The effects of chain length,embedded polar groups,pressure, and pore shape on structure and retention in reversed-phase liquid chromatography: Molecular-level insights from Monte Carlo simulations

Particle-based simulations using the configurational-bias and Gibbs ensemble Monte Carlo techniques are carried out to probe the effects of various chromatographic parameters on bonded-phase chain conformation, solvent penetration, and retention in reversed-phase liquid chromatography (RPLC). Specifically, we investigate the effects due to the length of the bonded-phase chains (C₁₈, C₈, and C₁), the inclusion of embedded polar groups (amide and ether) near the base of the bonded-phase chains, the column pressure (1, 400, and 1000 atm), and the pore shape (planar slit pore versus cylindrical pore with a 60 Å diameter). These simulations utilize a bonded-phase coverage of 2.9 μ$\mu$mol/m²and a mobile phase containing methanol at a molfraction of 33% (about 50% by volume). The simulations show that chain length, embedded polar groups, and pore shape significantly alter structural and retentive properties of the model RPLC system, whereas the column pressure has a relatively small effect. The simulation results are extensively compared to retention measurements. A molecular view of the RPLC retention mechanism emerges that is more complex than can be inferred from thermodynamic measurements.     

Sampling and Analysis of Odorous Compounds in Water Treatment Plants

Abstract

The following analytical method has been used to identify some odorous compounds in the air of the water treatment plant Werdholzli, Zurich: sampling of contaminated air with the help of activated carbon, desorption by the solvents carbon disulphide and methylene chloride, separation of the carbon disulfide extract into a polar and a nonpolar fraction by adsorption column chromatography on silica. Hereafter gaschromatographic analysis of the polar fraction on glass capillary column (Ucon HB 5100); detection and identification were achieved by flame ionisation, thermoionic nitrogen selective detector and computerized mass spectrometry (Finnigan 3200 F, data system 6110).

The results show the presence of sulfur compounds: thiophenes, thiazoles; nitrogen compounds: pyrazines; oxygen compounds: phenols, alcohols and some unsatured hydrocarbons. The malodorous compounds were sulfur and nitrogen compounds in the range of 0.01-0.1 ppm.     

Influence of a water vapor admixture in the carrier gas on capacity coefficients of polar organic compounds

The influence of a water vapor admixture in helium, nitrogen, and carbon dioxide on capacity coefficients of C₃−C₅ alcohols and pyridine during chromatography process in capillary columns with polar (PEG-20M) and nonpolar (SE-30) stationary phases was studied. The introduction of a water admixture into the carrier gas, increases the capacity coefficient of polar organic compounds on the capillary column with PEG-20M and has almost no effect on this value in the case of SE-30. The change in retention of polar organic compounds on the capillary column with the PEG-20M polar phase occurs due to a change in the properties of the stationary phase when it adsorbs water from the mobile phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2258–2261, November, 1998.     

Chromatographic Separation of Cresol Isomers by a β‐Cyclodextrin: Application for the Determination of Volatile Phenols in Alcoholic Beverages

Abstract

The chromatographic separation of o‐cresol, m‐cresol, and p‐cresol by using β‐cyclodextrin as a chiral reagent has been studied. Conditions for the chromatographic separation of these isomers by using the cyclodextrin in the mobile phase or bonded in the stationary phase were optimized, and both procedures provided good results for the resolution of the chromatographic peaks. The use of fluorimetric detection (λ_exc 275 nm λ_em 300 nm) allows detection and quantification limits of the µg/L for eight studied phenols by using both procedures. The determination of volatile phenols in alcoholic beverages must be carried out using the cyclodextrin in the mobile phase because of the co‐elution of phenol and ethylguaiacol with other compounds of some studied matrix.     

Comparison of common mobile‐phase volume markers with polar‐group‐containing reversed‐phase stationary phases

A systematic study of the behavior of several common mobile‐phase volume markers using traditional and polar‐group‐containing reversed‐phase stationary phases is presented. Examined mobile‐phase volume markers include two neutral molecules, uracil and thiourea, concentrated (0.10 M) and dilute (0.0001 M) KNO₃, and D₂O. Mobile‐phase volumes are examined over the entire reversed‐phase mobile‐phase range of 100% water to 100% methanol or acetonitrile. The behavior of these mobile‐phase volume markers is compared with a maximum theoretical value (i.e. the void volume), as determined by pycnometry. The data suggest that: (i) uracil begins to fail as a mobile‐phase volume marker in mobile phases below about 40% strong solvent for polar group containing phases; (ii) in nearly all cases, the mobile‐phase volume measured dynamically is smaller than the pycnometric void volume; (iii) a significant dependence of measured mobile‐phase volume on salt concentration is seen on the polar endcapped phase, which is not observed on the traditional and embedded polar group phase; and (iv) D₂O does not work well as a mobile‐phase volume marker with polar‐group‐containing phases, possibly due to interaction with the stationary phase polar group.     

