Development of electronic nose method for evaluation of residual solvents in low‐density polyethylene films

Flexible packaging films containing high levels of residual volatile organic compounds (VOCs) can alter the flavour and odour of packaged foods. Currently, a range of gas chromatographic techniques and sensory evaluations are used for assessing the residual VOCs in packaging films. An objective method for assessing the residual solvents from low‐density polyethylene (LDPE) was developed using an Alpha MOS Fox 3000 electronic nose (e‐nose) equipped with 12 metal oxide semiconductor sensors. Three VOCs, ethyl acetate, ethyl alcohol and toluene, were chosen as models for solvents of interest in flexible food packaging analysis. LDPE film samples were spiked with single and binary mixtures of solvents and analysed using the e‐nose and by GC–FID (HP 6890; Hewlett‐Packard Co., Wilmington, DE). The responses obtained from the e‐nose were processed using principal component analysis (PCA) and discriminate factorial analysis (DFA) in order to identify the residual solvents. Partial least squares analysis (PLS) was also used to quantify the amount of residual solvent and to correlate the e‐nose results with gas chromatography, which is currently the standard method for determining residual VOCs in packaging films. There was good agreement between the e‐nose responses and gas chromatography results for single solvents (r = 0.90–0.98). The technique also worked for binary solvent mixtures (r = 0.84–0.99). The electronic nose can be a viable alternative to traditional techniques while providing simplicity and objectivity, which would be extremely advantageous in routine quality control of residual solvents. Copyright © 2006 John Wiley & Sons, Ltd.     

Atomic force microscopic observation of the molecular orientation in ultrathin films of alkanoic acid-derivatized porphyrins on a mica surface

The observation of the molecular orientation of alkanoic acid-derivatized porphyrins in ultrathin films deposited on mica was carried out by atomic force microscopy. It was observed that the tetraacid derivative 5, 10, 15, 20-tetra(N-10-carboxydecyl-pyridinium-4-yl) porphyrin was arranged as a monolayer with the porphyrin macrocycle oriented coplanar to the mica surface. On the other hand, the diacid derivative protoporphyrin IX Zn(II) formed a bilayer with the hydrophobic part inside and the hydrophilic part in the periphery. Therefore, in this case, the porphyrin macrocycle is roughly perpendicular to the mica surface.     

A Cryotrap Membrane Introduction Mass Spectrometry System for Analysis of Volatile Organic Compounds in Water at the Low Parts-per-Trillion Level

Detection of volatile organic compounds (VOCs) in aqueous solution at low parts-per-trillion (ppt) levels is accomplished using a very simple and efficient on-line preconcentration cryotrap membrane introduction mass spectrometry (CT-MIMS) system. The conventional MIMS probe is modified so that the membrane interface is placed about 15 cm away from the ion source. A U-shaped trap tube is then inserted between the membrane interface and the ion source. Cryotrapping is performed with liquid nitrogen for 15 min, followed by fast heating at ～15 °C s(-)(1), which thermally releases the condensed VOCs almost at once into the ion source region of a quadrupole mass spectrometer. By applying electron ionization and a selective ion monitoring scan mode, a very sharp and intense peak is obtained. The performance of the CT-MIMS system was compared with that of conventional MIMS, and after reaching the best conditions for the trapping and heating cycles, an improvement factor in signal intensity of about 100 was observed for a series of VOCs. The extraordinary sensitivity of CT-MIMS system allows VOCs to be detected at very low concentrations, detection limits being typically on the order of 10-20 ppt. The results also show excellent linearity and reproducibility for the system.     

Preparation,characterization and catalytic oxidation properties of silica composites immobilized with cationic metalloporphyrins

