A membraneless gas-diffusion unit-multisyringe flow injection spectrophotometric method for ammonium determination in untreated environmental samples

A new design of a membraneless gas-diffusion (MGD) unit coupled to a multisyringe flow injection system is proposed. The spectrophotometric determination of ammonium using an acid-base indicator was chosen to show the feasibility of this approach. Hence, in alkaline medium, ammonium ions are transformed into ammonia (donor channel) which diffuses through the headspace into the acceptor stream (bromothymol blue solution), causing a pH change and subsequently a colour change. The exploitation of the enhanced potentialities of this re-designed MGD device was the main purpose of the present work. Hence, several strategies concerning flow management were studied seeking to characterize and improve the analytical features of the methodology and moreover, untreated environmental samples were analysed without previous filtration. Consequently, stopped flow in acceptor channel with continuous flow in donor channel was chosen for the application to wastewater and spiked river water samples. A linear concentration range between 10.0 and 50.0 mg L⁻¹ of NH₄⁺, a limit of detection of 2.20 mg L⁻¹ and a determination frequency of 11 h⁻¹ were obtained.     

Optosensing at active surfaces — a new detection principle in flow injection analysis

Reflectance spectrophotometry is applied to flow-injection measurements of pH and the assays of ammonia and urea with the aim of demonstrating the principle and testing the performance of optosensors integrated into microconduits. A novel injection approach, the split-loop technique, is applied. For pH measurements, detection is based on commercial non-bleeding acid-base indicator papers situated in the flow stream at the tip of the fibre optic. Measurements of pH in the range 4–10 are possible at a rate of 120 h^?1. Special attention is given to the physiological pH range; the standard deviation is 0.004 at pH 7.2. For the determinations of ammonia and urea (via urease), a bromothymol blue stream is used with a miniature gas-diffusion device.     

Potentiometric analytical microsystem based on the integration of a gas-diffusion step for on-line ammonium determination in water recycling processes in manned space missions

The design, construction and evaluation of a versatile cyclic olefin copolymer (COC)-based continuous flow potentiometric microanalyzer to monitor the presence of ammonium ion in recycling water processes for future manned space missions is presented. The microsystem integrates microfluidics, a gas-diffusion module and a detection system in a single substrate. The gas-diffusion module was integrated by a hydrophobic polyvinylidene fluoride (PVDF) membrane. The potentiometric detection system is based on an all-solid state ammonium selective electrode and a screen-printed Ag/AgCl reference electrode. The analytical features provided by the analytical microsystem after the optimization process were a linear range from 0.15 to 500 mg L⁻¹ and a detection limit of 0.07 ± 0.01 mg L⁻¹. Nevertheless, the operational features can be easily adapted to other applications through the modification of the hydrodynamic variables of the microfluidic platform.     

Gas-diffusion separation and flow injection potentiometry

Summary The similarities and differences of the operation principle of gas-sensing electrodes and potentiometric detection coupled to gas-diffusion separation in flow injection analysis are discussed with special emphasis on selectivity and sensitivity aspects. Several examples of application are presented highlighting the improvements in detectability obtained by gas-diffusion flow injection potentiometry. High sensitivity determination of ammonium is achieved through accumulation of ammonia released from the sample stream in the small recipient volume of the gas-diffusion unit. A method for almost specific determination of cyanide is presented making use of gas-diffusion separation of hydrogen cyanide and potentiometric detection with a selective AgI membrane electrode. The interference of sulfide is totally prevented by its oxidation in the donor line. If applied to potentiometric measurement following gas diffusion separation an intrinsically non-selective metallic silver wire electrode turns out to enable the selective detection of sulfide with high sensitivity and fast response. A new approach for diffusive sampling and on-line detection of gas-phase contaminants is exemplified by the determination of NO_x.     

Potentiometric and conductometric determination of ammonium by gas-diffusion flow injection analysis

Summary Considerable enhancement of selectivity in the potentiometric and conductometric determination of ammonium is provided by gas-diffusion separation in flow injection analysis. Ammonium and potassium selective liquid membrane electrodes can be used for determinations in the concentration range 10^–7–10^–2 mol/l with high precision and fast sample throughput. No interferences are encountered in the presence of ionic species and molecules that likely adsorb when the sensors are in direct contact with the sample. The selectivity over volatile amines is enhanced due to kinetic discrimination. Conductometric detection is shown to be as sensitive as the potentiometric detection. A major advantage, however, is the linear rather than logarithmic relationship between concentration and conductivity.     

