Molecular and vibrational structure of the extracellular bacterial signal compound N-butyryl-homoserine lactone (C4-HSL)

The molecular and vibrational structure of the title compound (C4-HSL) was studied by experimental and theoretical methods. The infrared (IR) absorption spectrum was measured in the solid state and in CCl₄ suspension. The observed absorption bands were compared with transitions obtained with B3LYP/cc-pVTZ density functional theory (DFT) calculations. Two stable molecular conformations were predicted, corresponding to an endo- and an exo-conformer with similar energies. Intermolecular amide–amide hydrogen bonding in the crystal state was approximated by a simple cluster model, leading to excellent agreement with the observed solid state IR spectrum. Due to the low solubility of C4-HSL in common solvents for IR spectroscopy, such as CS₂ and CCl₄, a liquid solution spectrum of pure, monomeric C4-HSL was not obtained. However, absorbance peaks observed in oversaturated CCl₄ solution could be assigned to distinct contributions from suspended micro-crystalline aggregates and dissolved monomeric species. The key vibrational bands of the monomeric form of C4-HSL are reported here for the first time: 3425 cm^?1 [ν(N–H)], 1784 cm^?1 [ν(CO), lactone], 1688 cm^?1 [amide I], and 1494 cm^?1 [amide II] (CCl₄).     

Atomic and molecular spectra of vapours evolved in a graphite furnace. Part 2: Magnesium chloride

Electrothermal atomic and molecular absorption spectrometry was applied to investigate the vaporization of magnesium chloride. Using a CCD linear array detector, atomic and molecular absorption spectra were simultaneously measured in the range 200–400 nm. Vaporization was performed from pyrocoated and tantalum-lined graphite tubes; Ar and He were employed as furnace gas. A broad molecular band was first observed at 210 nm and attributed to MgCl₂(g). The signal was followed by a partially resolved system at 266, 269, 273 nm and a three bands at 369, 376 and 382 nm, which are characteristic of MgCl(g). The release of MgCl vapours was accompanied by Mg atomic absorption and by light scattering. MgCl₂·6H₂O partially vaporizes as MgCl₂(g) and partially reacts with the water of crystallization (hydrolysis reaction), leading to a mixture of magnesium hydroxychloride and hydroxide. By further heating of the condensed phase MgO(s) and MgCl vapours are formed. The hydrolysis process was favoured by long pyrolysis treatments or by stopping the gas flow during the pyrolysis step. In He atmosphere or when a tantalum-lined tube was used, the fraction of salt vaporized as MgCl₂ was increased, while scattering effects were not observed.     

Evaluation of tungsten–rhodium coating on an integrated platform as a permanent chemical modifier for cadmium,lead, and selenium determination by electrothermal atomic absorption spectrometry

A tungsten–rhodium coating on the integrated platform of a transversely heated graphite atomizer is proposed as a permanent chemical modifier for the determination of Cd, Pb, and Se by electrothermal atomic absorption spectrometry. It was demonstrated that coating with 250 μg W+200 μg Rh is as efficient as the conventional Mg(NO₃)₂+NH₄H₂PO₄ or Pd+Mg(NO₃)₂ modifiers for avoiding most serious interferences. The permanent W–Rh modifier remains stable for 300–350 firings of the furnace, and increases tube lifetime by 50%–100% when compared to pyrolytic carbon integrated platforms. Also, there is less degradation of sensitivity during the atomizer lifetime when compared with the conventional modifiers, resulting in a decreased need of re-calibration during routine analysis. The characteristic masses and detection limits achieved using the permanent modifier were respectively: Cd 1.1±0.4 pg and 0.020 μgL⁻¹; Pb 30±3 pg and 0.58 μgL⁻¹ and Se 42±5 pg and 0.64μgL⁻¹. Results from the determination of these elements in water reference materials were in agreement with the certified values, since no statistical differences were found by the paired t-test at the 95% level.     

Vapor pressures and vaporization enthalpy of codlemone by correlation gas chromatography

The vapor pressure and vaporization enthalpy of codlemone (trans, trans 8,10-dodecadien-1-ol), the female sex hormone of the codling moth is evaluated by correlation gas chromatography using a series of saturated primary alcohols as standards. A vaporization enthalpy of (92.3 ± 2.6) kJ · mol⁻¹ and a vapor pressure, p/Pa = (0.083 ± 0.012) were evaluated at T = 298.15 K. An equation for the evaluation of vapor pressure from ambient temperature to boiling has been derived by correlation for codlemone. The calculated boiling temperature of T_B = 389 K at p = 267 Pa is within the temperature range reported in the literature. A normal boiling temperature of T_B = (549.1 ± 0.1) K is also estimated by extrapolation.     