Herein, the metalloporphyrins Co(II) (5, 10, 15, 20-tetra (4-(3-(N-ethyl-4-pyridyl)pyrazolyl)phenyl)porphyrin) (CoTEtPyP) and Mn(III) (5, 10, 15, 20-tetra(4-(3-(N-ethyl-4-pyridyl)pyrazolyl)phenyl)porphyrin) (MnTEtPyP(OAc)) were synthesized and characterized spectroscopically. The cationic metalloporphyrins were firstly immobilized on the surface of SiO₂ by electrostatic attractions with hydrothermal method to get the gels. Then, the gels were extracted by supercritical CO₂ to remove the redundant solvent molecules and the unreacted metal salt. The structures and properties of porphyrin–SiO₂ porous composites (PSC1 and PSC2) were characterized by Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, scanning electron microscopy, transmission electron microscope, powder X-ray diffraction, thermalgravimetric analysis and nitrogen sorption measurements. N₂ absorptions have verified that the porous materials have large BET surface area and big N₂ uptake capacity. The composites also have shown higher specific surface area and superior thermal stability. The catalytic activities of the new PSCs to the ethylbenzene oxidation carried out indicated that both of them exhibit highly selectivity of acetophenone (>?99%) with the conversion of 87.6% (PSC1) and 93.0% (PSC2), respectively.     

A Method of Feature Extraction on Recovery Curves for Fast Recognition Application With Metal Oxide Gas Sensor Array

In the fast recognition applications of electronic nose, not only the recognition time is important, another parameter response-recovery time also needs to be considered. The response-recovery time could be defined as the time from the beginning of measuring one sample to the state of being ready for new sample measurement. An electronic nose with nine metal oxide (MOX) gas sensors and a method of feature extraction on sensor recovery curves were presented in this paper. The electronic nose was designed to reduce the recognition time and the response-recovery time synchronously. In the sampling module of the electronic nose, there were two pumps, which could let the sensor quickly recovered. The feature extraction method could rapidly extract features from sensor recovery curves with robust information. Nine volatile organic compounds (VOCs) gas samples were measured with the electronic nose. The correct recognition ratios under 10 and 15 s recognition time are 91.0% and 95.8%, respectively. The mean response-recovery time of these sensors in the measurements was 33.5 s, which was about 42.7% of the response-recovery time in typical traditional gas sample measurements. The results show that the proposed feature extraction method could extract robust information with short recognition time and response-recovery time.     

Hybrid optical-electrochemical electronic nose system based on Zn-porphyrin and multi-walled carbon nanotube composite

In this work, we have enhanced the capability of an e-nose system based on combined optical and electrochemical transduction within a single gas sensor array. The optical part of this e-nose is based on detection of the absorption changes of light emitted from eight light emitting diodes (LEDs) as measured by a CMOS photo-detector. The electrochemical part works by measuring the change in electrical resistivity of the sensing materials upon contact with the sample vapor. Zinc-5,10,15,20-tetra-phenyl-21H,23H-porphyrin (ZnTPP) and multi-walled carbon nanotube (MWCNT) composite was used as the sensing materials based on its good optoelectronic properties. This sensing layer was characterized by UV-Vis spectroscopy and atomic force microscope and tested with various VOC vapors. Density functional theory (DFT) calculations were performed to investigate the electronic properties and interaction energies between ZnTPP and analyte molecules. It can be clearly seen that this hybrid optical-electrochemical electronic nose system can classify the vapor of different volatile organic compounds.     

Evaluation of an electronic nose to assess fruit ripeness

The main goal of our study was to see whether an artificial olfactory system can be used as a nondestructive instrument to measure fruit maturity. In order to make an objective comparison, samples measured with our electronic nose prototype were later characterized using fruit quality techniques. The cultivars chosen for the study were peaches, nectarines, apples, and pears. With peaches and nectarines, a PCA analysis on the electronic nose measurements helped to guess optimal harvest dates that were in good agreement with the ones obtained with fruit quality techniques. A good correlation between sensor signals and some fruit quality indicators was also found. With pears, the study addressed the possibility of classifying samples regarding their ripeness state after different cold storage and shelf-life periods. A PCA analysis showed good separation between samples measured after a shelf-life period of seven days and samples with four or less days. Finally, the electronic nose monitored the shelf-life ripening of apples. A good correlation between electronic nose signals and firmness, starch index, and acidity parameters was found. These results prove that electronic noses have the potential of becoming a reliable instrument to assess fruit ripeness.     

Convergent, self-encoded bead sensor arrays in the design of an artificial nose.

