Apolar Stationary Phases Research Articles

Facile chemical hydrophobization of thin-layer plates by vapor deposition of methyltrimethoxysilane for reversed-phase chromatography

Reversed-phase chromatography is based on a polar mobile phase and an apolar stationary phase. This separation mode is regularly used in planar (thin-layer) chromatography, and the necessary plates are commercially available. We investigated the preparation of hydrophobic plates for thin-layer chromatography by chemical vapor sorption of methyltrimethoxysilane directly onto a normal-phase plate. For this, a commercial normal-phase plate is simply exposed to the vapors of the reagent in a closed vessel. The obtained plates were characterized by infrared spectroscopy and dynamic water vapor sorption, which reported an extensive conversion of free hydroxyl groups by the reagent. The obtained plates were hydrophobic with a water contact angle close to 135°. The extent of hydrophobization precluded the use of pure water as an eluent while mixtures with organic solvents were perfectly adequate. The plates’ chromatographic performance was compared with that of C18 and paraffin-coated plates. For this, a set of parabens was separated with mixtures of acetone and water. The height of a theoretical plate was similar for the hydrophobized and the C18 plates (50–90 µm) and larger for the paraffin-coated ones. In contrast to the C18 plate, the hydrophobized and the paraffin-coated plates showed some selectivity for the analyte pair n-butylparaben and iso-butylparaben, which indicates a separation mechanism with the potential for regioselectivity.

The Chemistry of Olive Oil: an endless story

Because of their value, Olive oils, undergo to several attempts of adulteration and this improved the performances of analytical methods developed to check their authenticity. After the eve of the “Chemistry of indexes”, the development of separation techniques greatly improved the knowledge of the composition of lipids; the earlier gas chromatographic separations of fatty acid, could last even more than 40 minutes and the evaluation of minor fatty acids were problematic, at Bologna University, their previous separation on silver nitrate silica gel TLC was applied in 1974–1975, in order to concentrate them and obtain a better chromatogram. The gas chromatographic evaluation of minor compounds was another interesting challenge, packed columns with apolar stationary phase, admit the separation of few peaks; in 1975, the use of a slightly more polar stationary phase (OV17) highlighted the presence of Δ-5-avenasterol in olive oils. 1981 is a milestone for olive oil analytical control: Regulation (CEE) 2568/91 made the use of capillary columns mandatory. Official analytical methods consider many parameters, enclosed sensory evaluation, that is not matter for chemists, however, chemists are moved by curiosity and several papers had been published on the relationship between the volatile compounds of head space of olive oil and sensory characteristics. Despite being one of the more studied food, olive oils’ composition continues to give the chemists the possibility to discover new compounds, e.g. degradation products of sterols that leads to the standardization of the method for sterenes evaluation. From 1973 to nowadays, the laboratory performances greatly improved, nowadays it is possible to generate a huge number of data in a short time, then the problem is how to correctly interpretate them, surely chemometric and lipidomic greatly can help.

Characterization and analysis of structural isomers of dimethyl methoxypyrazines in cork stoppers and ladybugs (Harmonia axyridis and Coccinella septempunctata).

The three constitutional isomers of dimethyl-substituted methoxypyrazines: 3,5-dimethyl-2-methoxypyrazine 1; 2,5-dimethyl-3-methoxypyrazine 2; and 2,3-dimethyl-5-methoxypyrazine 3 are potent flavor compounds with similar mass spectrometric, gas chromatographic, and nuclear magnetic resonance spectroscopic behavior. Therefore, unambiguous analytical determination is critical, particularly in complex matrices. The unequivocal identification of 1-3 could be achieved by homo- and heteronuclear NMR correlation experiments. The observed mass fragmentation for 1-3 is proposed and discussed, benefitting from synthesized partially deuterated 1 and 2. On common polar and apolar stationary phases used in gas chromatography (GC) 1 and 2 show similar behavior whereas 3 can be separated. In our focus on off-flavor analysis with respect to wine aroma, 1 has been described as a "moldy" off-flavor compound in cork and 2 as a constituent in Harmonia axyridis contributing to the so-called "ladybug taint," whereas 3 has not yet been described as a constituent of wine aroma. A successful separation of 1 and 2 could be achieved on octakis-(2,3-di-O-pentyl-6-O-methyl)-γ-cyclodextrin as stationary phase in GC. Applying heart-cut multidimensional GC analysis with tandem mass spectrometric detection we could confirm the presence of 1 as a "moldy" off-flavor compound in cork. However, in the case of Harmonia axyridis, a previous identification of 2 has to be reconsidered. In our experiments we identified the constitutional isomer 1, which was also found in Coccinella septempunctata, another species discussed with respect to the "ladybug taint." The analysis of such structurally related compounds is a demonstrative example for the importance of a chromatographic separation, as mass spectrometric data by itself could not guarantee the unequivocal identification.

