Characterization and use of a Raman liquid-core waveguide sensor using preconcentration principles

A novel Raman sensor using a liquid-core optical waveguide is reported, implementing a Teflon-AF 2400 tube filled with water. An aqueous analyte mixture of benzene, toluene and p-xylene was introduced using a 1000 μl sample loop to the liquid-core waveguide (LCW) sensor and the analytes were preconcentrated on the inside surface of the waveguide tubing. The analytes were then eluted from the waveguide using an acetonitrile-water solvent mixture injected via a 30 μl eluting solvent loop. The preconcentration factor was experimentally determined to be 14-fold, in reasonable agreement with the theoretical preconcentration factor of 33 based upon the sample volume to elution volume ratio. Raman spectra of benzene, toluene and p-xylene were obtained during elution. It was found that analytically useful Raman signals for benzene, toluene and p-xylene were obtained at 992, 1004 and 1206 cm⁻¹, respectively. The relative standard deviation of the method was 3% for three replicate measurements. The limit of detection (LOD) was determined to be 730 ppb (parts per billion by volume) for benzene, exceptional for a system that does not resort to surface enhancement or resonance Raman approaches. The Raman spectra of these test analytes were evaluated for qualitative and quantitative analysis utility.     

Liquid-liquid equilibria for mixtures of (ethylene carbonate + aromatic hydrocarbon + cyclohexane)

Liquid-liquid equilibrium data for mixtures of (ethylene carbonate + benzene + cyclohexane) at temperatures 303.15 and 313.15 K and (ethylene carbonate + BTX + cyclohexane) at temperature 313.15 K are reported, where the BTX is benzene, toluene and m-xylene. The compositions of liquid phases at equilibrium were determined by gas liquid chromatography. The selectivity factors and partition coefficients of ethylene carbonate for the extraction of benzene, toluene and m-xylene from (ethylene carbonate + BTX + cyclohexane) are calculated and presented. The obtained results are compared with the selectivity factors and partition coefficients of ethylene carbonate for the extraction of benzene from (ethylene carbonate + benzene + cyclohexane). The liquid-liquid equilibrium data were correlated with the UNIQUAC and NRTL activity coefficient models. The phase diagrams for the studied mixtures are presented and the correlated tie line results have been compared with the experimental data. The comparisons indicate the applicability of the UNIQUAC and NRTL activity coefficients model for liquid-liquid equilibrium calculations of the studied mixtures. The tie line data of the studied mixtures also were correlated using the Hand method.     

Carbon nanotube field-effect transistor detector associated to gas chromatography for speciation of benzene,toluene, ethylbenzene, (o-, m- and p-)xylene

An analytical methodology based on a field-effect transistor detector using carbon nanotubes (NTFET) coupled to a gas chromatograph has been developed for the speciation of the following aromatic compounds: benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene (BTEX). This methodology combines the proven separation capability of gas chromatography (GC) with the potential for detection of a NTFET. The developed analyzer shows a high and stable analytical response upon repeated analysis of BTEX during 4 weeks, with detection limit less than 4 μg/L. The GC–NTFET system also shows a great suitability for actual monitoring of indoor atmospheres and no significant difference was observed between the results obtained by the developed analyzer and a more classical analytical methodology, namely gas chromatography–flame ionization detection (GC–FID).     

Headspace-mass spectrometry determination of benzene, toluene and the mixture of ethylbenzene and xylene isomers in soil samples using chemometrics

A simple and fast method has been developed for the determination of benzene, toluene and the mixture of ethylbenzene and xylene isomers (BTEX) in soils. Samples were introduced in 10 mL standard glass vials of a headspace (HS) autosampler together with 150 μL of 2,6,10,14-tetramethylpentadecane, heated at 90 °C for 10 min and introduced in the mass spectrometer by using a transfer line heated at 250 °C as interface. The volatile fraction of samples was directly introduced into the source of the mass spectrometer which was scanned from m/z 75 to 110. A partial least squares (PLS) multivariate calibration approach based on a classical 3³ calibration model was build with mixtures of benzene, toluene and o-xylene in 2,6,10,14-tetramethylpentadecane for BTEX determination. Results obtained for BTEX analysis by HS-MS in different types of soil samples were comparables to those obtained by the reference HS-GC-MS procedure. So, the developed procedure allowed a fast identification and prediction of BTEX present in the samples without a prior chromatographic separation.     