The partition coefficients of ethylene between vapor and hydrate phase for methane + ethylene + THF + water systems

The vapor-hydrate equilibria were studied experimentally in detail for CH₄ + C₂H₄ + tetrahydrofuran (THF) + water systems in the temperature range of 273.15–282.15 K, pressure range of 2.0–4.5 MPa, the initial gas–liquid volume ratio range of 45–170 standard volumes of gas per volume of liquid and THF concentration range of 4–12 mol%. The results demonstrated that, because of the presence of THF, ethylene was remarkably enriched in vapor phase instead of being enriched in hydrate phase for CH₄ + C₂H₄ + water system. This conclusion is of industrial significance; it implies that it is feasible to enrich ethylene from gas mixture, e.g., various kinds of refinery gases or cracking gases in ethylene plant, by forming hydrate.     

Optimization of the electrothermal vaporization conditions for inductively coupled plasma excitation spectrometry: Selective volatilization versus covolatilization approaches

A new version of the outlet port of a graphite furnace electrothermal vaporizer (upward streaming system) is described, in which the hot sample vapour is mixed with an auxiliary carrier argon stream of ambient temperature. The operation procedures using carrier volatilization of organic liquids as gas phase additives are also outlined. The selective volatilization and transport efficiency for As, Cd, Hg, Pb, Sb, Se and Zn could be increased by applying sodium thiosulfate as chemical modifier to solution samples with controlled nitric acid content. On the other hand, a near simultaneous vaporization of 16 elements using chlorination with CCl₄ vapour at 2100° C could be performed for a multielement analysis. By wetting the auxiliary carrier argon stream, the linearity of the analytical curves was improved (except for chromium), when applying multielement standards. Linear analytical curves could also be obtained in the presence of alkali and alkaline earth metal matrices in multielement standards using halocarbon assisted electrothermal vaporization sample introduction.     

Effect of magnesium nitrate vaporization on gas temperature in the graphite furnace

The vaporization of magnesium nitrate was observed in longitudinally-heated graphite atomizers, using pyrocoated and Ta-lined tubes and filter furnace, Ar or He as purge gas and 10–200-μg samples. A charge coupled device (CCD) spectrometer and atomic absorption spectrometer were employed to follow the evolution of absorption spectra (200–400 nm), light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Magnesium atomic absorption at 285.2 nm appeared at approximately 1500°C in all types of furnaces. The intensity and shape of Mg atomization peak indicated a faster vapor release in pyrocoated than in Ta-lined tubes. Light scattering occurred only in the pyrocoated tube with Ar purge gas. At 1500–1800°C it was observed together with Mg absorption using either gas-flow or gas-stop mode. At 2200–2400°C the scattering was persistent with gas-stop mode. Light scattering at low temperature showed maximum intensity near the center of the tube axis. Magnesium emission at 382.9, 383.2 and 383.8 nm was observed simultaneously with Mg absorption only in the pyrocoated tube, using Ar or He purge gas. The emission lines were identified as Mg ³P°–³D triplet having 3.24 eV excitation energy. The emitting species were distributed close to the furnace wall. The emitting layer was thinner in He than in Ar. The experimental data show that a radial thermal gradient occurs in the cross section of the pyrocoated tube contemporaneously to the vaporization of MgO. This behavior is attributed to the reaction of the sample vapor with the graphite on the tube wall. The estimated variation of temperature within the cross section of the tube reaches more than 300–400°C for 10 μg of magnesium nitrate sampled. The increase of gas temperature above the sample originates a corresponding increase of the vaporization rate. Fast vaporization and thermal gradient together cause the spatial condensation of sample vapor that induces the light scattering.     

Atomic and molecular spectra of vapours evolved in a graphite furnace. Part 1. Alkali halides

Molecular Absorption Spectrometry (MAS) with electrothermal vaporization was applied to the measurement of absorption by alkali halides. The MAS system, consisting of a deuterium lamp primary source, a tubular graphite furnace, a grating polychromator and a linear array of Charge-Coupled-Device (CCD) detectors, allowed the simultaneous determination of atomic and molecular absorption in the range 200–400 nm. Vaporization was carried out in a pyrocoated graphite tube and absorption was measured during the heating of the furnace from 500°C to 2000°C in 100 s.Alkali halides vaporize as molecular compounds which absorb radiation in the whole ultraviolet range. The complexity of the molecular bands as well as the extent of the absorption increases from fluorides to iodides. The limit of absorption at long wavelengths is 254 nm for NaF, 287 nm for NaCl, 320 nm for NaBr and 370 nm for NaI. The appearance of vapors was observed between 680°C (RbI) and 1220°C (LiF), while the maximum absorption was reached between 800°C (CsI) and 1440°C (LiF); the characteristic temperatures of the vaporization peak were shifted towards lower values going from fluorides to iodides.     

