Screening Brazilian commercial gasoline quality by hydrogen nuclear magnetic resonance spectroscopic fingerprintings and pattern-recognition multivariate chemometric analysis

The identification of gasoline adulteration by organic solvents is not an easy task, because compounds that constitute the solvents are already in gasoline composition. In this work, the combination of Hydrogen Nuclear Magnetic Resonance (¹H NMR) spectroscopic fingerprintings with pattern-recognition multivariate Soft Independent Modeling of Class Analogy (SIMCA) chemometric analysis provides an original and alternative approach to screening Brazilian commercial gasoline quality in a Monitoring Program for Quality Control of Automotive Fuels. SIMCA was performed on spectroscopic fingerprints to classify the quality of representative commercial gasoline samples selected by Hierarchical Cluster Analysis (HCA) and collected over a 6-month period from different gas stations in the São Paulo state, Brazil. Following optimized the ¹H NMR-SIMCA algorithm, it was possible to correctly classify 92.0% of commercial gasoline samples, which is considered acceptable. The chemometric method is recommended for routine applications in Quality-Control Monitoring Programs, since its measurements are fast and can be easily automated. Also, police laboratories could employ this method for rapid screening analysis to discourage adulteration practices.     

Screening Brazilian C gasoline quality: application of the SIMCA chemometric method to gas chromatographic data

A total of 2400 samples of commercial Brazilian C gasoline were collected over a 6-month period from different gas stations in the S?o Paulo state, Brazil, and analysed with respect to 12 physicochemical parameters according to regulation 309 of the Brazilian Government Petroleum, Natural Gas and Biofuels Agency (ANP). The percentages (v/v) of hydrocarbons (olefins, aromatics and saturated) were also determined. Hierarchical cluster analysis (HCA) was employed to select 150 representative samples that exhibited least similarity on the basis of their physicochemical parameters and hydrocarbon compositions. The chromatographic profiles of the selected samples were measured by gas chromatography with flame ionisation detection and analysed using soft independent modelling of class analogy (SIMCA) method in order to create a classification scheme to identify conform gasolines according to ANP 309 regulation. Following the optimisation of the SIMCA algorithm, it was possible to classify correctly 96% of the commercial gasoline samples present in the training set of 100. In order to check the quality of the model, an external group of 50 gasoline samples (the prediction set) were analysed and the developed SIMCA model classified 94% of these correctly. The developed chemometric method is recommended for screening commercial gasoline quality and detection of potential adulteration.     

Characterization of Gasoline by 1H Nuclear Magnetic Resonance and Chemometrics

Automotive fuel adulteration is an old and significant problem. One common type of fuel adulteration is the addition of diesel to gasoline. Unsupervised models were developed through hierarchical cluster and principal component analysis models. Supervised models through partial least square discriminant analysis using ¹H nuclear magnetic resonance spectra as the input were used to classify samples as adulterated or unadulterated. Quantitative models were developed using partial least squares to determine the gasoline and diesel concentrations in the samples. This set contained samples composed of pure gasoline and anhydrous ethanol reproducing commercial gasoline and other samples treated with diesel. Hierarchical cluster and principal component analysis did not distinguish between adulterated and unadulterated samples except for the most adulterated materials. However, partial least square discriminant analysis classified 100% of the samples correctly. The partial least square algorithm provided excellent regression models for the gasoline and diesel content. The determination coefficient was 0.9920 for both models, whereas the root mean square error of cross-validation and root mean square error of prediction for the diesel model were 2.32 and 1.42%, respectively, and 2.40 and 1.38% for the gasoline model.     

Use of cetyltrimethylammonium bromide as surfactant for the determination of copper and chromium in gasoline emulsions by electrothermal atomic absorption spectrometry

