A facile preparation of nanocrystalline Mo2C from graphite or carbon nanotubes

Nanocrystalline Mo₂C powders were successfully synthesized at 500 °C by reacting molybdenum chloride (MoCl₅) with C (graphite or carbon nanotube) in metallic sodium medium. X-ray powder diffractometer (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS) and surface area analyzer (BET method) were used to characterize the samples. Experiments reveal that the carbon source used for the carbide synthesis has a great effect on the particle size and the surface area of the samples. When micro-sized graphite was used as C source the obtained nanocrystalline Mo₂C powder consists of particles of 30∼100 nm, with a surface area of 2.311 m²/g. When carbon nanotubes were used as C source, the as-synthesized Mo₂C sample is composed of particles of 20∼50 nm, with a surface area of 23.458 m²/g, which is an order of magnitude larger than that of the carbide prepared from the graphite.     

Surface structure and electrochemical characteristics of natural graphite fluorinated by ClF3

Natural graphite samples with average particle sizes of 5, 10 and 15 μm (NG5 μm, NG10 μm and NG15 μm, respectively) were fluorinated by ClF₃ (3 × 10⁴ Pa) at 200 and 300 °C for 2 min. X-ray photoelectron spectra of surface-fluorinated samples showed that surface fluorine concentration increased with increase in the particle size of graphite and reaction temperature. Small amounts of chlorine were also detected in all the fluorinated samples. Raman spectra of original and surface-fluorinated samples indicated that the surface disordering was increased for NG10 μm and NG15 μm. Surface areas were decreased by the fluorination for NG5 μm and NG10 μm but unchanged for NG15 μm. The mesopores with diameter of 1.5-2 nm increased while those of 2-3 nm decreased for all the samples. First coulombic efficiencies for NG10 μm and NG15 μm were highly increased by surface fluorination in 1 mol/dm³ LiClO₄-EC/DEC/PC (EC: ethylene carbonate, DEC: diethyl carbonate, PC: propylene carbonate) solution.     

Novel molecularly imprinted polymer prepared by nanoattapulgite as matrix for selective solid-phase extraction of diethylstilbestrol

Using nanoattapulgite as matrix, both diethylstilbestrol surface molecularly imprinted polymer and non-imprinted polymer were synthesized in this work. Compared with each other, the diethylstilbestrol surface molecularly imprinted polymer is superior to non-imprinted polymer in adsorption capacity, selectivity and mass transfer property. The maximum static adsorption capacities of diethylstilbestrol surface molecularly imprinted polymer, non-imprinted polymer and nanoattapulgite for diethylstilbestrol was 105.14, 78.54 and 28.50 mg g⁻¹, respectively. As the packing material of solid-phase extraction, the diethylstilbestrol surface molecularly imprinted polymer has been applied to concentrating diethylstilbestrol in pond water and fish samples. A corresponding analytical method to determine diethylstilbestrol has been developed. The limit of detection for diethylstilbestrol in pond water sample and fish samples were 3 μg L⁻¹ and 15 μg kg⁻¹.     

Determination of mercury in gasoline diluted in ethanol by GF AAS after cold vapor generation, pre-concentration in gold column and trapping on graphite tube

A simple method for the determination of mercury in gasoline samples diluted with ethanol by graphite furnace atomic absorption spectrometry (GF AAS) after cold vapor (CV) generation, pre-concentration in a gold column and trapping on a graphite tube is proposed. The methodology is based upon conventional analytical processes that can be performed by any laboratory with a chemical generation and gold amalgamation systems coupled to the atomic absorption spectrometer. The GF AAS temperature was optimized, being the retention, pyrolysis and atomization temperatures, respectively, 100 °C, 150 °C and 800 °C. Gasoline samples were prepared simply by forming a 2-fold diluted solution in ethanol. The mercury formed vapors by reacting the sample with the reducing agent were pre-concentrated in a gold column and further retained on a graphite tube, coated with gold as permanent modifier. Five samples from different gas stations around the UFSC Campus (Florianópolis, Brazil) were analyzed and the Hg concentrations were found to be in the range from 0.40 µg L^− 1 to 0.90 µg L^− 1. Calibration against aqueous standard solutions in acidic medium was carried out. The standard solutions had about the same viscosity as the gasoline diluted in ethanol. The relative standard deviations were lower than 2.4% for the samples. The limits of detection in the samples were 0.08 and 0.14 µg L^− 1, with and without pre-concentration in the gold column, respectively. The accuracy of the method was estimated by applying the recovery test and recovery values between 92 and 100% were obtained. A sample throughput of 4 h^− 1 was achieved. Simplicity and high detection capability are some of the qualities of the method.     