We report a new approach to designing an artificial nose based on high-density optical arrays that directly incorporate a number of structural and operational features of the olfactory system. The arrays are comprised of thousands of microsphere (bead) sensors, each belonging to a discrete class, randomly dispersed across the face of an etched optical imaging fiber. Beads are recognized and classified after array assembly by their unique, "self-encoded" response pattern to a selected vapor pulse. The high degree of redundancy built into the array parallels that found in nature and affords new opportunities for chemical-sensor signal amplification. Since each bead is independently addressable through its own light channel, it is possible to combine responses from same-type beads randomly distributed throughout the array in a manner reminiscent of the sensory-neuron convergence observed in the mammalian olfactory system. Signal-to-noise improvements of approximately n1/2 have been achieved using this method.     

A colorimetric array of metalloporphyrin derivatives for the detection of volatile organic compounds

Spin-coated layers of two metalloporphyrin derivatives (2-nitro-5,10,15,20-tetraphenylporphyrinato) zinc (II) (ZnTPP-NO₂) and (2,3-dioxo-5,10,15,20-tetraphenylporphyrinato) zinc (II) (ZnTPPO) have been used as sensing materials for the detection of volatile organic compounds (VOCs) in the UV-vis spectral range. An optical characterization in controlled atmosphere has been carried out by acquiring the absorption spectra of the two metalloporphyrin derivatives both in dry air and in the presence of different VOCs. The results of the optical VOCs sensing tests show interesting differentiation in the optical responses depending on the metal ion and the peripheral substituent of the macromolecules. A 2 × 1 colorimetric array detection of VOCs has been achieved using ZnTPP-NO₂ and ZnTPPO as immobilized on a reverse phase silica gel. The gas molecules were detected by the colorimetric array and the obtained responsive images were decomposed to RGB color components using a CMOS image sensor. RGB color components change patterns obtained from the colorimetric array give a good identification of VOCs. Morphological characterization of the sensing layer was also performed by scanning electron microscopy.     

High-brightness mid-infrared photonic-crystal distributed-feedback lasers

The far-field emission and spectral characteristics of a photonic-crystal distributed-feedback (PCDFB) laser with an antimonide type-II ‘W’ active region were studied theoretically and experimentally. A 2nd-order grating was defined on a 2D rectangular lattice tilted by 20° relative to the facet normal, using optical lithography and dry etching. For pulsed optical pumping, the emission line centred on λ = 4.6–4.7 μm was up to an order of magnitude narrower (7–10 nm) than those of Fabry-Pérot and angled-grating DFB (α-DFB) lasers fabricated from the same wafer. The PCDFB beam quality was also substantially enhanced, for example, by a factor of 5 compared to the α-DFB at a pump-stripe width of 200 μm. Theoretical simulations based on a self-consistent time-domain Fourier-transform simulation confirm significant advantages of the PCDFB laser over an α-DFB geometry.     

Crystallization of carbon fibre reinforced polypropylene

Polypropylene (PP) was compounded with carbon fibre of various contents (0, 5, 10, 15, 20 vol%) using a single and a twin screw extruder. The influence of both the carbon fibre content and the compounding method on the thermal behaviour and characteristics of crystallization was studied using differential scanning calorimetry (DSC), polarizing optical microscopy and scanning electron microscopy (SEM). Melting and crystallization temperatures increased with the amount of carbon fibre. Isothermal crystallization was observed using DSC and it was found that crystallization was accelerated by the presence of carbon fibres. Using polarizing optical microscopy, it was found that the nucleation of polypropylene started at the crossing point of two or more fibres.     

Characteristics of an artificial graphite anode material for rapid charging: manufactured with different coke particle sizes

An artificial graphite anode material (10–15 μm) is produced using coke at two sizes (10–15 μm, 2–5 μm) and the electrochemical properties are compared and discussed. We produce and measure an artificial graphite anode material using coke with a particle size of 10–15 μm, limited lithium ion insertion–desorption pathways, increased migration pathways, and low-speed charge–discharge characteristics. When a block is manufactured using coke at a particle size of 2–5 μm and an anode material is created with a particle size of 10–15 μm, voids capable of storing lithium ions between the coke particles form inside the anode material. These spaces are utilized and the capacity was measured. In addition, the lithium ion insertion-deintercalation path and lithium ion diffusion distance are controlled and the high-speed discharge properties were measured (78.3%) at low temperatures (5C/0.1C, ? 10 °C). At the same time, the high specific surface area due to the small size of the coke was controlled by the binder pitch used in the block, leading to excellent initial efficiency performance.

