End-functionalized polyethylene oxide coated silica particles for packed capillary column supercritical fluid chromatography

Summary Polyethylene oxide (PEO)-based polymers with hydroxy, methoxy, and aminopropoxy terminal groups were coated on diol functionalized and hexamethyldisilazane end-capped silica particles. Proton-donor and proton-acceptor test solutes, including carboxylic acids, hydroxy-containing compounds, arylamines, and alkylamines were used to evaluate the chromatographic performances of these polymer coated particles under SFC conditions with neat CO₂ as mobile phase. It was found that the particles coated with hydroxy-terminated PEO were suitable for the separation of proton-donor compounds such as hydroxy-containing compounds and carboxylic acids, and the particles coated with aminopropoxy-terminated PEO could be used for the separation of amines. That is, the proton-accepting stationary phase is suitable for the separation of proton accepting solutes, including strong basic alkylamines (pK_b4), using neat CO₂ as mobile phase, while the protondonating stationary phase is suitable for the separation of proton-donating compounds such as carboxylic acids (pK_a4). Hydrogen bond basicity was found to be a critical factor for the chromatography of basic amines. Low volatility acidic and basic drugs were chromatographed using the new stationary phases. The stability of the PEO coated particles was determined by measuring the loss of organic carbon under SFC conditions. It was found that approximately 18 % of the coating (average molecular weight of 15,000) was washed out of the particles by supercritical CO₂ after 7 h at 350 atm and 50°C     

Separation behavior of basic compounds on unbonded silicon oxynitride and silica high‐performance liquid chromatography stationary phases with reversed‐phase eluents

Unbonded silicon oxynitride and silica high‐performance liquid chromatography stationary phases have been evaluated and compared for the separation of basic compounds of differing molecular weight, pK_a, and log D using aqueous/organic mobile phases. The influences of percentage of organic modifier, buffer pH, and concentration in the mobile phase on base retention were investigated on unbonded silicon oxynitride and silica phases. The results confirmed that unbonded silicon oxynitride and silica phases demonstrated excellent separation performance for model basic compounds and both the unbonded phases examined possessed a hydrophobic/adsorption and ion‐exchange character. The silicon oxynitride stationary phase exhibited high hydrophilicity compared with silica with a reversed‐phase mobile phase. An ion‐exclusion‐type mechanism becomes predominant for the separation of three aimed bases on the silicon oxynitride column at pH 2.8. Different from silicon oxynitride stationary phase, no obvious change for the retention time of three model bases on silica stationary phase at pH 2.8 can be observed.     

Characterization,evaluation and in-situ cross-linking of new polar phases in capillary gas chromatography

Summary The possibilities of OV-1701 and RSL-310, two new stationary phases, have been evaluated for capillary gas chromatography in fused silica columns. OV-1701 is a cyanopropylphenyldimethyl polysiloxane of moderate polarity possessing excellent chromatographic characteristics. The phase exhibits high coating and chromatographic efficiencies, high temperature stability and is suitable for cross-linking. RSL-310 is a polar liquid stationary phase yet to be permanently bonded in a capillary column. The selectivity of both phases extends the applicability of fused silica columns.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Optimization of the surface modification process of cross‐linked polythiol‐coated chiral stationary phases synthesized by a two‐step thiol‐ene click reaction

A new platform technology for the preparation of stable chiral stationary phases was successfully optimized. The chiral selector tert‐butylcarbamoylquinine was firstly covalently connected to the polymer poly(3‐mercaptopropyl)methylsiloxane by thiol‐ene click reaction. Secondly, the quinine carbamate functionalized polysiloxane conjugate was coated onto the surface of vinyl modified silica particles and cross‐linked via thiol‐ene click reaction. The amount of polysiloxane, chiral selector, radical initiator, reaction solvent (chloroform and methanol), reaction time, and pore size of the supporting silica particles were varied and systematically optimized in terms of achievable plate numbers while maintaining simultaneously enantioselectivity. The optimization was based on elemental analysis data, chromatographic results, and H/u‐curves (Van Deemter) of the resultant chiral stationary phases. The results suggest that better chromatographic efficiency (higher plate numbers) at equal enantioselectivity can be achieved with methanol (a poor solvent for the polysiloxane that is dispersed rather than dissolved) and a lower film thickness of quinine carbamate functionalized polysiloxane. In this study, chiral stationary phases based on 100 Å silica slightly outperformed 200 Å silica particles (each 5 μm). The optimized two step material exhibited significantly reduced mass transfer resistance compared to the one step material and equal performance as a brush‐type chiral stationary phase.     

