Tubular Detector Research Articles

Flow-injection analysis of Kjeldahl nitrogen in milk and dairy products by potentiometric detection

A flow-injection analysis (FIA) system with a potentiometric tubular detector and a gas diffusion unit was developed for the determination of Kjeldahl nitrogen in milk and dairy products. The samples were digested by the Kjeldahl method and afterwards injected into the FIA system without prior treatment. NaOH solution was introduced into the system to promote the conversion of ammonium ions to ammonia gas. The gas produced diffused through a gas-permeable membrane towards an acceptor channel (Tris–HCl, pH=7.5). When in contact with this solution, ammonia gas was reconverted to amonium cation and carried to a tubular electrode sensitive to this species. Therefore, a PVC tubular ammonium ion-selective electrode was prepared without inner reference solution and with the sensor system based on nonactine and monactine dissoved in Tris–(2-ethylhexyl) phosphate. The results obtained with the developed FIA system were in good agreement with those of the reference method the relative deviations being less than 4%. Reproducibility of the method was assessed by the relative standard deviation, which was always less than 0.5% for 12 consecutive injections of sample digests. A sampling rate of 100 samples h −1 was obtained.

Ion-Selective Electrodes for Promethazine Determinations in Pharmaceutical Preparations and Application to Flow Injection Analysis

The construction and evaluation of PM-selective electrodes with a conventional configuration and without internal reference solution are described. Two different PVC membranes were prepared with PM tetraphenylborate dissolved in 2-nitrophenyl octyl ether (type A) or bis(2- ethylhexyl)sebacate (type B). When the electrodes were evaluated in 0.01M ionic strength solutions, both presented linear responses from 5 X 10−5 to 1 X 10−2M, slopes close to the theoretical value, practical detection limits of ~2 x 10−5M, reproducibilities of ±0.2mV/day, and a quick response (<20s). Using phosphate buffer solutions (pH6.0), type B electrodes presented a lower practical detection limit and a better reproducibility than type A. Selectivity against several cations was assessed by the separated solutions method, and the type B electrodes were generally more selective. The type B membrane was also used for the construction of tubular electrodes for the automatic analysis of pharmaceutical products by flow injection analysis (FIA). These tubular detectors, evaluated in a low dispersion FIA system, presented better selectivity and sensitivity than the corresponding conventional ones. Several pharmaceutical preparations were analyzed with both conventional and tubular electrodes. The results obtained by FIA presented relative deviations of <2% when compared with those obtained from the United States Pharmacopoeia monographs.

Potentiometric FIA System with Reactor Based on Natural Urease Source and Tubular Detector of Ammonium Ions. Determination of Urea in Fertilizers.

An FIA system was developed to analyze urea by monitoring ammonium ions generated by enzymatic degradation with a tubular ion-selective electrode containing nonactin on polyvinyl chloride (PVC) membrane. As an urease source, 120mg of the leguminous canavalia brasiliensis were immobilized in an enzymatic reactor with glutaraldehyde. Ammonium ions are converted to ammonia by an alkaline solution, 1mol l-1 NaOH, and permeates through a polytetrafluoroethylene membrane until the detector. The method can be used in an urea concentration range of 2.5×10-4 to 2.5×10-3mol 1-1(25.0°C). Analysis of 25μl commercial samples of fertilizers, showed a relative error of 0.2% with a high selectivity to the species present.

Construction and evaluation of PVC conventional and tubular tripelennamine-selective electrodes: their application in pharmaceutical preparations