Determination of molecular masses for petroleum distillates by simualted distillation

Determination of Molecular Mass (MM) for petroleum distillates is explored for selected samples viaHigh Temperature Simulated Distillation (HTSD). MM is determined as a by-product from routineHTSD carried out using open wall capillary columns coated with apolar stationary phases. No samplepre-separation into hydrocarbon group-types is required. Determined MM values were validated with resultsachieved via correlations based on specific gravity and refractive index. Furthermore, determined MM withthe former methodologies were found to match mass spectrometric determinations carried out for the basicpolar components present within the studied samples. Moreover, HTSD analyzed petroleum distillates areproposed as more representative calibration standards for size exclusion chromatographic separations, thusopening the feasibility of extrapolating MM determination for non volatile petroleum residua.

Occurrence and biomarker potential of 23-methyl steroids in diatoms and sediments

23-Methyl sterols have been reported to be synthesized by a few marine algae, but unambiguous identification of 23-methyl steroids in sediments and petroleum is lacking. We report the presence of 23-methylcholesta-5,22E-dien-3β-ol in 14 out of 106 diatom cultures, thereby showing that diatoms, together with dinoflagellates, may be an important environmental source for such steroids. Synthesis of authentic 23-methylcholestanes showed that their mass spectra are identical to those of 24-methylcholestanes, but that they elute earlier on apolar stationary phases during gas chromatography (GC) analysis. Co-injection of the authentic standards with sediment extracts revealed the presence of these compounds in the Skole unit of the Oligocene Menelite Formation, the Miocene Monterey Formation and the Messinian Vena del Gesso Formation. In addition, we tentatively identified 23,24-dimethyl-27-norcholestanes in some of these sediments. Molecular clock calculations suggest that diatoms are a possible source for 23-methyl steroids in sediments and petroleum from the late Jurassic onwards.

Fast gas chromatography characterisation of purified semiochemicals from essential oils of Matricaria chamomilla L. (Asteraceae) and Nepeta cataria L. (Lamiaceae)

The chemical composition of Matricaria chamomilla L. and Nepeta cataria L. essential oils was determined by GC–MS on an apolar stationary phase by comparison of the characteristic fragmentation patterns with those of the Wiley 275L database. The GC–MS chromatograms were compared with those obtained by fast GC equipped with a direct resistively heated column (Ultra Fast Module 5% phenyl, 5 m × 0.1 mm, 0.1 μm film thickness). Analytical conditions were optimised to reach a good peak resolution (split ratio = 1:100), with analysis time lower than 5 min versus 35–45 min required by conventional GC–MS. The fast chromatographic method was completely validated for the analysis of mono- and sesquiterpene compounds. Essential oils were then fractionated by column chromatography packed with silica gel. Three main fractions with high degree of purity in E-β-farnesene were isolated from the oil of M. chamomilla. One fraction enriched in ( Z, E)-nepetalactone and one enriched in β-caryophyllene were obtained from the oil of N. cataria. These semiochemical compounds could act as attractants of aphid’s predators and parasitoids.

