Study of polar and nonpolar substituted benzenes and aromatic isomers on carbon-coated zirconia and alkyl bonded phases

Entropy driven separation of xylene isomers on graphitic carbon adsorbents

Separation of xylene isomers remains one of the most important and challenging applications of adsorption-based separations in petrochemical industry. Despite the sustainable success of zeolite-based separations a search for efficient adsorbents selective for xylenes, especially para-xylene, is constantly ongoing. In this work, a potentially scalable chromatographic separation of all three xylenes was achieved on graphitic carbon sorbents, including a self-packed sorbent based on an oligo-graphene. A curious feature of this separation is stronger retention of para-xylene than meta- and, in some conditions, even than ortho-xylene. Noticeably, separation selectivity between para- and meta-isomers does not depend on temperature. Apparently, lower entropy of para-xylene in solution due to its higher molecular symmetry leads to a lesser adsorption entropy loss, which makes its adsorption statistically more likely. The concept of using carbon adsorbents for entropy driven chromatography separations may be useful for the isolation of xylenes from their mixture and, possibly, for other positional isomers separation.

Impact of reversed phase column pairs in comprehensive two-dimensional liquid chromatography

A major issue in optimizing the resolving power of two-dimensional chromatographic separations is the choice of the two phases so as to maximize the distribution of the analytes over the separation space. In this work, we studied the choice of appropriate reversed phases to use in on-line comprehensive two-dimensional liquid chromatography (LC×LC). A set of four chemically different conventional bonded reversed phases was used in the first dimension. The second dimension column was either a conventional bonded C18 phase or a carbon-clad phase (CCP). The LC×LC chromatograms and contour plots were all rather similar indicating that the selectivities of the two phases were also similar regardless of the reverse phase column used in the first dimension. Further, the spatial coverage seen with all four first dimension stationary phases when paired with a second dimension C18 phase were low and the retention times were strongly correlated. However, when the C18 column was replaced with the CCP column much improved separations were observed with higher spatial coverages, greater orthogonalities and significant increases in the number of observed peaks.

Fabrication of monodisperse porous zirconia microspheres and their phosphorylation for Friedel-Crafts alkylation of indoles

Monodisperse porous zirconia (ZrO2) microspheres with nanocrystallized framework were fabricated by impregnation of porous polymer microspheres as a novel hard template with zirconia precursors followed by calcination to remove the template. Porous phosphorylated zirconia (PhZr) microspheres were prepared by further treating porous zirconia microspheres with phosphoric acid. The morphology, structure, and properties of these microspheres were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption/desorption measurement, FT-IR, and X-ray powder diffraction. The as-prepared zirconia and phosphorylated zirconia microspheres showed uniform particle size and well-defined morphology. The phosphorylated zirconia microspheres served as highly active solid acid catalysts for Friedel-Crafts alkylation of indoles with chalcones and could be reused for 22 cycles with negligible loss of activity. In situ pyridine-adsorbed FT-IR analysis of the best performing PhZr microspheres suggested the presence of both Lewis and Brønsted acid sites, and the total acidity as measured by temperature-programmed desorption of ammonia (NH3-TPD) was 328 μmol·g(-1).

Development of a carbon clad core-shell silica for high speed two-dimensional liquid chromatography

We recently introduced a new method to deposit carbon on fully porous silicas (5 μm) to address some of the shortcomings of carbon clad zirconia (C/ZrO(2)), which has rather low retention due to its low surface area (20-30 m(2)/g). The method enables the introduction of a thin, homogeneous layer of Al(III) on silica to serve as catalytic sites for carbon deposition without damaging the silica's native pore structure. Subsequent carbon deposition by chemical vapor deposition resulted in chromatographically useful carbon phases as shown by good efficiencies and higher retentivity relative to C/ZrO(2). Herein, we use the above method to develop a novel carbon phase on superficially porous silica (2.7 μm). This small, new form of silica offers better mass transfer properties and higher efficiency with lower column back pressures as compared to sub 2 μm silica packings, which should make it attractive for use as the second dimension in fast two-dimensional LC (LC × LC). After carbon deposition, several studies were conducted to compare the new packing with C/ZrO(2). Consistent with work on 5 μm fully porous silica, the metal cladding did not cause pore blockage. Subsequent carbon deposition maintained the good mass transfer properties as shown by the effect of velocity on HETP. The new packing exhibits efficiencies up to ∼5.6-fold higher than C/ZrO(2) for polar compounds. We observed similar chromatographic selectivity for all carbon phases tested. Consequently, the use of the new packing as the second dimension in fast LC×LC improved the peak capacity of fast LC × LC. The new material gave loading capacities similar to C/ZrO(2), which is rather as expected based on the surface areas of the two phases.

