Improving selectivity in gas chromatography by using chemically modified multi-walled carbon nanotubes as stationary phase

Amino-terminated alkyl MWCNTs (MWCNTs-R-NH₂), synthesized by chemical modification of the nanotube skeleton by nucleophilic substitution with 2,2′-(ethylenedioxy)diethylamine, were successfully used as stationary phases for gas chromatographic separation of esters and chloroaromatics. The presence of alkyl chains with polar embedded groups made the functionalized MWCNTs (f-MWCNTs) a mixed-mode GC separation material able to interact in different ways with the analytes. Compared with non-functionalized MWCNTs (nf-MWCNTs), MWCNTs-R-NH₂ had higher selectivity, enhanced resolution, and optimum retention behaviour, and they were proved to perform better than the commercial stationary phase Porapak QS (PQS), claimed to be suitable for similar applications. The so-prepared stationary phase was used for analysis of a synthetic mixture containing different classes of analytes, viz. esters, ketones, alcohols, alkanes, and aromatic hydrocarbons, and finally used for investigation of similar real matrices. In particular, the constituents of a commercial paint thinner were determined by direct injection of the sample, with good reproducibility (inter-day precision RSDs from 5 to 19%). Two unknown samples of commercial white spirit were also analysed for determination of the aromatic hydrocarbon content, and their composition was profiled on the basis of the different compounds identified.     

Aromatization of a C3–C4 alkane mixture over Zn-pentasil modified with tin and lead

Modification of Zn-containing H-ZSM-5 pentasil by tin and lead reduces the yield of C₁₀₊ aromatic hydrocarbons from alkanes C₃–C₄. The high selectivity of formation of the aromatization products is retained.     

Membrane separation of multicomponent mixture of alkanes C1–C4

The membrane separation of the four-component mixture of gaseous alkanes C₁–C₄ is studied. Homogeneous films based on two high-permeable polymers, namely, addition-type poly[3-(trimethylsilyl)tricyclononene-7] and poly[3,4-bis(trimethylsilyl)tricyclononene-7], are used as membranes. Separation of the multicomponent mixture of hydrocarbons on these polymers follows the same trends as separation of binary mixtures CH₄-C₄H₁₀ on polyacetylenes. In the presence of higher hydrocarbons, the permeability coefficients of methane decrease and the permeates become enriched with higher hydrocarbons. During separation of the multicomponent mixture, permeability coefficients P(C₄H₁₀) attain high values (up to 12000 Barrers).     

A Supported Bismuth Halide Perovskite Photocatalyst for Selective Aliphatic and Aromatic C–H Bond Activation

Direct selective oxidation of hydrocarbons to oxygenates by O₂ is challenging. Catalysts are limited by the low activity and narrow application scope, and the main focus is on active C−H bonds at benzylic positions. In this work, stable, lead-free, Cs₃Bi₂Br₉ halide perovskites are integrated within the pore channels of mesoporous SBA-15 silica and demonstrate their photocatalytic potentials for C−H bond activation. The composite photocatalysts can effectively oxidize hydrocarbons (C₅ to C₁₆ including aromatic and aliphatic alkanes) with a conversion rate up to 32900 μmol g_cat⁻¹ h⁻¹ and excellent selectivity (>99 %) towards aldehydes and ketones under visible-light irradiation. Isotopic labeling, in situ spectroscopic studies, and DFT calculations reveal that well-dispersed small perovskite nanoparticles (2–5 nm) possess enhanced electron–hole separation and a close contact with hydrocarbons that facilitates C(sp³)−H bond activation by photoinduced charges.     

Polymerized ionic liquid functionalized multi-walled carbon nanotubes/polyetherimide composites

Thin polyetherimide (PEI) films containing 0.1–3 wt.% multi-walled carbon nanotubes (MWCNTs), have been prepared from three types of MWCNTs, namely pristine, oxidized and polymerized ionic liquid (PIL) functionalized CNTs. Oxidized and PIL functionalized CNTs (CNT–PIL) showed better dispersion in the matrix compared to pristine CNTs. For CNT–PIL, alignment of CNTs has been observed in the matrix. Regardless of the type of CNTs, their incorporation led to an increased thermal stability of the polymer matrix. Dynamic mechanical analysis showed that storage modulus increased by up to 25% (3 wt.% CNT–PIL) and an increase in the height of the damping peaks (tan δ). The addition of CNTs did not have any significant influence on the tensile properties and T_g of the polymer, and the electrical conductivity did not decrease in the case of modified CNTs.     