Analysis of carbonyl compounds via headspace solid-phase microextraction with on-fiber derivatization and gas chromatographic–ion trap tandem mass spectrometric determination of their O-(2,3,4,5,6-pentafluorobenzyl)oxime derivatives

An improved method for the analysis of carbonyls is described utilizing a headspace solid-phase microextraction (HS-SPME) step and on-fiber derivatization with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) hydrochloride. Thermal desorption of the oxime derivatives formed on the fiber is followed by gas chromatographic separation coupled to an ion trap tandem mass spectrometer (GC–ITMS). Selecting specific fragment ions within the electron ionization (EI +) mass spectra of these oxime derivatives as precursor ions for MS–MS fragmentation provides a suitable method for the target analysis of individual carbonyl classes, such as alkanals, ( E)-2-alkenals, ( E, E)-2,4-alkadienals, and others. Retention indices on polar as well as on apolar stationary phases along with EI + mass spectra patterns are presented for a large set of oxime derivatives, giving valuable information needed for unambiguous assignment of substances in complex sample matrices. The fast sample preparation and derivatization step via HS-SPME can be automated and is applicable to a variety of biological samples and foodstuffs, allowing rapid and sensitive screening analyses of important aldehydic biomarkers and aroma active compounds.

Evaluation of solubility parameters for nonvolatile branched hydrocarbons by inverse gas chromatography

AbstractFlory‐Huggins interaction parameters determined by inverse gas chromatographic (IGC) technique have been used for the estimation of solubility parameters of nonvolatile branched hydrocarbon solvents. A family of isosteric branched hydrocarbons consists of one apolar stationary phase, 19,24‐dioctadecyldotetracontane (C78), and four polar stationary solvents formed by replacing one of the CH3 groups of C78 by OH (POH), CN (PCN), SH (PSH), CF3 (MTF), and two other polar solvents formed by replacing four CH3 groups of C78 with four CF3 (TTF) and OCH3 (TMO) are investigated. The three‐dimensional Hansen solubility parameters of the solvents have been estimated following the approaches of Voelkel and the method of Huang at five different temperatures. In the three components estimated by Voelkel method, the dispersion component is decreasing rapidly, whereas the polar, and hydrogen bonding components of the solubility parameters are increasing with temperature. In the components of solubility parameter estimated by Huang method, the dispersion and hydrogen bonding components are slowly decreasing with temperature, however, the polar components are almost constant with temperature. The components of solubility parameters for different solvents have been discussed in terms of solvent polarity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Reversed-phase liquid chromatography analysis of alkyl-imidazolium ionic liquids: II. Effects of different added salts and stationary phase influence

Two mixtures of four 1-alkyl-3-methylimidazolium ionic liquids (ILs) salts associated to the anions tetrafluoroborate or hexafluorophosphate were analyzed by reversed-phase liquid chromatography with three different stationary phases: Kromasil C 8, Zorbax Extend C 18 and Zorbax Sb-Aq. The effect on retention of various inorganic salts (NaCl, NaH 2PO 4, NaBF 4, NaClO 4 and NaPF 6) added to acetonitrile/water mobile phases was studied. The three columns gave similar separation profiles. In all cases, the retention of ILs increased with the increasing affinity of the inorganic anions for the apolar stationary phases; a phenomenon called chaotropicity. The chaotropic anion order is Cl − ∼ H 2PO 4 − < BF 4 − ∼ ClO 4 − < PF 6 −. It is established that the presence of chaotropic anions in the mobile phase do not permit to differentiate between ILs associated to different anions. However, chloride or dihydrogenphosphate added salts do not fully screen the retention differences between ILs associated with different anions. Distorted and even split peaks may appear in the chromatogram depending on the nature and concentration of the injected ILs. In the RPLC analysis of imidazolium-based IL, it is recommended to add to the mobile phase significant amounts of a salt containing a chaotropic anion. This salt addition will improve the IL peak shapes and give reproducible retention factors. LODs in the low microgram range (∼5 nmol) were obtained with the Kromasil C 8 column with a 50/50 acetonitrile–water mobile phase containing 0.01 M NaPF 6 added salt and 230 nm UV detection.

