Determination of total nitrogen in food by flow injection analysis (FIA) with a potentiometric differential detection system

A flow injection set-up based on potentiometric detection and gas diffusion device for the determination of total nitrogen in food is described. The detection system consisted of two ammonium-sensitive electrodes placed sequentially and each alternately operating as reference electrode. Tubular electrodes without an inner reference solution were prepared with a PVC membrane composed of nonactin in Tris (2-ethylhexyl) phosphate and potassium tetrakis (4-chlorophenyl) borate to reduce the membrane resistance. The food sample digests were inserted into the system, and the ammonium present was converted into ammonia gas. The gas diffused through a gas-permeable membrane to a buffer acceptor stream with a pH that ensured transformation to the ammonium cation, which was potentiometrically detected. Good agreement between FIA results and those provided by the reference procedure was obtained, with relative deviation errors below 5%. Using the proposed system, low reagent consumption is possible, a sampling rate of about 30 samples/h was achieved, as well as a good reproducibility for consecutive injections of the same sample (variation coefficient < 2%). Received: 8 October 1998 / Revised: 7 January 1999 / Accepted: 12 January 1999     

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.     

Flow injection colorimetric method using acidic ceric nitrate as reagent for determination of ethanol

Ceric ammonium nitrate has been used for qualitative analysis of ethanol. It forms an intensely colored unstable complex with alcohol. In this work, a simple flow injection (FI) colorimetric method was developed for the determination of ethanol, based on the reaction of ethanol with ceric ion in acidic medium to produce a red colored product having maximum absorption at 415 nm. Absorbance of this complex could be precisely measured in the FI system. A standard or sample solution was injected into a deionized water donor stream and flowed to a gas diffusion unit, where the ethanol diffused through a gas permeable membrane made of plumbing PTFE tape into an acceptor stream to react with ceric ammonium nitrate in nitric acid. Color intensity of the reddish product was monitored by a laboratory made LED based colorimeter and the signal was recorded on a computer as a peak. Peak height obtained was linearly proportional to the concentration of ethanol originally presented in the injected solution in the range of 0.1-10.0% (v/v) (r² = 0.9993), with detection limit of 0.03% (v/v). With the use of gas diffusion membrane, most of the interferences could be eliminated. The proposed method was successfully applied for determination of ethanol in some alcoholic beverages, validating by gas chromatographic method.     

微流控芯片流动注射气体扩散分离光度测定系统的研究

提出了纳升级进样量的微流控芯片流动注射气体扩散分离光度检测系统. 制作三层结构微流控芯片, 在玻璃片上加工微反应通道, 用聚二甲基硅氧烷[Poly(dimethylsiloxane), PDMS]加工气体渗透膜和具有接收气体微通道的底片, 实现了生成气体的化学反应、气-液分离和检测在同一微芯片上的集成化. 采用缝管阵列纳升流动注射进样系统连续进样, 用吸光度法测定NH+4以验证系统性能. 结果表明， 该系统对NH+4的检出限为140 μmol/L(3σ), 峰高精度为3.7%(n=9). 在进样时间12 s、注入载流48 s和每次进样消耗200 nL试样条件下, 系统分析通量可达60样/h. 若加大样品量到800 nL, 使接收溶液停流1 min, 该系统对NH+4的检出限可达到35 μmol/L(3σ), 但分析通量降低到20样/h.     

A direct comparison of amperometric gas sensors with gas-diffusion and ion-exchange membrane based electrodes

The effect of the nature of the working electrode used in amperometric gas sensors on the performance criteria of sensitivity, detection limit, gas flow rate and humidity dependence was evaluated. The arrangement based on metallized ion-exchange membranes (Nafion) was compared with gas-diffusion electrodes based on porous poly(tetrafluoroethylene) (PTFE) with metallic electrodes deposited on the rear side. Two representative analyte gases were chosen: SO2, which has fast reaction kinetics, and NO, which has slow reaction kinetics. It was found that both types of electrodes showed a similar performance. A dependence on the flow rate of the sample gas was found in both cases. The sensitivities were higher for the ion-exchange membrane-backed electrodes; however, the 3sigma detection limits were all in the lower ppb range and for NO were significantly lower on the Nafion membrane than on the PTFE membrane. The Nafion electrode was found to show a dependence on the relative humidity of the gas stream, but not the PTFE-based electrode.     

