Unmodified Silica Gel Research Articles

Noncovalent linking of a chiral selector to unmodified silica gel through amine functional groups

The noncovalent linking of Pirkle's dinitrobenzoyl- l-leucine chiral selector to unmodified silica gel by means of polar moieties such as amines and quaternary ammonium ions was studied. Selectors linked with amine groups provided resolution of racemic 2,2,2-trifluoro-1(9-anthryl)ethanol comparable to that of Pirkle's column having a covalently bonded selector. Non-covalent linking by means of the quaternary ammonium ion group was much less efficient in resolving optical isomers.

Investigation of chromatographic behaviour of ethoxylated alcohol surfactants in normal-phase and reversed-phase systems using high-performance liquid chromatography–mass spectrometry

Retention behaviour of ethoxylated alcohols was investigated on octadecyl silica, unmodified silica and aminosilica columns. Good separation according to the alcoholic alkyl length is achieved in reversed-phase systems. The elution order of the oligomers with different numbers of oxyethylene groups depends on the type of the organic solvent (methanol or acetonitrile) and on its concentration in the mobile phase. Different retention behaviour of lower and higher oligomers was observed. The distribution of the oligomers according to the number of oxyethylene units is suppressed, but it overlaps with the peak distribution according to the alkyl length in mobile phases containing high concentrations of acetonitrile. In normal-phase systems, the alkyl length affects the retention much less than the number of oxyethylene units. Better separation than on unmodified silica gel columns can be achieved on a chemically bonded aminosilica column in 2-propanol– n-hexane and especially in acetonitrile–water–dichloromethane mobile phases. Possible retention mechanisms in the systems studied are discussed. The retention is strongly affected by solvation of the oxyethylene groups by the mobile phase. The retention factors of higher oligomers with bimodal mass distribution can be described using a simple equation. HPLC–MS with atmospheric pressure chemical ionization allows sensitive detection, easy identification and reconstruction of the chromatographic peaks in chromatograms of complex mixtures of oligomers with different numbers of methylene and oxyethylene groups.

Ion chromatographic behavior of alkali and alkaline earth metal cations on silica gel columns with cation exchange characteristics

Ion chromatographic behavior of alkali metal cations and ammonium ion on various unmodified silica gel columns by elution with 2 mM nitric acid (pH 2.7) was investigated. As a result it was found that one silica gel (Develosil 30-5) column acted as a cation exchange column under acidic eluent conditions and an excellent separation of these cations was achieved. Therefore, in order to produce silica gel, which acts as a cation exchanger for ion chromatography under strongly acidic conditions, the main cause of the cation exchange characteristics of the Develosil 30-5 silica gel was investigated. As a result it was found that the main cause could be attributed to aluminium present as an impurity in the silica gel. This finding suggests that the modification of silica gel with aluminium is very useful for the production of new advanced silica-based cation exchangers for ion chromatography.

Characterisation and prediction of retention in isocratic and gradient-elution normal-phase high-performance liquid chromatography on polar bonded stationary phases with binary and ternary solvent systems

The retention of phenylurea herbicides on bonded nitrile- and amino-silica gel columns was studied in mixed mobile phases containing 2-propanol or dioxane in n-hexane. Under isocratic conditions, a three-parameter retention equation describes adequately the dependence of the retention factors on the concentration of a polar solvent in binary mobile phases. Parameters of this equation can be used for the prediction of elution volumes in gradient elution with binary solvent gradients. If dry solvents and controlled temperature are used, if the migration of sample compounds corresponding to the gradient dwell volume is accounted for and if the initial concentration of the polar solvent in gradient elution is 3% or more, the differences between the predicted gradient elution volumes and the experimental data were, in most cases, in between 0.1 and 0.25 ml, i.e., less than 5%. With gradients starting from pure non-polar solvents, the differences are more significant because of preferential adsorption of the polar solvents during the gradient. The adsorption isotherms of 2-propanol and of dioxane in n-hexane on the nitrile-bonded phase are less steep and column saturation occurs at a higher concentration of the polar solvent than with the unmodified silica gel column. Based on three experimental retention factors, retention can be predicted in isocratic and gradient elution with ternary mobile phases.