The construction and evaluation of tripelennamine conventionally-shaped ion-selective electrodes and tubular detectors for the determination of this compound in pharmaceutical formulations are described. Electrodes with conventional configuration have been constructed without an internal reference solution, using several types of immobilized ionic sensors in PVC. The different electrode membranes were prepared by using tripelennamine tetraphenylborate as ionic-exchanger, dissolved in 2-nitrophenyl octyl ether (type A), dibutylphthalate (type B) and bis-(2-ethylhexyl)sebacate (type C) as plasticizer solvents. The general working characteristics of the different types of conventional electrodes were evaluated in tripelennamine solutions, with adjusted ionic strength, showing a linear response in the concentration range of about 4 × 10 −5·1 × 10 −1 M and a slope near the theoretical value. The electrodes presented a fast response (>20 s) and a high reproducibility (±0.2 mV per day). The electrode selectivity in the presence of some interferents, such as sodium, potassium, lithium, ammonium, chlorpheniramine, diphenydramine, promethazine, meclizine and pentazocine, was good, particularly for those whose sensor membrane was prepared with tripelennamine tetraphenylborate dissolved in 2-nitrophenyl octyl ether (type A). Tubular detectors were also prepared using the same sensor membrane and were evaluated in a low-dispersion flow-injection manifold. Under these conditions the detectors presented response characteristics similar to those of the corresponding conventionally-shaped electrodes. The analysis of different pharmaceutical forms (creams, syrups and gels) gave good results with mean recoveries of 99.8–100.6% when the experiments were conducted by direct potentiometry and 99.9–100.4% where the same determinations were conducted by flow-injection analysis with tubular detectors.

Simultaneous assay of nitrite, nitrate and chloride in meat products by flow injection.

A flow injection (FI) analytical method for the simultaneous assay of nitrite, nitrate and chloride in meat products is reported. The method is based on the potentiometric determination of chloride using a tubular ISE and on the spectrophotometric determination of nitrite. The FI system consisted in splitting the flow after potentiometric detection using a tubular detector and the subsequent confluence of the flow before reaching the spectrophotometric detector. This allowed the reduction of nitrate to nitrite in part of the sample plug on an on-line copper cadmium reductor column. Since each channel had a different residence time, two peaks were obtained for nitrite and nitrite plus nitrate. Spectrophotometric determination was made after a diazotization-coupling reaction. The results obtained were in good agreement with reference procedures and showed adequate precision (RSDs less than 6% for chloride and nitrite and 2% for nitrate). A high sampling rate was obtained (120 determinations per hour corresponding to 40 samples per hour).

Tubular detectors for flow-injection potentiometric determination of tetrafluoroborate in electroplating baths

PVC tetrafluoroborate tubular electrodes without an inner reference solution, based on tetraoctylammonium as the ion exchanger and 2-nitrophenyloctyl ether as the mediator solvent, were constructed. The general operating characteristics of the tubular detectors were evaluated by a single line flow-injection system with low sample dispersion and compared to those of corresponding conventionally-shaped electrodes, determined in batch conditions. In flow-injection analysis (FIA), the detector response was linear in the concentration range between 4 × 10 −6 and 10 −1 M, with a mean slope of 59.2 ± 0.9 mV/dec in the pH range between of 2.7 and 11.5 The reproducibility of the detector potentials was approximately 0.2 mV during a day of continuous operation. The membrane used showed a high degree of selectivity. The tubular detectors remained operational over a long period of time (approximately 36 months) and had a response time that enabled a sampling rate of 360 h −1. The electrodes were later used in the determination of tetrafluoroborate in concentrated electrolytic baths for which a high dispersion FIA system was developed. The analyses of seven baths with different compositions presented results with mean recovery values of 98.8% with a coefficient variation of 1.9%.

Tubular potentiometric detector for flow injection based on homogeneous crystalline membranes sensitive to copper, cadmium and lead

Copper, cadmium and lead potentiometric tubular detectors, based on homogeneous crystalline membranes obtained by high-pressure pressing of XS–Ag2S mixtures (X = Cu, Cd or Pb), were prepared and tested. Response characteristics (linear response range, limit of detection, stability and time of response, pH range and potentiometric selectivity coefficients) were established and compared with those of conventionally shaped electrodes prepared with the same membranes. The standards of performance of the tubular potentiometric detectors when used in low-dispersion manifolds were found to be similar to those of the corresponding conventional electrodes in batch conditions.

Evaluation of a mathematical model to simulate dynamic response of tubular potentiometric sensors in flow-injection systems

Abstract Potentiometric detectors based on ion-selective electrodes are specially suited to flow measurements because of their low cost and high selectivity. Simultaneously in flow analysis the use of tubular-electrode configuration seems ideal as the flow characteristics can be kept constant throughout the system. The design of very simple manifolds, which increases the robustness and the system reliability, is constrained by the knowledge of the influence of hydrodynamic parameters in the detector response. With the aim to optimize the design of this kind of systems a mathematical model, that simulates the response of tubular potentiometric detectors in flow-injection systems, has been developed and tested. Results provided by simulation are compared with experimental ones, and the model capabilities are discussed.

