Configuration and optimization of semiconductor gas sensors as gas chromatographic detectors

A tin oxide, gas-sensitive semiconductor sensor was configured as a gas chromatographic detector and its performance was optimized. Two sensor housings were compared but little difference was found in performance. The flow rate and temperature of the column and the internal heater voltage of the sensor affected both the sensitivity and peak shape. The temperature of the sensor surface was the most critical parameter. Optimal conditions for the gas chromatographic detection of a mixture of alkanes (C₁–C₅) and hydrogen were identified by using the simplex technique. The detection limit for hydrogen was improved by a factor of five to 20 ppb (v/v), illustrating the value of optimization and the excellent sensitivity of the detector. It is concluded that semiconductor gas sensors offer significant advantages as gas chromatographic detectors for the determination of reducing gases.     

Detection of organophosphorus compounds with semiconductor gas sensors using adsorption preconcentration

Effective concentrators for organophosphorus compounds, highly sensitive semiconductor sensors, and a prototype of a detector for specific determination of low concentrations of organophosphorus compounds in air were developed. It is preferable to use as concentrating sorbents silicates with low density of surface hydroxy groups, ensuring high degree of recovery of the target compound by thermal desorption without its decomposition. SnO₂ modified with RuO₂ is the most sensitive sensor material.     

Fullerene‐Crown Ether Coated Piezoelectric Crystal Liquid Chromatographic Detector for Metal Ions and Polar/Nonpolar Organic Molecules

Fullerene(C60)‐dibenzo‐16‐crown‐5‐oxyacetic acid (DBI6C5‐OCH₂‐COOC₆₀) was prepared and applied as the coating material on piezoelectric quartz crystals for detection of various metal ions and polar/nonpolar organic molecules. The C60‐crown ether‐coated piezoelectric crystal sensor with a home‐made computer interface for signal acquisition and data processing was applied as an ion chromatographic (IC) detector for various metal ions, e.g., alkali metal, alkaline earth metal and transition‐metal ions. The piezoelectric detector exhibited quite good sensitivity of 10⁴ ～ 10⁶ Hz/M and good detection limit of 10^?3 ～ 10^?4 M for these metal ions. The C60‐crown ether piezoelectric detector compared well with the commercial conductivity detector conventionally used for metal ions. The ionic size and ionic charge seemed to have significant effect on the frequency response of the piezoelectric detector. The C60‐crown ether coated piezoelectric crystal sensor was also employed as a high performance liquid chromatographic (HPLC) detector for various polar organic molecules with frequency responses in the order: amines > carboxylic acids > alcohols > ketones. Furthermore, nonpolar organic molecules, e.g., n‐hexane, 1‐hexene and 1‐hexyne, were also detected with this piezoelectric crystal detector. The frequency responses of the piezoelectric crystal detector for these nonpolar organic molecules were in the following order: alkynes > alkenes > alkanes. The effects of solvents and flow rate on the frequency responses of the piezoelectric crystal detector were investigated. The C60‐crown ether coated piezoelectric crystal detector also showed short response time (< 1 min.) and good reproducibility.     

手持数显式乙醇测试仪的研制

用半导体氧化物气敏传感器及微控制器８９Ｃ５１研制的手持式数显乙醇测试仪能对气体中的乙醇含量进行现场实时检测。该文从硬件、软件两方面介绍其优化电路和抗干扰技术。该仪器具有灵敏度高、选择性好、抗干扰能力强等特点,可用于交通安全检测、酒厂和食品工厂发酵监控。     

Liquid Chromatographic Determination of Glutathione with Electrochemically Pretreated Glassy Carbon Electrode

Abstract

Simple electrochemical pretreatment of a glassy carbon electrode used as a working electrode in an electrochemical detector has been found to enhance the analytical capability of the detector for the determination of glutathione by lowering the required operating potential as well as by increasing the maximum oxidation current. The electrochemical pretreatment of the electrode was made in a 0.2 M phosphate buffer (KH₂PO₄-KOH, pH 6.5) at +1.9 V vs. Ag/AgCl for 2 min. The minimum detectable quantity of glutathione was found to be about 100 pg, when eluate from a reversed-phase column was amperometrically monitored by aid of the detector with the electrochemically pretreated glassy carbon electrode. Amount of glutathione in a rat lens was determined by the chromatographic method developed in this study.     