Characterization of dextrans by size-exclusion chromatography on unmodified silica gel columns, with light-scattering detection, and capillary electrophoresis with laser-induced fluorescence detection

Unmodified silica gel size-exclusion columns were used in an on-line combination with light-scattering detection for a size characterization of dextrans. The influence of electrostatic interactions on analyte retention was briefly investigated. Size-exclusion chromatography was also used for evaluation of the fluorescence labeling procedure for dextrans with 8-aminonaphthalene-1,3,6-trisulfonic acid. The derivatives obtained through this procedure were used for electrophoretic measurements with laser-induced fluorescence detection. A comparison between the size-exclusion data and capillary electrophoresis indicates that the effectiveness of fluorescent labeling decreases with molecular mass of the dextran analytes.

Ion-exclusion chromatography of benzenecarboxylic acids on an unmodified silica-gel column

In order to demonstrate the effectiveness of two unmodified silica-gel (Develosil 30-5) columns (300×7.8 mm I.D.) used in series for the ion-exclusion chromatographic separation of mono-, di, tri- and tetrabenzenecarboxylic acids, the elution behavior of benzenecarboxylic acids and phenol was investigated. When using water as an eluent, many benzenecarboxylic acids were co-eluted and the separation was not satisfactory. However, when using 5 m M sulfuric acid (pH 2.0) as the eluent, excellent separation and high detection sensitivities for many benzenecarboxylic acids and phenol were achieved in 40 min using UV detection at 200 nm.

Retention behavior of alkali, alkaline earth and transition metal cations in ion chromatography with an unmodified silica gel column

An unmodified silica gel (Develosil 30-5) column (150×4.6 mm I.D.) has been applied to the ion chromatographic separation of alkali, alkaline earth and transition metal cations. The retention behavior of the above cations on the bare substrate was investigated using a number of weak inorganic and organic acid eluents. During this investigation, several separations were achieved and the most suitable eluent conditions were identified. It was concluded that: (a) 1.5 m M HNO 3-0.5mM pyridine-2,6-dicar☐ylic acid eluent was the most effective for the simultaneous separation of common alkali and alkaline earth metal cations, (b) 1.5 m M oxalic acid eluent resulted in the best separation of alkali, alkaline earth, and transition metal cations, (c) 0.5 m M CuSO 4 eluent could be used for the separation of alkali metal cations alone and (d) 0.5 m M ethylenediamine-oxalic acid eluent at pH 5.5 resulted in themost efficient separation of both alkaline earth and transition metal cations.

Reactions of Group IV Metallocene Dichlorides with Modified Silica Gel Surfaces

Reactions of bis(cyclopentadienyl)titanium dichloride, bis(cyclopentadienyl)zirconium dichloride, and bis(cyclopentadienyl)hafnium dichloride with premodified silica gel surfaces were studied. The silica gel surface was pretreated to selectively eliminate nonhydrogen bonded silanols, hydrogen bonded silanols, or both before reaction. By study of the metallocene reactions with these silicas and unmodified silica gel, it was found that the metallocenes react predominantly with nonhydrogen bonded silanols. Although none of the metallocenes are very reactive with silica as compared to titanium tetrachloride, reactivity increases in the order Cp 2TiCl 2< Cp 2ZrCl 2< Cp 2HfCl 2.

Simultaneous determination of common mono- and divalent cations by ion chromatography with an unmodified silica gel column

In order to characterize the cation-exchange properties of a silica gel column in the acidic region, the retention behavior of common mono- and divalent cations (Na +, NH 4 +, K +, Mg 2+ and Ca 2+) was investigated. When using 5 mM nitric acid (pH 2.3) as an eluent, these cations were retained on a silica gel column (150×4.6 mm I.D.) packed with Develosil 30-5 unmodified silica gel that has a very large surface area (ca. 770 m 2/g), showing that such a stationary phase could be used as a cation-exchanger for ion chromatography with acidic eluents. When using 1.5 mM 2,6-pyridinedicarboxylic acid as an eluent, a good separation of these cations was obtained in 12 min using conductimetric detection. This separation was applied successfully to the simultaneous determination of the five cations in various environmental water samples.

