Determination of chlorinated hydrocarbons in water by fiber-optic evanescent wave spectroscopy and partial least-squares regression

An application of the multivariate calibration technique of partial least-squares (PLS) regression to near-infrared spectra of a fiber-optic sensor based on the evanescent wave principle is presented. The sensing element consists of a quartz glass fiber with a silicone cladding which enriches nonpolar water contaminants. Due to the interaction of the extracted molecules with the part of the light which is transmitted in the evanescent wave zone of the cladding, absorbance spectra of the contaminants can be collected. In view of a sensor application for in-situ environmental analysis, aqueous solutions of chlorinated hydrocarbon solvents (CHS), which often can be found as major water contaminants, have been measured. PLS regression was applied to three sets of CHS samples, representing typical features of NIR evanescent wave spectral data. These are, e.g., strong overlapping of the absorption bands of different CHS components, peak distortions due to temperature variations between reference and sample measurement and noisy data at analyte concentrations near to the limit of detection, respectively. For trichloroethene and 1,1-dichloroethene, where the calibration model was built for samples within a small concentration range of 1–9 mg l^–1, satisfactory prediction results could be obtained with a relatively small root-mean-square error of 0.3 mg l^–1 compared to analytical reference measurements. In contrast to this, for a three component system of dichloromethane, trichloromethane and trichloroethene with strongly overlapping absorption bands, where samples over a very broad concentration range from 3–4940 mg l^–1 were included in the PLS model, the prediction accuracy decreased enormously and for some samples strong deviations between real and predicted data occurred. Nevertheless, applying multivariate calibration to this difficult system with similar spectral features and huge differences in the concentration of the species allowed an acceptable spectral distinction and at least a semi-quantitative determination of the CHS species.     

Integrated optical NIR-evanescent wave absorbance sensorfor chemical analysis

A new, long-path integrated optical (IO) sensor for the detection of non-polar organic substances is described. The sensing layer deposited on a planar multimode IO structure is built by a suitable silicone polymer with lower refractive index (RI). It acts as a hydrophobic matrix for the reversible enrichment of non-polar organic contaminants from water or air. Light from the near-infrared (NIR) range is coupled into the planar structure and the evanescent wave part of the light field penetrating into the silicone layer interacts with the enriched organic species. As a result, light is absorbed at the characteristic frequencies of the corresponding C-H, N-H or O-H overtone and combination band vibrations of the analytes. To perform evanescent field absorbance (EFA) measurements, the arc-shaped strip waveguide structure of 172 mm interaction length was adapted to a tungsten-halogen lamp and an InGaAs diode array spectrograph over gradient index fibers. Dimethyl-co-methly(phenyl)polysiloxanes with varying degrees of phenylation were prepared and used as sensitive coating materials for the IO structure. Light attenuation in the arc-shaped waveguides is high and typical insertion losses in the range of 14–18 dB were obtained. When the coated sensors were brought in contact with aqueous samples, the light transmission decreases, which is due to the formation of H₂O micro-emulsions in the silicone superstrates. Nevertheless, after reaching constant light transmissions, absorbance spectra of aqueous trichloroethene samples were successfully collected. For gas measurements, where water cross sensitivity problems are absent, the sensitivity of the IO device for trichloroethene was tested as a function of the RI of the silicone superstrate. The slope of the TCE calibration function increases by a factor of 10 by using a poly(methylphenylsiloxane) layer with a RI of 1.449 instead of poly(dimethylsiloxane) (RI: 1.41). A comparison of the IO-EFA and an earlier developed fiber-optic EFA sensor for trichloroethene measurements in the gas phase showed an increase in sensitivity per unit length of the waveguide by a factor of up to 120.     