Determination of α-tocopherol and ergocalciferol by thin-layer chromatography

A procedure was developed for determining α-tocopherol (vitamin E) and ergocalciferol (vitamin D₂) in oils. The procedure involved the alkaline hydrolysis of an oil according to GOST (State Standard) 30417 followed by the independent recovery of vitamins by extraction with organic solvents. The extracts were separated on Sorbfil plates with the stationary phase of silica gel with a particle size of 5–12 μm by eluting vitamin E with an octane-diethyl ether mixture (7: 1) and vitamin D₂ with a tetrachloromethane-diethyl ether mixture (4: 1). The zones of α-tocopherol and ergocalciferol were developed with conc. HNO₃ and conc. H₂SO₄, respectively. The plate scanning and image processing were performed with a Sorbfil Videodensitometer software. The calibration plots were linear in the range of 2.5–15 and 15–50 mg/mL for α-tocopherol and 0.08–0.13 mg/mL for ergocalciferol. The procedure was used to determine vitamins in bur marigold oily extract and grape seed oil.     

Crosslinking of alkylpolysiloxane filsm on various types of glass surfaces including fused silica using γ-radiation of a60cobalt-source. Comparison to crosslinking by thermal peroxid treatment

Summary Crosslinking of alkylpolysiloxane stationary phases, especially in thick film capillary columns, is useful for the homogenous coating of fused silica and pretreated alkaliglass surfaces. The films of the stationary liquid are immobilized against solvent rinsing using CH₂Cl₂, pentane, and acetone, and maintain homogenity even at high temperature. Various doses of -radiation from a⁶⁰Co. source were used for the crosslinking instead of the thermal peroxid treatment recently described by other authors. The effect of the -radiation crosslinking-procedure was investigated in comparison to the peroxid method in regard of: decrease of stationary phase content by solvent rinsing, separation efficiency, tailing behaviour, and bleeding of the columns obtained. Similar results as with the cumylperoxid-treatment were achieved using the -radiation-method. By -radiation no polar functional groups or moleculs are introduced into the stationary phase, however, as with the peroxid method. Less than 20% of the various stationary liquids are usually removed from the columns by solvent rising after crosslinking using both methods depending on the doses of radiation and the cumylperoxid concentration applied respectively.Dedicated to Dr. L. S. Ettre on the occasion of his 60th birthday.     

Surface Energy of Silica-TEOS-PDMS Ormosils

Inverse gas chromatography (IGC) was applied to characterize the surface energy of organically modified silicates (ormosils) by measuring the interaction of molecular organic probes with the ormosil surface. Ormosils were prepared by the sol-gel method by the reaction of TEOS (tetraethoxysilane), PDMS (polydimethylsiloxane) and different types of silica (Aerosil 130, Aerosil 200 and Aerosil 380). The isosteric heat of adsorption, q _st, and the dispersive component of the surface energy, _s ^D, were estimated by using the retention volume of different nonpolar and polar probes at infinite dilution. The dispersive component shows an increase as the specific surface area of the silica is increased from 29.6 mJ/m² to 51.4 mJ/m² at 60°C. Such values are lower than that obtained for aerosil particles meaning that PDMS chains impede the interaction with silanol groups located on the silica surface. The specific interaction parameter, I^SP, and the enthalpy of specific adsorption, H _a ^SP, of polar probes on the ormosil surface were also measured in order to obtain the acid-base character of ormosil surface. The H _a ^SP, was correlated with the donor, DN, and the acceptor, AN, numbers of the probes to quantify the acidic, K _A, and the basic, K _B, parameters of the substrate surface. The obtained results suggest that the silica particles were covered by PDMS chains in a different way depending on the type of silica used. The values of K _A and K _B suggest that the ormosil surface is amphoteric, with predominantly acceptor electron sites.     

