Membrane Focusing - A New Injection Method For Capillary Gas Chromatography

Abstract

The principles of a new sample focusing technique in capillary gas chromatography are described. A solidsorbent trapped sample is thermally released from the sample tube, transfered to a membrane chamber by a carrier gas and there retained by a gas separation membrane which is highly permeable to the carrier gas. After focusing in the membrane chamber the sample is brane chamber geometry to increase the membrane surface and thereby the carrier gas flow is promising too.

The major advantage of a membrane focusing system as compared to the cryofocusing technique would be no need for a cooling agent. Such a system would be favourable for hydrocarbon analyses at remote sites.     

Potentiometric flow injection sensing system for determination of heparin based on current-controlled release of protamine

A flow injection system incorporated with a polycation-sensitive polymeric membrane electrode in the flow cell is proposed for potentiometric determination of heparin. An external current in nano-ampere scale is continuously applied across the polymeric membrane for controlled release of protamine from the inner filling solution to the sample solution, which makes the electrode membrane regenerate quickly after each measurement. The protamine released at membrane–sample interface is consumed by heparin injected into the flow cell via their strong electrostatic interaction, thus decreasing the measured potential, by which heparin can be detected. Under optimized conditions, a linear relationship between the potential peak height and the concentration of heparin in the sample solution can be obtained in the range of 0.1–2.0 U mL⁻¹, and the detection limit is 0.06 U mL⁻¹. The proposed potentiometric sensing system has been successfully applied to the determination of heparin in undiluted sheep whole blood.     

Flow injection analysis for glucose and urea with enzyme reactors and on-line dialysis

A flow injection system for glucose and urea determination is described. The glucose determination uses immobilized glucose oxidase in a reactor designed to give 100% substrate conversion. The hydrogen peroxide formed is converted to a coloured complex with 4-aminophenazone and N,N-dimethylaniline. The coupling is catalysed by a reactor containing immobilized peroxidase. The coloured complex is measured in a flow-through spectrophotometric cell. Urea is converted to ammonia in a reactor with immobilized urease and detected with an ammonia gas membrane electrode. Proteins and other interfering species from serum samples are removed in an on-line dialyzer. Calibration curves are linear for glucose in the range 1.6 × 10^-4–1.6 × 10^-2 M and for urea in the range 10^-4–10^-1 M. The samples are 25 μl for glucose determination and 100 μl for urea determination. Linear ranges can be changed by varying the sample sizes. The effects of the dialyser, enzyme reactors and detectors on dispersion are evaluated.     

Preparation and Response Properties of Selective Bio-Electrodes Utilizing Polymer Membrane Electrode-Based Ammonia Gas Sensors

Abstract

A new type of potentiometric ammonia gas sensor is employed in the preparation of selective bio-electrodes for urea and glutamine. The bio-electrodes are constructed by immobilizing the enzyme urease and intact porcine kidney cells, respectively, at the surface of a disposable ammonium selective polymer membrane electrode-based ammonia gas sensor. The resulting electrodes have favorable response properties when compared to corresponding devices previously assembled with costly commercial gas sensors. Preliminary studies with the urea electrode demonstrate its usefulness for the rapid determination of urea in serum samples.     

Enzyme-Based Fiber Optic Sensor for Glucose Determination

Abstract

A new fiber-optic probe for the determination of glucose is developed. Glucose oxidase is immobilized on a preactivated Immunodyne membrane by direct application of the enzymatic solution to one side of the membrane. The membrane is then placed around the common end of a bifurcated glass fiber optic bundle, and immersed in the sample cell which contains the glucose sample, peroxidase and a colorless dye. Glucose is quantified by the color change when the reaction takes place.     

Environmentally friendly method for determination of ammonia nitrogen in fertilisers and wastewaters based on flow injection-spectrophotometric detection using natural reagent from orchid flower