Vitamins B1 and B6 tubular electrodes as FIA detectors; their use in the analysis of pharmaceutical products

Ion-selective electrodes without an inner reference solution and tubular potentiometric detectors for the determination of vitamins B1 and B6 in pharmaceutical preparations by flow injection analysis (FIA) are reported. The membranes were prepared with the vitamin tetra(2-chlorophenyl)borate (TCPB) dissolved in o-nitrophenyloctyl ether (o-NPOE) and immobilized on PVC. Intrinsic behaviour of the tubular detectors was assessed using a low-dispersion single-channel FIA manifold and was compared with conventionally-shaped electrodes using the same membrane. Data obtained in the determination of vitamins B1 and B6 in pharmaceutical preparations with a double channel flow injection manifold incorporating the tubular detectors are presented and compared with those obtained by the U.S. Pharmacopeia method and by direct potentiometry with conventionally-shaped electrodes.

FIA Tubular Potentiometric Detectors Based on Homogeneous Crystalline Membranes. Their Use in the Determination of Chloride and Sulphide Ions in Water

Abstract The construction techniques and behaviour of tubular potentiometric detectors based on homogeneous crystalline membranes to be used in flow injection manifolds are reported. Tubular detectors for chloride and sulphide anions were prepared using pressed membranes with silver salts. The results of the assessment of their behaviour against that of the conventional selective electrodes using the same membrane are reported. Data obtained with FIA manifolds with tubular detectors in the determination of chloride and sulphide anions in natural and waste waters are also presented.

Solid-state PVC flow-through benzoate electrode

A general construction procedure for conventional shape ion-selective electrodes based on a conductive support prepared with a mixture of a non-conductive epoxy and graphite was used in the preparation of a flow-through benzoate electrode to be used as potentiometric detector in flow injection analysis manifolds. The membranes were prepared from tetraoctylammonium benzoate in o-nitrophenyloctylether immobilized on PVC. The results of the assessment of the tubular electrodes behaviour in low dispersion manifolds against that of conventional electrodes with the same membrane are reported. Data obtained with a double-channel flow injection manifold incorporating these tubular detectors for benzoate determinations in several commercially available pharmaceutical preparations are also presented.

Observations of the giant radio lobes region of Centaurus A with SAS 3

Observations of the X-ray flux from the extended radio lobes of the galaxy Centaurus A obtained with the SAS 3 observatory are reported. The 15 x 15 deg region that includes the lobes was mapped by superposing data from raster scans with the horizontal tubular collimator detectors, which have a 1.7 deg FWHM circular field of view. Data obtained is consistent with the presence of superposed emission from the point sources NGC 5128 and MX 1312-42, a previously unreported source detected by the Einstein Observatory IPC. Fluxes of 4.63 + or - 0.14 x 10 to the -10th and 4.6 + or - 0.6 x 10 to the -11th ergs/sq cm per sec were measured for NGC 5128 and MX 1312-42, respectively, with best fit power law spectra of indices 0.83 + or - 0.04 and 0.62 + or - 0.10 and hydrogen column densities of 1.23 + or - 0.10 x 10 to the 23rd and 9.7 + or - 15.0 x 10 to the 21st atoms/sq cm. Results imply an upper limit of 2.9 x 10 to the 11th for the X-ray emission from the extended radio lobes of Cen A between 2 and 10 keV, which indicates a lower limit to the magnetic field in the lobes of 1.6 x 10 to the -6th gauss.

Comparison of several voltammetric detectors for high-performance liquid chromatography

The performance of several voltammetric detecors, namely, a tubular detector with a platinum working electrode and thin-layer and wall-jet detectors with glassy carbon and graphite paste, was compared using d.c. and differential pulse polarization and adrenaline as the test substance. The parameters studied were the residual current, the accessible potential range, the detection limit, sensitivity, reproducibiltiy, the time constant, the response volume and the dependence of the signal on the eluate flow-rate. The theoretical detector response was calculated and compared with the experimental values.

The Manufacture of Vacuum Detectors

Experiments with three-electrode vacuum detectors are described. Various filament, grid and plate metals were tested. Different degrees of exhaustion were used. The paper then describes in detail the manufacture of a tubular detector and the testing thereof.