On-line trace enrichment of phenolic compounds from water using a pyrrole-based polymer as the solid-phase extraction sorbent coupled with high-performance liquid chromatography

A pyrrole-based conductive polymer was prepared and applied as new sorbent for on-line solid-phase extraction (SPE) of phenol and chlorophenols from water samples. Polypyrrole (PPy) was synthesized by chemical oxidation of the monomer in non-aqueous solution. The efficiency of this polymer for extraction of phenol and chlorophenols was evaluated using 35 mg of PPy as the sorbent in an on-line SPE system coupled to reversed-phase liquid chromatography with UV detection. The mobile phase were mixture of phosphate buffer-acetonitrile and compounds were eluted by the mobile phase using a six-port switching valve. High volumes of water, up to 160 ml, could be preconcentrated without the loss of phenols, except for the more polar ones. The R.S.D. for a river water sample spiked with phenol and chlorophenols at sub-ppb level was lower than 7% (n=5) and detection limits of 15-100 and 35-150 ng l⁻¹ for tap and river water were obtained, respectively.     

A New Anionic Micellar Mobile Phase System for the Normal-Phase Thin-Layer Chromatography of Amino Compounds: Simultaneous Separation of Aminophenol Isomers

The chromatography of some amino compounds was performed on high-performance thin-layer plates (silica-gel 60 F₂₅₄, catalog no. 1.05554, Merck, Germany) using aqueous and alcoholic (MeOH) and aqueous–organic solutions of cationic, anionic, and nonionic surfactants. The results obtained with 0.01 M aqueous sodium dodecyl sulfate (SDS) have been compared with distilled water (i.e., zero SDS) as the eluent. The effects of the surfactant concentration, the nature of the alcohol, and the presence of inorganic salts (NaCl, LiCl, CaCl₂, and CoCl₂) in the mobile phase were examined in order to understand the mobility pattern of amino compounds. Among the added salts, CaCl₂ was found to be the most effective for facilitating an analytically important separation of coexisting ortho-, meta-, and para-aminophenols. The TLC system consisting of a precoated high-performance silica layer as the stationary phase and 0.01 M methanolic SDS plus 0.1 M CaCl₂ in a 3 : 7 ratio as the mobile phase was identified as the most suitable system for the separation of o-, m-, and p-APHS from their mixtures. The influence of various impurities, such as amines, phenols, and inorganic cations, on the mobility and the separation of coexisting aminophenol isomers has also been examined. The lower limits of detection of aminophenols were determined on HPTLC plates using I₂ vapors as the detection reagent.     

Solvent optimization in normal-phase liquid chromatography of selected steroids

Summary Resolution (R_s) is a function of three factors: efficiency, retentivity and selectivity. A procedure for optimization of mobile phase selectivity was demonstrated using a reversed-phase (RP) separation of compounds with various functionalities. In this paper, the same procedure is used with normal bonded-phase (NP) chromatography to develop a separation of structurally similar steroids. The optimization of mobile-phase selectivity provided increased problem-solving capability and decreased the analysis time.A brief summary of this work has been published in September 1981, in the form of a du Pont Application Study.     

Porous graphitic carbon (PGC) high-performance liquid chromatography of diphosphine-bridged complexes with heteronuclear Au-M (M=Mn, Re) bonds

Summary The isocratic high-performance liquid chromatography of 1,1′-bis(diphenylphosphino)ferrocene (dppf), 1,1′-bis(diphenylphosphino)ruthenocene (dppr), bis(diphenylphosphino)methane (dppm) and triphenylphosphine (PPh₃)-substituted heterometallic Au-Mn or Au-Re carbonyl complexes is reported. A column packed with PGC (porous graphitic carbon) was used after preliminary experiments had shown that silica- and bonded-phase (silica-based) stationary phases were unsatisfactory for separation. The PGC column exhibited unique selectivity for the complexes studied. The mobile phases used were water-acetonitrile, dichloromethanehexane and tetrahydrofuran-hexane. The retention behaviour of the compounds was governed by the polar character and size of molecules, and influenced by metal-metal bond polarity. Separation of isomorphous structures with different metallocenyl moieties was achieved.     