Development of membrane electrodes for the selective determination of hyoscine butylbromide

Four polyvinyl chloride (PVC) membrane sensors for the determination of hyoscine butylbromide are described and characterized. The sensors are based on the use of the ion association complexes of hyoscine cation with ammonium reineckate counter anions as ion exchange sites in the PVC matrix. The membranes incorporate ion association complexes of hyoscine with dibutylsebathete (sensor 1), dioctylphthalate (sensor 2), nitrophenyl octyl ether (sensor 3) and β-cyclodextrin (sensor 4). The performance characteristics of these sensors were evaluated according to IUPAC recommendations, which reveal a fast, stable and linear response for hyoscine over the concentration range of 10⁻⁵-10⁻² M for sensors 1 and 2 and 10⁻⁶-10⁻² for sensors 3 and 4 with cationic slopes of −53.19, −55.17, −51.44 and −51.51 mV per concentration decade for the four sensors, respectively. The direct potentiometric determination of hyoscine butylbromide using the proposed sensors gave average recoveries % of 99.92 ± 1.11, 99.93 ± 1.00, 99.94 ± 1.18 and 99.87 ± 1.39 for the four sensors, respectively. The sensors are used for determination of hyoscine butylbromide in laboratory prepared mixtures, pharmaceutical formulations in combination with ketoprofen and in plasma. Validation of the method shows suitability of the proposed sensors for use in the quality control assessment of hyoscine butylbromide. The developed method was found to be simple, accurate and precise when compared with a reported HPLC method.     

Development of an enzymeless biosensor for the determination of phenolic compounds

The construction of amperometric enzymeless biosensors for phenolic compounds determination, using carbon paste electrode modified with copper phtalocyanine (CuPc) and histidine (His), based on the chemistry of the dopamine β-monooxygenase (DβM) enzyme that catalyzes the hydroxylation of the dopamine and its analogs is shown. The modified carbon paste was evaluated on electrodes constructed in two ways: putting the paste into a cavity of a rotating disk electrode and a platinum slide electrode fixed into a glass tube. The sensor in hydrodynamic conditions presented a linear response range between 30 and 250 μmol l⁻¹, with a sensitivity of 4.6±0.1 nA l μmol⁻¹ cm⁻² for catechol, response time of 3 s and lifetime of about 50 days when stored at room temperature. The sensor in static conditions showed a linear response range from 40 to 250 μmol l⁻¹, with a sensitivity of 0.30±0.01 nA l μmol⁻¹ cm⁻² for catechol. The sensors presented the following relative response order for dopamine and some analog species: catechol>dopamine>guaiacol>serotonin>phenol.     

Preparation and characterization of metal-organic framework MIL-101(Cr)-coated solid-phase microextraction fiber

Metal-organic frameworks (MOFs) have received great attention as novel sorbents due to their fascinating structures and intriguing potential applications in various fields. In this work, a MIL-101(Cr)-coated solid-phase microextraction (SPME) fiber was fabricated by a simple direct coating method and applied to the determination of volatile compounds (BTEX, benzene, toluene, ethylbenzene, m-xylene and o-xylene) and semi-volatile compounds (PAHs, polycyclic aromatic hydrocarbons) from water samples. The extraction and desorption conditions of headspace SPME (HS-SPME) were optimized. Under the optimized conditions, the established methods exhibited excellent extraction performance. Good precision (<7.7%) and low detection limits (0.32–1.7 ng L⁻¹ and 0.12–2.1 ng L⁻¹ for BTEX and PAHs, respectively) were achieved. In addition, the MIL-101(Cr)-coated fiber possessed good thermal stability, and the fiber can be reused over 150 times. The fiber was successfully applied to the analysis of BTEX and PAHs in river water by coupling with gas chromatography–mass spectrometry (GC–MS). The analytes at low concentrations (1.7 and 10 ng L⁻¹) were detected, and the recoveries obtained with the spiked river water samples were in the range of 80.0–113% and 84.8–106% for BTEX and PAHs, respectively, which demonstrated the applicability of the self-made fiber.     