Crude aqueous extract from the orchid ‘Dendrobium Sonia earsakul’ was utilised as a natural product reagent in flow injection analysis (FIA) incorporating a gas diffusion unit (GD) for the determination of ammonia nitrogen. Sample solution was injected into a NaOH donor stream to generate ammonia gas (NH₃). In the GD unit, NH₃ diffused across a PTFE gas-permeable membrane into the acceptor stream of the orchid extract. As the result, the aqueous orchid reagent became more alkaline and its colour changed from purple to green. The change in the colour of orchid acceptor correlated with the concentration of ammonia nitrogen in the sample and its absorbance monitored by a spectrophotometer at 600 nm. Ammonia nitrogen in chemical fertiliser samples and wastewater samples from agricultural fields were determined and reported as %N (w/w) and mg N L^?1, respectively. For chemical fertilisers which contained high content of ammonia nitrogen, a flow rate of 1.0 mL min^?1 and injection volume of 100 µL were used with a linear range of 5–40 mmol L^?1 and detection limit of 2.12 mmol L^?1. However, a higher sensitivity was required for wastewater samples having low ammonia nitrogen content. The flow rate was reduced to 0.3 mL min^?1 and the injection volume increased to 1000 µL. As a result, detection limit of 0.76 mmol L^?1 was achieved with linear range of 1–5 mmol L^?1. The results of our method agreed well with that using the OPA method employing fluorescence detection.     

Abstracts

Abstract

The developed potentiometric urea biosensor is based on a modified PVC-nonactin NH₃-sensitive gas electrode. Membrane resistance has been optimised by incorporating lipophilic salt. The coefficient of variation for the standard urea solution is 2.45. The sensor performance has been compared with a photometric method for blood samples.     

Enzyme Urea Biosensor Based on a Modified Potentiometric PVC-Nonactin Membrane Electrode for Assay of Urea in Blood

Abstract

The developed potentiometric urea biosensor is based on a modified PVC-nonactin NH₃ -sensitive gas electrode. Membrane resistance has been optimised by incorporating lipophilic salt. The coefficient of variation for the standard urea solution is 2.45. The sensor performance has been compared with a photometric method for blood samples.     

An enzyme reactor electrode for urea determinations.

An enzyme reactor electrode system for the determination of urea is described. A buffer is pumped through an enzyme reactor (0.4 ml) containing urease immobilized with glutaraldehyde to glass. The effluent is mixed with sodium hydroxide pumped through a second channel and fed through an ammonia gas electrode. Samples are introduced via a third flow channel and mixed with the buffer. The conversion of urea to ammonia is quantitative for sample concentrations of less than 0.03 M for a flow rate of 40 ml h^-1. The reactor electrode shows a Nernstian slope of 57 mV/decade for 5·10^-5–3·10^-2 M urea. The response is independent of variations in the flow rate, enzyme activity or temperature of the reactor.     

A Urea Electrode Based on the Ammonia Probe

Abstract

An enzyme electrode specific for the substrate urea is described. The electrode consists of a layer of urease polymerized directly on to the surface of the gas diffusion membrane of an ammonia probe by means of glutaraldehyde. The calibration curves obtained when the electrode is allowed to measure in buffered solutions are discussed in terms of enzyme activity and pH-gradients in the membrane.     

Dual Adsorber-Capillary Column System for Gas Chromatographic Analysis of Air Samples

Abstract

An interface which allows thermal desorption and subsequent capillary gas chromatographic analysis of air samples is described. A small solid-sorbent trap is positioned between the sampling tube and capillary column. A sample thermally released from the sampling tube is transferred by a carrier gas at high flow rate to the trap and retained. From there it is again thermally released and transferred to the capillary column by carrier gas at a low flow rate, as required by capillary GC. The transfer and injection steps are effected by means of externally placed solenoid valves. The performance of the system depends on the desorption temperature and time allowed for transfer of the sample between the two adsorbers and the column. These parameters are programmable and can be changed to suit the requirements of a particular analysis. The system allows the analysis of sub-parts-per-billion concentrations of organic compounds in a comparatively simple and reproducible manner. Operation of the system does not require cryogenic cooling of either the trap or the GC oven. Chromatograms of a variety of air samples are presented and discussed.     

An Extractive Spectrophotometric Method for the Determination of Nitrite Using Fluorescein Amine Isomer I

ABSTRACT

A sensitive spectrophotometric method for the determination of nitrite using fluorescein amine isomer I is described. The method is based on the formation of azido derivative of fluorescein amine isomer I, which is selectively extracted into 20% iso amyl alcohol in toluene and is stripped to aqueous phase using NaOH. Formation of the azido derivative depends on nitrite concentration and the system obeys Beer's law in the range 0-0.4 ppm of nitrite at 495 nm. The molar absorptivity of the colour system is 6.67 × 10⁴ L mol^-1 cm^-1 with a relative standard deviation of 3% for 10 determinations at 1 μg of nitrite. The proposed method is successfully applied for the determination of nitrite and nitrate in soil, water and radiator coolant sample, NO₂ gas in a laboratory fume cupboard is determined after fixing it as nitrite in sodium arsenite absorber solution.     