A New Sol-Gel Derived Optical Fiber Sensor for High Acidity Measurements: Application in Nuclear Fuel Reprocessing

Nuclear fuel reprocessing requires high acidity levels that should be accurately and rapidly controlled in order to optimize extraction yields and process efficiency. A new optical fiber sensor based on a sol-gel film doped with an acid-sensitive indicator has been developed to improve remote and on-line monitoring.The organic molecule was physically incorporated in an acid-catalyzed sol-gel matrix and coated onto the core of a denuded optical fiber. The sensor response was evaluated in synthetic solutions of nitric acid, concentrations ranging from 1 to 10 mol · l^–1. Changes in absorption properties and response characteristics of the sensor, when compared to the indicator in solution, have been discussed. Analytical performances of the sensor, i.e. sensitivity, response time, reversibility and repeatability are very encouraging for the on-line analysis. Viability of the sensor for process measurements depends on the stability which might be improved by a strict control of the sensitive film porosity.     

Development of a Fiber Optic Evanescent Wave Sensor for Anionic Surfactants Using Ethyl Violet

A rapid and simple method for the determination of anionic surfactants based on an evanescent wave fiber optic was developed using ethyl violet. The sensor was prepared by removing the middle of the multimode fiber cladding. The optical signal from ethyl violet decreased with an increase in the sodium dodecyl sulfate concentration. The calibration curve was linear from 4 to 15 milligrams per liter with a limit of detection of 3.3 milligrams per liter. This simple fiber optic sensor requires a low volume of sample and does not employ extraction with organic solvents compared with conventional methods.     

Fiber optic evanescent field sensors for gaseous species using MIR transparent fibers

A new method of optical gas sensing is presented which uses evanescent waves of optical fibers. Gas detection can be done in a simple, flexible, and possibly distributed way using a single optical fiber as both the sensor with an active element of 10 cm and the transmission line for optical signals. The detection of several single gaseous species — such as hexane, trichloro-trifluoro-ethane, methane, acetone, etc. — as well as gaseous mixtures in a concentration range of 1–10% could be achieved with an average reproducibility of 2–3% with a chalcogenide fiber optic coupled to an IFS 88 Bruker spectrometer.     

A fiber-optic evanescent wave sensor for dissolved oxygen detection based on novel hybrid fluorinated xerogels immobilized with [Ru(bpy)3]2+

We have prepared a novel fiber-optic evanescent wave sensor (FEWS) for dissolved oxygen (DO) detection. The sensor fabrication was based on coating a decladded portion of an optical fiber with a microporous coating, which was prepared from 3,3,3-trifluoropropyltrimethoxysilane and n-propyltrimethoxysilane. The fluorophores were immobilized in the porous coating and excited by the evanescent wave field produced on the core surface of the optical fiber. The sensitivity of the sensor was quantified by the ratio of the fluorescence intensities in pure deoxygenated (I ₀) and in pure oxygenated environments (I). Results show that the quenching response of DO is increased with the enhancement of the coating surface hydrophobicity using the presented hybrid fluorinated ORMOSILs. The calibration curve of I ₀/I to [O₂] is linear from 0 to 40 ppm and the detection limit is 0.05 ppm (3σ) with a short response time of 15 s for DO detection. Figure       

Optimisation of an integrated optical evanescent wave absorbance sensor for the determination of chlorinated hydrocarbons in water

The suitability of an integrated optical chemical sensor for the determination of highly volatile chlorinated hydrocarbons in aqueous solutions has been proven. The analytes are detected by NIR absorption spectrometry in the evanescent field of an integrated optical strip waveguide generated in a BGG31 (Schott, Germany) glass substrate, which is coated with a hydrophobic polymer superstrate as sensing layer. It has been shown that the sensitivity increases when the refractive index of the superstrate is increased from 1.333 up to 1.46. Different UV-cured polysiloxanes with low cross sensitivity to water have been prepared. Due to the good light transmission properties of the IO-sensors prepared by this method, quantitative measurements have been performed with the model system trichloroethene (TCE) in water. A detection limit of 22 ppm has been found and the sensor response times (t₉₀-value) are between five and fourteen minutes for a coating thickness of around 30 m. The sensor response is totally reversible. The analyte desorbes in air within 2 min. The enrichment of trichloroethene in the polysiloxane coating can be described by film diffusion through the aqueous boundary layer as rate determining step.     