Gaseous catalysts for end-point indication in titrimetric analysis in the microgram range

The method of end-point detection described is very sensitive. In titrations with sodium sulphide, the first drop of titrant in excess cause evolution of hydrogen sulphide which acts as a catalyst for the indicating iodine—azide reaction. The gas formed at the end-point is transferred by a nitrogen stream to the indicator vessel. Microgram amounts of metal ions (Sb, Ni, Fe, Hg⁺ Cd, Pb⁺ Cu, Ag⁺ Cu) and permanganate can be titrated in acidic solution with standard sulphide solution. In a variation of this method, acid-base titrations are possible.     

Low cost and compact analytical microsystem for carbon dioxide determination in production processes of wine and beer

The design, construction and evaluation of a low cost, cyclic olefin copolymer (COC)-based continuous flow microanalyzer, with optical detection, to monitor carbon dioxide in bottled wines and beers as well as in fermentation processes, is presented. The microsystem, constructed by computer numerically controlled (CNC) micromilling and using a multilayer approach, integrates microfluidics, gas-diffusion module and an optical flow-cell in a single polymeric substrate. Its size is slightly bigger than a credit card, exactly 45 × 60 × 4 mm in the microfluidic and diffusion module zone and 22.5 × 40 × 3 mm in the flow-cell zone. The gas-diffusion module is based on a hydrophobic polyvinylidene fluoride (PVDF) membrane, which allows the transfer of the carbon dioxide present in the sample to a bromothymol blue (BTB) pH-sensitive acceptor solution, where the color change is measured optically. The detection system consisted of a LED with an emission peak at 607 nm and a photodiode integrated in a printed circuit board (PCB). The obtained analytical features after the optimization of the microfluidic platform and hydrodynamic variables are a linear range from 255 to 10000 mg L⁻¹ of CO₂ and a detection limit of 83 mg L⁻¹ with a sampling rate of 30 samples h⁻¹.     

Hybrid sequential injection-flow injection manifold for the spectrophotometric determination of total sulfite in wines using o-phthalaldehyde and gas-diffusion

A new automated spectrophotometric method for the determination of total sulfite in white and red wines is reported. The assay is based on the reaction of o-phthalaldehyde (OPA) and ammonium chloride with the analyte in basic medium under SI conditions. Upon on-line alkalization with NaOH, a blue product is formed having an absorption maximum at 630 nm. The parameters affecting the reaction - temperature, pH, ionic strength, amount concentration and volume of OPA, amount concentration of ammonium chloride, flow rate and reaction coil length - and the gas-diffusion process - sample and HCl volumes, length of mixing coil, donor flow rate - were studied. The proposed method was validated in terms of linearity (1-40 mg L⁻¹, r = 0.9997), limit of detection (c_L = 0.3 mg L⁻¹) and quantitation (c_Q = 1.0 mg L⁻¹), precision (s_r = 2.2% at 20 mg L⁻¹ sulfite, n = 12) and selectivity. The applicability of the analytical procedure was evaluated by analyzing white and red wine samples, while the accuracy as expressed by recovery experiments ranged between 96% and 106%.     

Determination of ammonium ion in a flow-injection system with a gas-diffusion membrane : Selection of Optimal Conditions for the pH Indicator

The spectrophotometric determination of ammonium ion in water by flow-injection analysis with a membrane-separator and a pH indicator for detection is studied in detail. The relations derived facilitate the selection of appropriate solution compositions or the prediction of sensitivity. It is shown that 1.5×10^?5 M bromocresol purple (pH 6.8) as acceptor solution gives the maximal sensitivity in the flow system with a laboratory-made separation unit. Application of ultrasonic radiation in the separation step and the use of different flow rates for the donor and acceptor streams may result in increased permeation of ammonia and a correspondingly high sensitivity. By modifying the acceptor solution so that the sensitivity is decreased, more concentrated samples such as urine can be analyzed by direct injection without prior dilution. In this procedure, the sample rate was 60 h^?1 for ammonium concentration of more than 10^?5 M and 30–40 h^?1 for concentrations in the range 3×10^?7?10^?5 M. The detection limit was about 3×10^?7 M.     