Decolorization Reactions Between Chromium (VI) and Three Colour Reagents. Determination of Chromium (VI) by Flow Injection Analysis

Abstract

Chromium (VI) can oxidize and decolor three colour reagents, i.e. 1,8-dihydroxy-2-(4′-chloro-2′-phosphonophenylazo)-7-(6″.8″-disulfonaphthylazo)-3,6-disulfonaphthalene (RI): 1,8-dihydroxy-2-(4′-chloro-2′-phosphonophenylazo)-7-(4″-sulfonamidephenylazo)-3,6-disulfonaphthalene (RII): and 1,8-dihydroxy-2-(4′-chloro-2′-phosphonophenylazo)-7-(p-hippuric acid azo)-3,6-disulfonaphthalene (RIII). Loss of absorbance of three colour reagents at maximum absorption wavelengths is proportional to the concentration of chromium (VI) in solution. We have made use of the decolorization reactions between chromium (VI) and three colour reagents to determine chromium in steel by flow injection analysis, and relative error in sample determination is less than 5.0%. Response is linear from 0.84 to 6.72 μg/mL and optimum measuring acidity is 1.3–1.7 mol/L H₂SO₄ in three systems. The effect of interference ions on determination has been studied.     

Flow injection conductometric system with gas diffusion separation for the determination of Kjeldahl nitrogen in milk and chicken meat

A simple flow injection (FI) conductometric system with gas diffusion separation was developed for the determination of Kjeldahl nitrogen (or proteins) in milk and chicken meat. The sample was digested according to the Kjeldahl standard method and the digest was diluted and directly injected into the donor stream consisting of 4 M NaOH. In alkaline medium, ammonium was converted to ammonia, which diffused through the PTFE membrane to dissolve in an acceptor stream (water). Dissociation of ammonia caused a change in conductance of the acceptor solution, which was linearly proportional to the concentration of ammonium originally present in the injected solution. A conductometric flow through cell and an amplifier circuit was fabricated, which helped improve sensitivity of the conductometric detection system. With using a plumbing Teflon tape as a gas diffusion membrane and without thermostating control of the system, a linear calibration graph in range of 10-100 mg L⁻¹ N-NH₄ was obtained, with detection limit of 1 mg L⁻¹ and good precision (relative standard deviation of 0.3% for 11 replicate injections of 50 mg L⁻¹ N-NH₄). The developed method was validated by the standard Kjeldahl distillation/titration method for the analysis of milk and chicken meat samples. The proposed system had sample throughput of 35 h⁻¹ and consumed much smaller amounts of chemical than the standard method (275 mg vs 17.5 g of NaOH per analysis, respectively).     

Selective detection of F− by chromogenic tetrazole receptor

A chromogenic anion host 4, containing two amide functionalities linked to azo dye and tetrazole rings, was synthesised and its complexes with various anions were investigated. The results show that receptor 4 can recognise selectively biologically important F^?  ion. The binding affinity for F^?  was investigated by naked-eye colour change, UV–vis and ¹H NMR spectroscopy. Addition of F^?  ion in CH₃CN and Dimethyl sulfoxide to receptor 4 causes a change in colour of the solution from colourless to yellow. The stoichiometry for host:anion is 1:1. Furthermore, receptor 4 was used as an ion carrier in ion-selective membrane electrodes. Selectivity of this membrane was studied towards various anions in water solution. Binding behaviour of receptor 4 towards several anions (Cl^? , F^? , Br^? , I^? ) has been investigated using density functional theory calculations.     

Gas permeability of combined membrane systems with mobile liquid carrier

Gas transfer in a membrane system called a selective membrane valve (SMV) is studied. The SMV is a system consisting of two mobile gas phases, one mobile liquid phase, and two membranes acting as interfaces between gas and liquid. Such a membrane system has supplementary variable parameters and is designated for the separation of multicomponent gas mixtures. System permeability for individual gases (CO₂, O₂, and H₂) and its dependence on a flow rate of a liquid phase are studied. Time dependences of the non-steady state transfer of CO₂ through the immobile layer of chemisorbent (aqueous K₂CO₃ solution) at its different concentrations are studied for the first time. Two theoretical models are developed: the model of gas transfer through a selective membrane valve system with a mobile liquid absorbent (in the absence of chemical interaction) and the model of a non-steady-state transfer of CO₂ through the immobile layer of aqueous potassium carbonate solution. The first model makes it possible to determine gas-to-liquid diffusion coefficients; the second model permits us to plot kinetic permeability curves and to calculate system permeability with allowance for the CO₂ transfer accompanied by reversible chemical reaction with the carrier. The model dependences agree well with the experimental data.     