Comparison of liquid chromatographic separations of geometrical isomers of substituted phenols with β- and γ-cyclodextrin bonded-phase columns

The capacity factors (k′) of several substituted phenols were measured by using β- and γ-cyclodextrin bonded-phase columns with mobile phases varying from the classical normal-phase conditions (e.g., heptane/2-propanol) to the reversedl-phase conditions (e.g. water/2-propanol). The cyclodextrin columns have unusual selectivities in both normal- and reversed-phase separations because of their large number of hydroxyl functional groups and their ability to form inclusion complexes with substrates. The occurrences of minima in plots of log k′ vs. percent organic modifier for various substituted phenols are considered to result from solute/solvent competition for interaction with the stationary phase and from the relative hydrophobicity of the stationary and mobile phases.     

A simple molecular model for adsorption chromatography. VII. Relationship between the RM value and the composition for phenols in systems of the type (cyclohexane+polar solvent)/silica

Summary The effect of the composition of binary developing solvents on R_M values of phenols with one, two or three hydroxyl groups was investigated. In most cases linear R_M vs. (log X_S) relationship were obtained for wide composition ranges (X_S — mole fraction of polar solvent; diluting solvent: cyclohexane), the slope of the lines being related to the molecular mechanism of adsorption. The slopes of some derivatives of quinoline, aniline and phenols are compared and the effects of the molecular structure of the solute and the eluen strength of solvent are discussed. The experimental data for phenols are interpreted in terms of competitive solvation in the mobile phase by electron donor solvents; practical rules concerning optimization of TLC systems for the analysis of the compounds investigated are formulated.     

Magnetic solid‐phase extraction based on a polydopamine‐coated Fe3O4 nanoparticles absorbent for the determination of bisphenol A,tetrabromobisphenol A, 2,4,6‐tribromophenol,and (S)‐1,1’‐bi‐2‐naphthol in environmental waters by HPLC

Polydopamine‐coated Fe₃O₄ magnetic nanoparticles synthesized through a facile solvothermal reaction and the self‐polymerization of dopamine have been employed as a magnetic solid‐phase extraction sorbent to enrich four phenolic compounds, bisphenol A, tetrabromobisphenol A, (S)‐1,1′‐bi‐2‐naphthol and 2,4,6‐tribromophenol, from environmental waters followed by high‐performance liquid chromatographic detection. Various parameters of the extraction were optimized, including the pH of the sample matrix, the amount of polydopamine‐coated Fe₃O₄ sorbent, the adsorption time, the enrichment factor of analytes, the elution solvent, and the reusability of the nanoparticles sorbent. The recoveries of these phenols in spiked water samples were 62.0–112.0% with relative standard deviations of 0.8–7.7%, indicating the good reliability of the magnetic solid‐phase extraction with high‐performance liquid chromatography method. In addition, the extraction characteristics of the magnetic polydopamine‐coated Fe₃O₄ nanoparticles were elucidated comprehensively. It is found that there are hydrophobic, π–π stacking and hydrogen bonding interactions between phenols and more dispersible polydopamine‐coated Fe₃O₄ in water, among which hydrophobic interaction dominates the magnetic solid‐phase extraction performance.     

Evaluation of different solvent systems for the isolation of Sparattosperma leucanthum flavonoids by counter-current chromatography

Three flavonoids - 2′,4′,6′-trihydroxy-4′-O-β-d-glucopiranosyl dihydrochalcone, 1, pinocembrin-7-O-(-neohesperidoside, 2 and pinocembrin-7-O-(-(6″-O-acetyl) neohesperidoside, 3 - were successfully isolated from the EtOAc extract of leaves of Sparattosperma leucanthum (Vell.) K. Schum (Bignoniaceae) using a two-step counter-current chromatography (CCC). Two different CCC machines were used, with different column axes (P.C. Inc., vertical orientation axis and AECS Quattro HTPrep, horizontal orientation axis). Detailed studies of flavonoids behaviour in several solvent systems made possible the use of the best system for their isolation. HEMWat and its modifications - exchange of alcohol and addition of a fifth solvent - were tested for isolation of the three compounds in a single run, but good K and α values were not achieved. So, HEMWat 4:10:4:10, with upper phase as mobile, was used to isolate compound 3. A mixture of compounds 1 and 2 was recovered and submitted to a new CCC fractionation using a more polar solvent system: EBuWat 8:2:10, upper phase as mobile. Butironitrile-acetonitrile-water (BuCN-ACN-H₂O) 5:10:10, upper phase as mobile, was also used for the isolation of the mixture containing compounds 1 and 2, in order to increase the solubility of the compounds in the CCC solvent system. It is the first time that the system BuCN-ACN-H₂O is described in literature.     