Semiquantitative determination of ten vitamins by the nichrome wire ring chamber

Summary A method for microidentification and semiquantitative determination of vitamins A, B₁, B₂, B₆, C, D₂, D₃, E, K₃, andp-aminobenzoic acid from a single drop of solution using the all glass nichrome wire ring chamber is described. The accuracy is± 5% and the method is selective and rapid and other vitamins do not interfere. The application of the method is demonstrated by the determination of vitamins from commercially available multivitamin tablets and injections.

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Zusammenfassung Der Mikronachweis und die semiquantitative Bestimmung der Vitamine A, B₁, B₂, B₆, C, D₂, D₃, E, K₃ und p-Aminobenzoesäure in einem Tropfen Lösung mit Hilfe der aus Glas gefertigten, mit einem Nichromdraht versehenen Ringkammer wurde beschrieben. Die Genauigkeit beträgt ± 5%das Verfahren ist selektiv, andere Vitamine stören nicht. Es wurde an handelsüblichen Multivitamintabletten und -Injektionslösungen erprobt.

     

Raman spectroscopic study of the conformational order of octadecylsilane stationary phases: effects of electrolyte and pH

This study investigates effects of the electrolyte, of acidic and basic compounds, and of pH on the rotational and conformational order of octadecylsilane stationary phases with surface coverages of 3.09 and 6.45 mol/m². Both phases exhibit an increase in alkyl chain rotational and conformational order in 5–200 mM aqueous electrolyte solutions relative to water. These stationary phases are effectively salted-out of aqueous electrolyte solutions, thereby causing alkyl chain intermolecular interactions to increase with a concomitant increase in alkyl chain order. Although the presence of acidic and basic compounds generally has no effect on the conformational order of either stationary phase as a function of pH, the higher coverage stationary phase does exhibit pH-dependent changes in aqueous solutions of benzoic acid. At pH values below the pK_a of benzoic acid, the conformational order of this stationary phase is unchanged relative to that observed in the same pH solution in the absence of benzoic acid. In light of independent evidence that such monosubstituted aromatics interact with the octadecylsilane stationary phase under these conditions, the absence of a measurable effect on alkyl chain order for these conditions is attributed to benzoic acid self-association at the stationary phase-mobile phase interface. In contrast, at pH values above the pK_a of benzoic acid, slight disordering of the alkyl chains is observed and is attributed to repulsive interactions between retained benzoate anions.Electronic Supplementary Material Supplementary material is available for this article at     

Study on the chromatographic behavior of water-soluble vitamins on p-tert-butyl-calix[8]arene-bonded silica gel stationary phase by HPLC

In this paper, the chromatographic behavior of some water-soluble vitamins was studied on a new p-tert-butyl-calix[8]arene-bonded silica gel stationary phase (CABS, 5 μm particle size, the bonded amount 0.071 mmol g⁻¹) by using vitamin standards as probes for HPLC. The comparative study of the separation of these compounds was done by using CABS and ODS as stationary phases under the same chromatographic conditions. The better separation of six vitamins including: B₁, B₂, B₆, B₁₂, C, and nicotinic acid (B₅), on CABS can be achieved by using isocratic mode with methanol-phosphate buffer (25:75, (v/v)) as mobile phase within 20 min. The results show that the calix[8]arene-bonded phase exhibits high selectivity for water-soluble vitamins. We found that the elution order of B₂ (12.08 min) and B₁₂ (16.42 min) on CABS was very different from that of B₁₂ (7.76 min) and B₂ (18.47 min) on ODS, which indicate that different retention mechanisms exist in the chromatographic processes of the two stationary phases. According to the chromatographic data, it can be concluded that various chromatographic retention mechanisms are responsible for the separation of above compounds on CABS, such as hydrophobic interaction, hydrogen bonding interaction, and π-π interaction. The new packing has two advantages over ODS. On one hand, the polar and ionized analytes, such as C and B₅, exhibited stronger affinities to CABS because of hydrogen bonding interaction. On the other hand, the retention of B₂ and B₁₂ became shorter on CABS with weaker hydrophobicity in comparison with ODS. The new material exhibits the promising application in the separation of water-soluble vitamins.     