Highly sensitive determination method for total carbonate in water samples by flow injection analysis coupled with gas-diffusion separation.

A spectrophotometric method for the determination of total carbonate in water samples was developed. The method is based on the color change of an acid-base indicator in relation to the concentration of permeable gas substances through a membrane. By using a new portable FIA system equipped with a gas-diffusion unit, a highly sensitive and on-site determination of total carbonate in aqueous solutions was investigated. A new color-change system with 4-(2',4'-dinitrophenylazo)-1-naphthol-5-sulfonic acid (DNN5S) was developed. Absorbance changes of the reagent solution were measured at 450 nm with a light-emitting diode (LED) as a light source. A new type of gas-diffusion unit was used, and was constructed with double tubing: the inner tubing was a micro porous PTFE (polytetrafluoroethylene) tubing (1.0 mm inner diameter and 1.8 mm outer diameter, pore size 2 microns, porosity 50%); the outer tubing was made of glass with 2.0 mm inner diameter. The optimized system conditions were as follows: the sample size was 200 microliters, the temperature of the air bath for the gas-diffusion unit was 25 degrees C, and the length of the gas-diffusion unit was 15 cm; each flow rate was 0.3 ml min-1. For measuring carbonate at low concentrations, a method for preparing water with less carbonate was proposed: the carbonate content of the water was decreased down to 5 x 10(-7) M. The calibration graph was rectilinear from 1 x 10(-6) M to 10(-3) M, and the detection limit (corresponding to a signal-to-noise ratio of 3) was 1 x 10(-6) M of carbonate. The relative standard deviation (RSD) of ten measurements of 2.3 x 10(-5) M Na2CO3 solution was 1.9%. The total carbonate in various kinds of water (such as river, sea, rain, distilled and ultra purified) was determined.     

Gas-diffusion flow injection determination of Hg(II) with chemiluminescence detection

A gas-diffusion flow injection method for the chemiluminescence detection of Hg(II) based on the luminol-H₂O₂ reaction was developed. The analytical procedure involved the injection of Hg(II) samples and standards into a 1.50 M H₂SO₄ carrier stream, which was subsequently merged with a reagent stream of 0.60% (w/v) SnCl₂ in 1.50 M H₂SO₄ to reduce Hg(II) to metallic Hg. The gas-diffusion cell was thermostated at 85 °C to enhance the vaporisation of metallic Hg. Mercury vapour, transported across the Teflon membrane of the gas-diffusion cell into the acceptor stream containing 1.00 × 10⁻⁴ M KMnO₄ in 0.30 M H₂SO₄, was oxidised back to Hg(II). The acceptor stream was merged with a reagent stream containing 2.50 M H₂O₂ in deionised water and then the combined stream was merged with another reagent stream containing 7.50 × 10⁻³ M luminol in 3.00 M NaOH at a confluence point opposite to the photomultiplier tube of the detection system. The chemiluminescence intensity of the luminol-H₂O₂ reaction was enhanced by the presence of Hg(II) in the acceptor stream. The corresponding increase was related to the original concentration of Hg(II) in the samples and standards. Under optimal conditions, the chemiluminescence gas-diffusion flow injection method was characterised by a linear calibration range between 1 μg L⁻¹ and 100 μg L⁻¹, a detection limit of 0.8 μg L⁻¹ and a sampling rate of 12 samples per hour. It was successfully applied to the determination of mercury in seawater and river samples.     