New device for controlling the amount of methanol in carbon dioxide mobile phase for supercritical fluid chromatography

A new device to accurately deliver a small amount of methanol into supercritical carbon dioxide fluid is described. Carbon dioxide, the most widely used mobile phase in supercritical fluid chromatography, is a relatively non-polar fluid, and hence the addition of a small amount of methanol could change the solvent strength of the mobile phase. In this work, supercritical CO₂ and methanol are delivered from the pump to a 100-μl mixing chamber in which a small magnetic bar is rotating. After passing through the mixing chamber, supercritical CO₂ is changed to a new mobile phase with different polarity. The modified mobile phase was successfully used for the separations of polar compounds and polyaromatic hydrocarbons (PAHs).     

Optical isomer separation of flavanones and flavanone glycosides by nano-liquid chromatography using a phenyl-carbamate-propyl-β-cyclodextrin chiral stationary phase

In this paper a phenyl-carbamate-propyl-β-cyclodextrin stationary phase was employed for the enantioseparation of several flavonoids, including flavanones and methoxyflavanones by using nano-liquid chromatography (nano-LC). The same stationary phase was also used for the diastereoisomeric separation of two flavanone glycosides. The compounds: flavanone, 2′-hydroxyflavanone, 4′-hydroxyflavanone, 6-hydroxyflavanone, 7-hydroxyflavanone, 4′-methoxyflavanone, 6-methoxyflavanone, 7-methoxyflavanone, hesperetin, hesperidin, naringenin and naringin were studied using reversed, polar organic and normal elution modes. The effect of the nature and composition of the mobile phase (organic modifier type, buffer and water content in the reversed phase mode) on the enantioresolution (R_s), retention factor (k) and enantioselectivity (α) were investigated. Baseline resolution of all studied flavonoids, with the exception of 2′-hydroxyflavanone and naringin, was achieved in reversed phase mode using a mixture of MeOH/H₂O at different ratios as mobile phase. Good results, in terms of peak efficiency and short analysis time, were obtained adding 1% triethylammonium acetate pH 4.5 buffer to MeOH/H₂O mixture. The separation of the studied compounds was also performed in polar organic mode. By using 100% of MeOH as mobile phase, the resolution was achieved for the studied analytes, except for 7-hydroxyflavanone, 2′-hydroxyflavanone, naringenin, hesperidin and naringin. Normal mode was tested employing a mixture of EtOH/hexane/TFA as mobile phase achieving the enantiomeric and diastereomeric separation of only hesperetin and hesperidin, respectively. The use of nano-LC technique for the resolution of flavanones optical isomers allowed to achieve good resolutions in shorter analysis time compared to the results reported in literature with conventional HPLC.     

Speciation of phenyltin(IV) compounds using high‐performance liquid chromatography. Part 2: The direct analysis of mixed standard solutions of triphenyltin halides/pseudohalide and of triphenyltin carboxylates containing functional group variations in the esteryl moiety

Simultaneous speciation of mixed standard solutions of triphenyltin halides (triphenyltin chloride, bromide, iodide) and pseudohalide (triphenyltin isothiocyanate) has been achieved with reversed‐phase high‐performance liquid chromatography on a Waters Spherisorb S5W ODS‐2 (octadecyl‐silica) column. An isocratic mixture of 95:5 (v/v) acetonitrile:water was used as the mobile phase at a flow rate of 1 ml min^?1. A series of selected triphenyltin carboxylates, Ph₃SnOCOZ, where Z = Me, Ph, CH:CHPh, CH:NOMe, CH₂SC₅H₄N and CH₂SC(S)NMe₂, was also similarly analysed using this system with two separate isocratic elutions using 100% acetonitrile and 96:4 (v/v) acetonitrile:water as the mobile phase. UV detection was done at 254 nm and the total run time for each analysis was less than 3 min. The detection limits for all the phenyltin(IV) compounds were in the range 0.01–0.03 ppm. Spiked water samples containing the triphenyltin carboxylates could also be simultaneously analysed by the above method without the need for any prior derivatization, following extraction with hexane. Pretreatment of the aqueous sample with NaCl/HCl and of the organic phase with hexamethylphosphoramide enabled recoveries of about 80% of the triphenyltins. Copyright © 2002 John Wiley & Sons, Ltd.     

