The use of a piezoelectric crystal to determine sulphur dioxide in gases

This work describes the detection of sulphur dioxide with a piezoelectric crystal, at a resonance frequency of 9 MHz, covered with an active coating containing trioctylmethylammonium dichromate which irreversibly bonds sulphur dioxide in an oxidation-reduction reaction. The decrease in the vibration frequency of the crystal is directly proportional to the mass of sulphur dioxide bonded. The detector works on the basis of integration of the decrease in the frequency over a given time interval (at least 10 min), resulting in a high measuring sensitivity. The calibration curve is linear for sulphur dioxide concentrations from a few to 500 microg/m(3). Partial exhaustion of the capacity of the active coating, occurring after prolonged use, appears as nonlinearity of the detector concentration response. This nonlinearity can be compensated by a simple mathematical correction that permits the lifetime of the active coating to be lengthened by several orders of magnitude.     

Relative humidity measurement using a coated piezoelectric quartz crystal sensor

Miniature piezoelectric quartz crystals, which are manufactured cheaply by photolithographic and chemical etching techniques, have a high mass sensitivity. AT-cut 10 MHz quartz crystals have been coated with four materials and used to measure the relative humidity in various gases. The coated crystal is used as a resonator in an oscillator circuit, the frequency of which varies as a function of the change in mass. Characteristics that determine the usefulness of the coatings, such as sensitivity, response linearity, response time, selectivity, hysteresis and ageing, were evaluated. The detector has potential for use as a hygrometer, provided that the appropriate coating is chosen for a specific application.     

Determination of bioelectrochemical parameters of red cell using a piezoelectric crystal sensor

Based on the impedance behavior of red cell at high frequency, the frequency response of series piezoelectric crystal sensor in the red cell suspension was derived and verified experimentally. A method of using piezoelectric crystal sensor to determine the conductivity of the interior of the cell was proposed. The experimental results show that the mean conductivity of rabbit red cell cytoplasm was 0.269 S/m and the mean shape factor of red cell was 2.05.     

A coated piezoelectric crystal sensor for acetic acid vapour determination

A novel sensor for acetic acid vapour determination is proposed. This sensor is based on a piezoelectric crystal covered with a film of diethylenetriamine. For the sensor development a system of our own design-consisting of testing chamber, oscillator circuit and measure instruments-has been employed. The sensor shows its activity to the acetic acid vapours for more than 60 days. The selectivity is adequate although some vapours interfere: hydrochloric acid, formic acid, formaldehyde, tributyl phosphate, chloroform, chlorobenzene, acetone and isobutylmethylketone. The sensor described can be applied to detect acetic acid vapours in the presence of other vapours: acetonitrile, acrolein, benzene, n-hexane, ethanol, propanol, n-butyl acetate, isopropyl ether, isoamyl alcohol, ethyl ether, methylene chloride, carbon tetrachloride and toluene. The major advantages of the proposed sensor over other existing techniques are its simplicity, reduced cost and capacity for use in situ.     

A quartz crystal piezoelectric device for monitoring organic gaseous pollutants

The behavioural pattern of a coated piezoelectric crystal detector for the monitoring or traces of atmospheric gases partitioning into the coating on the crystal is predicted. The effects of variable experimental parameters such as detector cell volume, gas flow rate, analyte concentration, distribution and size of detector coating are described and the intrinsic properties of the coating material are discussed. The behaviour of model systems set up arbitrarily for traces of chloroform and toluene is in agreement with expectations.     

Silver-selective sensor using an electrode-separated piezoelectric quartz crystal modified with a chitosan derivative.

Silver(I) adsorbed selectively onto a quartz plate modified with N-(2-pyridylmethyl)chitosan in an ammonium chloride buffer solution containing EDTA, and the frequency of the quartz plate increased. It was supposed that the increasing frequency was caused by the desorption of adsorbed water on the chitosan derivative, which was induced from the reaction of silver(I) with the chitosan derivative. The concentration of the buffer, pH, temperature, conductivity and eluent affected the frequency shift resulting from the adsorption of silver(I). The frequency decreased at a conductivity lower than 2.2 mS/cm, and increased with increasing conductivity above this value. The frequency shifts caused by the adsorption of silver(I) were proportional to the concentration over the range 10-80 nM of silver(I), and the correlation coefficient was 0.9969. The detection limit and the relative standard deviation at 50 nM for five times were 6 nM and 3.4%, respectively. The proposed method was simple while showing higher sensitivity and selectivity.     

Identification of combustible material with piezoelectric crystal sensor array using pattern-recognition techniques

A promising way of increasing the selectivity and sensitivity of gas sensors is to treat the signals from a number of different gas sensors with pattern recognition (PR) method. A gas sensor array with seven piezoelectric crystals each coated with a different partially selective coating material was constructed to identify four kinds of combustible materials which generate smoke containing different components. The signals from the sensors were analyzed with both conventional multivariate analysis, stepwise discriminant analysis (SDA), and artificial neural networks (ANN) models. The results show that the predictions were even better with ANN models. In our experiment, we have reported a new method for training data selection, 'training set stepwise expending method' to solve the problem that the network can not converge at the beginning of the training. We also discussed how the parameters of neural networks, learning rate eta, momentum term alpha and few bad training data affect the performance of neural networks.     