Photodissociation/gas-diffusion separation and fluorimetric detection for the analysis of total and labile cyanide in a flow system

Total cyanide species are determined in a flow injection system which includes UV-photodissociation, gas-diffusion separation and spectrofluorimetric detection. Without the irradiation step, only cyanide easily released in acid medium, i.e. labile cyanide, is determined. Cyanide diffuses through a microporous PTFE membrane from an acid donor stream to a sodium hydroxide acceptor stream. Then, the transferred cyanide reacts with ¶o-phthalaldehyde and glycine to form a highly fluorescent isoindole derivative. Complete cyanide recoveries were obtained for the most important metal cyanide complexes found in environmental samples, excepting cobaltocyanide. The sampling frequency for total cyanide was 4 samples h^–1 and the detection limit was 0.4 μg L^–1. Recoveries of total cyanide from river water obtained with this method are about 90% of those obtained with APHA Method 4500-CN C for total cyanide.     

Conductometric determination of ammonium by a multisyringe flow injection system applying gas diffusion

A multisyringe flow injection system (MSFIA) coupled to a gas-diffusion cell has been developed for the conductometric determination of ammonium in different water samples. Operation strategies, membrane, reagent concentrations, and flow rates have been studied to optimize the sensitivity of detection and to fit the required working range. The proposed MSFIA system has been compared with former FIA and SIA systems using gas diffusion. The system was applied to the determination of ammonium in water samples of different matrices in order to evaluate its performance. These samples were coastal waters, pond waters, and compost aqueous extracts. Good recoveries of 102?±?13% were obtained and no significant differences with the reference methods were found. The system can be used for a wide concentration range of ammonia, from 0.075 to 360?mg?L^?1, without sample dilution and with a precision better than 2% of RSD. The throughput of the method was 32 injections per hour.     

Sensitive and ultra-fast determination of arsenic(III) by gas-diffusion flow injection analysis with chemiluminescence detection

A novel chemiluminescence gas-diffusion flow injection system for the determination of arsenic(III) in aqueous samples is described. The analytical procedure involves injection of arsenic(III) samples and standards into a 0.3 mol L⁻¹ hydrochloric acid carrier stream which is merged with a reagent stream containing 0.2% (w/v) sodium borohydride and 0.015 mol L⁻¹ sodium hydroxide. Arsine, generated in the combined carrier/reagent donor stream, diffuses across the hydrophobic Teflon membrane of the gas-diffusion cell into an argon acceptor stream and then reacts with ozone in the flow-through chemiluminescence measuring cell of the flow system. Under optimal conditions, the method is characterized by a wide linear calibration range from 0.6 μg L⁻¹ to 25 mg L⁻¹, a detection limit of 0.6 μg L⁻¹ and a sample throughput of 300 samples per hour at 25 mg L⁻¹ and 450 samples per hour at 25 μg L⁻¹.     

Gas-diffusion dilution flow-injection method for the determination of ethanol in beverages without sample pretreatment

The combination of immobilized alcohol oxidase (AOD) with a gas-diffusion membrane in a flow-injection system greatly enhanced the specificity of ethanol determination. This gas-diffusion flow-injection system with amperometric detection of hydrogen peroxide had a linear range of analysis from 0.0006 up to 60% (v/v) ethanol. Silicone-modified polypropylene membranes of different thickness were used. Undiluted samples of beer, wine, spirits and medicine could be investigated with excellent correlation with standard methods (r > 0.997). The frequency of analysis was 120–180 samples h^?1 and the operational half-life of the immobilized enzyme was 8000 injections within 44 h.     

Photodissociation/gas-diffusion separation and fluorimetric detection for the analysis of total and labile cyanide in a flow system

Total cyanide species are determined in a flow injection system which includes UV-photodissociation, gas-diffusion separation and spectrofluorimetric detection. Without the irradiation step, only cyanide easily released in acid medium, i.e. labile cyanide, is determined. Cyanide diffuses through a microporous PTFE membrane from an acid donor stream to a sodium hydroxide acceptor stream. Then, the transferred cyanide reacts with ?o-phthalaldehyde and glycine to form a highly fluorescent isoindole derivative. Complete cyanide recoveries were obtained for the most important metal cyanide complexes found in environmental samples, excepting cobaltocyanide. The sampling frequency for total cyanide was 4 samples h^–1 and the detection limit was 0.4 μg L^–1. Recoveries of total cyanide from river water obtained with this method are about 90% of those obtained with APHA Method 4500-CN C for total cyanide. Received: 10 February 1999 / Revised: 7 May 1999 / Accepted: 13 May 1999     

