Octadecyl-modified Silica Research Articles

Monitoring of selected metabolites and biotransformation products from fermentation broths by high-performance liquid chromatography

Reversed-phase high-performance liquid chromatographic methods with UV-photodiode array or multiwavelength detection were developed for the analysis of secondary fungal metabolites mevinolin (also known as monacolin K, lovastatin or Mevacor) and fumagillin in fermentation broths. Samples obtained from screening experiments of Aspergillus fumigatus and Aspergillus terreus strains were analyzed for fumagillin and mevinolin, respectively. Isocratic and gradient elution was used with acetonitrile-aqueous phosphoric acid or acetonitrile-phosphate buffer eluents. Sample preparation was accomplished by extraction of the whole fermentation broth with an acid-organic solvent mixture. Similarly, bacterial degradation products of 2,4,6-trichlorophenol obtained from microbial remediation processes were analyzed by reversed-phase HPLC with UV-photodiode array detection. Extraction of the analyte from solid samples was done by solvent extraction and solid-phase extraction clean-up using octadecyl-modified silica sample preparation cartridges.

Retention behaviour of metal complexes with 5-sulphoquinoline-8-ol in reversed-phase ion-pair liquid chromatography

The reversed-phase ion-pair liquid chromatographic behaviour of several metal complexes with 5-sulphoquinoline-8-ol (HQS) on an octadecyl-modified silica column was investigated by using a fluorescence detector. The aluminum complex gave two peaks on the chromatograms when eluents containing phosphate buffer were used. The effects of the pH of the eluent, HQS concentration, the nature of anions in the eluent, and the column temperature on the retention behaviour of the aluminum complex are discussed on the basis of the equilibria and the kinetics of the complexation of the aluminum ion with HQS and anions in the eluent. The equilibrium constant for the interconversion reaction between the two Al(III)-HQS species which correspond to the two peaks observed and the second stepwise formation constant for the Zn(II)-HQS complex were evaluated from the HPLC data obtained.

Stationary phase degradation in reversed-phase liquid chromatography: a possible cause of bad predictions in experimental design

The degradation in function of time of cyanopropyl- and octadecyl-modified silica stationary phases, a polymer coated silica phase and a polymer phase is investigated as this could be a cause of bad predictions in experimental design. By simulating routine conditions on both types of silica columns, considerable changes of retention, peak width and asymmetry in function of time were observed when switching mobile phase pH but also when mobile phase pH is held constant. The initial column performance was best maintained on the polymer coated silica column. On most of the other columns, changes in elution characteristics are rapid enough to make them a major cause of bad predictions in experimental design.

Determination of chemical composition distribution of poly(methyl methacrylate)-graft-polystyrene by adsorption high-performance liquid chromatography

Three poly(methylmethacrylate)-graft-polystyrene samples of different compositions at low conversions and four samples obtained at different conversions from the same monomer feed compositions were separated and analyzed by HPLC using reversed-phase combination of an octadecyl-modified silica gel column and a linear gradient of THF and acetonitrile. The samples were eluted from components of high MMA content to components of low MMA content. The chromatograms were converted to chemical composition distributions using three samples of different compositions as standards by the optimization method

Ion-pair reversed-phase liquid chromatography of arsenic species on polymeric styrene-divinylbenzene packed columns with an alkaline aqueous mobile phase

The resolving power of an ion-pair reversed-phase liquid chromatography method, on a polymeric stationary phase (PRP-1) with an alkaline aqueous mobile phase (pH=9) containing tetrabutylammonium cation as ion-pairing agent, is compared with a traditional ion-pairing system using an octadecyl-modified silica column and a neutral aqueous mobile phase (pH=7.3). This chromatographic system (PRP-1 column) provides a rapid means for the determination of two of the most important arsenic species, arsenobetaine and arsenite. Addition of methanol (5%) to the aqueous mobile phase allows the differentation of arsenobetaine from the system peak when using UV detection.

On-line liquid chromatography/thermospray mass spectrometry in the analysis of oligosaccharides.

The analysis of intact neutral oligosaccharides by on-line liquid chromatography/thermospray mass spectrometry is described. Molecular-weight information on oligomers up to a degree of polymerization of 10 is obtained using an aqueous mobile phase containing 10(-4) mol/L sodium acetate, which was found to be compatible with thermospray interfacing and ionization. Ions due to sodiated and disodiated oligosaccharides are observed under these conditions without fragmentation. The aqueous 10(-4) mol/L sodium acetate mobile phase is demonstrated to be applicable in the separation of mixtures of oligosaccharides on a reversed-phase octadecyl-modified silica column.