Synthesis of titanium dioxide nanocrystalline layers using hexaprismatic shaped μ-oxo Ti(IV) alkoxo carboxylates as precursors

The reaction between Ti(OR)₄ (R = ⁿBu, ⁱBu, SiMe₃) and 2,2-dimethylpropionic acid lead to the formation of hexanuclear μ-oxo titanium(IV) alkoxo carboxylato complexes of the general formula [Ti₆O₆(OR)₆(OOC^tBu)₆]. Thermal decomposition pathways of these compounds and their potential application in the preparation of TiO₂ nanolayers using chemical vapor deposition (CVD) methods have been discussed. The type of the alkoxide ligands causes differences in the thermolysis pathway, and the type of the volatile decomposition products. Among the examined complexes only [Ti₆O₆(OR)₆(OOC^tBu)₆] (R = ⁱBu, SiMe₃) show promising properties for their application as precursors in CVD methods. The TiO₂ films were grown in a wide range of substrate temperatures (653–873 °K), under the total reactor pressure 2.0–3.0 mbar. The crystallinity and the composition of layers were analyzed by X-ray diffraction (XRD). It was found that the formation of TiO₂ amorphous, anatase or rutile films depends on the deposition temperature and gas phase composition.     

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.     

Trace Cd,Co, and Pb elements distribution during Sulcis coal pyrolysis: GFAAS determination with slurry sampling technique

A simple and fast method based on graphite furnace atomic absorption spectrometry (GF AAS) and slurry sampling technique (SlS) was developed to determine trace Cd, Co and Pb in high-sulphur coal (Sulcis, Italy) and coal chars derived at 600, 750 and 950 °C under N₂ atmosphere for developing a clean coal for electricity production. The proposed method was then coupled to a four-step sequential chemical extraction method for assessment of metals distribution in coaled samples. The slurries were prepared by varying sample mass (1–50 mg), volume (1–3 mL) and kind of dispersing medium (1% v/v Triton X-100 and 2 N HNO₃), and sonication time (5–30 min). Pyrolysis/atomization temperatures as well carrier gas flow rate were optimised. Pd(NO₃)₂ and NH₄H₂PO₄ were employed to stabilize Cd and Pb, respectively, in the pyrolysis stage of furnace program. The use of HNO₃ as dispersing agent was found to be effective in lowering the high level of background absorption on the Cd analytical signal produced by raw coal matrix. Conversely, coal charred samples did not show significantly level of background interferences, so that Triton X-100 dispersing agent could be used for all analytes. Calibration curve against acid-matched standards was allowed for Cd, whereas the standard addition calibration was used for Co and Pb owing to chemical matrix interferences. The precision, expressed as relative standard deviation (% RSD, n = 5), was better than 5% for Cd, Co, and Pb at 1, 10, and 15 μg L^? 1 levels, respectively. The accuracy of the analytical method was checked by analyzing a BCR No. 182 steam coal certificated reference material and the results were in good agreement with certificated and informed values. The solid detection limits (3σ_blank) were as low as 0.001 Cd, 0.01 Co, and 0.01 Pb mg kg^? 1, using 30 mg sample mass and slurry concentration of 30 m v^? 1 for Cd, and 50 mg sample mass and 50 m v^? 1 slurry concentration for Co and Pb. The content of elements in Sulcis coal was found to be 0.33 Cd, 4.0 Co, and 3.8 Pb mg kg^? 1 and mostly associated to sulphates and di-sulphides as indicated by the leaching test. Under pyrolysis conditions Cd significantly volatilised (about 64%) at temperature higher than 600 °C, whereas residue chars at 950 °C are enriched in Co and Pb up to 20%. The proposed method is suitable for routine metals monitoring in coaled samples.     