Microband Sensor for As(III) Analysis: Reduced Matrix Interference

A portable sensor based on a microband design for arsenic detection in drinking water is presented. The work was focused to minimize interference encountered with a standard screen‐printed electrodes featuring an onboard gold working electrode, carbon counter and silver−silver chloride pseudo‐reference electrodes as composite coatings on plastic surface. The interference effect was identified as chloride ions interacting with the silver surface of the reference electrode and formation of soluble silver chloride complexes such as AgCl₄³⁻. By modification of the reference electrodes with Nafion membrane (5 % in alcohols), the interference was entirely eliminated. However, membrane coverage and uniformity can impact the electrodes reproducibility and performance. Hence, the sensor design was further considered and a microband format was produced lending favorable diffusive to capacitive current characteristics. Using the microband electrodes allowed As(III) detection with limit of detection of 0.8 ppb (in 4 M HCl electrolyte), inherently avoiding the problems of electrode fouling and maximizing analyte signal in river water samples. This is below the World Health Organization limit of 10 μg L⁻¹ (ppb). The electrolyte system was chosen so as to avoid problems from other common metal ions, most notably Cu(II). The presented electrode system is cost effective and offers a viable alternative to the colorimetric test kits presently employed for arsenic analysis in drinking water.     

The synthesis and membrane transport characteristics of macrocyclic polyether ligands composed of 1,10-phenanthroline as carriers for primary amine species

Four macrocyclic polyether derivatives of o-phenanthroline were synthesized and used as neutral carriers for preparing poly(vinyl chloride) (PVC) membrane electrodes to sense primary amine species. The potentiometric response characteristics of the electrodes prepared were investigated. The electrode sensitive to benzyl amine as model analyte showed a linear response range of 8.0 x 10(-6)-0.1 mol/l with a detection limit of 8.9 x 10(-7) mol/l and a slope of 56.5 mV/decade. The linear potentiometric response of the mexiletine-sensitive electrode was 4.7 x 10(-6)-0.1 mol/l, and the detection limit was 5.0 x 10(-7)1 mol/l with a slope of 59.0 mV/decade. The transfer behavior of amines and ammonium ions through an organic phase was investigated by means of the bulk liquid membrane transport experiment. The effects of pH, counter anions and other factors on the transfer of the amine and ammonium species were studied. The mass transfer rates of the test species facilitated by macrocyclic polyether derivatives of o-phenanthroline were determined and the following sequence was found: benzyl amine > ethyl amine > tetramethyl ammonium > triethyl amine > diethyl amine > K(+) > ammonium > Na(+) > Ca(2+) > Mg(2+). This was exactly the potentiometric selectivity sequence of the membrane electrodes prepared by using these carriers. The mechanism of transfer of benzyl amine through a membrane phase induced by the carriers has been elucidated on the basis of experimental observations.     

Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series

An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 μl) are injected into a TISAB II (pH=5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 1.94×10⁻⁶ mol l⁻¹ recovery 99.22%, RSD<0.5%) and fluoride (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 4.83×10⁻⁶ mol l⁻¹ recovery 98.63%, RSD=0.3%) at a sampling rate of 60 samples h⁻¹.     

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).     

Evaluation of Water Ammonium Ion Test Kit and Its Feasibility for the Analysis of Protein in Food

The traditional method for the determination of protein in food needs the operations of digestion, distillation, absorption, and titration; therefore, it is complicated and time-consuming and requires professional personnel. Is there a more convenient and faster detection method that can directly determine the ammonium ions in protein digestion solution to obtain the protein content of food and avoid the distillation–absorption–titration process? The feasibility of water ammonium ion test kits for food protein rapid detection was discussed here. After digestion, the protein in food transforms into ammonium ions in the digestion solution. Because of the variety of food, there are many different inorganic ions left in the food digestion solution, and at the same time, digestion agents are added in the digestion process and become potential interference factors in ammonium determination. Therefore, the detection accuracy of ammonium test kits needs to be evaluated first, including their anti-interference ability. The standard curve of ammonium was established by the test kit. When the ammonium concentration was 0.00–2.50 mg/L, the absorbance at 620 nm was linearly related to the ammonium concentration, the determination coefficient R² was 0.9995, and the detection limit of this method was 0.01 mg/L. The influences of temperature, pH value, and reaction time on the test kit method were discussed. The precision was 0.90–3.33%; the repeatability was 1.71–4.86%; and the recovery rate of tap water, river water, and sea water was controlled within 90–103%. The anti-interference ability of the evaluated test kit was better than that of the national standard detection method. The test kit, combined with sample pretreatment and protein conversion formula, was used to detect protein in different types of food (milk powder, rice flour, wheat flour, soy, banana, milk, fish food, chicken food, and dog food). The results showed that there were no significant differences (ρ > 0.05) between the national method and the test kit method. The ammonium ion test kit method shortened the determination time and had higher sensitivity, showing its potential for the rapid determination of food protein.     