A Supported Bismuth Halide Perovskite Photocatalyst for Selective Aliphatic and Aromatic C–H Bond Activation

Direct selective oxidation of hydrocarbons to oxygenates by O₂ is challenging. Catalysts are limited by the low activity and narrow application scope, and the main focus is on active C?H bonds at benzylic positions. In this work, stable, lead‐free, Cs₃Bi₂Br₉ halide perovskites are integrated within the pore channels of mesoporous SBA‐15 silica and demonstrate their photocatalytic potentials for C?H bond activation. The composite photocatalysts can effectively oxidize hydrocarbons (C₅ to C₁₆ including aromatic and aliphatic alkanes) with a conversion rate up to 32900 μmol g_cat^?1 h^?1 and excellent selectivity (>99 %) towards aldehydes and ketones under visible‐light irradiation. Isotopic labeling, in situ spectroscopic studies, and DFT calculations reveal that well‐dispersed small perovskite nanoparticles (2–5 nm) possess enhanced electron–hole separation and a close contact with hydrocarbons that facilitates C(sp³)?H bond activation by photoinduced charges.     

Multiwalled carbon nanotubes/cryogel composite, a new sorbent for determination of trace polycyclic aromatic hydrocarbons

A new cost-effective sorbent, multiwalled carbon nanotubes/poly (vinyl alcohol) cryogel composite (MWCNTs/PVA), was prepared under frozen conditions for the extraction and preconcentration of trace polycyclic aromatic hydrocarbons (PAHs) in water samples. This was followed by high performance liquid chromatography (HPLC) with fluorescence detection. The proposed method provided a high enrichment factor with an extremely high extraction efficiency (89–98%) of three spiked levels of three standard PAHs with relative standard deviations of less than 8%. The low detection limits of the method were 5, 8 and 5 ng L^{− 1} for benzo(a)anthracene, benzo(b)fluoranthene and benzo(a)pyrene, respectively. This method was successfully applied for the determination of the three PAHs in real water samples where they were found in the range of 7 to 22 ng L^{− 1}. The major advantages of MWCNTs/PVA over the commercial C₁₈ is that it can be operated at a higher loading flow rate without sorbent clogging and requires a shorter time for completion without any loss of extraction efficiency.     

Depolymerization and hydrodeoxygenation of lignin to aromatic hydrocarbons with a Ru catalyst on a variety of Nb-based supports

Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbO_x species played a key role in the activation and cleavage of C–O bonds in lignin and its model compounds. In this study, commercial niobic acid (HY-340), niobium phosphate (NbPO-CBMM) and lab-made layered niobium oxide (Nb₂O₅-Layer) were chosen as supports to study the effect of Brönsted and Lewis acids on the activation of C–O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C₇–C₉ hydrocarbons (82.3–99.1%). What's more, Ru/Nb₂O₅-Layer affords the best mole yield of C₇–C₉ hydrocarbons and selectivity for C₇–C₉ aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C–O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.     

Characterization of military fog oil by comprehensive two-dimensional gas chromatography