Flow Injection Analysis of Mercury/Cold Vapor Atomic Fluorescence Spectrophotometry

Abstract

A new flow injection system for the determination of mercury by the cold vapor atomic fluorescence method is described. A sample solution (64 μ1) is injected into a stream of 0.1 M hydrochloric acid, which is mixed with a stream of 3% tin (II) chloride solution in a mixing joint. The combined stream is carried through a reaction coil for reduction of Hg (II) to Hg (0) and subsequently introduced into a specially designed gas-liquid separation vessel. Then the vaporized mercury is swept into a flow type fluorescence cell with a continuous flow of argon after removal of water in the gas phase through a condenser. Mercury is excited with an electrodeless discharge lamp as a source and the mercury fluorescence at both 184.9 and 253.7 nm is measured with a solar-blind photomultiplier. This method allows about 35 determinations of mercury in aqueous samples per hour. The calibration curve is linear over the 0–20 ppb range of mercury. The limit of detection (S/N = 3) is 0.008 ng (0.12 ppb × 64 μ1) and the coefficient of variation is below 1% for the 1–20 ppb solutions (n=10).     

Fluoride-responsive hydrogel of cholesterol appended pyridinium urea and its metal detecting ability and semi-conducting behaviour

Cholesterol appended pyridinium urea 1 acts as low molecular weight gelator in DMSO:H₂O (1:1, v/v) showing distinct colour change in the presence of aqueous solution of KF as well as tetrabutylammonium fluoride and recognises F^?  specifically. In addition, this hydrogel is noted to detect aqueous solution of Cu²⁺ and Pb²⁺ ions over a series of other metal ions and exhibits good semi-conducting property.     

High Sensitive and Selective Spectrophotometric Determination of Aluminium after Collection on a Membrane Filter Using 2,3‐Dichloro‐6‐(3‐carboxy‐2‐hydroxy‐1‐naphthylazo)quinoxaline and Zephiramine

Abstract

A high sensitive and selective spectrophotometric method for the determination of aluminium(III) using 2,3‐dichloro‐6‐(3‐carboxy‐2‐hydroxy‐1‐naphthylazo)quinoxaline (DCHNAQ) and zephiramine (zeph) is described. The formed ion pair precipitate between zephiramine and perchlorate ions is effective for the enrichment of aluminium(III) on a membrane filter as its ternary complex with DCHNAQ and zephiramine. The solid–state absorbance of the complex on the membrane filter is measured at 655 nm against a blank thin layer and the difference is calculated. The colour system obeys Beer's law from 5.0–150 ng ml^?1 of aluminium. The detection and quantification limits were calculated. The relative standard deviation for 60 ng of aluminium(III) in 20‐ml sample volume amounts 0.84% (n=10). A ligand buffer solution, composed of transcyclohexane‐1,2‐diaminetetraacetic acid with an excess of zinc(II), is effective for masking interferences from foreign ions, particularly iron(III). The proposed method was applied successfully to tap and environmental water, biological (human blood, urine, and gallstone), and soil samples.     

Flow‐Injection Amperometric Detection with Solvent Polymeric Membrane Ion Sensors

An amperometric detector for hydrophobic ions based on a plasticized poly(vinyl) chloride (PVC) membrane incorporated in a flow‐injection system was developed. A four‐electrode potentiostat with ohmic drop compensation was used, while a flow‐through cell incorporated the four electrodes and the membrane, which contained tetrabutylammonium tetraphenylborate. When the influence of the applied potential and of the flow‐injection variables on the determination of tetrabutylammonium was studied, a linear relationship was observed between current peak height and ion concentration over a range of 5×10^?6–6×10^?5 M tetrabutylammonium. Good repeatability and between‐day reproducibility and high sample frequency were obtained. The effect of other ions was studied. Two different amperometric methods, indirect and direct, were also developed for the determination of dodecylsulfate in the concentration range 3×10^?5–9×10^?4 M.