In this work, the use of cetyltrimethylammonium bromide as surfactant for the preparation of oil-in-water emulsions for the determination of Cu and Cr in gasoline by electrothermal atomic absorption spectrometry (ET AAS) was evaluated. The surfactant amount was tested in the range of 25 to 300 mg, added to 2 ml of gasoline, and completed to 10 mL with 0.1% (v/v) nitric acid solution. 150 mg of surfactant was found optimum, and a sonication time of 10 min sufficient to form an oil-in-water emulsion that was stable for several hours. The ET AAS temperature program was established based on pyrolysis and atomization curves. The pyrolysis temperatures were set at 700 and 1300 °C for Cu and Cr, respectively and the selected atomization temperatures were 2400 and 2500 °C. The time and temperature of the drying stage and the atomization time were experimentally tested to provide optimum conditions. The limits of detection were found to be 5 μg L^− 1 and 1.5 μg L^− 1 for Cu and Cr, respectively in the original gasoline samples. The relative standard deviation (RSD) ranged from 4 to 9% in oil-in-water emulsions spiked with 5 μg L^− 1 and 15 μg L^− 1 of each metal, respectively. Recoveries varied from 90 to 98%. The accuracy of the proposed method was tested by an alternate procedure using complete evaporation of the gasoline sample. The method was adequate for the determination of Cu and Cr in gasoline samples collected from different gas stations in Salvador, BA, Brazil.     

Determination of arsenic in diesel, gasoline and naphtha by graphite furnace atomic absorption spectrometry using microemulsion medium for sample stabilization

A procedure for the determination of As in diesel, gasoline and naphtha at μg L⁻¹ levels by GFAAS is proposed. Sample stabilization was achieved by the formation of three component solutions prepared by mixing appropriate volumes of the samples propan-1-ol and nitric acid aqueous solution. This mixture resulted in a one-phase medium, which was indefinitely stable. No changes in the analyte signals were observed over several days in spiked samples, proving long-term stabilization ability. The use of conventional (Pd) and permanent (Ir) modification was investigated and the former was preferred. Central composite design multivariate optimization defined the optimum microemulsion composition as well as the temperature program. In this way, calibration using aqueous analytical solutions was possible, since the same sensitivity was observed in the investigated microemulsion media and in 0.2% v/v HNO₃. Coefficients of correlation larger than 0.999 and an As characteristic mass of 22 pg were observed. Recoveries (n=4) obtained from spiked samples were 98±4, 99±3 and 103±5%, and the limits of detection in the original samples were 1.8, 1.2 and 1.5 μg L⁻¹ for diesel, gasoline and naphtha, respectively. Validation was performed by the analysis of a set of commercial samples by independent comparative procedures. No significant difference (Student’s t-test, p<0.05) was observed between comparative and proposed procedure results. The total determination cycle lasted 4 min for diesel and 3 min for gasoline and naphtha, equivalent to a sample throughput of 7 h⁻¹ for diesel and 10 h⁻¹ for gasoline and naphtha.     

13C-NMR spectroscopic analysis of vinyl groups in styrene–divinylbenzene networks

p-Divinylbenzene (DVB) ¹³C-labeled at the methine carbon of the vinyl group was copolymerized in suspension with styrene at 70, 70–95, and 135–155°C using 2,2′-azobisisobutyronitrile (AIBN) as the initiator. The number of unreacted vinyl groups in each copolymer was determined by ¹³C CP–MAS NMR analysis of solid samples, direct polarization ¹³C-NMR analysis of CDCl₃-swollen gels, and bromination. Results from the three methods agree methods agree qualitatively. Even the 1% DVB-crosslinked networks contained 40% unreacted DVB-vinyl groups when prepared by high conversion polymerization at 70°C and 16% unreacted DVB-vinyl groups when polymerization was finished at 95°C. The analyses were also applied to some commercial crosslinked polystyrenes. Every sample examined contained pendent vinyl groups     

Calculated and experimental 1H and 13C NMR assignments for cannabicitran

Cannabicitran is an important cannabinoid natural product produced by Cannabis sativa and is often found at surprisingly high levels (up to ~10%) in “purified” commercial cannabidiol (CBD) extract preparations. Despite the prevalence of this molecule in CBD oil and other cannabinoid-related products, and the rapidly expanding interest in cannabinoids for treatment of a wide range of physiological conditions, only unassigned ¹H NMR data and partial unambiguous ¹³C assignments have been published. Herein, we report the complete ¹H and ¹³C NMR assignments of cannabicitran and comparatively evaluate the performance of several density functional theory (DFT) methods with varying levels of theory for the calculation of NMR chemical shifts.     