Development of a tandem thin-layer chromatography-high-performance liquid chromatography method for the identification and determination of corticosteroids in cosmetic products

A solid-phase extraction clean-up and and a liquid chromatographic method with ultraviolet detection were developed for the analysis of 51 corticosteroids in cosmetic samples in order to screen commercial samples for the presence of undeclared synthetic corticosteroids. A thin-layer chromatographic analysis was carried out on silica gel plates, using different eluants and detection reagents. When such a preliminary chromatographic separation gave some indications about the presence of steroid compounds, the methanol extracts from real samples were applied to a solid-phase extraction C₁₈ cartridge, and the analytes eluted with ethyl ether. The high-performance liquid chromatographic separation was then carried out for the identification and determination of the analytes using a Purospher RP-18 column, an isocratic or a gradient elution with a mixture acetonitrile-water and a photodiode-array detector. The accuracy of the method was determined by spiking experiments on home-made cosmetic samples. The analytical recoveries were satisfactory.     

Characterization of a TeflonÒcoated semiconductor detector flow cell for monitoring of pertechnetate in groundwater

Summary A new flow-cell detection system using a Teflon^òcoated passivated ion-implanted planar silicon semiconductor detector was developed for detection of pertechnetate (⁹⁹TcO₄^-E_b     

A Simple Design to Realize Micro-column Separation by Conventional Analytical HPLC

The conventional analytical HPLC was successfully developed for micro‐column separation by using a simple eluate splitting system, self‐preparation of packing column and on‐capillary column detector in our laboratory. Porous inlet frit in fused silica capillary was rapidly prepared by sintering stainless steel powders under 500 meshes for about 20 s. The use of such frits or metal meshes in capillary to retain C₁₈ particles of chromatographic packing was demonstrated to be stable and specially robust with continuous packing and long chromatographic runs. Furthermore, the chromatographic behavior was detailedly evaluated by changing the flow rate and the percentage of mobile phase using the prepared capillary column. Under the optimal experimental conditions, baseline separation of the model analytes including thiourea, benzene, toluene, ethylbenzene was obtained with a high column efficiency near 70000N (plates/m) by the developed capillary‐HPLC.     

Distinguishing Among Gases with a Semiconductor Sensor Depending on the Frequency Modulation of a Cyclic Temperature

A gas‐sensing system based on a dynamic nonlinear response shows enhanced selectivity of the sensor response toward sample vapors. A cyclic temperature composed of fundamental and 1.5‐th harmonics was applied to a SnO₂ semiconductor gas sensor and the resulting conductance of the sensor was evaluated by a polynomial approximation. The dynamic nonlinear responses to the samples were further characterized by the addition of 1.5‐th harmonic perturbation as a frequency modulation. These characteristic sensor responses under frequency modulation were considered theoretically based on a reaction‐diffusion model for the semiconductor surface.     

Piezoelectric Crystal Membrane Chemical Sensors Based on Fullerene and Macrocyclic Polyethers

Various reusable and sensitive piezoelectric (PZ) quartz crystal membrane sensors with home‐made computer interfaces for signal acquisition and data processing were developed to detect organic/inorganic vapors and organic/inorganic/biologic species in solutions, respectively. Fullerene(C60), fullerene derivatives and artificial macrocyclic polyethers, e.g., crown ethers and cryptands, were synthesized and applied as coating materials on quartz crystals of the PZ crystal sensors. The oscillating frequency of the quartz crystal decreased due to the adsorption of organic or inorganic species onto coating material molecules on the crystal surface. The crown ether‐coated PZ crystal gas detector exhibited high sensitivity with a frequency shift range of 10–340 Hz/(mg/L) for polar organic gases, a short response time (< 2.0 min.), good selectivity, and good reproducibility. The Ag(I)/crptand22 and Ru(III) / crptand22 coated PZ gas detectors were also prepared for nonpolar organic vapors, e.g., alkynes and alkenes. The frequency shifts of the nonpolar PZ sensors were in the order: alkynes > alkenes > alkanes. A Ti(IV)/Cryptand22‐coated PZ crystal sensor was also developed to detect the inorganic air pollutants, e.g., CO and NO₂. A piezoelectric gas sensor for both polar/nonpolar organic vapors based on C60‐cryptand22 was also prepared. The cryptand22‐coated PZ gas sensor was also employed as a GC detector for organic molecules. The cryptand22‐coated piezoelectric GC detectors compared well with the commercial thermal conductivity detector (TCD). The interaction between fullerene C60 and organic molecules was studied with a fullerene coated PZ gas detector. A multi‐channel PZ organic gas detector with PCA(Principal Component Analysis) and BPN (Back Propagation Neural) analysis methods was developed. Various liquid piezoelectric crystal sensors based on long‐chain macrocyclic polyethers, e.g., C₁₀H₂₁‐dibenzo‐16‐crown‐5, C₁₈H₃₇‐benzo‐15‐crown‐5, (C₁₇CO)₂‐cyptand22 and fullerene derivatives, e.g., C60‐NH‐cryptand22 and dibenzo‐16‐crown‐5‐C60, were also developed as HPLC detectors for metal ions, anions, and various organic compounds in solutions. The sensitive and highly selective PZ bio‐sensors based on enzymes, polyvinylaldehyde, polycinnaldehyde‐C60 and C60‐cryptand22 were developed to detect various biologic species, e.g., proteins, glucose, and urea. A quite sensitive EQCM (Electrochemical Quartz Crystal Micro‐balance) detection system was also developed for detection of trace heavy metal ions.     