Effect of first dimension phase selectivity in online comprehensive two dimensional liquid chromatography (LC×LC)

In this study, we examined the effect of first dimension column selectivity in reversed phase (RP) online comprehensive two dimensional liquid chromatography (LC×LC). The second dimension was always a carbon clad metal oxide reversed phase material. The hydrophobic subtraction model (HSM) and the related phase selective triangles were used to guide the selection of six different RP first dimension columns. Various kinds of samples were investigated and thus two different elution conditions were needed to cause full elution from the first dimension columns. We compared LC×LC chromatograms, contours plots, and fcoverage plots by measuring peak capacities, peak numbers, relative spatial coverage, correlation values, etc. The major finding of this study is that the carbon phase due to its rather different selectivity from other reversed phases is reasonably orthogonal to a variety of common types of bonded reversed phases. Thus quite surprisingly the six different first dimension stationary phases all showed generally similar separation patterns when paired to the second dimension carbon phase. This result greatly simplifies the task of choosing the correct pair of phases for RP×RP.

A new, validated HPLC-MS/MS method for the simultaneous determination of the anti-cancer agent capecitabine and its metabolites: 5'-deoxy-5-fluorocytidine, 5'-deoxy-5-fluorouridine, 5-fluorouracil and 5-fluorodihydrouracil, in human plasma

A rapid and selective liquid chromatography/tandem mass spectrometric method was developed for the simultaneous determination of capecitabine and its metabolites 5'-deoxy-5-fluorocytidine (5'-DFCR), 5'-deoxy-5-fluorouracil (5'-DFUR), 5-fluorouracil (5-FU) and dihydro-5-fluorouracil (FUH(2)) in human plasma. A 200 microL human plasma aliquot was spiked with a mixture of internal standards fludarabine and 5-chlorouracil. A single-step protein precipitation method was employed using 10% (v/v) trichloroacetic acid in water to separate analytes from bio-matrices. Volumes of 20 microL of the supernatant were directly injected onto the HPLC system. Separation was achieved on a 30 x 2.1 mm Hypercarb (porous graphitic carbon) column using a gradient by mixing 10 mm ammonium acetate and acetonitrile-2-propanol-tetrahydrofuran (1 : 3 : 2.25, v/v/v). The detection was performed using a Finnigan TSQ Quantum Ultra equipped with the electrospray ion source operated in positive and negative mode. The assay quantifies a range from 10 to 1000 ng/mL for capecitabine, from 10 to 5000 ng/mL for 5'-DFCR and 5'-DFUR, and from 50 to 5000 ng/mL for 5-FU and FUH(2) using a plasma sample of 200 microL. Correlation coefficients (r(2)) of the calibration curves in human plasma were better than 0.99 for all compounds. At all concentration levels, deviations of measured concentrations from nominal concentration were between -4.41 and 3.65% with CV values less than 12.0% for capecitabine, between -7.00 and 6.59% with CV values less than 13.0 for 5'-DFUR, between -3.25 and 4.11% with CV values less than 9.34% for 5'-DFCR, between -5.54 and 5.91% with CV values less than 9.69% for 5-FU and between -4.26 and 6.86% with CV values less than 14.9% for FUH(2). The described method was successfully applied for the evaluation of the pharmacokinetic profile of capecitabine and its metabolites in plasma of treated cancer patients.

Preparation and evaluation of carbon coated alumina as a high surface area packing material for high performance liquid chromatography

The retention of polar compounds, the separation of structural isomers and thermal stability make carbonaceous materials very attractive stationary phases for liquid chromatography (LC). Carbon clad zirconia (C/ZrO(2)), one of the most interesting, exhibits unparalleled chemical and thermal stability, but its characteristically low surface area (20-30 m(2)/g) limits broader application as a second dimension separation in two-dimensional liquid chromatography (2DLC) where high retentivity and therefore high stationary phase surface area are required. In this work, we used a high surface area commercial HPLC alumina (153 m(2)/g) as a support material to develop a carbon phase by chemical vapor deposition (CVD) at elevated temperature using hexane vapor as the carbon source. The loading of carbon was varied by changing the CVD time and temperature, and the carbon coated alumina (C/Al(2)O(3)) was characterized both physically and chromatographically. The resulting carbon phases behaved as a reversed phase similar to C/ZrO(2). At all carbon loadings, C/Al(2)O(3) closely matched the unique chromatographic selectivity of carbon phases, and as expected the retentivity was increased over C/ZrO(2). Excess carbon - the amount equivalent to 5 monolayers--was required to fully cover the oxide support in C/Al(2)O(3), but this was less excess than needed with C/ZrO(2). Plate counts were 60,000-76,000/m for 5 μm particles. Spectroscopic studies (XPS and FT-IR) were also conducted; they showed that the two materials were chemically very similar.