The most commonly used military fog oil is characterized by comprehensive two-dimensional gas chromatography (GC×GC) coupled to either Flame Ionization Detection (FID) or Time-of-Flight Mass Spectrometric Detection (TOFMS) to advance the knowledge regarding the complete chemical makeup of this complex matrix. Two different GC×GC column sets were investigated, one employing a non-polar column combined with a shape selective column and the other an inverse column set (medium-polar/non-polar). The inverse set maximizes the use of the two-dimensional separation space and segregates aliphatic from aromatic fractions. The shape selective column best separates individual polycyclic aromatic hydrocarbons (PAHs) from the bulk oil. The results reveal that fog oil (FO) is composed mainly of aliphatic compounds ranging from C₁₀ to C₃₀, where naphthenes comprise the major fraction. Although many different species of aromatics are present, they constitute only a minor fraction in this oil, and no conjugated PAHs are found. The composition of chemically similar aliphatic constituents limits the analytical power of silica gel fractionation and GC–MS analysis to characterize FO. Among the aliphatic compounds identified are alkanes, cyclohexanes, hexahydroindanes, decalins, adamantanes, and bicyclohexane. The aromatic fraction is composed of alkylbenzene compounds, indanes, tetrahydronaphthalenes, partially hydrogenated PAHs, biphenyls, dibenzofurans and dibenzothiophenes. This work represents the best characterization of military fog oil to date. As the characterization process shows, information on such complex samples can only be parsed using a combination of sample preprocessing steps, multiple detection schemes, and an intelligent selection of column chemistries.     

Application of hollow fiber liquid-phase microextraction in identification of oil spill sources

In this study, hollow fiber based liquid-phase microextraction (HF-LPME), coupled with GC, GC–MS and GC–IRMS detections, was employed to determine petroleum hydrocarbons in spilled oils. According to the results, the HF-LPME method collected more low-molecular weight components, such as C₇–C₁₁n-alkanes, naphthalene, and phenanthrene, than those collected in conventional liquid–liquid extraction (LLE). The results also showed that this method had no remarkable effect on the distributions of high-molecular weight compounds such as >C₁₈n-alkanes, C₁–C₃ phenanthrene, and hopanes. Also, the carbon isotopic compositions of individual n-alkanes in the two preparation processes were identical. Accordingly, HF-LPME, as a simple, fast, and inexpensive sample preparation technique, could become a promising method for the identification of oil spill sources.     

Influence of modification of MWCNTs on the structure and performance of MWCNT‐Poly(MMA‐AM) hybrid membranes

Hybrid membranes containing multi‐walled carbon nanotubes (MWCNTs) were initially prepared to separate benzene/cyclohexane mixtures. Subsequently, MWCNT surfaces were chemically modified using two methods to change the surface polarity of the MWCNTs and improve the distribution thereof in Poly(methylmethacrylate) (PMMA). This change consequently enhanced the separation performance of hybrid membranes with MWCNTs. Raman spectroscopy was used to characterize the structure of the pristine MWCNTs and the modified MWCNTs. The morphology and distribution of the MWCNTs in PMMA were investigated by transmission electron microscopy. The results showed that the addition of MWCNTs clearly improved the separation performance of the hybrid membranes. Surface modification introduced polar groups onto the MWCNT surface, which significantly improved the distribution of MWCNTs in the PMMA membranes and the performance of hybrid membranes. MWCNTs with higher surface polarity also increased the amount of MWCNTs distributed homogeneously in PMMA. Aminated MWCNTs (MWCNT‐NH₂) showed the highest surface polarity. Thus, the content of MWCNT‐NH₂ well distributed in PMMA was the highest among the three types of MWCNTs. The highest separation factor for the hybrid membranes with 1.0 wt% MWCNT‐NH₂ was about seven times that of membranes containing pristine MWCNTs. Copyright © 2013 John Wiley & Sons, Ltd.     

Measurements of dissolved nonmethane hydrocarbons in sea water

Summary An automated stripping technique for the measurement of dissolved hydrocarbons in sea water is presented together with some results obtained during a ship cruise from Europe to Brazil. The sea water concentrations of NMHC were determined in a three step process: degassing, preconcentration, and gas chromatographic analysis. In a stripping chamber the dissolved gases were purged from sea water with helium. The stripped hydrocarbons were cryogenically concentrated, and after thermal desorption they were injected into the gas chromatograph. The light fraction (C₂–C₄) was separated on a packed and the heavy fraction (C₅–C₁₀) on a capillary column. All valves were microprocessor controlled in order to achieve an automated process. For the C₂–C₄ hydrocarbons the stripping efficiencies exceeded 90% except for acetylene (80%), the lower limit of detection was 1 to 4.5 pmol hydrocarbon per liter of sea water and the accuracy of the method was better than 25%, depending on the individual hydrocarbons. Typical oceanic concentrations were in the 10 and 100 pmol/l range. Alkenes were generally more abundant than the corresponding alkanes and within the homologous series the concentrations decreased with increasing number of carbon atoms.     