Feasibility of high-resolution continuum source molecular absorption spectrometry in flame and furnace for sulphur determination in petroleum products

For the first time, high-resolution molecular absorption spectrometry with a high-intensity xenon lamp as radiation source has been applied for the determination of sulphur in crude oil and petroleum products. The samples were analysed as xylene solutions using vaporisation in acetylene-air flame or in an electrothermally heated graphite furnace. The sensitive rotational lines of the CS molecule, belonging to the ?ν = 0 vibrational sequence within the electronic transition X¹∑⁺ → A¹П, were applied. For graphite furnace molecular absorption spectrometry, the Pd + Mg organic modifier was selected. Strong interactions with Pd atoms enable easier decomposition of sulphur-containing compounds, likely through the temporal formation of Pd_xS_y molecules. At the 258.056 nm line, with the wavelength range covering central pixel ± 5 pixels and with application of interactive background correction, the detection limit was 14 ng in graphite furnace molecular absorption spectrometry and 18 mg kg⁻¹ in flame molecular absorption spectrometry. Meanwhile, application of 2-points background correction found a characteristic mass of 12 ng in graphite furnace molecular absorption spectrometry and a characteristic concentration of 104 mg kg⁻¹ in flame molecular absorption spectrometry.The range of application of the proposed methods turned out to be significantly limited by the properties of the sulphur compounds of interest. In the case of volatile sulphur compounds, which can be present in light petroleum products, severe difficulties were encountered. On the contrary, heavy oils and residues from distillation as well as crude oil could be analysed using both flame and graphite furnace vaporisation. The good accuracy of the proposed methods for these samples was confirmed by their mutual consistency and the results from analysis of reference samples (certified reference materials and home reference materials with sulphur content determined by X-ray fluorescence spectrometry).     

Effect of surface fluorination on the electrochemical behavior of petroleum cokes for lithium ion battery

Surface structure change and electrochemical behavior of fluorinated petroleum coke samples (petroleum cokes: petroleum coke and those heat-treated at 1860 °C, 2300 °C and 2800 °C, abbreviated to PC, PC1860, PC2300 and PC2800, respectively) have been investigated. Surface oxygen of petroleum coke was decreased by the fluorination using elemental fluorine. Raman and EPR spectroscopies revealed that surface fluorination increased surface disorder and lattice defects. ¹⁹F NMR spectrum suggests that distribution of fluorine atoms in PC fluorinated 300 °C was similar to that in graphite fluoride with covalent CF bonds. Surface areas of fluorinated petroleum cokes were nearly the same as those of non-fluorinated ones or only slightly increased by fluorination, except PC fluorinated at 300 °C. It is noted that first coulombic efficiencies of PC2300 and PC2800 were highly increased to 80-84% by the fluorination at 300 °C. These values of 80-84% were 12-18% higher than those of non-fluorinated PC2300 and PC2800.     