Stability indicating method for famotidine in pharmaceuticals using porous graphitic carbon column

A simple, sensitive and rapid HPLC method was developed and validated for the simultaneous determination of famotidine (FMT) and related impurities in pharmaceuticals. Chromatographic separation was accomplished within 10 min on a porous graphitic carbon (PGC) column using 50:50 v/v ACN-water containing 0.5% pentane sulphonic acid (PSA) as the mobile phase. Separation was achieved with a flow rate of 1 mL/min and a detection wavelength of 265 nm. The calibration curves were linear over a concentration range of 1.5-100 microg/mL. The intra- and interday RSDs (n = 5) for the retention times and peak area were all less than 2%. The method was sensitive with an LOD (S/N = 3) of 0.1 microg/mL for FMT, imp. C and 0.05 microg/mL for imp. 2, A and D. All recoveries were greater than 98%. The method was demonstrated to be precise, accurate and specific with no interference from the tablet ingredients and separation of the drug peak from the peaks of the degradation products (oxidative degradation and acid and base degradation). The results indicated that the proposed method could be used for the determination of FMT in commercial dosage forms and as a stability-indicating assay.

Determination and characterization of organic explosives using porous graphitic carbon and liquid chromatography–atmospheric pressure chemical ionization mass spectrometry

A new LC-MS method for the determination and characterization of three groups of commonly used organic explosives (nitroaromatic compounds, cyclic nitroamines and nitrate esters) was developed using a porous graphitic carbon (PGC) (Hypercarb) column. Twenty-one different explosive-related compounds--including 2,4,6-trinitrotoluene, its by-products and its degradation products--were chromatographically separated in a single analysis. This efficient separation facilitates the identification of the manufacturer of the explosive using the identified analytes as a fingerprint. A final, conclusive identification of the analytes can be obtained using LC-MS equipped with an atmospheric pressure chemical ionization (APCI) interface. Solvent effects on chromatographic behaviour were investigated, as were the effects of solvent mixtures and mobile phase additives. The number and the relative positions of the nitro groups within analyte molecules influence their order of elution; these effects were investigated. The data thus generated can be interpreted to support a hypothesis concerning the retention mechanism of nitro-containing compounds when using PGC. Limits of detection ranged from 0.5 to 41.2 ng. The new methodology described herein improves the sensitivity and selectivity of explosive detection. The effectiveness of the method is demonstrated by the analysis of soil samples containing explosives residue from test fields in Sweden and Afghanistan.

Separation of substituted aromatic isomers with porous graphitic carbon in subcritical fluid chromatography

The ability of porous graphitic carbon (PGC) to separate structural isomers has been reported in high-performance liquid chromatography (HPLC). This paper presents studies carried out in subcritical fluid chromatography (SubFC). Various polar and nonpolar modifiers were added to the carbon dioxide mobile phase, in proportions ranging from 5 to 40%. The effects of both the nature and the percentage of the modifier on aromatic isomer separations were studied. Two types of selectivity behaviour appear. The first one, related to steric recognition, is due to the number of contact points between the compounds and the flat surface of PGC. In this case, retention orders are often identical to that reported in HPLC. The second is related to the favourable interaction between the polar moieties of the solutes and the stationary phase. In this case, the retention and selectivity strongly depend on the mobile phase composition. Thus, the separations obtained are greatly enhanced, compared to those obtained in HPLC. The retention and selectivity variations observed when the composition of the mobile phase is changed are discussed based on linear solvation energy relationships (LSERs). Practical applications are presented, namely benzene, toluene, ethylbenzene and xylenes (BTEX) and flavour molecules separations.