Measuring quaternary ammonium cleaning agents with ion selective electrodes

Data for coated-wire, ion selective electrodes (ISEs) are presented for cationic surfactant ions found in common cleaners including benzyldimethyltetradecylammonium, benzyldimethyldodecylammonium, and benzyldimethylhexadecylammonium. The ion exchangers dinonylnaphthalene sulfonic acid, tetraphenyborate, and tetrakis(4-chlorophenyl)borate are examined, showing dinonylnaphthalene sulfonic acid to be the favored species. The ISEs exhibit approximately Nernstian behavior down to the 10⁻⁶ M limit of detection with lifetimes in excess of 50 days when used continuously, and a shelf life of over 100 days. Reaching the upper detection limit at the critical micelle concentration requires use of polymeric-membrane reference electrodes including a new membrane cocktail, which allow response measurements of an order of magnitude higher than the traditional fritted-glass reference electrode. The surfactant ISEs show excellent selectivity over the common metal ions Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cu²⁺ with selectivity coefficients less than 10^−5.3. The ISEs are also selective over the lower molecular weight quaternary ammonium ions tetradecyltrimethylammonium, dodecyltrimethylammonium, benzyldimethyl(2-hydroxyethyl)ammonium, and tetrabutylammonium with selectivity coefficients ranging from 10^−1.7 to 10^−5.5. Use of a single electrode to determine accurately the total cationic surfactant concentration in common cleaning solutions is accomplished with information about concentration dependent interferences and a modified Nikolsky–Eisenman model. Finally, quaternary ammonium surfactants have a deleterious effect on the measurements of pH and common ions like K⁺, Mg²⁺ and Ca²⁺ with polymeric ISEs. This makes it critical to include surfactant electrodes in a detector array when cleaning agents are present.     

Flow methodology for methanol determination in biodiesel exploiting membrane-based extraction

A methodology based in flow analysis and membrane-based extraction has been applied to the determination of methanol in biodiesel samples. A hydrophilic membrane was used to perform the liquid-liquid extraction in the system with the organic sample fed to the donor side of the membrane and the methanol transfer to an aqueous acceptor buffer solution. The quantification of the methanol was then achieved in aqueous solution by the combined use of immobilised alcohol oxidase (AOD), soluble peroxidase and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The optimization of parameters such as the type of membrane, the groove volume and configuration of the membrane unit, the appropriate organic solvent, sample injection volume, as well as immobilised packed AOD reactor was performed. Two dynamic analytical working ranges were achieved, up to 0.015% and up to 0.200% (m/m) methanol concentrations, just by changing the volume of acceptor aqueous solution. Detection limits of 0.0002% (m/m) and 0.007% (m/m) methanol were estimated, respectively. The decision limit (CCα) and the detection capacity (CCβ) were 0.206 and 0.211% (m/m), respectively. The developed methodology showed good precision, with a relative standard deviation (R.S.D.) <5.0% (n = 10). Biodiesel samples from different sources were then directly analyzed without any sample pre-treatment. Statistical evaluation showed good compliance, for a 95% confidence level, between the results obtained with the flow system and those furnished by the gas chromatography reference method. The proposed methodology turns out to be more environmental friendly and cost-effective than the reference method.     

Gas Diffusion Electrodes for Use in an Amperometric Enzyme Biosensor

The preparation of gas diffusion electrodes and their use in an amperometric enzyme biosensor for the direct detection of a gaseous analyte is described. The gas diffusion electrodes are prepared by covering a PTFE membrane (thickness 250 μm, pore size 2 μm, porosity 35%) with gold, platinum, or a graphite/PTFE mixture. Gold and platinum are deposited by e‐beam sputtering, whereas the graphite/PTFE layer is prepared by vacuum filtration of a respective aqueous suspension. These gas diffusion electrodes are exemplarily implemented as working electrodes in an amperometric biosensor for gaseous formaldehyde containing NAD‐dependent formaldehyde dehydrogenase from P. putida [EC. 1.2.1.46] as enzyme and 1,2‐naphthoquinone‐4‐sulfonic acid as electrochemical mediator. The resulting sensors are compared with regard to background current, signal noise, linear range, sensitivity, and detection limit. In this respect, sensors with gold or graphite/PTFE covered membranes outclass ones with platinum for this particular analyte and sensor configuration.     

A flow injection analyser conductometric coupled system for the field analysis of free dissolved CO<Subscript>2</Subscript> and total dissolved inorganic carbon in natural waters