Humidity independent mass spectrometry for gas phase chemical analysis via ambient proton transfer reaction

In this work, a humidity independent mass spectrometric method was developed for rapid analysis of gas phase chemicals. This method is based upon ambient proton transfer reaction between gas phase chemicals and charged water droplets, in a reaction chamber with nearly saturate humidity under atmospheric pressure. The humidity independent nature enables direct and rapid analysis of raw gas phase samples, avoiding time- and sample-consuming sample pretreatments in conventional mass spectrometry methods to control sample humidity. Acetone, benzene, toluene, ethylbenzene and meta-xylene were used to evaluate the analytical performance of present method. The limits of detection for benzene, toluene, ethylbenzene and meta-xylene are in the range of ∼0.1 to ∼0.3 ppbV; that of benzene is well below the present European Union permissible exposure limit for benzene vapor (5 μg m⁻³, ∼1.44 ppbV), with linear ranges of approximately two orders of magnitude. The majority of the homemade device contains a stainless steel tube as reaction chamber and an ultrasonic humidifier as the source of charged water droplets, which makes this cheap device easy to assemble and facile to operate. In addition, potential application of this method was illustrated by the real time identification of raw gas phase chemicals released from plants at different physiological stages.     

Pore size dynamics in interpenetrated metal organic frameworks for selective sensing of aromatic compounds

The two-fold interpenetrated metal-organic framework, [Zn₂(bdc)₂(dpNDI)]_n (bdc = 1,4-benzenedicarboxylate, dpNDI = N′N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide) can undergo structural re-arrangement upon adsorption of chemical species changing its pore structure. For a competitive binding process with multiple analytes of different sizes and geometries, the interpenetrated framework will adopt a conformation to maximize the overall binding interactions. In this study, we show for binary mixtures that there is a high selectivity for the larger methylated aromatic compounds, toluene and p-xylene, over the small non-methylated benzene. The dpNDI moiety within [Zn₂(bdc)₂(dpNDI)]_n forms an exciplex with these aromatic compounds. The emission wavelength is dependent on the strength of the host-guest CT interaction allowing these compounds to be distinguished. We show that the sorption selectivity characteristics can have a significant impact on the fluorescence sensor response of [Zn₂(bdc)₂(dpNDI)]_n towards environmentally important hydrocarbons based contaminants (i.e., BTEX, PAH).     

Separation of nonpolar analytes using methanol-water mixtures at elevated temperatures

Pure subcritical water has been found to be an efficient mobile phase for reversed-phase separations of both polar and moderately polar compounds. However, subcritical water must be modified with organic solvent in order to elute nonpolar analytes in an efficient manner. In this study, the separation of polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and benzene, toluene, and p-xylene (BTX) was performed by using heated methanol-water mixtures as the mobile phase. Temperatures employed in this study ranged from 21 to 140 °C, while the percentage of methanol in the mobile phase ranged from 52 to 90%. The retention times of analytes were matched under different mobile phase conditions by increasing the temperature and decreasing the percentage of methanol in the mobile phase.     

Viscometric, Acoustical and Spectroscopic Investigation of β-Pinene with Benzene, Toluene, m-Xylene and Mesitylene at 303.15, 308.15 and 313.15 K and Atmospheric Pressure

The densities (ρ), viscosities (η), ultrasonic speeds (u) and spectroscopic data of binary mixtures of benzene, toluene, m-xylene and mesitylene with β-pinene as a common component, over the whole composition range of mole fraction of β-pinene including those of pure components, have been measured at 303.15, 308.15 and 313.15 K, except for the spectroscopic study where the temperature was maintained at 298.15 K. The experimental results deviation in viscosity, deviation in ultrasonic velocity, isentropic compressibility and deviation in isentropic compressibility are discussed in terms of molecular interactions between unlike molecules. The variation of these excess parameters indicates the presence of weak interactions between β-pinene and benzene, toluene, m-xylene and mesitylene molecules. Moreover, the viscosity data are discussed in terms of interaction parameters. The theoretical ultrasonic speed was computed using the Nomoto model, ideal mixing relation, Jacobson’s free length theory and compared with the experimentally measured values. The experimental values are also discussed in terms of FTIR spectroscopy.     