Modelling retention in reversed-phase liquid chromatography as a function of pH and solvent composition

Models describing the concomitant effects of pH and organic modifier concentration on retention in reversed-phase liquid chromatography are established. Two different octadecyl-modified silica columns were used. The retention behaviour of several acidic, basic and neutral solutes were studied, using methanol as the organic modifier. The suggested models accurately describe retention as a function of pH and composition. A unified formalism for retention modelling that is applicable to all ionogenic solutes is also established. This formalism is tested with the modelling of retention for several weak bases. The resulting (general) model describes retention accurately and is applicable to all solutes studied.

Effects of pH in reversed-phase liquid chromatography

The effects of concomitant variations in pH and organic modifier concentration on retention, efficiency and peak symmetry are considered for reversed-phase liquid chromatography (RPLC) on octadecyl-modified silica (ODS) columns. A number of factors are discussed, which make the systematic exploitation of pH effects in RPLC more complicated than the optimization of solvent composition. If the pH is varied, a second factor (usually the concentration of organic modifier) will need to be varied simultaneously to maintain retention (capacity factors) in the optimum range. When pH is considered as a parameter in RPLC, not only its effects on retention, but also the variations in efficiency (plate count) and peak shape (asymmetry) need to be considered. These parameters turn out to vary drastically between individual solutes and between different experimental conditions. The results of a study involving a number of acidic, basic and neutral solutes, two different ODS columns and mixtures of either methanol or acetonitrile with aqueous buffers are reported. In the earlier part of the study, using methanol as the organic modifier, reproducible data for retention, peak width and peak symmetry were obtained and these data are reported. In the later part of the study, using acetonitrile, a gradual change in retention as a function of time was observed, this effect coinciding with a decrease in column efficiency. It is concluded that ODS columns are subject to considerable degradation during studies in which the pH is varied. Although this effect can be described mathematically, the preferred solution is thought to be the use of pH-stable columns.

Sample remaining in an ODS column after compositional fractionation of copolymers by high-performance liquid chromatography

High-performance liquid chromatography is very effective for the compositional fractionation of copolymers. Octadecyl-modified silica gel (ODS) has been used as a packing material for fractionation in the reversed-phase (RP) adsorption mode. In the compositional fractionation of the samples of copoly-(methyl methacrylate-styrene) by RP gradient elution through ODS column, it was found that small peaks were observed with blank elution just after the sample measurement and a perfect baseline was recovered after several blank elutions.

Use of membrane extraction disks for on-line trace enrichment of organic compounds from aqueous samples

Membrane extraction disks have been recently introduced for the solid-phase extraction of organic compounds from aqueous samples. The material consists of alkyl-modified silica particles enmeshed in an inert PTEE matrix. Aqueous samples containing polar pesticides and herbicides, are preconcentrated on-line from acidic solutions on membrane extraction disks containing immobilized octadecyl-modified silica and analyzed with an isocratic liquid chromatographic system using PLRP-S as the stationary phase and aqueous acetonitrile mixtures (pH 3) as the eluent. Data on the lifetime and the dimensions of the preconcentration disks, and the efficiency and the repeatability of the procedure are reported. For 10 ml samples, the detection limits of the analytes atrazine, simazine and 2,3,4-trichlorophenol in tap water are 0.1–1 ppb.

Liquid chromatographic determination of azidothymidine in human plasma using on-line dialysis and preconcentration on a silver(I)-thiol stationary phase

An automated method is described for the routine determination of 3′-azido-2′,3′-dideoxythymidine (AZT), the best known drug against acquired immunodeficiency syndrome (AIDS). The method is based on on-line dialysis to remove matrix macromolecules, followed by selective preconcentration and clean-up with a silver(I)-thiol stationary phase. After desorption of the solute with a small plug of perchloric acid, chromatography is applied using an octadecyl-modified silica column. Using UV absorbance detection at 269 nm, the minimum detectable concentration in plasma is 20 ng ml −1 (600-μl sample). The within-day reproducibility at the 20 ng ml −1 level is 4.4% and at least 128 samples can be analysed unattendedly without exchanging the dialysis membrane, the precolumn or the analytical column.