Photoelectrochemical pK Scale for weak CH-acids

The method of laser photoelectron emission from metals in electrolyte solutions has been used to measure the rate constants w₃ for the electrochemical reduction of simple organic radicals R to carbanions R^–. The following empirical rule has been established: the changes in the standard redox potential E° for R^–/R in series of organic radicals are equal to the changes in the potential corresponding to individual values of w₃. On the basis of this rule and the value of E° for CH₃ ^–/CH₃ E° values were obtained for C₂H₅, n-C₃H₇, n-C₄H₉, CH₂OH, CH₃CHOH, (CH₃)₂COH, CH₂Cl, CHF₂, CHFCl, CHCl₂, CF₂Cl, CFCl₂, CF₃, CCl₃, C₆H₅, C₆Cl₅, and a scale of pK for the CH acids conjugate to R^–. Consideration is given to the nature of the changes in E° and p^K on passing from aqueous solutions to solutions in water-dioxane mixtures and in acetonitrile.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1508–1514, July, 1990.     

Characterization of manganese–gallium mixed oxide powders

Mn---Ga mixed oxides have been prepared by coprecipitation of the corresponding oxo-hydroxides as powders and have been characterized in relation to their structural and optical properties. The materials have been characterized by XRD, TG-DTA, skeletal IR and UV–visible–NIR spectroscopies. Large solubility of Mn in the diaspore type α-GaOOH oxo-hydroxide has been found. The spinel related structures of hausmannite Mn₃O₄ and of β-gallia present large reciprocal solubilities at least in a metastable form. At high temperature also bixbyite-type α-Mn₂O₃ solid solutions containing up to 20% at. Ga have been observed.     

Potentiometric study using chemically modified silica gel with pyridinium as membrane for ClO 4 − ions

A potentiometric sensor for the perchlorate anion was developed by mixing chemically modified silicagel with pyridinium perchlorate, with an epoxy polymer and graphite. The electrode showed Nernstian response between 1.0 × 10^–2 and 1.0 × 10^–3 M perchlorate concentrations. The electrode showed high selectivity to this ion at solutions pH between 5.5 and 8.0. The presence of IO ₄ ^– , NO ₃ ^– ,Br^–, IO ₃ ^– , Cl^– and SO ₄ ^2– ions in the solutions, had only small interference in the electrode response in the range mentioned.     

IR absorption spectra of 1-aryl-3-pyrazolidones

An examination of the frequencies and intensities of the valence vibration bands of carbonyl groups established that the phenyl group interacts with the C=O group of 1-phenyl-3-pyrazolidone and its m- and p-tolyl derivatives in solution. It is assumed that the interaction is accomplished through the N₁ and N₂ atoms in the sp² state. 1-Phenylpyrazolidone derivatives are strongly associated in CC1₄ and CHCl₃ solutions. The association decreases on passing from CCl₄ to CHCl₃ solutions and when there are methyl groups in the ortho positions of the phenyl rings. The energy of association between the 1-phenylpyrazolidones and organic bases (acetonitrile, ethyl acetate, and dioxane), evaluated from the shift in _nh, is 1.36–3.5 kcal/mole. The frequencies and integral intensities of the bands of the C=O and NH groups in chloroform were measured.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1678–1682, December, 1970.     

Infrared and Raman spectroscopic investigations of δ2-oxazolin-5-ones

The infrared and Raman Spectra of saturated and unsaturated ²-oxazolin-5-ones (azlactones) have been studied for CCl₂, CHCl₃, i-PrOH, and t-BuOH solutions and for the solid phase. Frequency assignments have been made for C=O bond stretching in the region 1825–1770 cm^–1 and for a mixed C=C and C=N vibration at 1690–1550 cm^–1. All of the azlactones showed an intense, characteristic absorption in a narrow range near 900 cm^–1 which was assigned to a deformation vibration of the oxazolone ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1133–1136, August, 1989.     

