lipophilicity difference of the two CSPs. The effect of the column temperature on the chromatographic resolution behaviors on the new CSP was consistent with that on the original CSP.     

Separation of basic proteins by capillary zone electrophoresis with coatings of a copolymer of vinylpyrrolidone and vinylimidazole

Capillary zone electrophoretic (CZE) separation of basic proteins has been achieved with capillary columns modified with copolymers of vinylpyrrolidone (VP) and vinylimidazole (VI). The copolymerization reaction is performed inside the capillary column and involves chemical bonding of the polymer to silica. The electroosmotic flow (EOF) is greatly decreased by this surface modification. The presence of positive charges on the coating surface, due to the cationic property of vinylimidazole at pH below 7, reduces the adsorption of basic proteins onto the silanol groups of the capillary surface. Acidic proteins are irreversibly adsorbed, but rapid separation and good performance reproducibility are obtained with basic proteins. In the case of capillaries modified with VP, the acidic and basic proteins are eluted within 10 min. In this work, we studied the effects of pH and buffer concentration on the magnitude of the EOF, as well as the effect of copolymer composition on the separation efficiency.     

基于L-谷氨酸的手性硅胶球的制备及其应用

无机介孔硅球因其具有足够的机械强度、热稳定性,以及适应多种流动相的优点,成为高效液相色谱(HPLC)柱填料中使用最广泛和最重要的材料。但在此研究领域中,并未见球形的全无机手性硅胶用作HPLC手性固定相。该文以无机球形介孔硅胶作为研究对象,通过堆砌硅珠法,以硅溶胶为原料,L-谷氨酸(L-Glu)为手性源,在手性环境中制造出脲醛树脂与胶体二氧化硅混合的小球,在550 ℃高温下煅烧除去树脂部分,制备基于L-Glu的无机介孔硅胶球。通过元素分析、红外光谱、扫描电镜、透射电镜和氮气吸附等表征证明这是一种具有规则球形的手性硅胶球,其手性来源于硅胶球自身的骨架和孔结构。将L-Glu手性硅胶球作为固定相制备了HPLC色谱柱,以正己烷-异丙醇(9∶1, v/v)作为流动相,流速为0.1 mL/min,考察了该手性柱对一系列外消旋化合物的拆分性能。实验表明,该手性柱拆分了15种外消旋化合物,其中特罗格尔碱、吡喹酮、3-苄氧基-1,2-丙二醇、1,2-环氧己烷、3-羟基-2-丁酮、2-甲基四氢呋喃-3-酮、异丙基缩水甘油醚达到基线分离;还分离了10种苯系位置异构体,o,m,p-氨基苯酚、o,p-氯苯酚、o,m,p-碘苯胺、o,m,p-甲苯胺、o,m,p-二硝基苯、o,m,p-氯苯胺、o,m,p-硝基苯酚、o,m,p-溴苯胺达到基线分离。实验表明,L-Glu手性硅胶球在手性分离方面具有良好的可行性,与普通硅胶相比不需要进一步修饰就可以有较好的手性分离效果,是一种低成本、制备便捷的手性无机硅胶固定相。     

HPLC of synthetic polymers

Summary Statistical copolymers of styrene and acrylonitrile have been separated by high performance precipitation chromatography according to the increase in acrylonitrile content. The separation mechanism was strongly depending on the combination of stationary phase and eluent composition. By applying a typical normal phase gradient from n-heptane (precipitation) to dichloromethane the polymers were adsorbed, after dissolution of the polar groups of the stationary phases. The elution curves became broader with increasing acrylonitrile content of the polymer. By the addition of 20% methanol to the dichloromethane the adsorption could be minimized and copolymer elution was dependent solely on solubility and independent of stationary phase polarity. Separation according to chemical composition was also possible without precipitation with a large pore silica and a normal phase gradient from dichloromethane to the same eluent containing 2.75% methanol.Part I see reference 19     

CMC型高分子表面活性剂在固/液界面上的吸附

在润湿、乳化、洗涤、分散等应用领域中,表面活性剂分子在界面上的吸附状态对性能有重要影响.另一方面,在化学驱油过程中,表面活性剂分子在氧化物矿物上的吸附是引起表面活性剂损失的主要原因,表面活性剂的损耗量大,将降低采收率及经济效益[1].高分子表面活性剂作为一种多功能的新型表面活性剂在许多领域有广阔的应用前景,但对其性能研究尚处于起步阶段,特别是结构复杂的高分子双亲性共聚物,在吸附、乳化等方面研究尚少报导.羧甲基纤维素系列高分子表面活性剂是采用独特的超声波辐照技术合成的嵌段型共聚物,具有优良的表/界面活性[2],可望用…     

Silanol group effect of alkylnitrile (cyano) bonded phase in high-performance liquid chromatography

Summary When a cyano bonded phase is used together with a nonaqueous eluent, it is universal that the silanol groups which remain at the surface of the silica gel after bonding affect the retention of solutes. With solutes containing such atoms as N, S and O, hydrogen bond may form between the solute and the residual silanol group, leading to dual retention mechanism. Based on the understanding of retention mechanism, methods were developed to separate metal-diethyldithiocarbamate (DDTC) chelates and crown ethers on cyano bonded phase, the mobile phase being a nonaqueous solvent containing triethylamine (TEA). The work was supported by the National Science Foundation of China.