Supercritical ammonia as mobile phase in capillary chromatography

Instrumentation was assembled that allows the use of supercritical ammonia as mobile phase in capillary supercritical fluid chromatography. Several modifications of the typical chromatographic system were necessary, especially with respect to injection and detection. In addition, the stabilities of various polysiloxane stationary phases were studied. The chromatography of polarizable and polar basic materials was demonstrated using a nonpolar polysiloxane stationary phase.     

Laterally attached liquid-crystalline polymers as stationary phases. Effect of temperature in RP HPLC and comparison with a commercial C18 stationary phase

Summary Specific side-on-fixed liquid-crystalline polymers (SOLCP) have been synthesized for use in silica-modified stationary phases in high-performance liquid chromatography (HPLC). The mesogenic side group of the SOLCP is composed of three benzoate-type phenyl rings with terminal alkoxy chains and is laterally linked to a polysiloxane backbone via an alkyl ester spacer arm. The dependence of the logarithm of the retention factor on the reciprocal temperature showed that the liquid-crystalline anisotropic order was conserved in the small pores (200 ? diameter) of the silica gel. The first-order nematic-isotropic transition is lost and probably becomes second-order. Adsorption enthalpies for the liquid-crystalline stationary phases have been measurement for three polycyclic aromatic hydrocarbon isomers (ortho-terphenyl, triphenylene, and chrysene) and compared with those for a commercial C₁₈ phase. The adsorption enthalpies never exceeded 30 kJ mol⁻¹, i.e. ten times the thermal agitation energy,RT. They were always less on the SOLCP stationary phase than on the C₁₈ column, emphasizing the more rigid structure of the liquid crystalline phase and its mechanism based upon adsorption. Better separation of steroids, pesticides and amino acids were obtained with the LCP-coated silica than the commercial bonded C₁₈ column. Four small peptides were successfully separated by using pure water as mobile phase.     

Study of the retention behavior of small polar molecules on different types of stationary phases used in hydrophilic interaction liquid chromatography

The retention behavior of a large group of analytes (35) with varied properties (pK_a and logP) was studied on eight hydrophilic interaction LC columns with different surfaces, stationary phase chemistries, and types of particles. The acetonitrile content (5–95%), buffer concentration (0.5–200 mM), and pH of the mobile phase (3.8 and 6.8) were evaluated for their effects on the retention behavior. The type of stationary phase had a significant impact on the selectivity and retention time of the tested analytes. Completely different selectivity was observed on the aminopropyl stationary phase. In this study, the influence of the buffer concentration was similar for all tested columns, except for the aminopropyl stationary phase. Increasing the buffer concentration led to decreased retention times for the basic compounds and increased retention times for the acidic compounds, while the inverse behavior was observed on the aminopropyl stationary phase. The selectivity of the individual stationary phases was evaluated at pH 3.8 and 6.8. Much lower selectivity differences between the stationary phases were observed at pH 6.8 than pH 3.8. Bare silica stationary phases were used in the comparison of the particles (fused‐core and fully porous particles of 3 and 1.7 μm) and the columns provided by different manufacturers.     

Separation of Monovalent and Divalent Cations on a Pure Silica Gel Column with Dilute Oxalic Acid Containing 18-Crown-6 as Mobile Phase