A flow injection analyser coupled with a gas diffusion membrane and a conductometric microdetector was adapted for the field analysis of natural concentrations of free dissolved CO₂ and dissolved inorganic carbon in natural waters and used in a number of field campaigns for marine water monitoring. The dissolved gaseous CO₂ presents naturally, or that generated by acidification of the sample, is separated by diffusion using a hydrophobic semipermeable gas porous membrane, and the permeating gas is incorporated into a stream of deionised water and measured by means of an electrical conductometric microdetector. In order to make the system suitable and easy to use for in-field measurements aboard oceanographic ships, the single components of the analyser were compacted into a robust and easy to use system. The calibration of the system is carried out by using standard solutions of potassium bicarbonate at two concentration ranges. Calibration and sample measurements are carried out inside a temperature-constant chamber at 25 °C and in an inert atmosphere (N₂). The detection and quantification limits of the method, evaluated as 3 and 10 times the standard deviation of a series of measurements of the matrix solution were 2.9 and 9.6 μmol/kg of CO₂, respectively. Data quality for dissolved inorganic carbon was checked with replicate measurements of a certified reference material (A. Dickson, Scripps Institution of Oceanography, University of California, San Diego), both accuracy and repeatability were −3.3% and 10%, respectively. Optimization, performance qualification of the system and its application in various natural water samples are reported and discussed. In the future, the calibration step will be operated automatically in order to improve the analytical performance and the applicability will be increased in the course of experimental surveys carried out both in marine and freshwater ecosystems. Considering the present stage of development of the method, it can only be applied for studying of the carbon cycle in oxic environments.     

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.     

Flow injection system with gas diffusion for the sequential determination of total nitrogen and phosphorus in vegetables

 A flow injection system was developed for the sequential determination of total nitrogen and phosphorus in digests of vegetables using potentiometric and spectrophotometric detection systems, respectively. A tubular ammonium selective electrode with a sensor system composed of nonactin/monactin in tris(2-ethylhexyl) phosphate was used. The selectivity limitations of this electrode were overcome by the inclusion of a gas-diffusion unit in the system that separated ammonium from the rest of the sample matrix and allowed the determination of total nitrogen and phosphorus by the partition of the sample plug between two streams. The results obtained with the developed FIA system were in good agreement with those of the reference methods. Sampling rates from 40 to 60 samples per hour and relative standard deviations below 3.5% were achieved. Received: 17 October 1996/Revised: 21 November 1996/Accepted: 27 November 1996     

Flow injection system with gas diffusion for the sequential determination of total nitrogen and phosphorus in vegetables

 A flow injection system was developed for the sequential determination of total nitrogen and phosphorus in digests of vegetables using potentiometric and spectrophotometric detection systems, respectively. A tubular ammonium selective electrode with a sensor system composed of nonactin/monactin in tris(2-ethylhexyl) phosphate was used. The selectivity limitations of this electrode were overcome by the inclusion of a gas-diffusion unit in the system that separated ammonium from the rest of the sample matrix and allowed the determination of total nitrogen and phosphorus by the partition of the sample plug between two streams. The results obtained with the developed FIA system were in good agreement with those of the reference methods. Sampling rates from 40 to 60 samples per hour and relative standard deviations below 3.5% were achieved. Received: 17 October 1996/Revised: 21 November 1996/Accepted: 27 November 1996     

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.     

The bilayer lipid membrane as a basis for a selective sensor for ammonia

The development of an electrochemical method for the selective sensing of ammonia gas, based on a modified bilayer lipid membrane, is described. Membrane selectivity for ammonium ion is achieved through incorporation of the antibiotic nonactin as ion-carrier. The detection limits compare favourably with those for conventional ammonia gas-sensing electrodes, but the selectivity is much superior. Theoretical evaluation of the potential sensitivity of the new gas-sensor with respect to design parameters is described.     

Ionic Liquid‐Based,Liquid‐Junction‐Free Reference Electrode

In this paper, we describe a new type of polymer membrane‐based reference electrode (RE) based on ionic liquids (ILs), in both liquid‐contact (LCRE) and solid‐contact reference electrode (SCRE) forms. The ILs used were bis(trifluoromethane sulfonyl)amid with 1‐alkyl‐3‐methyl‐imidazolium as well as phosphonium and ammonium cations. In addition to their charge stabilisation role, it was found that the ILs also functioned as effective plasticizers in the PVC matrix. The LCREs and SCREs were prepared using the same design as their corresponding indicator electrodes. LCREs were prepared by casting in glass rings while SCREs were prepared on platforms made using screen‐printing technology, with poly(3‐octylthiophene‐2,5 diyl) (POT) as the intermediate polymer. After potentiometric characterization of the response mechanism, the practical performance of the REs was studied using potentiometric titrations (Pb²⁺ and pH), and characterised using cyclic voltammetry and impedance spectroscopy. All results were compared via parallel experiments in which the novel RE was substituted by a conventional double junction Ag/AgCl reference electrode. The mechanism of response is most likely based on a limited degree of partitioning of IL ions into the sample thereby defining aquo‐membrane interfacial potential. Despite their simple nature and construction, the REs showed excellent signal stability, and performed well in the analytical experiments. The identical mode of fabrication to that of the equivalent indicator (or Ion‐Selective Electrode, ISE) will facilitate mass‐production of both indicator and reference electrode using the same fabrication line, the only difference being the final capping membrane composition.