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Altres ajuts: acords transformatius de la UAB Some inborn errors of metabolism and other diseases can result in increasing blood ammonium (hyperammonemia episodes), which can cause serious neurological complications in patients or even death. Early diagnosis, follow up and treatment are essential to minimize irreversible damages in brain. Currently, adequate analytical instrumentation for the necessary ammonium bedside determination is not available in all health centers but only in clinical laboratories of reference hospitals. We therefore have developed a low cost and portable potentiometric Point-of-Care microanalyzer (POC) to address this problem. It consists of a cyclic olefin copolymer-based microanalyzer, the size of a credit card and working in continuous flow, which integrates microfluidics, a gas-diffusion module and a potentiometric detection system. The analytical features achieved are a linear range from 30 to 1000 μmol L NH , a detection limit of 18 μmol L NH and a required sample volume of 100 μL, which comply with the medical requirements. Plasma and blood samples are analyzed with no significant differences observed between ammonium concentrations obtained with both the proposed microanalyzer and the reference method. This demonstrates the value of the developed POC for bedside clinical applications.

The design, construction and evaluation of a low-cost cyclic olefin copolymer (COC)-based continuous flow microanalyzer with optical detection to determine the titratable acidity content of wine is here presented. The analysis method is based on the monitoring of the blue coloration decrease of a buffered bromothymol blue (BTB) solution in the presence of the acidic compounds of wine. The microanalyzer monolithically integrates the required microfluidic motifs as well as an optical flow cell where the measurements are performed by using a miniaturized and versatile photometric detection system. Fluid management is totally automated by the use of computer-controlled microvalves, permitting the automatic calibration of the system as well as the automatic sampling, including in-line dilution and analysis. The reduced size of the whole system along with its high simplicity and automation make it suitable for its application to the on-line monitoring of titratable acidity during wine-making processes. With the optimal conditions, a linear range up to 0.50 g L−1 tartaric acid, a quantification limit (LOQ) of 0.01 g L−1 and a detection limit (LOD) of 0.004 g L−1 were obtained, covering the most common acidity content of musts and wines. A sampling rate up to 26 h−1 could be achieved, consuming less than 3 mL of inexpensive reagents per analysis and requiring no pretreatment of the sample. The microsystem has been successfully applied to the quantification of the titratable acidity content of several wine samples, being the results in excellent agreement with the ones obtained by the reference method.

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.

The construction and evaluation of a Cyclic Olefin Copolymer (COC)-based continuous flow potentiometric microanalyzer to simultaneously monitor potassium, chloride and nitrate ions in samples from an on-board water recycling process expected to be installed in future manned space missions is presented. The main goals accomplished in this work address the specific required characteristics for a miniaturized on-line monitoring system to control water quality in such missions. To begin with, the integration of three ion-selective electrodes (ISEs) and a reference electrode in a compact microfluidic platform that incorporates a simple automatic autocalibration process allows obtaining information about the concentration of the three ions with optimal analytical response characteristics, but moreover with low reagents consumption and therefore with few waste generation, which is critical for this specific application. By a simple signal processing (signal removal) the chloride ion interference on the nitrate electrode response can be eliminated. Furthermore, all fluidics management is performed by computer-controlled microvalves and micropumps, so no manual intervention of the crew is necessary. The analytical features provided by the microsystem after the optimization process were a linear range from 6.3 to 630 mg L−1 and a detection limit of 0.51 mg L−1 for the potassium electrode, a linear range from 10 to 1000 mg L−1 and a detection limit of 1.58 mg L−1 for the chloride electrode and a linear range from 10 to 1000 mg L−1 and a detection limit of 3.37 mg L−1 for the nitrate electrode with a reproducibility (RSD) of 4%, 2% and 3% respectively. Sample throughput was 12 h−1 with a reagent consumptions lower than 2 mL per analysis.