Using the cationic surfactants N-cetyl-N-methylpyrrolidinium bromide and 1-cetyl-3-methylimidazolium bromide for sweeping–micellar electrokinetic chromatography

This paper describes a sweeping–micellar electrokinetic chromatography (sweeping–MEKC) technique for the determination of seven benzodiazepines, using, as sweeping carriers, the ionic liquid-type cationic surfactants 1-cetyl-3-methylimidazolium bromide (C₁₆MIMBr) and N-cetyl-N-methylpyrrolidinium bromide (C₁₆MPYB). These surfactants resemble the commonly employed cationic surfactant cetyltrimethylammonium bromide (CTAB), but they provide different separation efficiencies. We optimized the separation and sweeping conditions, including the pH, the concentrations of organic modifier and surfactant, and the sample injection volume. Adding C₁₆MIMBr or C₁₆MPYB to the background electrolyte enhanced the separation efficiency and detection sensitivity during the sweeping–MEKC analyses of the benzodiazepines. C₁₆MIMBr enhanced the sensitivity for each benzodiazepine 31–59-fold; C₁₆MPYB, 86–165-fold. In the presence of C₁₆MPYB, the limits of detection for the seven analytes ranged from 4.68 to 9.75 ng/mL. We adopted the sweeping–MEKC conditions optimized for C₁₆MPYB to satisfactorily analyze a human urine sample spiked with the seven benzodiazepines. To minimize the matrix effects, we subjected this urine sample to off-line solid phase extraction (SPE) prior to analysis. The recoveries of the analytes after SPE were satisfactory (ca. 77.0–88.3%). Our experimental results reveal that the cationic surfactant C₁₆MPYB exhibits superior sweeping power relative to those of C₁₆MIMBr and CTAB and that it can be applied in sweeping–MEKC analyses for the on-line concentrating and analyzing of benzodiazepines present in real samples at nanogram-per-milliliter concentrations.     

Measurement of activity coefficients at infinite dilution in N-methyl-2-pyrrolidone and N-formylmorpholine and their mixtures with water using the dilutor technique

With the help of the dilutor technique activity coefficients at infinite dilution have been measured for 24 solutes (alkanes, alkenes, cyclic hydrocarbons, aromatic hydrocarbons, ketones, ethers and alcohols) in N-methyl-2-pyrrolidone (NMP), N-formylmorpholine (NFM) and their mixtures with water in the temperature range between 303.15 and 333.15 K (in a few cases for temperatures up to 423.15 K). The influence of water on the activity coefficient at infinite dilution is presented for different solutes. Furthermore, the selectivities at infinite dilution (S_ij^∞=γ_i^∞/γ_j^∞) for the separation of aliphatics from aromatics are discussed.     

Separation of aromatic hydrocarbons from alkanes using ammonium ionic liquid C2NTf2 at T = 298.15 K

(Liquid + liquid) equilibrium data are presented for four ternary systems of an alkane, or aromatic compound and ethyl(2-hydroxyethyl)dimethylammonium bis{(trifluomethyl)sulfonyl}imide (C₂NTf₂) at 298.15 K: [hexane + benzene + C₂NTf₂], [hexane + p-xylene + C₂NTf₂], and [hexane, or octane + m-xylene + C₂NTf₂]. The separation of aromatic hydrocarbons (benzene, or p-xylene, or m-xylene) from aliphatic hydrocarbons (hexane, or octane) is investigated by extraction with the ammonium ionic liquid. Selectivities and distribution ratios are discussed for these mixtures at constant temperature. The data were analysed and compared to those previously reported for other ionic liquids and especially for the system {hexane + benzene + [EMIM][NTf₂]}. The nonrandom two liquid NRTL model was successfully used to correlate the experimental tie-lines and to calculate the phase compositions of the ternary systems.     