High-speed gas chromatography in doping control: Fast-GC and fast-GC/MS determination of β-adrenoceptor ligands and diuretics

In official doping controls, about 300 drugs and metabolites have to be screened for each sample. Moreover, the number of determinations to be routinely processed increases continuously as the number of both samples and potential illicit drugs keeps growing. As a consequence, increasingly specific, sensitive, and, above all, fast methods for doping controls are needed. The present study presents an efficient fast-GC/MS approach to the routine screening of two different classes of doping agents, namely beta-adrenoceptor ligands and diuretics (belonging to the S3, P2, and S5 groups of the WADA list of prohibited substances). Narrow bore columns (100 mm id) of different lengths and coated with apolar stationary phases were successfully used to separate the derivatized analytes; preliminary experiments (results not shown) showed better performances with OV-1701 for the separation of beta-adrenoceptor ligands. On the same stationary phase some diuretics required too high a temperature or a long isothermal time for elution, in which case a DB1-MS column was preferred. Two methods of sample preparation, derivatization, and analysis were used on aqueous standard mixtures of, respectively, (i) eight beta-adrenoceptor ligands, including five beta-antagonists (acebutolol, alprenolol, atenolol, metoprolol, pindolol) and three beta2-agonists (salbutamol, clenbuterol, terbutaline) and (ii) seventeen diuretic drugs (acetazolamide, althiazide, bendroflumethiazide, bumethanide, canrenone, chlorothiazide, chlortalidone, clopamide, ethacrinic acid, furosemide, hydrochlorothiazide, hydroflumethiazide, indapamide, indomethacine, spironolactone, triamterene, trichloromethiazide) and one masking agent (probenecid). The mixture of beta-adrenoceptor ligand derivatives was efficiently separated in about 5.6 min, while the one of 18 diuretics and masking agents required less than 5 min for analysis. Limits of detection were from 1 microg/L for pindolol, ethacrinic acid, furosemide, indomethacine, and trichloromethiazide, to 20 microg/L for terbutaline, salbutamol, and metoprolol, and 50 microg/L for clopamide; the instrumental repeatability proved to be excellent (area RSD% <2 for almost all analytes). For this work a quadrupole MS with inert ion source has been used, demonstrating that the quadrupole technology is perfectly adequate to provide precise integration of 400 ms-wide GC peaks.

Influence of cetophenolic and diphenolic intramolecular hydrogen bonding on the chromatographic and spectroscopic properties of hydroxyanthraquinones

Anthraquinones have been widely employed for dyeing textiles in combination with various mordants. Many papers have dealt with the modification of optical properties induced by the formation of dye complexes under various pH conditions. This paper deals with the characterization of diphenolic intramolecular hydrogen bonding via molecular modeling and cetophenolic interactions by FT-IR spectroscopy to determine their effect on the spectroscopic and chromatographic properties of hydroxyanthraquinones (anthraflavic acid, alizarin, quinizarin and purpurin). The formation of cetophenolic hydrogen bonding induces a substantial bathochromic shift to the visible absorption band. Moreover, it implies that the constitution of hypercyclised aromatic systems is potentially responsible for the fluorescence of quinizarin and purpurin. This study also demonstrates the modification of the chromatographic retention of dihydroxyanthraquinones on apolar stationary phase consequent to the monopolization of polar groups involved in the formation of such interactions.

Quantitative structure and retention relationships for gas chromatographic data: Application to alkyl pyridines on apolar and polar phases

Quantitative structure and retention relationships (QSRR) have been developed to model gas chromatographic retention data of alkyl pyridines on apolar (branched alkane) and polar (primary alcohol) stationary phases. The retention properties analyzed are Kovats retention index, I; partial molar enthalpy, Δ H; partial molar entropy, Δ S and partition coefficient, log K. Using the seven valence molecular connectivity indices ( χ) calculated for the 18 alkyl pyridines, regression models are generated to predict the retention properties. The best model (model A) obtained with the descriptors χ P V 1 , χ P V 3 and χ CH V 6 was unable to produce a satisfactory statistical performance and correct order of elution. The model has been modified (model B) by including steric parameter s, which has been empirically derived by considering the steric effects due to the presence of alkyl groups at the ortho and meta positions. The modified model predicts the correct order of elution for all the alkyl pyridines and good correlation coefficients, r. The r values obtained for I, Δ H, Δ S and log K are: r = 0.955, 0.975, 0.984 and 0.955 (model A) become r = 0.999, 0.996, 0.990 and 0.999 (model B) on the apolar stationary phase and r = 0.931, 0.926, 0.911 and 0.931 (model A) become r = 0.999, 0.992, 0.958 and 0.999 (model B) on the polar stationary phase.

Effect of film thickness on the retention index of nitro and cyano compounds. Interpretation of the logarithmic equation.