Molar heat capacities for (1-butanol + 1,4-butanediol, 2,3-butanediol, 1,2-butanediol, and 2-methyl-2,4-pentanediol) as function of temperature

The isobaric molar heat capacities for the binary mixtures (1-butanol + 1,4-butanediol) were determined in the temperature range from (293 to 353) K from measurements of isobaric specific heat capacity in a differential scanning calorimeter. The composition dependencies of the excess molar isobaric heat capacities obtained from the experimental results were fitted by the Redlich-Kister polynomials. Above T = 303.15 K, the excess isobaric molar heat capacities are negative over the whole composition range and absolute values increase with temperature. For temperatures (293.15 and 298.15) K, the excess values show S-shaped character. These excesses are however in general very small; at the temperature 298.15 K smaller than 0.1 J · K⁻¹ · mol⁻¹.Additionally, the isobaric molar heat capacities of 2,3-butanediol, 1,2-butanediol, and 2-methyl-2,4-pentanediol were determined over a similar temperature range. The experimental data for all diols are compared with available literature data and values estimated from group additivity.     

Comparison of direct sampling and emulsion analysis using a filter furnace for the determination of lead in crude oil by graphite furnace atomic absorption spectrometry

The determination of trace elements in crude oil is difficult due to the complex nature of the sample and the various different chemical forms in which the metals can occur. The advantage of graphite furnace atomic absorption spectrometry is that only a minimum of sample pretreatment is required. In this work two techniques have been compared to establish a fast and reliable method for lead determination in crude oil. In the first one the crude oil samples were weighed directly onto solid sampling (SS) platforms and introduced into the graphite tube for analysis. In the second one the samples were prepared as oil-in-water emulsions and analyzed in a filter furnace (FF). Twenty μL of a mixture of 0.5 mg L^− 1 Pd + 0.3 mg L^− 1 Mg + Triton X-100 has been used as the modifier, and calibration against aqueous solutions has been used for both methods. The sensitivity obtained with the FF was more than a factor of two better than that with SS; however, as a larger sample mass could be introduced in the latter case, so that the limits of detection for both techniques were 0.004 mg kg^− 1. Seven crude oil samples were analyzed using the two procedures, and all results were in agreement at a 95% confidence level using a paired Student's t-test. For validation purposes, three crude oil samples have been mineralized using an open-vessel acid digestion, and the results were in agreement with those found with direct sampling and with emulsion sampling using FF according to ANOVA test. Both methods are simple, fast and reliable, being appropriated for routine analysis; however, the direct method using SS technology should be preferred because of its simplicity, speed and commercial availability.     

Electrothermal atomic absorption spectrometric determination of arsenic in essential lavender and rose oils

Analytical procedures for electrothermal atomic absorption spectrometric (ETAAS) determination of arsenic in essential oils from lavender (Lavendula angustifolia) and rose (Rosa damascena) are described. For direct ETAAS analysis, oil samples are diluted with ethanol or i-propanol for lavender and rose oil, respectively. Leveling off responses of four different arsenic species (arsenite, arsenate, monomethylarsonate and dimethylarsinate) is achieved by using a composite chemical modifier: l-cysteine (0.05 g l⁻¹) in combination with palladium (2.5 μg) and citric acid (100 μg). Transverse-heated graphite atomizer (THGA) with longitudinal Zeeman-effect background correction and ‘end-capped’ graphite tubes with integrated pyrolytic graphite platforms, pre-treated with Zr-Ir for permanent modification are employed as most appropriate atomizer. Calibration with solvent-matched standard solutions of As(III) is used for four- and five-fold diluted samples of lavender and rose oil, respectively. Lower dilution factors required standard addition calibration by using aqueous (for lavender oil) or i-propanol (for rose oil) solutions of As(III). The limits of detection (LOD) for the whole analytical procedure are 4.4 and 4.7 ng g⁻¹ As in levender and rose oil, respectively. The relative standard deviation (R.S.D.) for As at 6-30 ng g⁻¹ levels is between 8 and 17% for both oils. As an alternative, procedure based on low temperature plasma ashing in oxygen with ETAAS, providing LODs of 2.5 and 2.7 ng g⁻¹ As in levender and rose oil, respectively, and R.S.D. within 8-12% for both oils has been elaborated. Results obtained by both procedures are in good agreement.     