Catalytic efficiency of some novel nanostructured heterogeneous solid catalysts in pyrolysis of HDPE

In the present study, the catalytic conversion of high density polyethylene (HDPE) to useful products has been investigated in the presence of BaTiO₃ based catalysts in a micro steel reactor at 350 °C and 30 min reaction time. The catalysts, including BaTiO₃, Pb/BaTiO₃, Co/BaTiO₃ and Pb–Co/BaTiO₃ were prepared in the laboratory by reactive calcination method and characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-rays (EDX), Surface Area Analyzer (SAA) and X-rays Diffractometry (XRD). The product yields (over all yields and yields of liquid, gas and coke/residue) as a function of individual catalyst concentration was studied. The result indicated a promising effect of the catalysts used on conversion to liquid products and their composition in term of carbon range (C₆ – >C₃₀) & hydrocarbon group types (paraffin's, olefins, naphthenics, and aromatics). Among the catalysts used, Pb–Co/BaTiO₃ gave the maximum yield of liquid products (86%) when used in 1 wt % loading. The same catalyst gave the average yield (20–25%) of different range hydrocarbons i.e. C₆–C₁₂, C₁₃–C₁₆, C₁₇–C₂₀ and C₂₀–C₃₀. Inversely, the un-doped BaTiO_3, favored the formation of C₆–C₁₂ and C₁₃–C₁₆ range hydrocarbons, whereas Pb doped BaTiO₃ and Co doped BaTiO₃ enhanced the yield of C₁₃–C₁₆, and C₂₀–C₃₀ range hydrocarbons. Regarding the hydrocarbon group types, all catalysts significantly increased the formation of paraffins and reduced olefins and naphthenes.     

“Sock-Ball” Chromatographic Separation of High Molecular Weight Fullerenes with Sock-Shaped Stationary Phase

A method for “Sock-Bail” chromatographic separation of high molecular weight fullerenes is described. A prepared sock-shaped stationary phase (Sock-SAF-phase) was used for HPLC separation of several polycyclic aromatic hydrocarbons (PAHs) and fullerenes. Fullerenes, as ball-shaped molecules, are much more strongly retained than PAHs on this stationary phase and have the eluted order C₅₀ < C₇₀ < C₇₆ < C₇₈ < C₈₄ in the mobile phase of n-hexane/dichloromethane (100/0 ～ 80/20). In contrast, chromatography on the corresponding unmodified silica phase or SC-3OH-phase (an intermediate phase of Sock-SAF-phase) gave no separation of fullerenes. This fact indicated that the separation of fullerenes on Sock-SAF-phase was related to the selective interaction with the sock moiety.     

Activated aluminum oxide selectively retaining long chain n-alkanes: Part II. Integration into an on-line high performance liquid chromatography-liquid chromatography-gas chromatography-flame ionization detection method to remove plant paraffins for the determination of mineral paraffins in foods and environmental samples

Aluminum oxide activated by heating to 300-400 °C retains n-alkanes with more than about 20 carbon atoms, whereas iso-alkanes largely pass non-retained (with characteristics described in more detail in Part I). This property is useful for the analysis of mineral oil contamination of foods and other matrices: it enables the removal of plant n-alkanes, typically ranging from C₂₃ to C₃₃, when they disturb the analysis of mineral paraffins (usually almost exclusively consisting of iso-alkanes). An on-line HPLC-LC-GC-FID method is proposed in which a first silica gel HPLC column isolates the paraffins from the bulk of edible oils or extracts and is backflushed with dichloromethane. In a second separation step, a 10 cm × 2 mm i.d. column packed with activated aluminum oxide separates the long chain n-alkanes from the fraction of the iso-alkanes which is transferred to GC-FID by the on-column interface and the retention gap technique. The retained n-alkanes are removed by flushing with iso-octane.     