The construction and evaluation of a Low Temperature Co-fired Ceramics (LTCC)-based continuous flow potentiometric microanalyzer prototype to simultaneously monitor the presence of two ions (potassium and nitrate) in samples from the water recycling process for future manned space missions is presented. The microsystem integrates microfluidics and the detection system in a single substrate and it is smaller than a credit card. The detection system is based on two ion-selective electrodes (ISEs), which are built using all-solid state nitrate and potassium polymeric membranes, and a screen-printed Ag/AgCl reference electrode. The obtained analytical features after the optimization of the microfluidic design and hydrodynamics are a linear range from 10 to 1000 mg L−1 and from 1.9 to 155 mg L−1 and a detection limit of 9.56 mg L−1 and 0.81 mg L−1 for nitrate and potassium ions respectively.

In this paper, the universal determination of non-ionic surfactants (NIS) of the polyethoxylate type is attempted using flow-injection potentiometry. Two systems are proposed which use specifically developed tubular flow-through ion selective electrodes (ISEs). These are sensitive to NIS with an hydrophilic chain between 6 and 18 ethoxylate units, which are predominant species in the environment. An on-line pre-concentration system is designed with the aim of a possible application for the unattended monitoring of NIS in surface waters. This on-line pre-concentration is achieved by employing a column packed with a commercial solid phase extraction (SPE) sorbent for the enrichment and purification of the target analytes. The procedure outlined improves the detection limit of a direct system, decreasing it from 1×10 −4 to 3×10 −6 M by use of a pre-concentration volume of 40.0 ml and 200 μl of 75% acetonitrile in water as the eluent. Precision was estimated as 4% relative standard deviation (R.S.D.) ( n =25) for a 1×10 −6 M (0.7 ppm) nonylphenol polyethoxylate with 12 ethoxylate units when 10.0 ml of sample are pre-concentrated. Finally, the on-line pre-concentration system is applied for the total NIS determination in environmental samples from Llobregat river basin of Barcelona area (NE Spain).

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.

A method is described for the conductimetric determination of CO 2 . The proposed system was constructed with a pervaporator chamber coupled to a flow system comprised of a six-way solenoid valve, a conductivity cell, a conductivimeter and a PC microcomputer for data acquisition and external controls. An amount of 2 g of dry soil samples were directly weighted in the donor compartment of the pervaporation chamber. The CO 2 was determined from an analytical curve within 5.0 x 10 -3 -5.0 x 10 -2 mol kg -1 CaCO 3 . This curve was constructed with the matrix matching approach and presented a linear regression of y = 13.58 + 584.77r (r = 0.9962). Soils with different lime amounts were analyzed by the proposed methodology and with the official method, that uses the calcium difference between the total, extracted with HCl 1 mol l -1 and exchangeable, extracted with the KCl 1 mol l -1 After paired t-test application, no significant difference between the methods was found at the 95% probability level. The proposed method presented sampling rate of 17 h -1 , detection limit of 8.0 x 10 -4 mol kg -1 CaCO 3 , and R.S.D. < 3%. The use of the pervaporator allowed the in situ soil pretreatment for the residual lime determination.

A flow injection system for the determination of polyethoxylated nonionic surfactants is described. Potentiometric detection based on tubular flow-through ion-selective electrodes (ISEs) sensitive to this type of surfactants was used. As ion-exchanger, the plasticised PVC membrane incorporates the ion pair between the adduct of a nonionic surfactant with barium and tetraphenylborate. Two different membrane compositions were studied. They differed in the nonionic surfactant used in the ion-exchanger preparation: either nonylphenoxypolyethoxylate with 5 (Ig5) or 12 (Ig12) ethylene oxide units were used. Experimental results showed that the use of Ig5 membrane with a barium salt adjusting solution allowed lower detection limits, while Ig12 ISE with potassium salt solution presented enhanced sensitivity. Both FI systems were evaluated employing three different polyethoxylate standards, which could be determined down to ca. 10 −5 M. Precision was 3% R.S.D. for n = 50 using repetitive injections of a 3 × 10 −4 M Ig12 standard. Preliminary tests of an on-line preconcentration flow system employing the developed sensing devices are also shown. © 2001 Elsevier Science B.V. All rights reserved.