Excess and deviation properties for the binary mixtures of methylcyclohexane with benzene, toluene, p-xylene, mesitylene, and anisole at T = (298.15, 303.15, and 308.15) K

Experimental data on density, viscosity, and refractive index at T = (298.15, 303.15, and 308.15) K, while speed of sound values at T = 298.15 K are presented for the binary mixtures of (methylcyclohexane + benzene), methylbenzene (toluene), 1,4-dimethylbenzene (p-xylene), 1,3,5-trimethylbenzene (mesitylene), and methoxybenzene (anisole). From these data of density, viscosity, and refractive index, the excess molar volume, the deviations in viscosity, molar refraction, speed of sound, and isentropic compressibility have been calculated. The computed values have been fitted to Redlich-Kister polynomial equation to derive the coefficients and estimate the standard errors. Variations in the calculated excess quantities for these mixtures have been studied in terms of molecular interactions between the component liquids and the effects of methyl and methoxy group substitution on benzene ring.     

Comparative study of 2-hydroxy propyl beta cyclodextrin and calixarene as ionophores in potentiometric ion-selective electrodes for neostigmine bromide

Three novel neostigmine bromide (NEO) selective electrodes were investigated with 2-nitrophenyl octyl ether as a plasticiser in a polymeric matrix of polyvinyl chloride (PVC). Sensor 1 was fabricated using tetrakis(4-chlorophenyl)borate (TpClPB) as an anionic exchanger without incorporation of an ionophore. Sensor 2 used 2-hydroxy propyl β-cyclodextrin as an ionophore while sensor 3 was constructed using 4-sulfocalix-8-arene as an ionophore. Linear responses of NEO within the concentration ranges of 10⁻⁵ to 10⁻², 10⁻⁶ to 10⁻² and 10⁻⁷ to 10⁻² mol L⁻¹ were obtained using sensors 1, 2 and 3, respectively. Nernstian slopes of 51.6 ± 0.8, 52.9 ± 0.6 and 58.6 ± 0.4 mV/decade over the pH range of 4-9 were observed. The selectivity coefficients of the developed sensors indicated excellent selectivity for NEO. The utility of 2-hydroxy propyl β-cyclodextrin and 4-sulfocalix[8]arene as ionophores had a significant influence on increasing the membrane sensitivity and selectivity of sensors 2 and 3 compared to sensor 1. The proposed sensors displayed useful analytical characteristics for the determination of NEO in bulk powder, different pharmaceutical formulations, and biological fluids (plasma and cerebrospinal fluid (CSF)) and in the presence of its degradation product (3-hydroxyphenyltrimethyl ammonium bromide) and thus could be used for stability-indicating methods.     

Application of solid-phase microextraction to monitoring indoor air quality

Practical application of Solid-Phase Microextraction (SPME) for the assessment of the quality of indoor air is presented. SPME was used to sample selected organic pollutants (carbon tetrachloride, benzene, toluene, chlorobenzene, p-xylene and n-decane). An SPME fiber was coated with a 100 μm film of polydimethylsiloxane. The analytes extracted were analysed with a gas chromatograph directly coupled with a mass spectrometer (GC-MS). The method was used to assess the indoor air quality in a few selected flats. The concentrations ranged from below detection limits to 6.9 mg/m³ for benzene depending on the flat; they were relatively high for newly built or freshly renovated flats. Received: 14 July 1998 / Revised: 17 November 1998 / Accepted: 21 November 1998     

Excess volumes of mixing of 1,2-dichloroethane and aromatic hydrocarbons

Excess volumes of mixing, V^E, for binary mixtures of 1,2-dichloroethane with benzene, toluene, o^?, m^?, and p-xylenes have been determined at 308.15 K over the complete composition range. V^E is positive for all these mixtures and varies in the order m-xylene >o-xylene >p-xylene > benzene > toluene. The experimental data have been analyzed in terms of the Prigogine's average potential cell model coupled with Balescu's theory. The calculated V^E values do not agree with the corresponding experimental values.     