Recently, a minimum was observed on the temperature dependence of retention indices in case of polar solutes chromatographed on apolar stationary phases. Physical significance is attributed to the equation constants describing the minimum. The equation constants are explained using a thermodynamic and a kinetic approach. Statistical properties of the models for different film thicknesses are compared. Chemical potential for one methylene unit is calculated in different ways for various film thicknesses and compared with literature values. The activation enthalpy attributed to the solvation process decreases as the carbon atom number (molecular mass) of the compounds decreases. The activation enthalpies for cyanoalkanes are always higher than those for the respective nitroalkanes. The interpretation of the equation constants does not change if the film thickness of the stationary phase increases (it is varied). This fact eliminates adsorption on the surface of the column wall from the possible causes responsible for the minimum on the retention index versus temperature curve.

Contribution to the time balance in gas–liquid chromatography: New definition equations of the retention times and retention volumes

The numerous research groups and researchers, as well as IUPAC, that during the last half century have dealt with different theoretical and practical problems in gas–liquid chromatography (GLC), including its nomenclature, have failed in giving an exact definition equation of the net retention time. Using our earlier results and starting from a time balance of GLC we have solved this problem by introducing the so-called acceleration time, t ac, in the absence of which, the theoretical plate number concept, including the stationary phase transfer, is misinterpreted. The measurements were carried out both on support coated and on wall-coated open tubular columns with apolar and polar stationary phases. Different relationships of t ac with some solute properties and the column temperature for a series of n-alkanes on an apolar stationary phase under isothermal conditions were tested. The results obtained are presented in different tables and mathematical relationships.

Phenyldibenzofurans and phenyldibenzothiophenes in marine sedimentary rocks and hydrothermal petroleum

The phenyl derivatives of dibenzo[ b, d]furan and dibenzo[ b, d]thiophene, so far not reported in geological samples, have now been identified in some sedimentary rocks, bitumen and hydrothermal petroleum through a comparison of their retention times and mass spectra with those of authentic standards. All four possible positional isomers were detected for each phenyldibenzo[ b, d]furan (PhDBF) and phenydibenzo[ b, d]thiophene (PhDBT). Their elution sequence is 1-, 4-, 2- and 3-phenyldibenzo[ b, d]furan and 1-, 4-, 2- and 3-phenyldibenzo[ b, d]thiophene on apolar stationary phases. In contrast to phenyldibenzofurans, the distributions of the thiophene series is controlled mainly by the organic matter maturity. In the most mature samples 4-, 2- and 3-PhDBTs prevail over 1-PhDBT, while less mature samples contain mainly 4-PhDBT or 1-PhDBT and 2-, 3-PhDBTs occur at comparable concentrations. However, in samples of low maturity, in addition to the phenyldibenzo[ b, d]thiophene C 18H 12S series, other structural isomers of C 18H 12S possessing very similar mass spectra were detected. These are likely to be either naphthylbenzo[ b]thiophenes and/or phenylnaphtho[ b]thiophenes. PhDBFs and PhDBTs are likely to be intermediate products in the formation of more condensed heterocyclic polycyclic aromatic compounds like triphenyleno[1,12 −bcd]thiophene and triphenyleno[1,12 -bcd]furan, and benzobisbenzofurans and benzobisbenzothiophenes, which are commonly detected in geological samples.

Gas chromatography for in situ analysis of a cometary nucleus: III. Multi-capillary column system for the cometary sampling and composition experiment of the Rosetta lander probe