     

Microstructure and optical properties of Al2O3/ZrO2 nano multilayer thin films prepared by pulsed laser deposition

The aluminium oxide/zirconium oxide (Al₂O₃/ZrO₂) nanolaminate thin films (5/20 nm with 4 bilayers, 5/15 nm with 5 bilayers and 5/10 nm with 7 bilayers) were deposited on Si (100) and quartz substrates at an optimized oxygen partial pressure of 3 × 10⁻² mbar at room temperature using pulsed laser deposition. The multilayer films were characterized using X-ray diffraction, X-ray reflectivity, Atomic force microscopy and UV–Visible spectroscopy. The X-ray diffraction studies showed amorphous nature for 5/20 nm film, whereas 5/15 nm and 5/10 nm multilayers showed only tetragonal zirconia at room temperature. X-ray reflectivity studies showed the Kiessig fringes and Bragg peaks, indicating the well defined formation of individual layers and bilayer periodicity in the multilayer films. The AFM studies showed the RMS roughness values of 0.7 nm, 0.9 nm and 1.1 nm for 5/10 nm, 5/15 nm and 5/20 nm multilayers respectively. The optical performance of the combined Al₂O₃/ZrO₂ nanolaminates showed that the refractive indices of the films increased from 1.75 to 1.99 with the decrease of ZrO₂ layer thickness from 20 to 10 nm.     

Silver-island films as substrates for enhanced Raman scattering: effect of deposition rate on intensity.

The relationship between surface-enhanced resonance Raman scattering (SERRS) intensity and the rate of deposition during silver-island film preparation was examined, using zinc tetraphenylporphine (ZnTPP) as the adsorbate. The effect of the deposition rate on the optical properties of the films at specific wavelengths was also analyzed. The data show that the film extinction (the term extinction is used rather than absorption because the spectra have not been corrected for reflection or scattering losses) increases exponentially at 514 and 458 nm as the deposition rate is decreased. SERRS intensities also increase exponentially at these two excitation wavelengths with a decrease in the deposition rate. The optical density is linearly related to the SERRS intensity, and maximal enhancement is observed for films with the greatest extinction at these excitation wavelengths. In contrast, neither the extinction at 406 nm nor the SERRS scattering intensities resulting from excitation at this wavelength differ substantially. The surface-enhanced Raman scattering (SERS) intensity and the electronic spectra of 4,4'-bipyridine (BP) adsorbed onto silver films as a function deposition rate were also examined. The behavior of the nonresonantly enhanced BP is comparable to that of the resonantly enhanced ZnTPP samples. The effects of the surface morphology, as determined from transmission electron micrographs of the films at various deposition rates, on the corresponding electronic spectra and SERS/SERRS spectra are described.     

Microwave-assisted generation of standard gas mixtures

Microwave heating was employed for preparation of the standard gas of volatile organic compounds (VOCs) and semivolatile organic compounds (semi-VOCs) by using a 1000 W commercial domestic microwave oven and 1 L gas-sampling bulbs. The VOCs investigated were benzene, chloroform, 1,3-dichlorobenzene, tetrachloroethylene, toluene, and 1,1,2-trichloroethane, and the semi-VOCs used were the polychlorinated biphenyls (PCBs) PCB 1016 and PCB 1248. Since these weakly or nonpolar molecules are very poor absorbers of microwave energy, an appropriate amount of water was introduced to accept microwave radiation and act as the thermal source to accelerate their evaporation. The glass bulb may also contribute thermal energy to the VOCs/semi-VOCs by accepting microwave energy to a small degree. For 0.5 microL of liquid VOCs on 10 mg of glass wool, it was shown that 15 microL of H2O and 60 s of microwave heating yielded a very efficient evaporation [97.2-106.4%, compared with a classic method (Muller, L; Gorecki, T.; Pawliszyn, J. Fresenius' J. Anal. Chem. 1999, 364, 610-616)]. For 1 microL of PCB solution (1000 microg/mL in hexane), 15 microL of H2O and 90 s of microwave heating also provided a complete evaporation. The addition of water was particularly significant for microwave-assisted evaporation of PCBs because semi-VOCs are much more difficult to evaporate than VOCs. This developed microwave technique proved to be quite simple, powerful, rapid, accurate, and safe for the preparation of VOC/semi-VOC standard gas. Solid- phase microextraction combined with gas chromatography was used for the gas analysis.     