Forensic differentiation of biogenic organic compounds from petroleum hydrocarbons in biogenic and petrogenic compounds cross-contaminated soils and sediments

“Total petroleum hydrocarbons” (TPHs) or “petroleum hydrocarbons” (PHCs) are one of the most widespread soil pollutants in Canada, North America, and worldwide. Clean-up of PHC-contaminated soils and sediments costs the Canadian economy hundreds of million of dollars annually. Much of this activity is driven by the need to meet regulated levels of PHC in soil. These PHC values are legally required to be assessed using standard methods. The method most commonly used in Canada, specified by the Canadian Council of Ministers of the Environment (CCME), measures the total hydrocarbon concentrations in a soil by carbon range (Fraction 1: C₆–C₁₀; Fraction 2: C₁₀–C₁₆, Fraction 3: C₁₆–C₃₄: and Fraction 4: C₃₄+). Using the CCME method, all of the materials extractible by a mixture of 1:1 hexane:acetone are considered to be petroleum hydrocarbon contaminants. Many hydrocarbon compounds and other extractible materials in soil, however, may originate from non-petroleum sources. Biogenic organic compounds (BOCs) is a general term used to describe a mixture of organic compounds, including alkanes, sterols and sterones, fatty acids and fatty alcohols, and waxes and wax esters, biosynthesized by living organisms. BOCs are also produced during the early stages of diagenesis in recent aquatic sediments. BOC sources could include vascular plants, algae, bacteria and animals. Plants and algae produce BOCs as protective wax coating that are released back into the sediment at the end of their life cycle. BOCs are natural components of thriving plant communities. Many solvent-extraction methods for assessing soil hydrocarbons, however, such as the CCME method, do not differentiate PHCs from BOCs. The naturally occurring organics present in soils and wet sediments can be easily misidentified and quantified as regulated PHCs during analysis using such methods. In some cases, biogenic interferences can exceed regulatory levels, resulting in remediation of petroleum impacts that are not actually present. Consequently, reliance on these methods can trigger unnecessary and costly remediation, while also wasting valuable landfill space. Therefore, it is critically important to develop new protocols to characterize and differentiate PHCs and BOCs in contaminated sediments. In this study, a new reliable gas chromatography–mass spectrometry (GC–MS) method, in combination with a derivatization technique, for characterization of various biogenic compounds (including biogenic alkanes, sterols, fatty acids and fatty alcohols) and PHCs in the same sample has been developed. A multi-criteria approach has been developed to positively identify the presence of biogenic compounds in soil and sediment samples. More than thirty sediment samples were collected from city stormwater management (SWM) ponds and wetlands across Canada. In these wet sediment samples, abundant biogenic n-alkanes, thirteen biogenic sterols, nineteen fatty carboxylic acids, and fourteen fatty alcohols in a wide carbon range have been positively identified. Both PHCs and BOCs in these samples were quantitatively determined. The quantitation data will be used for assessment of the contamination sites and toxicity risks associated with the CCME Fraction 3 hydrocarbons.     

Activated aluminum oxide selectively retaining long chain n-alkanes. Part I, description of the retention properties

Aluminum oxide activated by heating to 350-400 °C retains n-alkanes with more than about 20 carbon atoms, whereas iso-alkanes largely pass the column non-retained. Retention of n-alkanes is strong with n-pentane or n-hexane as mobile phase, but weak or negligible with cyclohexane or iso-octane. It is strongly reduced with increasing column temperature. Even small amounts of polar components, such as modifiers or impurities in the mobile phase, cause the retention of n-alkanes to irreversibly collapse. Since n-alkanes are not more polar than iso-alkanes and long chain n-alkanes not more polar than those of shorter chains, retention by a mechanism based on steric properties is assumed. The sensitivity to deactivation by polar components indicates that polar components and n-alkanes are retained by the same sites. The capacity for retaining n-alkanes is low, with the effect that the retention of n-alkanes depends on the load with retained paraffins. These retention properties are useful for the pre-separation of hydrocarbons in the context of the analysis of mineral oil paraffins in foodstuffs and tissue, where plant n-alkanes, typically ranging from C₂₃ to C₃₃, may severely disturb the analysis (subject of Part II).