Carbon nanotube networks as gas sensors for NO2 detection

Networks of different carbon nanotube (CNT) materials were investigated as resistive gas sensors for NO₂ detection. Sensor films were fabricated by airbrushing dispersions of double-walled and multi-walled CNTs (DWNTs and MWNTs, respectively) on alumina substrates. Sensors were characterized by resistance measurements from 25 to 250 °C in air atmosphere in order to find the optimum detection temperature. Our results indicate that CNT networks were sensitive to NO₂ concentrations as low as 0.1 ppm. All tested sensors provided significantly lower response to interfering gases such as H₂, NH₃, toluene and octane. We demonstrate that the measured sensitivity upon exposure to NO₂ strongly depends on the employed CNT material. The highest sensitivity values were obtained at temperatures ranging between 100 and 200 °C. The best sensor performance, in terms of recovery time, was however achieved at 250 °C. Issues related to the gas detection mechanisms, as well as to CNT network thermal stability in detection experiments performed in air at high operation temperatures are also discussed.     

A comparison of phosphated and sulfated β-cyclodextrins as chiral selectors for capillary electrophoresis

The enantioseparation capabilities of three different functionalized β-cyclodextrins, two sulfated β-cyclodextrins with 4 and 15 nominal degrees of substitution and a phosphated β-cyclodextrin with 8 degrees of substitution, were compared. While anodic detection was used with both sulfated cyclodextrins, the phosphated cyclodextrin required cathodic detection suggesting either lower ionization of the phosphated cyclodextrin or generally lower affinity of the analytes for the phosphated cyclodextrin. The effects of several experimental parameters were evaluated with respect to enantioseparation. The degrees of substitution of the cyclodextrin, pH of the background electrolyte as well as the concentration of the functionalized β-cyclodextrin, each had a significant influence on the successful enantiomeric separation of the chiral drugs investigated.     

Absorbance characteristics of a liquid-phase gas sensor based on gas-permeable liquid core waveguides

The absorbance characteristics and influential factors on these characteristics for a liquid-phase gas sensor, which is based on gas–permeable liquid core waveguides (LCWs), are studied from theoretical and experimental viewpoints in this paper. According to theory, it is predicted that absorbance is proportional to the analyte concentration, sampling time, analyte diffusion coefficient, and geometric factor of this device when the depletion layer of the analyte is ignored. The experimental results are in agreement with the theoretical hypothesis. According to the experimental results, absorbance is time-dependent and increasing linearly over time after the requisite response time with a linear correlation coefficient r² > 0.999. In the linear region, the rate of absorbance change (RAC) indicates improved linearity with sample concentration and a relative higher sensitivity than instantaneous absorbance does. By using a core liquid that is more affinitive to the analyte, reducing wall thickness and the inner diameter of the tubing, or increasing sample flow rate limitedly, the response time can be decreased and the sensitivity can be increased. However, increasing the LCW length can only enhance sensitivity and has no effect on response time. For liquid phase detection, there is a maximum flow rate, and the absorbance will decrease beyond the stated limit. Under experimental conditions, hexane as the LCW core solvent, a tubing wall thickness of 0.1 mm, a length of 10 cm, and a flow rate of 12 mL min⁻¹, the detection results for the aqueous benzene sample demonstrate a response time of 4 min. Additionally, the standard curve for the RAC versus concentration is RAC = 0.0267 c + 0.0351 (AU min⁻¹), with r² = 0.9922 within concentrations of 0.5–3.0 mg L⁻¹. The relative error for 0.5 mg L⁻¹ benzene (n = 6) is 7.4 ± 3.7%, and the LOD is 0.04 mg L⁻¹. This research can provide theoretical and practical guides for liquid–phase gas sensor design and development based on a gas-permeable Teflon AF 2400 LCW.     

Highly sensitive optical humidity probe

A highly sensitive optical humidity probe based on reflectance measurements has been developed using Nafion^®-crystal violet (CV) films. This sensor can be used to calibrate relative humidity (RH) in the range 0-0.25% with a detection limit (blank signal + 3σ_b, where σ_b = the standard deviation (S.D.) of the blank signal) of 0.018% RH (∼4.37 ppm) and exhibited low hysteresis. The sensor films were fully reversible in dry nitrogen and reversal times were shown to be dependent on exposure time and % RH. The response to 1% RH was highly reproducible (S.D. = 1.67%, number of samples (n) = 5). Hydrogen chloride gas did not interfere with the response of the sensor to RH but did reduce sensor reversal times. This sensor displayed sufficient sensitivity that it could be used to detect ppm levels of moisture in process gases such as nitrogen and HCl.