Development of a fabric phase sorptive extraction with high‐performance liquid chromatography and ultraviolet detection method for the analysis of alkyl phenols in environmental samples

A novel analytical method has been developed and validated for the quantification of alkyl phenols in aqueous and soil samples. Fabric phase sorptive extraction, a new sorptive microextraction technique, has been employed for the preconcentration of some endocrine‐disruptor alkylphenol molecules, namely, 4‐tert‐butylphenol, 4‐sec‐butylphenol, 4‐tert‐amylphenol, and 4‐cumylphenol, followed by high‐performance liquid chromatography with ultraviolet detection. Various parameters influencing the fabric phase sorptive extraction performance, namely, extraction time, eluting solvent, elution time and pH of the sample matrix, were optimized. The chromatographic separation was carried out with a mobile phase of acetonitrile/water (60:40 v/v) at an isocratic flow rate of 1.0 mL/min using a reversed‐phase C₁₈ column at λ_max 225 nm. The calibration curves of target analytes were prepared in the concentration range 5–500 ng/mL with good coefficient of determination values (R² > 0.992). Extraction efficiency values were 74.0, 75.6, 78.0, and 78.3 for 4‐tert‐butylphenol, 4‐sec‐butylphenol, 4‐tert‐amylphenol, and 4‐cumylphenol, respectively. The limits of detection range from 0.161 to 0.192 ng/mL. Subsequently, the new fabric phase sorptive extraction with high‐performance liquid chromatography and ultraviolet detection was successfully applied for the recovery of alkyl phenols from spiked ground water, river water, and treated water from a sewage treatment plant, and soil and sludge samples.     

Determination of antimony species with high-performance liquid chromatography using element specific detection

A new method for the fast and simultaneous determination of Sb(III) and Sb(V) is presented involving the use of anion exchange high-performance liquid chromatography (HPLC), a complexing reagent in the mobile phase, and element specific detection with flame atomic absorption spectrometry (FAAS) or inductively coupled plasma mass spectrometry (ICP-MS). Chromatographic parameters such as nature and concentration of the complexing and eluting compounds and pH of the mobile phase were investigated in detail. Additionally, the separation of inorganic Sb(III) and Sb(V) from organically bounded antimony (as (CH₃)₃SbCl₂ and (CH₃)₃Sb(OH)₂) was investigated by using anion, and cation exchange, and reversed phase HPLC. Best separation was obtained with anion exchange HPLC under alkaline conditions. Cation exchange and reversed-phase HPLC were not useful for the separation of the above compounds. With FAAS concentrations in the upper mg L^–1 range are detectable, which is not sensitive enough for the analyses of environmental samples. When the chromatographic system was coupled to ICP-MS, the detection limits are in the lower μg L^–1 range. The method was applied to various environmental samples with anthropogenic and naturally elevated Sb concentrations.     

Comparison of packed and capillary columns for practical SFC separations

Summary A newly developed polysiloxane-type packing material shows promise for use in SFC. Relatively polar compounds were eluted from a microbore column with good peakshape using a mobile phase consisting of CO₂ modified with formic acid and water. The latter combination is an effective modifier suitable for use with pressure programming and FID detection.

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Vergleich von gepackten und Capillar-Säulen für praktische SFC-Trennungen

     

Determination of Fluoxetine and Norfluoxetine in Serum by Liquid Chromatography with Fluorescence Detection

Abstract

A reversed phase liquid chromatographic procedure with fluorescence detection for the simultaneous determination of fluoxetine and its active metabolite, norfluoxetine in human serum is described. A 0.5-mL aliquot of the sample after the addition of protriptyline as the internal standard is passed through a 1-mL BondElut C₁₈ silica extraction column. The column is selectively washed to remove polar, neutral, acidic and weakly basic compounds. The desired compounds are eluted with a 0.25-mL aliquot of a mixture of 0.1 N perchloric acid + acetonitrile (1:3). A 20-uL aliquot of the eluate is injected onto a 15 cm × 4.6 mm (i.d.) column packed with 5-um C₈-silica particles, which is eluted at ambient temperature with a mobile phase containing tetramethyl-ammonium perchlorate. The peaks are detected with a fluorescence detector (ex = 235 nm; em = 310 nm). In the resulting chromatogram, there are only few extraneous peaks, fluoxetines give sharp peaks which are well resolved from peaks for solvent and internal standard. The extraction recovery of fluoxetines and internal standard is in the range of 85%.