Summary Pure silica gel (Pia Seed 5S-60-SIL) has been investigated as a cation-exchange stationary phase for ion chromatography of common monovalent and divalent cations (Li⁺, Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) with conductimetric detection; dilute oxalic acid (0.05 mm oxalic acid, pH 4.1, to 1 mm oxalic acid, pH 3.0) was used as mobile phase. The Pia Seed 5S-60-SIL silica gel acted as a cation-exchange stationary phase for these cations when 0.2 mm oxalic acid at pH 3.6 was used as the mobile phase. Excellent simultaneous separation and highly sensitive indirect conductimetric detection of these cations were achieved in 20 min on a 150 mm × 4.6 mm i.d. Pia Seed 5S-60-SIL silica gel column with 0.2 mm oxalic acid containing 4 mm 18-crown-6 (1,4,7,10,13,16-hexaoxacycloctadecane), pH 3.7, as mobile phase (detection limits (signal-to-noise ratio, 3, injection volume, 20 L), were 0.15 m for Li⁺, 0.16 m for Na⁺, 0.21 m for NH₄⁺, 1.0 m for K⁺, 0.17 m for Mg²⁺, and 0.25 m for Ca²⁺). The proposed IC–CD method was successfully applied to the separation and detection of major cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) in rain and river water samples.     

Preparation of spherical,mixed SiO2/TiO2 particles by the sol technique

Preparation of silica, titania and mixed silica/titania particles has been studied. The region for formation of monodisperse SiO₂ particles in the phase diagram tetraethyl orthosilicate (TEOS)-ethanol-H₂O was studied as a function of NH₃ concentration at room temperature. Titania particles could be prepared at lowered temperatures and concentration of ammonia up to 0.01 M. The size of SiO₂ particles was 0.03–1 m whereas TiO₂ particles were size range 0.5–0.8 m. Mixed SiO₂/TiO₂ particles were prepared from prehydrolyzed TEOS/EtOH solutions by adding tetraethyl orthotitanate (TEOT). This was accomplished at 3°C and slightly alkaline solutions. The final particle size of the mixed particles was about 0.3 m.     

Selective Solid Phase Extraction and Pre-Concentration of Heavy Metals from Seawater by Physically and Chemically Immobilized 4-Amino-3-Hydroxy-2-(2-Chlorobenzene)-Azo-1-Naphthalene Sulfonic Acid Silica Gel

4-Amino-3-hydroxy-2-(2-chlorobenzene)-azo-1-naphthalene sulfonic acid (AHCANSA) was used as a chelating modifier to improve the reactivity of the silica gel surface in terms of selective binding and extraction of heavy metal ions. The surface coverage values were found to be 0.488 and 0.473mmolg^–1 for the newly modified physically adsorbed silica gel phase (I) and chemically immobilized-AHCANSA phase (II), respectively. The modified silica gel phases (I, II) were tested for stability in different acidic buffer solutions (pH 1–6) and found to be highly resistant to hydrolysis and leaching by buffer solutions above pH 2. The application of these two phases as solid extractors for a series of mono-, di-, and tri-valent metal ions from aqueous solutions was also performed with different controlling factors such as the pH value of metal ion solutions and equilibrium shaking time. The mmolg^–1 metal capacity values determined by silica gel phases (I, II) were found to confirm high affinity and selectivity characters for binding with heavy metal ions such as Cr³⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ in a range of 0.250–0.483. The tested alkali and alkaline earth metals, Na⁺, K⁺, Mg²⁺ and Ca²⁺, were found to exhibit little interaction and binding ability with the modified silica gel phases. The selectivity characters incorporated into the modified silica gel phases were further utilized and applied in solid phase extraction and pre-concentration of trace concentration levels (1.0µgmL^–1 and 2.00–2.50ngmL^–1) from real seawater samples. The percentage recovery values determined for Cr³⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ were found to be in the range of 95.2–98.1±2.0–5.0%, and the pre-concentration recovery values for the same tested heavy metal ions were found to be in the range of 92.5–97.1±3.0–6.0% for the two newly modified silica gel phases with a pre-concentration factor of 500.Received December 20, 2002; accepted May 14, 2003 published online September 1, 2003     

Determination of Vitamins E and D3 in Some Preparations by High-Performance Liquid Chromatography with Indirect Spectrophotometric Detection

The behavior of vitamins E and D₃ was studied under the conditions of reversed-phase high-performance liquid chromatography with the use of unmodified and modified mobile phases. Acetonitrile modified with chlorophyll (a + b) (Chl) and tetraphenylporfyrin (H₂TPP) was selected for the indirect spectrophotometric detection of vitamins. It was demonstrated that detection limits are 2.3 g (8.33 × 10^–6 M H₂TPP, = 404 nm) and 3.5 ng (8.6 × 10^–6 M Chl, = 430 nm) for vitamin E and 0.12 g (3.0 × 10^–7 M H₂TPP, = 420 nm) for vitamin D₃. Pharmaceutical preparations containing vitamins E and D₃ were analyzed.     