The fabrication of ion-sensitive electrochemical sensors for calcium is described in the present work. The membrane uses a photocurable polymer based on aliphatic diacrylated polyurethane. The use of photocurable polymers as the support matrix instead of PVC simplifies the preparation and the casting of the developed ion-sensitive membranes. Additionally, these polymers are compatible with the photolithographic fabrication techniques used in the microelectronics industry. This aspect permits the patterning of membranes on wafer level for semiconductor based sensors thus the automatization of the whole sensor fabrication. These polymer matrices show also better adhesion to silanised semiconductor surfaces, such as the gate surfaces of ion selective field effect transistors (ISFETs). Membranes sensitive to calcium ions were optimised according the type of plasticizer and the polymer/plasticizer ratio. Ion selective electrodes (ISEs) with a solid internal reference and ISFETs were used to evaluate the membrane. The resulting sensors were functional for periods of more than 8 months and the resulting sensitivities were quasi-Nernstian (26–27 mV/dec) in a range of 5×10 −6 –8×10 −2 M. These sensors were used to measure calcium activity in water samples extracted from agricultural soils. The results were compared with those yielded by standard methods, finding a good correlation between the two analytical procedures.

The development of a new electrochemical microsensor based on silicon technology with a photocurable enzyme membrane based on polyurethane acrylate is described. The use of photocurable polymers as the entrapping matrix for enzymes fulfils all the requirements expected for these materials without damaging the biological material. Additionally, the preparation of biosensor devices following this methodology is fast and simple, and this entails the main improvement shown by the new immobilisation method proposed here. The polymer used for enzyme immobilisation enhances the adhesion of the membrane to the previously silanised ion-sensitive field effect transistor (ISFET) gate and assures a long-term stability of more than one month. Additionally, this material provides a membrane deposition methodology compatible with photolithographic techniques allowing mass-production of low cost biosensors. The membrane has been optimised taking into account a good entrapment of the biological material and the effective transport of the substrate and the products through the membrane. After mixing the components of the prepolymer solution, a short exposure (30 s) to 365 nm UV-radiation produced a polymeric membrane without damaging the enzyme. The calibration parameters for the sensors prepared compared well with other methods yielding a slope of 58 mV/dec and a linear range of 0.04–36.0 mM urea in a NH4Cl pH 5.6 solution. The response time (t 95%) was approximately 2 min.

The application of optimised poly(vinyl chloride) matrix, anionic surfactant-sensitive membranes to ion-selective field-effect transistors (ISFETs) is described. The developed devices showed a lifetime longer than four months, improving reported values of PVC membrane-ISFETs. Other characteristics are Nernstian slopes from 59 to 62 mV/dec, detection limits of about 10 ˇ6 M and good linearity. They also showed response to several anionic surfactant species and to the reagent used for the potentiometric titration. This allowed the measurement of the overall anionic surfactant content in different samples. Apart from the basic characteristics of the membranes, this report also shows the results of potentiometric titrations using the developed ISFETs with known samples. The devices were useful in a range from 0.02 to 10 mM for dodecylbenzenesulphonate solutions. End-point potential jumps up to 250 mV were obtained in the titrations. The reproducibility, expressed as relative standard deviation, was estimated, using a standard 4 mM sodium dodecylsulphate solution, as 1.45% (na14). Finally, in a comparative study, there were no significant differences between the results produced with the standard, two-phase titration method and the proposed potentiometric titration method using surfactant ISFETs as end-point indicators. # 1999 Elsevier Science B.V. All rights reserved.