Determination of palladium in gasoline by neutron activation analysis and automated column extraction

^? Palladium in gasoline was determined by means of neutron activation analysis (NAA) and selective sorbent extraction. Unleaded gasoline consistent with DIN EN 228, RON 95 was irradiated at a thermal neutron flux of Φ_th = 1.68 · 10¹³s^–1cm^–2 and an epithermal neutron flux of 3.32 · 10¹¹s^–1cm^–2 for t_irr = 12 h. The irradiated gasoline was digested with nitric acid and palladium was then separated as N,N-diethyl-N’-benzoylthiourea complex by an automated column pre-concentration procedure. The eluate of 50 μL was dried on a filter paper and the 88.03 keV photons resulting from the decay of ¹⁰⁹Pd were detected in a low level HPGe spectrometer with an efficiency of 35.5%. Severe interferences with other matrix constituents, especially ⁸²Br could be overcome and the detection limit for palladium was improved to 3.4 ng/L at a confidence level of 90%. Although the analytical procedure applied yielded the lowest detection limit for palladium obtained in gasoline up to now, no indications for the presence of palladium were found.     

Proton and carbon-13 chemical shifts: Comparison between theory and experiment

A series of ab initio ¹H and ¹³C NMR chemical shifts are presented for all molecules for which gas-phase experimental measurements exist. Quantitative agreement with this large set of data is achieved by the use of gauge-invariant atomic orbitals in an SCF perturbation theory approach. The effect of basis set completeness on these ¹H and ¹³C chemical shifts is also examined. The 4-31G basis set is found to provide internally consistent results and give satisfactory agreement with gas-phase experimental data. Errors within 6% for ¹H shifts and 3% for ¹³C shifts result. Increasing the basis set to the 6-31G^* level does not significantly improve the agreement. For ¹H shifts only, the 3-21G basis set is adequate. The validity of the particular computational approach employed here is further substantiated by comparison to another ab initio magnetic shielding method.     

The first catalytic asymmetric addition of diethylzinc to aldehyde promoted by chiral thiourea

A series of C₂-symmetric and asymmetric chiral thiourea derivatives were synthesized from commercial L-phenylalanine.All of the new compounds have been fully characterized by IR,¹H NMR,¹³C NMR,MS spectra and elemental analyses.The chiral thioureas were used as chiral ligands in the catalytic enantioselective ethylation of aldehydes with diethylzinc,the corresponding sec-alcohols were gained with excellent enantioselectivities（up to 87.1%ee） and high yields（up to 76.7%） after the conditions were optimized.     

Liquid chromatography/mass spectrometry stable isotope analysis of dissolved organic carbon in stream and soil waters

A commercial interface coupling liquid chromatography (LC) to a continuous‐flow isotope ratio mass spectrometry (CF‐IRMS) instrument was used to determine the δ¹³C of dissolved organic carbon (DOC) in natural waters. Stream and soil waters from a farmland plot in a hedgerow landscape were studied. Based on wet chemical oxidation of dissolved organics the LC/IRMS interface allows the on‐line injection of small volumes of water samples, an oxidation reaction to produce CO₂ and gas transfer to the isotope ratio mass spectrometer. In flow injection analysis (FIA) mode, bulk DOC δ¹³C analysis was performed on aqueous samples of up to 100 μL in volume in the range of DOC concentration in fresh waters (1–10 mg C.L^–1). Mapping the DOC δ¹³C spatial distribution at the plot scale was made possible by this fairly quick method (10 min for triplicate analyses) with little sample manipulation. The relative contributions of different plot sectors to the DOC pool in the stream draining the plot were tentatively inferred on the basis of δ¹³C differences between the hydrophilic and hydrophobic components. Copyright © 2011 John Wiley & Sons, Ltd.     

NMR Crystallography of an Oxovanadium(V) Complex by an Approach Combining Multinuclear Magic Angle Spinning NMR,DFT, and Spin Dynamics Simulations

Bioinorganic vanadium(V) solids are often challenging for structural analysis. Here, we explore an NMR crystallography approach involving multinuclear ¹³C/⁵¹V solid‐state NMR spectroscopy, density functional theory (DFT), and spin dynamics numerical simulations, for the spectral assignment and the 3D structural analysis of an isotopically unmodified oxovanadium(V) complex, containing 17 crystallographically inequivalent ¹³C sites. In particular, we report the first NMR determination of C–V distances. So far, the NMR observation of ¹³C–⁵¹V proximities has been precluded by the specification of commercial NMR probes, which cannot be tuned simultaneously to the close Larmor frequencies of these isotopes (100.6 and 105.2 MHz for ¹³C and ⁵¹V, respectively, at 9.4 T). By combining DFT calculations and ¹³C–⁵¹V NMR experiments, we propose a complete assignment of the ¹³C spectrum of this oxovanadium(V) complex. Furthermore, we show how ¹³C–⁵¹V distances can be quantitatively estimated.     