The cometary sampling and composition (COSAC) experiment is one of the principal experiments of the surface lander probe of the European Space Agency Rosetta mission to be launched in January 2003. The instrument is designed for the in situ chemical analysis of a cometary nucleus as the details of the nucleus composition are of primary importance for understanding both the formation of the solar system, and the origin of life on Earth. The COSAC experiment consists of an evaporation/pyrolysis device and two analytical systems: a multi-column gas chromatograph and a high-resolution time-of-flight mass spectrometer which may either be operated alone or in a coupled mode. The gas chromatograph includes five general purpose chromatographic columns and three chiral ones, all mounted in parallel. Taking into account the chemical species potentially present in the cometary nucleus as well as the space constraints, a set of five complementary columns was selected to perform the separation and identification of the compounds present in the cometary nucleus. This set of columns includes a carbon molecular sieve porous-layer open tubular (PLOT) column used for the separation of both the noble and other permanent gases, and the C 1–C 2 hydrocarbons. A second PLOT column uses a divinylbenzene–ethylene glycol–dimethylacrylate porous polymer as stationary phase for the analysis of a wide range of C 1–C 2 organic molecules. Two complementary wall-coated open tubular (WCOT) columns with polydimethylsiloxane (PDMS) liquid stationary phases, one containing cyanopropyl–phenylsiloxane and the other diphenylsiloxane groups, are designed to target the same range of organic compounds (C 3–C 7) which could be representative of the widest range of cometary compounds. A third WCOT column with an apolar stationary phase made of non-substituted PDMS is used for the separation and identification of higher-molecular-mass compounds (up to C 10) and aromatic species (monoaromatic and polyaromatic). This paper describes these five general-purpose capillary PLOT and WCOT columns, selected to be used in the COSAC GC system. The analytical capabilities are examined with a special emphasis on the exobiological and planetological implications.

Gas chromatographic and IR spectroscopic characteristics of tri- and tetramethylcyclohexenyl butenyl ketones

Infrared spectroscopic characteristics and gas chromatographic retention indices of tri- and tetramethylcyclohexenyl butenyl ketones were determined at different temperatures of analysis. Thermodynamic characteristics of sorption on the apolar stationary phase were calculated. The sorption characteristics of tri- and tetramethylcyclohexenyl butenyl ketones show that under conditions of capillary gas chromatography these compounds are retained due to the dispersive energy. It was shown by FTIR spectroscopy that the formation of conjugation systems of double bonds is responsible for the higher retention of the β-isomers compared with that of the α-isomers. The shortening of the distance between the carbonyl and trimethylcyclohexenyl groups was found to be accompanied by a decrease in the retention of the isomeric compounds.

Improving the accuracy of Kováts’ retention indices in isothermal gas chromatography

Isothermal Kováts’ retention indices are currently reported as whole numbers, and are frequently deduced from a linear least mean squares fitting of the logarithms of adjusted retention times of a number of n-alkanes versus carbon number, following an iterative method that minimises errors. The currently accepted accuracy is about one retention index unit for apolar stationary phases, and lower for polar stationary phases. This paper presents results that show how the accuracy of the retention index may be safely reported to one-tenth of a retention index unit by the use of a non-linear equation, with present day gas chromatographs without electronic flow controllers. Results are presented that prove the correctness of the retention indices found for several substances on one particular capillary column. Hints on the minimum retention times needed to achieve the 0.1 retention index accuracy are mentioned, for retention times recorded in minutes and in seconds. According to results of this paper, two chromatograms, run under the appropriate conditions, are sufficient to obtain the desired accuracy. The method proposed in this paper does not require knowledge of the hold-up time of the chromatogram.

Stabilization of gas chromatographic stationary phases with nanosized particles

Stabilization of polar and apolar stationary phases with nanosized particles has been demonstrated for gas chromatographic capillary columns. Mixtures of small amounts of (2–10%) of solid particles and liquids form gels, which preserve the stability of the films of stationary phases on the column walls. The stabilization effect of polar and apolar particles has been studied by determining some rheological properties of gels (flow curves, temperature dependence of yield stress values) and validated by gas chromatographic measurements using the most polar and apolar stationary phases.

Quantitative analysis of hydrophobic pulmonary surfactant proteins by high-performance liquid chromatography with light-scattering detection

A new method for the separation and quantification of two hydrophobic lung surfactant proteins (SPs) is described. It is based on size-exclusion chromatography using the apolar stationary phase butyl silicagel with a pore size of 30 nm and isocratic elution with chloroform, methanol and trifluoroacetic acid. The samples were prepared from sheep lung lavage fluid by centrifugation and fractional extraction with butanol and chloroform–methanol. The chromatograms show three peaks in the elution order SP-B, SP-C and lipids. A small peak ahead of SP-B, which disappeared after reduction with 2-mercaptoethanol, was oligomeric SP-B. The response of the evaporative light-scattering detector was non-linear. For preparative high-performance liquid chromatography ultraviolet detection at 279 nm is recommended.