High temperature liquid chromatography of triazole fungicides on polybutadiene-coated zirconia stationary phase

High temperature liquid chromatography using water-rich and superheated water eluent is evaluated as a new approach for the separation of selected triazole fungicides, hexaconazole, tebuconazole, propiconazole, and difenoconazole. Using a polybutadiene-coated zirconia column at temperatures of 100-150 degrees C, clear separations were achieved when 100% purified water was utilized as organic-free eluent. Excellent limits of detection down to pg level were obtained for the separation of the triazole fungicides under optimum conditions. Van't Hoff plots for the separations were linear suggesting that no changes occurred in the retention mechanism over the temperature range studied.

Layer-by-Layer Self-Assembly of Multilayer Zirconia Nanoparticles on Silica Spheres for HPLC Packings

A novel zirconia-based HPLC packing material, ZrO2/SiO2, which consists of micrometer-sized silica spheres as core and nanometer-sized zirconia particles as surface coating, was prepared by a layer-by-layer self-assembly technique. The material exhibits favorable characteristics for HPLC applications, including high surface area and pore volume, good pore structure, narrow particle size, and pore size distribution. Not only the support ZrO2/SiO2 but also the stationary-phase C18 bonded ZrO2/SiO2 exhibits excellent chemical stability. In addition, good permeability was observed for both of them. High specific area surface and good permeability of ZrO2/SiO2 permit a high loading amount of chiral polymer on it and greatly improved the enantioselectivity and resolution for some chiral separations.

Thermally-treated clay as a stationary phase in liquid chromatography

Spray-dried, spherical synthetic hectorite particles have been thermally-treated at 500 degrees C for 16 h and used as adsorbent materials in reversed-phase liquid chromatography. The retention of a 22 mono and disubstituted aromatic compounds was evaluated to study the retention mechanisms on the clay mineral. The retention of solutes on the thermally-treated clays was markedly different than that measured on octadecylsilica (ODS) columns under identical conditions, but remarkably similar to retention characteristics of the same solutes on porous graphitic carbon columns. The clay columns exhibit an enhanced selectivity over the ODS column in separation of nitroaromatic positional isomers. Under identical mobile phase compositions, a selectivity, alpha, of 7.15 between ortho- and para-dinitrobenzene isomers was measured on the clay column compared to a alpha of 1.04 on the ODS column.

A Graphitized-Carbon Monolithic Column

The preparation of a novel carbon monolithic column for high performance liquid chromatography is described. A phenolic resin rod with embedded 10-microm silica beads was prepared by acid-catalyzed polymerization of a resorcinol/iron(III) complex and formaldehyde. This rod was carbonized and graphitized under inert atmosphere with a programmed temperature cycle from room temperature to 1250 degrees C. Subsequently, the silica beads along with iron catalysts were removed, leaving a porous carbon rod. Imaging of this monolithic rod by scanning and transmission electron microscopies revealed a highly interconnected bimodal porous structure. The porosity and pore size distribution of the mesopores were characterized by N2 absorption/desorption. Graphene sheets were found in the TEM images of the carbon rod, and the graphite index was characterized by Raman spectrum and X-ray diffraction. A monolithic column prepared with the aforementioned carbon rod was evaluated using a mixture of alkylbenzenes. It exhibited an excellent separation power and a low hydraulic resistance. The bundle-of-capillaries model was used to characterize the hydrodynamics of this monolith. Its permeability was found to agree well with the theoretical one.

Analysis of a ginger extract by high-performance liquid chromatography coupled to nuclear magnetic resonance spectroscopy using superheated deuterium oxide as the mobile phase

A methanolic extract of powdered ginger was separated on a Xterra RP 18 column using deuterium oxide as the eluent and a temperature gradient from 50 to 130 degrees C at 4 degrees C/min. On-line and off-line HPLC-NMR analysis yielded spectra for vanillin, dihydroferulic acid, zingerone and ferulic acid. The identification of dihydroferulic acid and zingerone were confirmed by mass spectroscopy.

Separation of polar compounds using carbon columns

The objective of this review with 122 references is to provide structure and retention mechanisms of porous graphitic carbon by chromatographic analysis and computational chemical analysis of retention mechanisms. Synthesis methods of porous graphitic carbon are described. Applications for use as matrix for dynamic coating on porous graphitic carbon and direct separation of polar compounds on porous graphitic carbon demonstrated that the physical and chemical stability of graphitic carbons performed in both chromatography and extraction, especially for polar compounds, those are difficult on both silica-based and organic polymer-based packing materials. The disadvantage is difficult desorption of non-polar compounds adsorbed on the surface. The development of 3.5-microm particles improves the separation power of graphitic carbon columns with the high theoretical plate number.