Chemical microanalytical systems: objectives and latest developments

Chemical microanalysis systems are at a very early state of development. At the present time a modular approach with subsystems carrying out the various functions such as sampling, preparation and measurement seems to be favored by the developers. The marketpotential of the systems can be seen primarily in the area of inexpensive monitoring equipment for environmental and process control. Reduction of size leads also to smaller sample volumes, less waste and shorter analysis time: Miniaturization is in many cases an essential feature for biological and in-vivo medical investigations, while some properties inherently associated with miniaturization can also lead to substantial improvements in the technical performance of the microsystem as compared to its macro version. This will be demonstrated by discussing some microsystems which include spectroscopic detection systems:– Miniature separation instruments coupled with advanced miniaturized detection device based on photothermal spectroscopy– Integrated-optical NIR evanescent wave sensor systems– Optochemical and biosensor systems     

An Optical Chemical Sensor Based on Swellable Dicarboxylate Functionalized Polymer Microspheres for pH Copper and Calcium Determination

An optical chemical sensor based on polymer swelling and shrinking has been studied by way of optical transmission. Polyvinylbenzyl chloride cross-linked with divinylbenzene and derivatized as the dicarboxylate was dispersed as microspheres in a hydrogel membrane. The absorbance was measured vs. the wavelength upon exposing the modified membrane to solutions of varying pH (3.0–9.0). At low pH (3.0), the absorbance had the highest value (1.34), while the absorbance decreased significantly (1.10) when the pH was increased to 9.0, indicating polymer swelling.The modified membrane was also used for sensing metal ions, in particular calcium and copper. Complex formation with the dicarboxylate functionality caused the polymer to shrink. This resulted in an increase in absorbance for a concentration ranging from 1.0×10^–3 to 4.0×10^–3M.     

Anionic surfactant sensor based on a hetero-core structured optical fiber

A simple method of fiber optic spectrophotometric measurement for anionic surfactant is described. The probe was fabricated from hetero-core structured fiber optic that consisted of multi mode fibers and a short length of single mode fiber which was inserted in the multi mode fibers. The sensor surface was treated with octyltrimethoxysilane to adsorb the surfactant-methylene blue complex via hydrophobic interaction between the octyl group on the fiber surface and the hydrophobic group of the surfactant. Methylene blue—anionic surfactant complex adsorbed on the sensor surface caused propagating light loss because of evanescent wave interaction that was generated at the cladding. The propagating loss spectrum of the fiber showed a peak at around 610 nm. Absorbance increases with the concentration of the surfactant to reach an upper level at a concentration of 30?μm. The response time is within 30 min.     

Optimisation of an integrated optical evanescent wave absorbance sensor for the determination of chlorinated hydrocarbons in water

The suitability of an integrated optical chemical sensor for the determination of highly volatile chlorinated hydrocarbons in aqueous solutions has been proven. The analytes are detected by NIR absorption spectrometry in the evanescent field of an integrated optical strip waveguide generated in a BGG31 (Schott, Germany) glass substrate, which is coated with a hydrophobic polymer superstrate as sensing layer. It has been shown that the sensitivity increases when the refractive index of the superstrate is increased from 1.333 up to 1.46. Different UV-cured polysiloxanes with low cross sensitivity to water have been prepared. Due to the good light transmission properties of the IO-sensors prepared by this method, quantitative measurements have been performed with the model system trichloroethene (TCE) in water. A detection limit of 22 ppm has been found and the sensor response times (t(90)-value) are between five and fourteen minutes for a coating thickness of around 30 microm. The sensor response is totally reversible. The analyte desorbes in air within 2 min. The enrichment of trichloroethene in the polysiloxane coating can be described by film diffusion through the aqueous boundary layer as rate determining step.     