模式识别用于压电晶体传感器阵列定量检测有机蒸汽混合物

本文应用9个压电晶体组成传感器阵列, 每片晶体上分别涂有不同种类的冠醚衍生物, 用它来定量检测二元及三元有机蒸汽混合物, 在数据处理中比较了两种模式识别方法---偏最小二乘法(PLS)和人工神经网络法(ANN), 实验证明, ANN法在预测准确度上明显优于PLS法, 本文还讨论了解决神经网络训练过拟合现象的方法。     

Enantioselective piezoelectric quartz crystal sensor for d-methamphetamine based on a molecularly imprinted polymer

A piezoelectric quartz crystal (PQC) sensor based on a molecularly imprinted polymer (MIP) has been developed for enantioselective and quantitative analysis of d-(+)-methamphetamine (d(+)-MA). The sensor was produced by bulk polymerization and the resulting MIP was then coated on the gold electrode of an AT-cut quartz crystal. Conditions such as volume of polymer coating, curing time, type of PQC, baseline solvent, pH, and buffer type were found to affect the sensor response and were therefore optimized. The PQC-MIP gave a stable response to different concentrations of d(+)-MA standard solutions (response time = 10 to 100 s) with good repeatability (RSD = 0.03 to 3.09%; n = 3), good reproducibility (RSD = 3.55%; n = 5), and good reversibility (RSD = 0.36%; n = 3). The linear range of the sensor covered five orders of magnitude of analyte concentration, ranging from 10⁻⁵ to 10⁻¹ μg mL⁻¹, and the limit of detection was calculated as 11.9 pg d(+)-MA mL⁻¹ . The sensor had a highly enantioselective response to d(+)-MA compared with its response to l(−)-MA, racemic MA, and phentermine. The developed sensor was validated by applying it to human urine samples from drug-free individuals spiked with standard d(+)-MA and from a confirmed MA user. Use of the standard addition method (SAM) and samples spiked with d(+)-MA at levels ranging from 1 × 10⁻³ to 1 × 10⁻² μg mL⁻¹ showed recovery was good (95.3 to 110.9%).     

High sensitive piezoelectric quartz crystal liquid density sensor

<正>We report for the first time a cleavage phenomenon in the resonant peak of a piezoelectric quartz crystal(PQC) in liquid phase.In the presence of a strong longitudinal wave effect,an additional resonant peak appears in the conductance-frequency curve.With gradually increasing liquid density,the additional peak moves from low to high frequency region then disappears.The frequency of the additional resonant peak is sensitive to the change in liquid density.The frequency shift of the additional peak is linear with the liquid density in a given range.For a 5 MHz PQC with a reflection distance of 16 mm for longitudinal wave,the sensitivity to liquid density is 2.61×10~6 Hz g~(-1) cm~3.The overlap between the primary resonant peak and the additional resonant peak causes a decrease in the intensity of the former and an increase in the intensity of the latter.In a combined impedance analysis method,the changes in surface mass loading,density and viscosity of the liquid were monitored simultaneously by a PQC sensor.     

Ratiometric fluorescence-based dissolved carbon dioxide sensor for use in environmental monitoring applications

The focus of this work is on the development and characterisation of a fluorescence-based ratiometric sol–gel-derived dissolved carbon dioxide (dCO₂) sensor for use in environmental monitoring applications. Fluorescence-based dCO₂ sensors are attractive as they facilitate the development of portable and low-cost systems that can be easily deployed outside the laboratory environment. The sensor developed for this work exploits a pH fluorescent dye 1-hydroxypyrene-3,6,8-trisulfonic acid, ion-paired with cetyltrimethylammonium bromide (HPTS-IP), which has been entrapped in a hybrid sol–gel-based matrix derived from n-propyltriethoxysilane along with the liphophilic organic base. The sensor spot deposited on a cover slip has been interrogated with a robust, ratiometric optical probe that combines effective fluorescence excitation and detection and thus facilitates the production of a highly sensitive sensor system using low-cost optoelectronic components. The probe design involves the use of dual-LED excitation in order to facilitate ratiometric operation and uses a silicon PIN photodiode. HPTS-IP exhibits two pH-dependent changes in excitation bands, which allows for dual excitation ratiometric detection as an indirect measure of the dCO₂. Such measurements are insensitive to changes in dye concentration, leaching and photobleaching of the fluorophore and instrument fluctuations unlike unreferenced fluorescence intensity measurements. The performance of the sensor system is characterised by a high degree of repeatability, reversibility and stability. Calculated limit of detection for the sensor was 35 ppb. The sensor probe was used to monitor dCO₂ levels in a laboratory-based aquatic habitat, and the expected diurnal pattern was clearly visible. The influence of temperature, biofouling and photobleaching on sensor performance has been also investigated.     