Kinetics of sorption and permeation of water in glassy polyimide

A vapor permeation experiment for water–ethanol mixtures was carried out using asymmetric Ube polyimide hollow-fiber membranes, which exhibit high selective permeability for water vapor, under the conditions of T=413 K, upstream gas pressure P_h=1.5×10⁵∼2.95×10⁵ Pa and downstream gas pressure P_l=400 Pa. To represent gas separation properties of the Ube polyimide membrane with a high transition temperature (570 K), the contribution of Henry's law part and Langmuir part modes on the diffusion through the membrane is studied on the basis of the dual-mode transport models. The results show that Henry's law penetrant controls the diffusion in the membrane. For the separation of water–ethanol mixtures by permeation through Ube polyimide membranes, the water trapped in microcavities can be assumed to be totally immobilized under the operating conditions applied here.     

Vapor deposited ethanol–H2O ice mixtures investigated by micro-Raman scattering

Solid deposits have been formed at 88 K and 10⁻¹ Torr from ethanol–water gas collected above aqueous solutions of ethanol (EtOH) (0.6, 2, 4.5, 9 and 17 mol%). The composition of different gas mixtures varying between 1:16 and 1:0.8 EtOH:H₂O are determined at 295 K using our experimental vapor–liquid equilibrium (VLE) data in combination with the Wilson model [28]. The Wilson constants derived at this temperature are Λ₁₂ = 0.37(4) and Λ₂₁ = 0.58(5). The concentration of EtOH in the ice mixture can be calculated using these data and a kinetic model of condensation. It is found to vary between 9 and 65 mol% EtOH. The ice mixtures are analyzed in situ in a modified cryostage by micro-Raman spectroscopy. The distinct vibrational signatures of pure EtOH, EtOH aqueous solutions and EtOH–ice mixtures are identified in the 400–3800 cm⁻¹ spectral range. Internal vibrational motions of EtOH molecules are affected by temperature and concentration. The presence of amorphous EtOH–ice phases at 88 K is demonstrated by the characteristic vibrational signatures of the ν_OH stretching modes. The crystallization of an EtOH hydrate is proposed during annealing at ∼140 K of a 65 mol% EtOH–ice mixture. According to our preliminary X-ray diffraction work, this phase has apparently a distinct structure from that of solid EtOH or from EtOH–clathtrate structures usually found in frozen aqueous solutions. For ice mixtures of lower EtOH content, a distinct hydrate phase crystallizes at ∼170 K. These results suggest that ice mixtures obtained by vapor deposition reflect the existence of EtOH clusters of a distinctive structural nature with respect to those encountered in frozen aqueous mixtures.     

Thermodynamic modeling for clathrate hydrates of ozone

We report a theoretical study to predict the phase-equilibrium properties of ozone-containing clathrate hydrates based on the statistical thermodynamics model developed by van der Waals and Platteeuw. The Patel–Teja–Valderrama equation of state is employed for an accurate estimation of the properties of gas phase ozone. We determined the three parameters of the Kihara intermolecular potential for ozone as a = 6.815 · 10⁻² nm, σ = 2.9909 · 10⁻¹ nm, and ε · k_B⁻¹ = 184.00 K. An infinite set of ε–σ parameters for ozone were determined, reproducing the experimental phase equilibrium pressure–temperature data of the (O₃ + O₂ + CO₂) clathrate hydrate. A unique parameter pair was chosen based on the experimental ozone storage capacity data for the (O₃ + O₂ + CCl₄) hydrate that we reported previously. The prediction with the developed model showed good agreement with the experimental phase equilibrium data within ±2% of the average deviation of the pressure. The Kihara parameters of ozone showed slightly better suitability for the structure-I hydrate than CO₂, which was used as a help guest. Our model suggests the possibility of increasing the ozone storage capacity of clathrate hydrates (∼7% on a mass basis) from the previously reported experimental capacity (∼1%).     

A simple method of electrochemical lithium intercalation within graphite from a propylene carbonate-based solution

Electrochemical lithium intercalation within graphite from 1 mol dm^− 3 solution of LiClO₄ in propylene carbonate (PC) was investigated at 25 and − 15 °C. Lithium ions were intercalated into and de-intercalated from graphite reversibly at − 15 °C despite the use of pure PC as the solvent. However, ceaseless solvent decomposition and intense exfoliation of graphene layers occurred at 25 °C. The results of the Raman spectroscopic analysis indicated that the interaction between PC molecules and lithium ions became weaker at − 15 °C by chemical exchange effects, which suggested that the thermodynamic stability of the solvated lithium ions was an important factor that determined the formation of a solid electrolyte interface (SEI) in PC-based solutions. Charge–discharge analysis revealed that the nature of the SEI formed at − 15 °C in 1 mol dm^− 3 of LiClO₄ in PC was significantly different from that formed at 25 °C in 1 mol dm^− 3 of LiClO₄ in PC containing vinylene carbonate, 3.27 mol kg^− 1 of LiClO₄ in PC, and 1 mol dm^− 3 of LiClO₄ in ethylene carbonate.     