Determination of mercury in gasoline by cold vapor atomic absorption spectrometry with direct reduction in microemulsion media

The determination of Hg in gasoline by cold vapor atomic absorption spectrometry, after direct aqueous NaBH₄ reduction in a three-component (microemulsion) medium, was investigated. Microemulsions were prepared by mixing gasoline with propan-1-ol and 50% v / v HNO₃ at a 20 : 15 : 1 volume ratio. A long-term homogeneous system was immediately formed this way. After reduction, the Hg vapor generated in a reaction flask was transported to an intermediate K₂Cr₂O₇/H₂SO₄ trap solution in order to avoid poisoning of the Au–Pt trap by the gasoline vapors. A second reduction step was then conducted and the generated Hg vapor transported to the Au–Pt trap, followed by thermal release of Hg⁰ and atomic absorption measurement. Purified N₂ was used as purge and transport gas. After multivariate optimization by central composite design calibration graphs showed coefficients of correlation of 0.9999 and a characteristic mass of 2 ng was obtained. Typical coefficients of variation of 5% and 6% were found for ten consecutive measurements at concentration levels of 1 and 8 μg L⁻¹ of Hg²⁺, respectively. The limit of detection was 0.10 μg L⁻¹ (0.14 μg kg⁻¹) in the original sample. A total measurement cycle took 11 min, permitting duplicate analysis of 3 samples per hour. The results obtained with the proposed procedure in the analysis of commercial gasoline samples were in agreement with those obtained by a comparative procedure. Gasoline samples of the Rio de Janeiro city have shown Hg concentrations below 0.27 μg L⁻¹.     

Stable isotope ratios of carbon and hydrogen to distinguish olive oil from shark squalene‐squalane

Squalene and its hydrogenated derivate squalane are widely used in the pharmaceutical and cosmetic fields. The two compounds are mainly produced from the liver oil of deep sea sharks and from olive oil distillates. Squalene and squalane from shark cost less than the same compounds derived from olive oil, and the use of these shark‐derived compounds is unethical in cosmetic formulations. In this work we investigate whether ¹³C/¹²C and ²H/¹H ratios can distinguish olive oil from shark squalene/squalane and can detect the presence of shark derivates in olive oil based products. The ¹³C/¹²C ratios (expressed as δ¹³C values) of bulk samples and of pure compounds measured using isotope ratio mass spectrometry (IRMS) were significantly lower in authentic olive oil squalene/squalane (N: 13; ?28.4 ± 0.5‰; ?28.3 ± 0.8‰) than in shark squalene/squalane samples (N: 15; ?20.5 ± 0.7‰; ?20.4 ± 0.6‰). By defining δ¹³C threshold values of ?27.4‰ and ?26.6‰ for olive oil bulk and pure squalene/squalane, respectively, illegal addition of shark products can be identified starting from a minimum of 10%. ²H/¹H analysis is not useful for distinguishing the two different origins. δ¹³C analysis is proposed as a suitable tool for detecting the authenticity of commercial olive oil squalene and squalane samples, using IRMS interfaced to an elemental analyser if the purity is higher than 80% and IRMS interfaced to a gas chromatography/combustion system for samples with lower purity, including solutions of squalane extracted from cosmetic products. Copyright © 2010 John Wiley & Sons, Ltd.     

NMR spectroscopic study of 3-nitrofurazans

3-Nitrofurazans with different substituents at position 4 were studied by ¹H, ¹³C, and ¹⁴N NMR spectroscopy. A correlation between chemical shifts (CS) in the ¹³C NMR spectra of furazans and benzene with similar substituents was revealed. Increments for a series of new substituents were determined. The ¹³C and ¹⁴N CS values presented can be used as a reliable set of reference data.     

Efficient Synthesis of Molecular Precursors for Para‐Hydrogen‐Induced Polarization of Ethyl Acetate‐1‐13C and Beyond

A scalable and versatile methodology for production of vinylated carboxylic compounds with ¹³C isotopic label in C1 position is described. It allowed synthesis of vinyl acetate‐1‐¹³C, which is a precursor for preparation of ¹³C hyperpolarized ethyl acetate‐1‐¹³C, which provides a convenient vehicle for potential in vivo delivery of hyperpolarized acetate to probe metabolism in living organisms. Kinetics of vinyl acetate molecular hydrogenation and polarization transfer from para‐hydrogen to ¹³C via magnetic field cycling were investigated. Nascent proton nuclear spin polarization (%P_H) of ca. 3.3 % and carbon‐13 polarization (%P_13C) of ca. 1.8 % were achieved in ethyl acetate utilizing 50 % para‐hydrogen corresponding to ca. 50 % polarization transfer efficiency. The use of nearly 100% para‐hydrogen and the improvements of %P_H of para‐hydrogen‐nascent protons may enable production of ¹³C hyperpolarized contrast agents with %P_13C of 20–50 % in seconds using this chemistry.     