Comparative study of microwave-induced plasma atomic emission spectrometry and atomic fluorescence spectrometry as gas-chromatographic detectors for the determination of methylmercury in biological samples

In order to attain a lower detection limit with the HS GC MIP analytical method (Head-Space Gas Chromatography with Microwave-Induced Plasma detection) recently developed for the analysis of methylmercury in biological samples, the quarter-wave Evenson-type cavity used until now was replaced by a TM₀₁₀ Beenakker-type cavity, which was used with both argon and helium as carrier gas. With an argon plasma, an eightfold increase in detection limit was gained compared with the argon plasma sustained by the Evenson cavity, while only a four-fold increase was gained with the helium plasma. In a second step of the study, the MIP detector was replaced by an AFS (atomic fluorescence) detector (CVAFS Model-2, Brooks Rand Ltd, Seattle, USA). With this AFS detector a detection limit of 1 ng methyl mercury per g biological tissue could be reached; i.e. measurements were 40 times more sensitive than those using the Evenson cavity. This detector has some other advantages compared with MIP detection: it is less expensive and easier to manipulate, while the same precision and accuracy are obtained. The use of AFS as detector in the headspace gas chromatographic system is therefore an important improvement for the analysis of methyl-mercury in biological samples.     

Determination of butyl- and phenyltin compounds in human urine by HS-SPME after derivatization with tetraethylborate and subsequent determination by capillary GC with microwave-induced plasma atomic emission and mass spectrometric detection

A headspace solid-phase micro-extraction (HS-SPME) method was developed and optimized for gas chromatographic separation and determination of commonly found organotin compounds in human urine after potential exposure. Butyl- and phenyltin compounds were in situ derivatized to ethylated derivatives by sodium tetraethylborate (NaBEt₄) directly in the urine matrix. The relevant parameters affecting the yield of the SPME procedure were examined using tetrabutyltin as internal standard. The method was optimized for direct use in the analysis of undiluted human urine samples and mono-, di- and tri-substituted butyl- and phenyltin compounds could be determined after a 15-min headspace extraction time at room temperature. The selectivity of the microwave-induced plasma atomic emission detector (MIP-AED) as an element specific detector in combination with the relatively selective sample preparation technique of HS-SPME allowed the interference-free detection of the organotin compounds in all cases. A quadrupole mass spectrometer was used in parallel experiments as a detector for the confirmation of the identity molecular structure of the eluted compounds. The performance characteristics of the developed method are given for the determination of mixtures of these compounds. Finally the proposed method was applied to the analysis of several human urine samples.     

A radiofrequency plasma in a polydimethylsiloxane (PDMS) microchip

This is the first report of an analytical plasma in a polymer (polydimethylsiloxane, PDMS) microchip. The plasma channel has dimensions 2 mm diameter × 50 mm long, is operated at atmospheric pressure in Ar, 27.12 MHz and 70 W, and is viewed axially through a purged fiber optic cable. CF₄ gas at 0.1% in argon yields mainly C₂ emission bands. This PDMS microchip is manufactured easily, inexpensive, and more tolerant to fluorocarbons than microchip plasmas in silica. Based on these initial results, this PDMS microchip plasma could become useful as a sensor for the fluorocarbon gases emitted in semiconductor process or as a gas chromatography (GC) detector for potential application.     

Improvement of the Detection Limit in Capillary Gas Chromatographic Trace Analysis of C2–C4 Hydrocarbons by Post Column Cryogenic Trapping

A method for the improvement of signal-to-noise ratios and hence detection limits in capillary gas chromatographic trace analysis of C₂–C₄ hydrocarbons was developed. It is based on the post column cryogenic trapping of the separated hydrocarbons using liquid nitrogen as coolant and its fast reinjection into the detector by rapid heating of the capillary tubing used for trapping. The improved signal-to-noise ratios were applied to decrease the sample volume and/or to lower the detection limit 10 to 27 times. The concentrations of these hydrocarbons in an urban air sample determined without and with post column cryogenic trapping were in good agreement but the precision was better when applying the cryogenic trapping mode.     