Determination of major antimony species in seawater by continuous flow injection hydride generation atomic absorption spectrometry

The capabilities and limitations of the continuous flow injection hydride generation technique, coupled to atomic absorption spectrometry, for the speciation of major antimony species in seawater, were investigated. Two pre-concentration techniques were examined. After continuous flow injection hydride generation and collection onto a graphite tube coated with iridium, antimony was determined by graphite furnace atomic absorption spectrometry. The low detection limits obtained (∼5 ng l⁻¹ for Sb(III) and ∼10 ng l⁻¹ for Sb(V) for 2.5 ml seawater samples) permitted the determination of Sb(III) and total antimony in seawater with the use of selective hydride generation and on-line UV photooxidation. The number of samples that can be analyzed is about 15 per hour for Sb(III) determinations and 10 per hour for total antimony determinations. The analysis of seawater samples showed that Sb(V) was the predominant species, even in the presence of important biological activity.     

Determination of Cd by electrothermal atomic absorption spectrometry after electrodeposition on a graphite probe modified with palladium

Traces of Cd were determined by electrothermal atomic absorption spectrometry after electrochemical preconcentration on a commercial graphite ridge probe modified with Pd. The Pd electrochemically deposited on the probe surface served not only as the modifier but it also protected the graphite surface. Cd was electrodeposited at a controlled potential − 1.2 V (vs. saturated calomel electrode) using the Pd-modified graphite probe as a working electrode. The sensitivity of Cd determination remained unchanged for 300 electrodeposition and atomization cycles. The detection limit (3σ_blank) was improved with increasing time of electrolysis and was 1.2 ng l^− 1 for a 10 min electrolysis time in the presence of 0.1 mol l^− 1 NaNO₃. The procedure was applied for the determination of Cd in (1 + 1) diluted seawater and in (1 + 1) diluted urine samples using the standard addition method.     

Preconcentration of palladium in a flow-through electrochemical cell for determination by graphite furnace atomic absorption spectrometry

An electrochemical preconcentration at a controlled potential on the electrode in a flow-through mode followed by graphite furnace atomic absorption spectrometric (GFAAS) detection is proposed for determination of trace amounts of palladium. After electrolysis the polarization of the electrodes was changed and deposited metal was dissolved electrochemically in the presence of an appropriate stripping reagent. Conditions for the electrodeposition, such as pH of the solutions, a deposition potential, dissolution potential and a composition of stripping solution were optimised. The graphite electrode (GE) and glassy carbon electrode (GCE) were tested for the palladium reduction process. The detection limit of 0.05 ng ml⁻¹ Pd (1 pg) was obtained after palladium preconcentration on the GCE and dissolution with 0.2 mol l⁻¹ thiourea in 0.1 mol l⁻¹ HCl followed by GFAAS detection. The method was applied for the determination of palladium in spiked tap water and road dust samples.     

Evaluation of a high sensitivity PbO(2) pH-sensor

In this paper, a new high sensitivity potentiometric pH-electrode with a response in the acid region is proposed. It consists of a PbO₂-paraffin matrix deposited on graphite. Its simple construction and studies of some variables at 0.5 mol L⁻¹ ionic strength are reported. A direct relationship between the electric potential difference and the solution pH was observed for pH values ranging from 1.2 to 7.5. A slope greater than 100 mV/decade and a conditional electric potential of about 1250 mV were obtained. The results presented a high correlation with those from a conventional glass pH-electrode in complex matrix samples.     

Surface-fluorinated graphite anode materials for Li-ion batteries

Surface modification of graphite powder has been performed by chemical fluorination using elemental fluorine at 200 °C and 300 °C. This process leads to an increase of the BET surface area due to partial CC bond breaking. Surface analyses performed by secondary ions mass spectrometry have shown that the H + O content at the surface of graphite is significantly decreased by this fluorination treatment. Fluorinated graphite powders have been tested as negative electrodes in Li-ion battery, chronopotentiometry measurements have shown that the fluorinated graphite exhibits better electrochemical performances than raw graphite powder notably due to an increase of the surface area which allows the storage of a higher amount of lithium into the host lattice. In addition, impedance measurements performed in a delithiated state have shown a significant decrease of the total cell resistance, i.e. a decrease of both the charge transfer resistance and the resistance related to the solid electrolyte interface (SEI) layer.     