Simple method for the analysis of food dyes on reversed-phase thin-layer plates

A technique for the determination of food dyes using reversed-phase thin-layer chromatography on octadecyl-modified silica is described. A solvent system containing 5.0% aqueous sodium sulphate solution enables good separation of the food dyes. Their separation is dependent on the pH of the solvent system, good separation among all dyes being obtained between pH 6.0 and 7.0. The determination of thirteen dyes can be achieved by a combination of methanol—acetonitrile—5.0% aqueous sodium sulphate solution (3:3:10) (solvent system A) and methanol—methyl ethyl ketone—5.0% aqueous sodium sulphate solution (1:1:1) (solvent system B). Solvent system A was used for the determination of Tartrazine, Amaranth, Indigo Carmine, New Coccine, Sunset Yellow FCF, Allura Red AC, Fast Green FCF and Brilliant Blue FCF and for the screening of Acid Red (R-106), Eosin (R-103), Erythrosin (R-3), Rose Bengale (R-105) and Phloxine (R-104). When the spots of R-3, R-103, R-104, R-105 and R-106 appear on the plate, their determination can be achieved by using solvent system B. Each calibration graph was linear between 0.2 and 1.0 μg.

Comparison of phenyl- and octadecyl-modified silica gel as stationary phase for the prediction of n-octanol-water partition coefficients by high-performance liquid chromatography

The correlation between the retention of 29 aromatic compounds on phenyl and octadecyl silica gel with their n-octanol-water partition coefficients has been studied. The work also covered the influence of mobile phase buffers and the effect on the correlation of using k′ values extrapolated to 100% water. The results show that the retention on phenyl silica correlates significantly better than that on octadecyl silica. The error in the prediction of log P, ranging from 1 to 8, from retention data on phenyl silica at a 95% confidence limit was found to be ±0.04 log units at log P = 4, and ±0.07 log units at log P = 7 when extrapolated k′ values were used. When k′ values measured at one fixed mobile phase composition were used, the error was found to be ±0.10 and ±0.16 log units at log P = 4 and 7, respectively.

C18 reversed-phase trace enrichment of organophosphorus pesticides and residues in water

The efficiency of accumulation of traces of dialkylphosphates, dialkylphosphorothioates and dialkylphosphorodithioates and at the same time of the parent organophosphorus pesticides, from aqueous solutions, was investigated by reversed-phase adsorption with octadecyl-modified silica gel (Sep-Pak C18 cartridge). For desorption, methanol was used as the most appropriate solvent for the subsequent methylation of dialkylphosphorus anions and for gas chromatographic analysis. The recoveries of dimethyl-and diethylphosphorothioates and-phosphorodithioates, both free and as ionassociation complexes with the tetraphenylarsonium cation, from water samples adjusted to pH 1.25 and 4 respectively, were comparable. The recovery of diethylphosphate was tripled by acidification of the water sample, whereas the accumulation of dimethylphosphate was unsuccessful. Although the anions were not quantitatively trapped on the cartridge, a detection limit of 1μg/l for dialkyl-phosphorothioates and-phosphorodithioates was obtained with 10–20 ml sample volumes. The sample volume should not exceed 20 ml because of significant breakthrough of dialkylphosphorus anions on the C18 cartridge with larger volumes. Under the optimum conditions for accumulation of organophosphorus residues the adsorption of organophosphorus pesticides is nearly quantitative with a detection limit 1–2 orders of magnitude higher than that for the residues. To enhance the detection limit to 1μg/l the sample volume had to be increased to 100 ml, but there was then no possibility of achieving efficient simultaneous accumulation of the corresponding dialkylphosphorus anions. The procedure provides simple and rapid trace enrichment of organophosphorus pesticides and residues, in the control of surface waters polluted with these compounds, and is applicable in the field.

C 18 reversed-phase trace enrichment of chlorinated phenols, guaiacols and catechols in water

Reversed-phase adsorption with octadecyl-modified silica gel was found to give quantitative recoveries of di-, tri-, tetra- and pentachlorophenol, tri- and tetrachloroguaiacol and tri- and tetrachlorocatechol from water. After desorption with acetone the chlorophenolic compounds were determined as their acetyl derivatives, using quartz capillary column gas chromatography. Different aqueous acetylation methods were compared, the highest yields being obtained with potassium carbonate solution. The procedure described has been applied to natural and industrial waste waters.