Ion-exclusion chromatography of aliphatic carboxylic acids on an unmodified silica gel column

In order to demonstrate the effectiveness of a silica gel column in ion-exclusion chromatography, the elution behavior of aliphatic carboxylic acids was investigated on an unmodified silica gel column having cation-exchange characteristics. When water was used as an eluent, the elution order of the carboxylic acids was dependent on the p K a and the hydrophobicity, but the resolution between strong acids, e.g. HNO 3, and carboxylic acids was not satisfactory. Therefore, the composition of eluents containing sulfuric acid and organic modifiers was investigated for the separation of HNO 3 and C 1–C 10 aliphatic carboxylic acids. When 0.5 m M sulfuric acid containing 0.05% heptanol was used as an eluent, an excellent ion-exclusion chromatographic separation was achieved in 30 min using UV-detection at 220 nm.

A comparison of nitrogen adsorption and thermogravimetric methods in the assessment of the pore size distribution of mesoporous silica gels

The pore dimensions are determined by using adsorption and thermogravimetry methods for unmodified silica gel Si-100 and silanized silica gel Lichrosorb RP-18. Thermal desorption measurements for various liquids were performed under quasi-isothermal conditions. Methanol, acetone, benzene and n-hexane were used as a wetting liquids. Pore size distributions and total pore volumes derived from thermal desorption compare reasonably well with results obtained from the analysis of adsorption isotherms of nitrogen. For different types of adsorbates the effects connected with the presence of the surface liquid film are discussed. The presence of long alkyl chains on the silica surface strongly influences the pore diameter calculated from thermogravimetric data.

High-performance liquid chromatography of some alkaloids on unmodified silica gel with aqueous-organic solvent mixtures

The application of eluents composed of methanol-aqueous phosphoric buffer was investigated for the high-performance liquid chromatographic separation of alkaloids on a silica stationary phase. These eluents were shown to be very useful for preliminary analytical chromatography and allow the conditions for the preconcentration of some alkaloids from crude extracts to be established. The effects of different eluent pH values and methanol concentrations were examined.

The investigation of the pore structure of silica gels with long-chain alkyl grafts by thermal analysis

The total pore volumes and the core size distributions were calculated for the silica gel Si-100 and chemically modified silica gels RP-8 and RP-18 on the basis of the thermal desorption data of various liquids. The results obtained are compared with those from the nitrogen adsorption/desorption method. Good consistency between the parameters obtained by using different methods is observed in the case of unmodified silica gel. The presence of long alkyl chains on the silica surface strongly influences the pore diameter calculated from thermogravimetric data.

Ion-exclusion chromatographic determination of hydrogen carbonate in natural waters using unmodified silica gel and conductimetric detection

A simple and rapid ion-exclusion chromatographic method for the determination of hydrogen carbonate has been developed. Unmodified silica gel was used for the separation column instead of H+-form cation-exchange resin. By elution with water and monitoring with a conductimetric detector, excellent separation can be achieved. The chromatographic conditions for the separation of hydrogen carbonate, pore and particle sizes of packings and the separation mechanism are discussed. For the determination of hydrogen carbonate, borate buffer solution (pH 7.3) was added in order to keep the pH of the sample solution constant. The calibration graph was found to be linear in the range 2–2 0 µg ml–1 of hydrogen carbonate. Common anions and cations such as chloride, nitrate, sulfate, sodium and calcium do not interfere. The method has been applied to the determination of hydrogen carbonate in river and lake waters.

Effect of the substituents of β-diketonato ligands on R F values of tris(chelate) transition metal complexes obtained by normal- and reversed-phase thin-layer chromatography on unmodified silica gel

Three series of tris(β-diketonato) complexes of cobalt(III), chromium(III) and ruthenium(III), in which acetylacetonato ligands were successively replaced with dibenzoylmethanato ligands, were chromatographed on silica gel thin layers and the corresponding mechanisms were considered. The R F values increased with non-aqueous and decreased with aqueous solvent systems. It was established that unmodified silica gel can be used as a sorbent for both normal- and reversed-phase chromatography of metal complexes. The earlier rule of a linear dependence between the R M values of tris(chelate) transition metal complexes and the number of substituted chelate ligands by others was extended.