A micro and ultramicro test for sulphur dioxide

Summary A very sensitive reaction for sulphur dioxide is proposed for use on the micro and ultramicro scales. Interference by cyanide, sulphide, and thiosulphate can be prevented. Sulphite can be distinguished from bisulphite.

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Zusammenfassung Eine sehr empfindliche Reaktion zum Nachweis von Schwefeldioxid im Mikro- und Ultramikromaßstab wurde vorgeschlagen. Störungen durch Cyanid, Sulfid und Thiosulfat lassen sich vermeiden. Sulfit kann von Hydrogensulfit unterschieden werden.

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Résumé On propose une réaction très sensible pour la recherche de l'anhydride sulfureux à l'échelle micro et ultramicro. On peut éviter l'interférence avec les cyanures, les sulfures et les thiosulfates. On peut distinguer les sulfites des bisulfites.

     

Fibre-optic fluorosensor for sulphur dioxide

A fibre-optic sensor for continuous measurement of sulphur dioxide is described. It is based on the dynamic quenching of the fluorescence of a polynuclear aromatic hydrocarbon [benzo(b)fluoranthene] which is immobilized in silicone polymer. Sulphur dioxide is shown to be an efficient quencher; Stern-Volmer graphs are given which describe the relation between SO₂ concentration and relative fluorescence. Detection limits are about 0.01% (v/v) SO₂ in air; the useful range is from 0.01–6% (v/v). Other gases likely to occur in air were found to be inert, except for oxygen which also acts as a dynamic quencher. Its interference is negligible for SO₂ levels below 6% in air at constant oxygen pressure, because the quenching efficiency of SO₂ is about 26 times higher than that of oxygen. For varying oxygen levels, a two-sensor technique is suggested.     

p-nitroaniline as a colorimetric reagent for sulphur dioxide

The merits of the nitroanilines as an alternative to p-rosaniline and fuchsine in the colorimetric determination of sulphur dioxide and sulphites have been investigated. p-Nitroaniline was found to be a stable, well-defined reagent, with which sulphur dioxide can be determined to a high level of precision, the sensitivity being of the same order of magnitude as for p-rosaniline.     

A sulphur dioxide adduct of triphenylindium

The product from the reaction of triphenylindium with sulphur dioxide, previously considered to be O-benzenesulphinatodiphenylindium(III), is now formulated as the $\text{1}\text{1}$ adduct, Ph₃In(O₂S).     

A coated piezoelectric crystal detector for phosgene

A piezoelectric quartz crystal coated with methyltrioctylphosphonium dimethylphosphate was found to be a good detector for phosgene in air. The coating is sensitive to phosgene in the μg 1^?1 range and has a reasonably long lifetime. The response curve is linear over the concentration range 5–140 μg 1^?1. The coated crystal can be used for more than six weeks without significant loss in sensitivity, provided that high ammonia concentrations are not encountered.     

A model for the absorption of sulphur dioxide into a limestone slurry

A model has been developed for the removal of sulphur dioxide from flue gas by absorption into a limestone slurry. The flue gas desulphurization unit consists of an absorber tower and an oxidation tank. Flue gas enters the absorption tower at the bottom and meets the limestone slurry. There are five important chemical reactions with a finite rate. The rate-limiting reactions are limestone dissolution, calcium sulphite precipitation and dissolution, gypsum precipitation, sulphur dioxide absorption and sulphite oxidation in the slurry. The model also accounts for the presence of chloride ions, magnesium ions and organic acids in the limestone slurry. The absorption rates of sulphur dioxide and carbon dioxide in the tower are calculated according to the two-film model. A non-uniform set of limestone particles is also included in the model. The model was tested against literature data and the agreement between the data and the model was satisfactory. A sensitivity analysis of the desulphurization process was carried out, the inputs to the model were changed and the results from the calculations were compared with the expected results. The response to the change in the inputs agreed well with the expected results.     

A method for determination of hydrogen content in uranium dioxide samples using a semiconductor sensor

Measurements of H₂ content in UO₂ samples have been carried out on the basis of a technique using a hydrogen sensitive sensor of Pd–SiO₂–Si. A specially made set up allowed to register the amount of H₂ in UO₂ samples up to 10^–10, g. Measured H₂ content in investigated samples was 8, 32 and 102 ppb (mass) at 400, 800 and 1600°C, respectively.     

Kinetic determination of organic vapor mixtures with single piezoelectric quartz crystal sensor using artificial neural networks

A single piezoelectric quartz crystal coated with one kind of crown ether was applied to the simultaneous determination of binary acid and amine vapor mixtures. From the adsorption and desorption curves of analytes, which were somewhat different in shape, frequency shifts from ten time windows were taken as inputs for artificial neural networks (ANN). Prediction results were satisfactory for ANN in both sample sets. The average relative errors, for formic acid and acrylic acid were 5%, for n-butylamine and aniline, they were 3% with ANN respectively. The effects of number of neurons in the hidden layer of ANN on the performance of the network are also discussed.