A flow-injection system based on potentiometric detection and designed for the monitoring of anionic surfactant content of printing plates washing solutions is described. Two new PVC membrane ion-selective electrodes, constructed with an all-solid-state tubular flow-through design, are used. The sample, with high ionic strength and extremely alkaline pH, is conditioned in a two-channel flow-injection system, allowing for surfactant determination in the 1 × 10 −4 to 1 × 10 −3 M range, with NaOH contents up to 0.3 M.

Results obtained in the evaluation of a mathematical model for flow-injection sandwich systems, in which the sample is inserted between two different carrier solutions with the aid of an eight-way injection valve, are presented. The model considers the system as a tubular reactor with axially dispersed plug flow, where chemical kinetics have been included. Only a minimum of parameters are needed for the model: the dispersion coefficients, which are obtained from experimental correlations for the different hydrodynamic conditions, and the rate constants, which are determined by univariate optimization. The behaviour of the model, when applied to the case of one analyte and one reagent, was evaluated for the enzymatic determinations of glucose or glycerol. The case of two analytes determined with two reagents was also studied in the combined determination of glucose and glycerol. Experimental profiles were compared with the simulated peaks and showed good agreement.

The miniaturization of analytical systems for different applications is currently a very active field of research. The inherent advantages of micro total analysis systems (muTASs) are well known. Although the fluidic platforms and the development of suitable miniaturized detection systems have been studied extensively, the integration in a single substrate of the electronics that is needed to manage the whole system using a single technology is still a sizeable challenge. In this overview, we discuss the role of the low-temperature co-fired ceramics (LTCC) as a potential alternative for miniaturizing analytical systems, since a single technology can easily combine fluidics and electronics to produce a number of novel chemical microanalyzers. The authors would like to thank the Spanish MEC for its financial support through: Consolider-Ingenio 2010 (CSD2006-00012), TEC2006-13907-C04-04/MIC and CIT-310200-2007-29. Publicado

A simple flow-injection metod for the determination of ammonium ions in water, based on a flow-through pH-ISFET, is described. Ammonium is converted to ammonia, which diffuses through a microporous hydrophic membrane into an ammonium chloride recipient stream, forming a pH buffer which is monitored by the semiconductor detector. The linear working range extends from 0.1 to 10 mmol l−1 with a relative standard deviation of ca. 1% at the 0.7 mmol l−1 level.

In this work a microflow structure, suitable for micro-FIA (micro flow injection analysis), will be described, evaluated and applied to real samples. Microchannels, the detector flow cell and input/output ports have been micromachined in silicon and sealed with anodically bonded Pyrex glass. The channels are defined by etching approximately 200 microm depth in silicon using a dry reactive ion etching (RIE) process. Optical windows integrated in the chip structure allow simple absorbance/transmission measurements to be made. The optical measurements were made using an LED as emitter (lambda=525 nm) and a photodiode as a detector. A Visual-Basic program has been developed to control an automatic burette, three-way solenoid valves and the data acquisition system. The micro-FIA for nitrite determination using the Griess-Ilosvay reaction has been implemented for the on-line monitoring of wastewater treatment plants (WWTPs). The multicommutation concept has been applied in order to enhance the mixing process inside the microsystem. Tandem streams of reagent and sample were generated and evaluated at different commutation frequencies. Two optimal frequencies, 400/200 ms and 150/450 ms, were found to be the most suitable ones. The first commutation ratio gave rise to wide linear working range (0-250 ppm), in spite of a high detection limit (0.35 ppm) and a low sensitivity (0.0041+/-0.0004 AU ppm-1). With the second ratio, the working linear range was smaller (0-50 ppm) but the detection limit (0.17 ppm) and the sensitivity (0.0091+/-0.0003 AU ppm-1) improved remarkably. Finally, real samples with a high nitrite concentration (0-1500 ppm) coming from a study of kinetic inhibition in the nitrification process at a WWTP has been analysed with the proposed micro-FIA system. The obtained results have allowed the corroboration of the model of inhibition by the nitrite ion with great exactitude.