Determination of chromium by GFAAS in slurries of fish feces to estimate the apparent digestibility of nutrients in feed used in pisciculture

This paper presents a simple, fast and sensitive method to determine chromic oxide (used as a biological marker of fish feed) in samples of fish feces by GFAAS through the direct introduction of slurries of the samples into the spectrometer's graphite tube. The standard samples of feces and of fish feed containing 0.10-1.00 mg kg⁻¹ of Cr₂O₃ were pre-frozen for 1 min in liquid nitrogen and then ground a cryogenic mill for 2 min, which reduced the samples’ grain size to less than 60 μm. The standard slurries were prepared by mixing 20 mg of standard samples of fish feed or feces with 1 mL of a solution containing 0.05% (v/v) of Triton X-100 and 0.50% (v/v) of suprapure HNO₃ directly in the spectrometer's automatic sampling glass. The final concentrations of Cr₂O₃ present in the standard slurries were 2, 4, 8, 16 and 20 μg L⁻¹. After sonicating the mixture for 20 s, 10 μL of standard slurries were injected into the graphite tube, whose internal wall was lined with a metallic palladium film that acted as a permanent chemical modifier. The limits of detection (LOD) and quantification (LOQ) calculated for 20 readings of the blank of the standard slurries (2%, m/v of feces or feed devoid of minerals) were 0.81 and 2.70 μg L⁻¹ of Cr₂O₃ for the standard feces slurries, 0.84 and 2.83 μg L⁻¹ of Cr₂O₃ for the standard feed slurries. The proposed method was applied in studies of nutrient digestibility of different fish feeds and its results proved compatible with the results obtained from samples pre-mineralized by acid digestion.     

Method development and optimization for the determination of selenium in bean and soil samples using hydride generation electrothermal atomic absorption spectrometry

The present investigation is the first part of an initiative to prepare a regional map of the natural abundance of selenium in various areas of Brazil, based on the analysis of bean and soil samples. Continuous-flow hydride generation electrothermal atomic absorption spectrometry (HG-ET AAS) with in situ trapping on an iridium-coated graphite tube has been chosen because of the high sensitivity and relative simplicity. The microwave-assisted acid digestion for bean and soil samples was tested for complete recovery of inorganic and organic selenium compounds (selenomethionine). The reduction of Se(VI) to Se(IV) was optimized in order to guarantee that there is no back-oxidation, which is of importance when digested samples are not analyzed immediately after the reduction step. The limits of detection and quantification of the method were 30 ng L⁻¹ Se and 101 ng L⁻¹ Se, respectively, corresponding to about 3 ng g⁻¹ and 10 ng g⁻¹, respectively, in the solid samples, considering a typical dilution factor of 100 for the digestion process. The results obtained for two certified food reference materials (CRM), soybean and rice, and for a soil and sediment CRM confirmed the validity of the investigated method. The selenium content found in a number of selected bean samples varied between 5.5 ± 0.4 ng g⁻¹ and 1726 ± 55 ng g⁻¹, and that in soil samples varied between 113 ± 6.5 ng g⁻¹ and 1692 ± 21 ng g⁻¹.     

Direct determination of Ge in hot spring waters and coal fly ash samples by hydride generation-ETAAS

A method for Ge determination in hot spring water and acid extracts from coal fly ash samples involving hydride generation, trapping and atomisation of the hydride generated from Ir-treated graphite tubes (GTs) has been developed. Hydride was generated from hydrochloric acid medium using sodium tetrahydroborate. Several factors affecting the hydride generation, transport, trapping and atomisation efficiency were studied by using a Plackett-Burman design. Results obtained from Plackett-Burman designs suggest that trapping and atomisation temperatures are the significant factors involved on the procedure. The accuracy was studied using NIST-1633a (coal fly ash) reference material. The detection limit of the proposed method was 2.4 μg l⁻¹ and the characteristic mass of 233 pg was achieved. The Ge concentrations in fly ash and hot spring samples were between 6.25-132 μg g⁻¹ and 12.84-36.2 μg l⁻¹.     