Chromatographic behavior of packing materials for capillary electrochromatography with a coating of different immobilized polysiloxanes

Octadecyl silyl silica (ODS) phase coated with immobilized polysiloxanes (OV1701, SE-54, SE-30) were synthesized, their characteristics as capillary electrochromatography (CEC) column packing materials were studied. It was found that, although the polysiloxane coatings were different in polarity, the resulting packing materials showed the highest efficiencies when the respective coating ratios (polysiloxane:ODS, w/w) were all 20-30%. As expected, packing materials coated with different polysiloxanes resulted in different selectivity on solute pairs. Separations on these stationary phases were studied with different factors such as pH values and acetonitrile contents of the mobile phases. It was found that all these kind of stationary phases could resist basic mobile phase with a pH value as high as 11.6. Tests were made to analyze polar, basic drugs with CEC using the stationary phases.     

Elution of Cationic Species with/without Ion Pair Reagents from Polar Stationary Phases via SFC

Traditionally, the application of supercritical fluid chromatography has been limited to compounds of relatively low polarity. Thus, we report here that one secondary amine and two quaternary amine salts were successfully eluted from a Deltabond Cyano-bonded silica column with the addition of sodium alkylsulfonate to the methanol-modified, carbon dioxide-based mobile phase. A possible ion-pairing interaction between the positively charged analytes and the anionic part of the sulfonate additive has been proposed. In another set of experiments, the three amine salts readily eluted from both Ethyl-pyridine-bonded silica and Amino-bonded silica phases without the need of additive although the peak shapes were less than desirable. The addition of sulfonate salt to the mobile phase again sharpened the peaks. In the presence of a mixture of methanol and CO₂, we suggest that these stationary phases are positively charged. We describe here also for the first time the employment of a strong silica-based anion exchange (SAX) column for supercritical fluid separation of cationic species. Two elution mechanisms were proposed for the three amine salts depending upon whether the ionic additive was incorporated into the mobile phase. Separation as the ion-pair was proposed with an ionic additive in the mobile phase; whereas separation of the intact amine salt from the positively charged basic stationary phases was suggested to be operational without the ionic additive.     

Sequential determination of fat- and water-soluble vitamins in Rhodiola imbricata root from trans-Himalaya with rapid resolution liquid chromatography/tandem mass spectrometry

A rapid method was developed to determine both types of vitamins in Rhodiola imbricata root for the accurate quantification of free vitamin forms. Rapid resolution liquid chromatography/tandem mass spectrometry (RRLC–MS/MS) with electrospray ionization (ESI) source operating in multiple reactions monitoring (MRM) mode was optimized for the sequential analysis of nine water-soluble vitamins (B₁, B₂, two B₃ vitamins, B₅, B₆, B₇, B₉, and B₁₂) and six fat-soluble vitamins (A, E, D₂, D₃, K₁, and K₂). Both types of vitamins were separated by ion-suppression reversed-phase liquid chromatography with gradient elution within 30 min and detected in positive ion mode. Deviations in the intra- and inter-day precision were always below 0.6% and 0.3% for recoveries and retention time. Intra- and inter-day relative standard deviation (RSD) values of retention time for water- and fat-soluble vitamin were ranged between 0.02–0.20% and 0.01–0.15%, respectively. The mean recoveries were ranged between 88.95 and 107.07%. Sensitivity and specificity of this method allowed the limits of detection (LOD) and limits of quantitation (LOQ) of the analytes at ppb levels. The linear range was achieved for fat- and water-soluble vitamins at 100–1000 ppb and 10–100 ppb. Vitamin B-complex and vitamin E were detected as the principle vitamins in the root of this adaptogen which would be of great interest to develop novel foods from the Indian trans-Himalaya.