Comparison of Different Compositions of Mixed Pentane/1,4-Dioxane Mobile Phases in SFC

Abstract The effective plate number has been chosen for a comparison between different mixed mobile phases composed of pentane and 1,4-dioxane. A column packed with unmodified silica gel and pyrene as the analyte was used. The dependence of the effective plate number on pressure and composition for a constant temperature, or a constant reduced temperature, both at constant mass flow, are shown in three dimensional network plots. The same dependence is shown at constant temperature and constant linear velocity. For a detailed discussion, the dependence of the effective plate number on pressure, composition, capacity ratio, and other chromatographic parameters is given in two dimensional graphs. The main result for the present chromatographic system is that a Financial support by the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen (AIF) is gratefully acknowledged.

Analysis of basic drugs by high-performance liquid chromatography using on-line solid-phase extraction and an unmodified silica column

A method is described for the high-performance liquid chromatographic analysis of a number of basic drugs on unmodified silica gel, which employs an aqueous methanol-rich eluent of pH 8. The analytical system was coupled to a column-switching assembly in order to facilitate on-line solid-phase extraction of the drugs from plasma. The method is reproducible, with a coefficient of variation of 3% or less for most compounds, even without internal standardisation. Drug recovery from plasma is efficient, being in excess of 90% in most instances. The drugs were detected by UV absorption at 254 nm.

Determination of vinca alkaloids in plasma and urine using ion-exchange chromatography on silica gel and fluorescence detection

The development of a method for the determination of the antineoplastic vinca alkaloids vinblastine and vindesine in biological samples is described. The selectivity of the assay is high owing to the use of solid-phase extraction on a cyanopropyl extraction column prior to isocratic chromatography on unmodified silica gel with fluorescence detection. The influence of acetonitrile concentration and mobile phase pH on the capacity factors of the drugs was studied in order to optimize the separation between the drugs and endogenous components. The effect of varying the type and concentration of competing cations in the mobile phase was also examined. The limot of determination (signal-to-noise ratio = 3) for vinblastine is 0.5 ng/ml in plasma and urine and for vindesine 2.5 ng/ml. The assay is suitable for determining the concentrations of both compounds in plasma and urine samples from patients.

Synthesis, characterisation and application of hydrolytically stable chemically bonded silica supports with metal-complexating chiral ligands

Abstract The preparation of immobilized ( R,R )-tartaric acid and ( S )-mandelic acid on spherical 5 μm silica gel via chiral alkoxy silanes is reported. The reactions of the chiral silanes were confirmed by elemental analysis, 13 C-NMR and FT-IR. Subtraction of the FT-IR spectra of the unmodified silica gels from those of the modified ones results in the IR spectra of the chemically bonded chiral ligands. These were in good agreement with those of the starting compounds. Complexation of all bonded phases with Cu(II) or Cd(II) was examined by measuring the sorption isotherms at constant pH as well as the dependence of the metal ion concentration in the eluent on the retention of racemic compounds. Separation of amino alcohols on these chiral packings using ligand-exchange chromatography was found to be mainly controlled by the pH and the ionic strength of the eluent.

Liquid Chromatographic Analysis of a New Antihypertensive Agent, PD 78,799, in Plasma on Silica Gel with Reversed-Phase Eluent

Abstract A sensitive and selective HPLC assay has been developed for the analysis of a new antihypertensive agent in human plasma. The drug and internal standard were isolated from plasma by liquid-liquid extraction. Separation was accomplished on unmodified silica gel with a mobile phase of 60:40 acetonitrile:10 mM dibasic ammonium phosphate. Detection was by UV absorbance at 291 nm. The method is linear over a range of 20 to 4000 ng/ml. Relative error of calibration and control standards ranged from −1.5 to 5.0% and precision, as indicated by relative standard deviation, ranged from 0.8 to 5.2%. The method has been successfully used for analysis of plasma samples from human volunteers following oral administration of PD 78,799.