Determination of lead in gasoline by a flow-injection technique with atomic absorption spectrometric detection

The gasoline sample is treated with iodine and Aliquat-336 and diluted with 4-methylpentan-2-one; 100 μl is injected into a flowing acetone stream for aspiration into an atomic absorption spectrometer. Calibration is linear in the range 0/2-16 mg l^?1 lead. Results for commercial gasoline samples agree well with those obtained by published titrimetric and atomic absorption methods. The precision for samples containing 300/2-400 mg l^?1 lead is ±1%; with increased recorder amplification, the limit of detection is 0.1 mg l^?1 lead. The method is rapid and economic.     

Determination of the alternative butanol/gasoline and butanol/diesel fuel blends heats of combustion by a heat-loss compensated semi-microcalorimeter

The heat of combustion (HOC) of butanol/gasoline and butanol/diesel fuel blends was systematically determined in a Parr 6725/6772 heat-loss compensated semi-microcalorimeter under controlled temperature and pressure conditions. A set of blends containing 15 and 30% of butanol, in mass fraction, was tested, and the results were compared to those obtained for pure ethanol, pure gasoline, pure diesel, and Brazilian commercial gasoline. In view of the high volatility of samples, the use of gelatin capsules was necessary to avoid evaporation losses during the critical step of sampling. Results evidenced that despite a slight energy reduction observed for all blends, HOC values remained quite close to those measured for gasoline and diesel, even when considering blends with 30% of butanol in mass fraction, which reduction does not exceed 8.5%. Compared to ethanol, a HOC up to 14.7% higher was achieved for butanol. The present work confirms that in mass fractions up to 30%, butanol can be satisfactorily blended with gasoline and diesel without causing major impacts on the fuel energy density and, more than that, can offer energy advantage compared to ethanol.

     

Quantum-chemical study of the effects of noncovalent interactions on the nuclear magnetic screening constants of pyrimidine base associates

The effects of weak noncovalent interactions on the nuclear magnetic screening (NMS) constants (σ ¹H), (σ ¹³C) and charge distribution (q _t ) on atoms in van der Waals model associates of unsubstituted and substituted pyrimidines and substituted uracil are considered. The NMS constants were calculated by the UB3LYP/6-31G(d,p) with GIAO functions. The correlation dependences of the ¹H and ¹³C σ constants on the charge q on atoms whre constructed. It were shown that they can be represented as polynomials that include the terms that are linear and quadratic relative to the charge. The relations obtained in this way are similar in form and close in magnitude to the coefficients of the known Buckingham and Augspurger functions that describe the electric field effects on the nuclear magnetic screening constants. It was found that the coefficients in these polynomials have a definite physical sense in that they characterize nuclear magnetic screening and the “screening polarizability” tensor in the unperturbed molecule and associate, respectively. The NMS constants and charge distribution in pyrimidine base associates and accordingly the coefficients that reflect their values in polynomials depend on the form, size, and composition of the associate and can vary significantly depending on the position of the pyrimidine base in the associate.     

Quick analysis of composition of semi-aromatic copolyamide via 13C NMR study

Standard high resolution ¹³C NMR spectra of PA10T, PA6T, PA106, and PA66 were obtained by a nonacidic solvent mixture of HFIP and CDCl₃. Several chemical shifts were found extremely sensitive to the polyamide type. According to the standard spectra, semi-aromatic copolyamides comprising PA10T, PA6T, PA106, and PA66 units could be distinguished. The ratio of each polyamide component in the copolyamide was determined through the integration of the methylene carbon peak associated with the amine group. ¹³C NMR analysis results were consistent with the theoretical values and copolyamide hydrolysis test results, making ¹³C NMR analysis quite reliable on the quick composition analysis of semi-aromatic copolyamides. Based on this technique, several commercial semi-aromatic copolyamides were further examined and their compositions were easily determined.