Fast heating induced impulse halogenation of refractory sample components in electrothermal atomic absorption spectrometry by direct injection of a liquid halogenating agent

A novel electrothermal atomic absorption spectrometry (ETAAS) method was developed for the halogenation of refractory sample components (Er, Nd and Nb) of lithium niobate (LiNbO₃) and bismuth tellurite (Bi₂TeO₅) optical single crystals to overcome memory effects and carry-over. For this purpose, the cleaning step of a regular graphite furnace heating program was replaced with a halogenation cycle. In this cycle, after the graphite tube cooled to room temperature, a 20 μL aliquot of liquid carbon tetrachloride (CCl₄) was dispensed with a conventional autosampler into the graphite tube. The CCl₄ was partially dried at 80 °C under the mini-flow (40 cm³ min⁻¹) condition of the Ar internal furnace gas (IFG), then the residue was decomposed (pyrolyzed) by fast furnace heating at 1900-2100 °C under interrupted flow of the IFG. This step was followed by a clean-out stage at 2100 °C under the maximum flow of the IFG. The advantage of the present method is that it does not require any alteration to the graphite furnace gas supply system in contrast to most of the formerly introduced halogenation techniques.The effectiveness of the halogenation method was verified with the determination of Er and Nd dopants in the optical crystals. In these analyses, a sensitivity decrease was observed, which was likely due to the enhanced deterioration of the graphite tube surface. Therefore, the application of mathematical correction (resloping) of the calibration was also required. The calibration curves were linear up to 1.5 and 10 μmol L⁻¹ for Er and Nd, respectively. Characteristic masses of 18 and 241 pg and the limit of detection (LOD) values of 0.017 and 0.27 μmol L⁻¹ were found for Er and Nd, respectively. These LOD data correspond to 0.68 μmol mol⁻¹ Er and 11 μmol mol⁻¹ Nd in solid bismuth tellurite samples. The analytical results were compared with those obtained by a conventional ETAAS method and validated with X-ray fluorescence spectrometry analysis.     

HPLC-UV and HPLC-MSn multiresidue determination of amidosulfuron, azimsulfuron, nicosulfuron, rimsulfuron, thifensulfuron methyl, tribenuron methyl and azoxystrobin in surface waters

The paper presents a new HPLC method, with UV and MSⁿ detection, for the determination of seven pesticides, including the sulfonylurea herbicides amidosulfuron, azimsulfuron, nicosulfuron, rimsulfuron, thifensulfuron methyl, tribenuron methyl, and the fungicide azoxystrobin characterised by a methoxyacrilate structure. The methodology consists of a preconcentration/SPE (solid phase extraction) step and HPLC-UV (240 nm detection wavelength)-MSⁿ analysis. Under the optimised conditions and after a 1000/1 preconcentration factor, the limits of detection were lower than 14.5 ng L⁻¹ for UV detection and lower than 8.1 ng L⁻¹ for MS detection. The limits of quantification were lower than 48.3 ng L⁻¹ in UV detection and than 26.9 ng L⁻¹ in MSⁿ detection. The analysis of two samples, spiked with a mixture of the pesticides at threshold level concentrations, gave more than 60% recovery.     

Determination of platinum and palladium in environmental samples by graphite furnace atomic absorption spectrometry after separation on dithizone sorbent

A graphite furnace atomic absorption method of platinum and palladium determination after their separation from environmental samples has been presented. The samples were digested by aqua regia and the analyte elements were separated on the dithizone sorbent. The procedure of sorbent preparation was described and their properties were established. Two various procedures of elution by thiourea and concentrated nitric acid were described and discussed. The low limit of detection was established as 1 ng g⁻¹ for platinum and 0.2 ng g⁻¹ for palladium.There was also investigated the behaviour of platinum and palladium introduced into the soil in various chemical forms.