Ultrasensitive determination of gold in geogas by laser excited atomic fluorescence spectrometry

Ultratrace gold (Au) in geogas samples has been determined by means of laser excited atomic fluorescence spectrometry combined with graphite electrothermal atomization and time-gate technique. Gold atoms were excited from the ground state to the 6p²P_3/2 state by a pulsed laser beam with a wavelength of 242.8 nm. Fluorescence photons with a wavelength of 312.3 nm were measured by a photon-counting unit. The time-gate technique was used to reduce the background radiation caused by the furnace. This method has proved to be highly sensitive with minimal background interference. Eighty-two geogas samples were analysed and the Au contents obtained were in the range of 0.002–0.182 ng l⁻¹. The study of Au concentration of geogas in soil is of great interest in prospecting gold deposits.     

Hydrodechlorination of CCl4 over carbon-supported palladium–gold catalysts prepared by the reverse “water-in-oil” microemulsion method

Two series of carbon-supported Pd–Au catalysts were prepared by the reverse “water-in-oil, W/O” method, characterized by various techniques and investigated in the reaction of tetrachloromethane with hydrogen at 423 K. The synthesized nanoparticles were reasonably monodispersed having an average diameter of 4–6 nm (Pd/C and Pd–Au/C) and 9 nm (Au/C). Monometallic palladium catalysts quickly deactivated during the hydrodehalogenation of CCl₄. Palladium–gold catalysts with molar ratio Pd:Au = 90:10 and 85:15 were stable and much more active than the monometallic palladium and Au-richer Pd–Au catalysts. The selectivity toward chlorine-free hydrocarbons (especially for C₂₊ hydrocarbons) was increased upon introducing small amounts of gold to palladium. Simultaneously, for the most active Pd–Au catalysts, the selectivity for undesired dimers C₂H_xCl_y, which are considered as coke precursors, was much lower than for monometallic Pd catalysts. Reasons for synergistic effects are discussed. During CCl₄ hydrodechlorination the Pd/C and Pd–Au/C catalysts were subjected to bulk carbiding.     

Electrothermal vaporization coupled with inductively coupled plasma array–detector mass spectrometry for the multielement analysis of Al2O3 ceramic powders

An electrothermal vaporization (ETV) system useful for the analysis of solutions and slurries has been coupled with a sector-field inductively coupled plasma mass spectrometer (ICP–MS) equipped with an array detector. The ability of this instrument to record the transient signals produced for a number of analytes in ETV–ICP–MS is demonstrated. Detection limits for Mn, Fe, Co, Ni, Cu, Zn and Ga are in the range of 4–60 pg μL^− 1 for aqueous solutions and in the low μg g^− 1 range for the analysis of 10 mg mL^− 1 slurries of Al₂O₃ powders. The dynamic ranges measured for Fe, Cu and Ga spanned 3–5 orders of magnitude when the detector was operated in the low-gain mode and appear to be limited by the ETV system. Trace amounts of Fe, Cu and Ga could be directly determined in Al₂O₃ powders at the 2–270 μg g^− 1 level without the use of thermochemical reagents. The results well agree with literature values for Fe and Cu, whereas deviations of 50% at the 90 μg g^− 1 level for Ga were found.     

Photochemical vapor generation of carbonyl for ultrasensitive atomic fluorescence spectrometric determination of cobalt

UV photochemical vapor generation (photo-CVG) as sample introduction was first adapted for determination of ultratrace cobalt by atomic fluorescence spectrometry (AFS). Cobalt volatile species can be generated when the buffer system of formic acid and formate containing Co (II) is exposed to UV radiation. The generated gaseous products were separated from liquid phase within a gas–liquid separator and then transported to AFS for determination of cobalt. Factors affecting the efficiency of photo-CVG were investigated in detail, including type and concentration of low molecular weight (LMW) organic acid, buffer system, UV irradiation time, reaction temperature, carrier gas flow rate and hydrogen flow rate. With 4% (v/v) HCOOH and 0.4 mol L^− 1 HCOONa buffer solution, 150 s irradiation time and 15 W low pressure mercury lamp, a generation efficiency of 23–25% was achieved. A limit of detection (LOD) of 0.08 ng mL^− 1 without any pre-concentration procedure and a precision of 2.2% (RSD, n = 11) at 20 ng mL^− 1 were obtained under the optimized conditions. The proposed method was successfully applied in the analysis of several simple matrix real water samples.