Simple, stable and sensitive electrogenerated chemiluminescence detector for high-performance liquid chromatography and its application in direct determination of multiple fluoroquinolone residues in milk

A simple, stable and sensitive electrogenerated chemiluminescence (ECL) detector was developed. It was based on tris(2,2-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) immobilized on the surface of a Pt wire with Nepem-105D ion exchange solution. The detector was prepared by inserting a Pt wire with immobilized Ru(bpy)₃²⁺ (working electrode) into a capillary tube, followed by inserting another Pt wire (counter electrode) in this tube and sealing. ECL behavior was investigated using ofloxacin as an analyte. Under optimal conditions, stable ECL intensity was obtained. This detector has been used in HPLC-ECL for the determination of multiple target fluoroquinolone residues in milk. There is no post column reagent addition, which would dilute the analytes, potentially leading to chromatographic band-broadening. The system is very simple with low dead volume, low baseline and background noise, together with high sensitivity and stability. The as-prepared ECL detector, when was used for the determination of ofloxacin, pefloxacin, enrofloxacin and difloxacin in milk, demonstrated adequate sensitivity to allow quantification of trace FQ levels in commercial milk samples. One or more of the target FQ analytes were present at levels above the LOD of the new ECL detector in each and every one of the 22 milk samples analysed.     

A semiconductor sensor based on indium antimonide for the potentiometric analysis of soils from the Chinarev oil and gas field

The ion?salt composition of soils from the Chinarev oil and gas field (West Kazakhstan Region) was studied. A semiconductor sensor based on indium antimonide (InSb) and classical ion-selective electrodes were used to analyze soil extracts. Comparative results of determinations of pH, HCO₃^- Cl^–, and SO₄^2- in soil samples with a semiconductor InSb-electrode and with glass, chloride-selective, and copper-selective electrodes are presented. The use of a single semiconductor sensor for the determination of four components in a soil extract allowed us to reduce the duration of analysis and to unify the determination. Based on the results obtained, the salt composition of soils from the studied field was estimated.     

An integrated low-power thin-film CO gas sensor on silicon

An approach to an integrated semiconductor gas sensor is presented. The major reasons considered for developing a semiconductor oxide gas sensor on silicon are the accurate local temperature control of the sensing area and the low level of the heating power required, together with an appropriate integrated structure. Thermal loss measurements show that the integrated gas sensor can operate up to 400 °C with less than 200 mW heating power. Depending on the deposition conditions, catalyst addition or surface conditioning, the SnO_x thin films are known to have an optimal sensitivity to CO between 250 °C and 400 °C. The sensitivity for CO gas and the response time of the device are presented for sputtered thin films of SnO_x, deposited on top of an isolated resistive heater, separated from silicon by a thin thermally-isolating membrane.     

Application of a New pH‐Sensitive Electrode as a Detector in Flow Injection Potentiometry

A new pH sensitive detector for flow‐through potentiometry was developed on the basis of a graphite/quinhydrone composite electrode. The detector was constructed of two polymethylmethacrylate plates between which a 0.1 mm thick sensitive layer is situated. After drilling a hole through the plates, a ring of the sensitive layer is exposed to the solution stream. A conventional calomel reference electrode was placed downstream following the flow‐through sensor. The response behavior in different electrolyte solutions was investigated. The detector shows a Nernstian behavior for injections of hydrochloric acid into a KCl background solution as well as for the steady state response in concentration step experiments using hydrochloric acid and buffer solutions. The response time is sufficiently short for FIA applications (T₉₅ between 3 and 5.4 s). The influences of flow rate and injection volume on the detector signal are discussed.     

Improving the Selectivity of Semiconductor Gas Sensors Using the Pulse Adsorption Mode

A procedure is proposed for improving the selectivity of semiconductor gas sensors by the pulsed supply of the test gas mixture, with the use of transient spectroscopy for the separation and assessment of component contributions to the response of an electrical conductivity detector. The experimental verification of the procedure showed that the selective determination of several compounds and their mixtures can be performed using the same SnO₂ detector. The simulation of the detector response as the sum of simplest relaxations revealed three types of surface sites on SnO₂ films, with energy levels of 0.97, 1.38, and 1.50 eV. This means that the number of gas components that can be selectively determined with a single detector is limited to three.     

Enhanced CO sensing properties of Pd modified ZnO porous nanosheets

Noble metal is usually used to improve the gas sensing performance of metal oxide semiconductor (MOS) due to its better catalytic properties. In this work, we reported a synthesis of Pd/ZnO nanocomposite by an in situ reduction with ascorbic acid (AA). It was found that Pd/ZnO sensor has excellent selectivity to CO and the response of the Pd/ZnO sensor towards 100 ppm CO was as high as 15 (R_a/R_g), obviously higher than that of the pristine ZnO sensor (1.4) when the working temperature is 220 °C. Moreover, the pure ZnO sensor almost has no selectivity to CO, but the Pd/ZnO sensor has excellent selectivity to CO, which may be ascribed to the electronic sensitization of Pd. Our present results demonstrate that the Pd can significantly improve the gas-sensing performance of metal oxide semiconductor and the obtained sensor has great potential in monitoring coal mine gas.