A response function (RF) in order to simultaneously optimize different variables by the SIMPLEX method in FIA systems, with the possibility of applying weights, is proposed. The optimization program developed to use this RF is assayed in the optimization of two different flow injection procedures. The obtained results are discussed.

Flow injection methodology based on sample insertion between two different standard solutions used as carrier streams is described. This approach provides a simple system for continuous recalibration in process control; spectrophotometric and ion-selective electrode procedures are outlined.

A simply constructed flow-through pH-ISFET detector suitable for flow-injection analysis is described. Its response characteristics are presented and discussed. The calibration plot is linear in the pH range 2–12; the slope is 53 mV per decade.

A flow-through sandwich potentiometric detector for FIA in which a PVC membrane sensor is applied on a conductive epoxy support is described. It allows simple replacement of the membrane when exhausted or substituition of the sensor system and shows good mechanical and electric stability. The device can be easily included in a block with a location for the reference electrode. The detector was activated with a commercial sensor selective to nitrate and its behaviour in FIA was studied. A comparison of its perfomance with that of a nitrate selective electrode of classic shape constructed by the same technique is presented.

Samples (1500 μl) are inserted between zones of water and ascorbic acid solution with subsequent addition of 1,10-phenanthroline at pH 5.0. The signal provides a plateau region corresponding to Fe(Il) followed by a peak corresponding to total iron. The proposed system allows up to 90 injections per hour. Linear working ranges are 0.1–9 and 0.3–12 mg l−1 Fe(II) and total iron, with relative standard deviations of < 0.6 and 1.2%, respectively. Results obtained for various ground waters agree well with those obtained by a standard method.

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.

An approach to automation, in routine control, for the determination of anionic surfactants in river water and treatment plant water in the concentration range 0.04–3.5 µg ml–1 using a continuous solvent extraction process with flow injection analysis, is described. The method is based on an ion-pair extraction reaction with Methylene Blue in chloroform. The separation of both phases is accomplished by means of a membrane phase separator and detection is based on injection of the coloured organic phase into a chloroform stream which passes to the detector. Methanol was added to the organic phase in order to enhance the extraction process and to ensure efficient extraction of the various anionic surfactants. A number of anionic species and non-ionic surfactants have been tested as possible interferents. The results obtained for samples of river water show good agreement with those obtained with the Methylene Blue batch method.

A tubular PVC membrane electrode for calcium without inner reference solution and a device for location of the reference electrode are described. In the flow-injection system, calcium is determined potentiometrically and then magnesium is determined by atomic absorption spectrometry. The electrode provides linear response to calcium in the range 5 × 10 −5 /2-10 −1 M. On-line dilution of the sample allows magnesium determination in the range 0/2-10 mg l − . Flow rates between 3 and 6 ml min −1 are possible. The sampling frequency is 60/2-90 h −1 .

A very simple flow injection analysis system for direct determination of glucose in blood serum based on Trinder's reaction is described. The sera samples (15 μl) can be injected directly to the system without the deproteinization or the use of a dialyzer. Calibration curves are linear in the range 50–400 mg/dl. The sampling frequency is 60 samples per hour. Results obtained by the proposed procedure are compared with those acquired at a local hospital using their routine glucose procedure also based on Trinder's reaction. It is shown that a better mix between sample and reagents is achieved using the single bead string reactor (SBSR).

A simply constructed, tubular, all-solid-state, flow-through silver electrode for flow-injection analysis (F.I.A.) is described. Use of a single line manifold accommodating the silver electrode, with a low level of silver ion (5 × 10 −4 M) in the carrier stream, is a useful method to determine chloride in serum, by means of a precipitation pseudotitration F.I.A. technique. The sampling frequency is about 60/h.