An optoelectronic nose: "seeing" smells by means of colorimetric sensor arrays

A new approach to general sensors for odors and volatile organic compounds (VOCs) using thin films of chemically responsive dyes as a colorimetric sensor array is described. This optoelectronic "nose," by using an array of multiple dyes whose colors change based on the full range of intermolecular interactions, provides enormous discriminatory power among odorants in a simple device that can be easily digitally imaged. High sensitivities (ppb) have been demonstrated for the detection of biologically important analytes such as amines, carboxylic acids, and thiols. By the proper choice of dyes and substrate, the array can be made essentially nonresponsive to changes in humidity.     

Microbiological quality of aseptic packaging and the effect of pin-holes on sterility of aseptic products

The microbiological quality of aseptic packaging (the board for long-life milk) was good and viable counts were under 50cfu per package. The percentage of defective samples that had viable microorganisms after 7–14 days of pre-incubation was 54% (one pin-hole) and 54% (ten pin-holes) for coffee beverage packed in board B and 17% (one pin-hole) and 17% (10 pin-holes) for milk packed in board A. The differences in the percentage of defective samples between milk and coffee beverage and between board A and board B were significant, but no difference was observed between one and ten pin-holes. An increase in fat content or a decrease in protein content increased the permeation of milk components through the pin-holes. The microorganisms isolated from the products with artificial pin-holes were f actobacillus sp. (52%), Streptococcus sp. (48 %) and Enterobacteriaceae (34%).     

Synthesis and physicochemical characterizations of nanostructured Pt/Al2O3-CeO2 catalysts for total oxidation of VOCs

Pt/Al(2)O(3)-CeO(2) nanocatalysts with Pt loading of 1% and ceria loading of 10, 20 and 30% were successfully prepared via wet impregnation method to be utilized in catalytic oxidation of BTX. The nanocatalysts were characterized using XRD, FESEM, TEM, N(2) adsorption, FTIR and TPR-H(2) techniques. The XRD patterns confirmed the formation of cerium oxide as the crystalline phase on alumina with the average crystallite size of 8.1-8.7 nm, derived by Scherrer equation. FESEM images confirmed that these nanocatalysts had ceria particles in nano-ranges. TEM analysis showed that platinum particles were fairly well dispersed on Al(2)O(3)-CeO(2) with an average size of 5-20 nm. BET surface area presented large surface area for nanocatalysts. TPR patterns showed that by adding 1% platinum to support, the reducibility is highly increased. These patterns also revealed the promoting effect of ceria on reducibility of Pt and Al(2)O(3). The results of toluene oxidation indicated that the synthesized nanocatalysts were highly active and able to remove nearly 100% of toluene and xylene and about 85% of benzene as representative VOCs. The presence of nanoparticles along with good characteristics of the synthesized nanocatalysts presented them as highly efficient materials for catalytic oxidation of VOCs.     

Accurate and rapid optical characterization of an anisotropic guided structure based on a neural method

Optimal performances of integrated optical devices are obtained by the use of an accurate and reliable characterization method. The parameters of interest, i.e., optical indices and thickness of the waveguide structure, are calculated from effective indices by means of an inversion procedure. We demonstrate how an artificial neural network can achieve such a process. The artificial neural network used is a multilayer perceptron. The first result concerns a simulated anisotropic waveguide. The accuracy in the determination of optical indices and waveguide thickness is 5 x 10(-5) and 4 nm, respectively. Then an experimental application on a silica-titania thin film is performed. In addition, effective indices are measured by m-lines spectroscopy. Finally, a comparison with a classical optimization algorithm demonstrates the robustness of the neural method.     

Cavity ringdown detection of losses in thin films in the telecommunication wavelength window

The method of cavity ringdown spectroscopy (when a tunable pulsed optical parametric oscillator was used) was extended for the loss evaluation in thin films (2-20-mum thickness). The technique was applied in two key telecommunication wavelength ranges of 1260-1330 and 1480-1650 nm. The measurement sensitivity was determined to be 50 ppm (5 x 10(-5)). The results for polymer films are in close correlation with conventional spectrophotometric data and propagation loss for planar waveguides. Films of greater thickness and better optical quality are expected to provide an even higher loss resolution.