Detection of short oligonucleotide sequences of hepatitis B virus using electrochemical DNA hybridisation biosensor

A novel, sensitive and selective electrochemical hybridisation biosensor was developed for the detection of the hepatitis B virus (HBV) using a manganese(II) complex as electrochemical indicator and a DNA probe-modified carbon paste electrode as the biosensor (DNA/CPE). The results showed that this complex could be accumulated electrochemically the immobilised dsDNA layer rather than in the single-stranded DNA (ssDNA) layer. On the basis of this, the manganese complex was used as an electrochemical hybridisation indicator for the detection of oligonucleotides related to HBV. The hybridisation event was evaluated on the basis of the difference between the reduction signals of the manganese(II) complex with the probe DNA prior to and post hybridisation with a target sequence using a differential pulse mode. Several factors affecting the immobilisation and hybridisation of oligonucleotides as well as the indicator’s accumulation were investigated. Experiments with a non-complementary and mismatch sequences demonstrated the good selectivity of the biosensor. Using this approach, the HBV target oligonucleotide’s sequence could be quantified over arange from 0.22 ng L^?1 to 5.40 ng L^?1, with a linear correlation coefficient of 0.9994 and the limit of detection of 0.07 ng L^?1.     

A molecular switch based on acid and base promoted, cation governed binding in a crown ether threaded rotaxane

A rotaxane, containing both oligo ethylene glycol and secondary ammonium cation binding sites for a threaded crown ether, has been prepared. ¹H NMR spectroscopy has been used to show that the crown ether moiety in the rotaxane undergoes acid-base and alkali metal cation dependent switch from binding at the ammonium cation position to cooperative binding to the metal cation at the oligo ethylene glycol site.     

Spectrophotometric determination of total cyanide in waste waters in a flow-injection system with gas-diffusion separation and preconcentration

An automated flow-injection system with gas diffusion separation and preconcentration and spectrophotometric detection is described for the determination of total cyanide in waste waters. An unstable red intermediate product of the reaction of cyanide with isonicotinic acid and 3-methyl-1-phenyl-2-pyrazolin-5-one is used instead of the conventional blue final product to improve the efficiency. A novel combination of a gas-diffusion separator with the sampling valve enables efficient on-line separation, preconcentration and sampling of cyanide. The sampling frequency is 40 h^?1 and the detection limit is 0.006 μg ml^?1 (3σ) when a 2-ml sample is taken and a preconcentration factor of 3.5 is achieved. The relative standard deviation is 1.4% (n = 22) at the 0.5 μg ml^?1 level. Results obtained with the proposed method are in good agreement with the standard manual spectrophotometric method. Interference studies show that in the presence of 1,10-phenanthroline, most potential interferents present in appreciable amounts do not interfere, but the interference from cobalt is not overcome in this system.     

On-line simultaneous monitoring of glucose and acetate with FIA during high cell density fermentation of recombinant E. coli

A two-channel flow injection analysis (FIA) system was developed for the simultaneous on-line monitoring of acetate and glucose during high cell density fed-batch fermentations of recombinant Escherichia coli. Acetate measurement was performed with a modified and optimised version of an existing method, based on acetate diffusion through a gas-diffusion chamber into a stream containing an acid-base indicator. The subsequent decrease in the absorbance was detected with an incorporated photometer. After method optimisation, it was possible to achieve linearity until 10 g/kg with no dilution step and with a detection level of 0.05 g/kg. Although some interferences were found, the performance of the method proved to be sufficiently reliable for on-line control purposes Commercially packed glucose oxidase (GOD) was used for the amperometric measurement of glucose. The method was linear up to 5 g/kg and it was possible to detect concentrations lower than 0.06 g/kg. For these measurements, no significant interferences were detected when the results were compared with other reference methods. The application of a simultaneous parallel configuration of the methods to a high cell density fed-batch E. coli fermentation was tested and reliable results were obtained within a 3 min delay. This information was made available to a supervisory computer running a developed LabVIEW™ programme via an Ethernet network, allowing the immediate implementation of control actions, improving the process performance.