The construction, assessment and pharmaceutical appalications of a tubular potentiometric detector sensitive to the 5,5-diethylbarbiturate anion based on quaternary ammonium salt (tetraoctylammonium5,5-diethylbarbiturate) dissolved in o-nitrophenyl octyl ether immobilized in PVC are described. The intrinsic characteristics of the tubular electrode were determined in a low-dispersion manifold and compared with those of a conventional-shaped electrode with no internal reference solution and incorporating the same sensor. The behaviour of a double-channel flow-injection manifold with the same potentiometric detector was evaluated to assess its applicability to the determination of 5,5-diethylbarbiturate in pharmaceutical preparations. Data obtained in the determination of the anion in the latter matrices are presented and compared with those obtained by the conventional British Pharmacopoeia method.

An automated spectrophotometric method for the determination of titanium(IV) is described based on the reaction between titanium(IV) and hydrogen peroxide in a flow injection system. The system was optimised by a suitable numerical treatment of the different variables using the simplex algorithm. A wide linear range up to 1000 p.p.m. can be achieved at a sampling rate of 240 samples h–1. The method was applied to the determination of low titanium(IV) concentrations in the presence of high concentrations of salts. Under these conditions, the calibration is linear up to 30 p.p.m. and a sampling rate of up to 120 samples h–1 can be achieved. A study of possible interferences demonstrated that the method has good selectivity, but vanadium(V) and molybdenum(VI) interfere.

A compact and robust flow injection system for the simultaneous determination of calcium and nitrate in waters, with sequential detection by potentiometric sensors, is described. The effects of silver ionic strength adjuster (ISA) or lead ISA carrier solutions on the response of the tubular electrodes used and on the ability of the system to control interferences were studied in detail. Under the selected experimental conditions, the response slope and lower limit of linear response were 53.4 mV per decade and 10–4.3 mol l–1 for the nitrate sensor and 31.3 mV per decade and 10–4.75 mol l–1 for the calcium sensor. The reproducibility (relative standard deviation) was better than 1% and the sample throughput was approximately 145 h–1. The results provided by the flow system were shown to be accurate, reproducible and free from interferences. The results obtained for calcium and nitrate in 23 samples with the flow injection system and with a reference method were in good agreement with each other. No significant interferences from other ions, especially chloride and hydrogencarbonate, were observed in surface waters, excluding those with a high saline content. The sequential method described was shown to be useful in routine determinations with a high sampling rate at a low cost.

The Powell algorithm is adapted for the optimization of a flow-injection system for the spectrophotometric determination of ammonia based on the indophenol blue reaction. Its performance is compared with that of the modified simplex method in experimental optimization, and in simulation of the course along the experimental response surface, which can be modelled conveniently for this analytical system. The Powell algorithm needed fewer evaluations of the objective function (maximal sensitivity and sample throughput), thus minimizing experimental work, particularly in the initial optimization.

A methodology to design a flow- injection system with optimized performance characteristics for different experimental requirements is presented which requires minimal experimental effort. Departing from basic experimental knowledge of the hydrodynamic parameters and rates of the reactions involved, the coupling of a mathematical model describing the flow system with the Powell's optimization algorithm allows the automated generation of a complete set of flow- injection configurations, each of them being optimal with respect to its objective function. From this set of configurations, the performance of the flow- injection system is easily visualized, allowing the user to select the system that better suits the requirements of his particular application and exploiting the potentiality of multiple flow- injection optimal configurations.

A simply constructed tubular PVC matrix membrane electrode without inner reference solution suitable for flow injection analysis is described. The nitrate-selective electrode developed allows simple replacement of an exhausted membrane, or change of function with a new sensor membrane. The compact design ensures mechanical stability and the tubular configuration minimizes distortion of the hydrodynamic flow in the detector. Nitrate can be determined in the range 10−1–10−5 M, under different flow conditions, with a sampling frequency of about 120 h−1.

A flow injection system for the simultaneous determination of total Fe and CrVI based on the sandwich technique is described. Both metals were determined spectrophotometrically at 526 nm with a single detector over the range 0.2–10 p.p.m. The sampling frequency achieved was 60 samples h–1 and the standard deviations were 0.09 and 0.08 p.p.m. for total Fe and CrVI, respectively.