Integrated optical NIR-evanescent wave absorbance sensorfor chemical analysis

A new, long-path integrated optical (IO) sensor for the detection of non-polar organic substances is described. The sensing layer deposited on a planar multimode IO structure is built by a suitable silicone polymer with lower refractive index (RI). It acts as a hydrophobic matrix for the reversible enrichment of non-polar organic contaminants from water or air. Light from the near-infrared (NIR) range is coupled into the planar structure and the evanescent wave part of the light field penetrating into the silicone layer interacts with the enriched organic species. As a result, light is absorbed at the characteristic frequencies of the corresponding C-H, N-H or O-H overtone and combination band vibrations of the analytes. To perform evanescent field absorbance (EFA) measurements, the arc-shaped strip waveguide structure of 172 mm interaction length was adapted to a tungsten-halogen lamp and an InGaAs diode array spectrograph over gradient index fibers. Dimethyl-co-methly(phenyl)polysiloxanes with varying degrees of phenylation were prepared and used as sensitive coating materials for the IO structure. Light attenuation in the arc-shaped waveguides is high and typical insertion losses in the range of 14-18 dB were obtained. When the coated sensors were brought in contact with aqueous samples, the light transmission decreases, which is due to the formation of H(2)O micro-emulsions in the silicone superstrates. Nevertheless, after reaching constant light transmissions, absorbance spectra of aqueous trichloroethene samples were successfully collected. For gas measurements, where water cross sensitivity problems are absent, the sensitivity of the IO device for trichloroethene was tested as a function of the RI of the silicone superstrate. The slope of the TCE calibration function increases by a factor of 10 by using a poly(methylphenylsiloxane) layer with a RI of 1.449 instead of poly(dimethylsiloxane) (RI: 1.41). A comparison of the IO-EFA and an earlier developed fiber-optic EFA sensor for trichloroethene measurements in the gas phase showed an increase in sensitivity per unit length of the waveguide by a factor of up to 120.     

LED-compatible fluorosensor for measurement of near-neutral pH values

We report on an optical sensor material suitable for fluorimetric measurement of pH in the 6–9 range using a new, fully LED-compatible fluorescent dye. Its base form has a strong absorption between 580 and 630 nm that matches the emission band of conventional yellow or orange light-emitting diodes. Two kinds of dye immobilization are reported. The first is based on covalent binding to a cellulosic matrix and the resulting material is intended for use in sensing membranes. The second involves physical entrapment of the dye in a sol-gel matrix which can be used for optical fiber tip coating as well as in evanescent wave type sensors. Both kinds of sensor materials are studied with respect to dynamic pH ranges, response times, sensitivity toward ion strength, and stability.Dedicated to Professor G. Werner, Leipzig, on the occasion of his 60th birthday.     

Solvent extraction studies on tetravalent selenium

A systematic study has been carried out on the extraction of SeI₄, by various water-immiscible organic solvents. Extraction has been investigated as a function of H₂SO₄ and Kl concentrations. It has been found that Se(IV) extraction is appreciably increased by addition of iodide ion to sulfuric acid solutions. On the other hand, the presence of water-miscible alcohols and acetone was found to enhance Se(IV) extraction from H₂SO₄–Kl solutions. In the light of the results, an extraction mechanism is suggested.     

Nickel(II) hexamethyleneiminedithiocarbamate complexes with triphenylphosphine or tributylphosphine

A series of square-planar nickel(II) hexamethylenedithiocarbamate complexes with heterogeneous coordination spheres of composition [NiX(hmidtc)Y] · nCHCl₃ [X = Cl, Br, I or NCS; hmi = C₆H₁₂, dtc = S₂CN^–; Y = PPh₃ or PBu₃, n = 0,1] have been synthesized and characterized by elemental analyses, i.r. and u.v.–v.i.s. spectroscopy, magnetochemical and conductivity measurements, and by thermal analysis. X-ray structures of [NiCl(hmidtc)(PPh₃)] · CHCl₃ and [NiBr(hmidtc)(PPh₃)] · CHCl₃ have been determined.