A flow analysis system with an amperometric detector for the determination of hydrogen sulphide in waters

 A flow analysis system with an amperometric H₂S detector and a gas extraction unit as well as an integrated coulometric calibration unit is described, which allows an on-line determination of hydrogen sulphide in aquatic samples. By variation of different parameters (e.g. flow rate, gas injection volume, pH of solution) a wide dynamic working range of concentrations from 1 μmol/l H₂S to 750 μmol/l is accessible. The sampling rate is about 36 samples h⁻¹ using an average flow rate of 1.78 ml/min and a gas injection volume of 28 μl. The measuring system is designed as a portable device. In combination with the polyethylene-tube of a PTFE-underwater pump field-measurements on board are possible. Received: 16 February 1995/Revised: 5 April 1995/Accepted: 11 April 1995 Acknowledgements. This work was supported by the Bundesminister für Forschung und Technologie under the project “DYSMON II” (03F0123D) and by the Fonds der Chemischen Industrie zur F?rderung von Chemie und Biologischer Chemie. Correspondence to: P. Jeroschewski     

Inorganic tin(II) determination by FIA with amperometric detection of its oxinate complex

A highly selective, rapid and direct amperometric method, based on the formation of a complex between tin(II) and 8-hydroxyquinoline (oxine), has been developed for the determination of trace levels of tin(II) using flow injection analysis. Tin(II) electro-oxidation was catalyzed by oxine; its oxidation peak occurred at +0.05 V vs. Ag/AgCl at a glassy carbon electrode in 0.1 mol 1⁻¹ acetate buffer (pH 6). A linear relationship was obtained between the peak current and the tin(II) concentration in the range 0.25-20 μmol 1⁻¹. The detection limit was 0.1 μmol 1⁻¹ and the relative standard deviation calculated by the injection of a 10 μmol 1⁻¹ tin(II) solution was 5% (n = 20). Optimization of several experimental parameters has been carried out and the influence of numerous cations and possible interfering molecules encountered in radiopharmaceuticals and in dental gels has been investigated. The method was applied to the determination of tin(II) in dental gels.     

Indirect determination of iron in a flow-injection system with amperometric detection

The change in peak current resulting from the reaction of Fe(II) with nitroso-R salt in a flow-injection system is used to quantify Fe(II) with either single- or dual-electrode amperometric detectors. The current change varies linearly with Fe(II) concentration from 0 to 200 mg 1^?1. The relative standard deviation was about 5% with the single-electrode detector and about 10% with the dual-electrode detector. The method is evaluated for the determination of iron in dietary supplements.     

Evaluation of the 'antioxidant power' of olive oils based on a FIA system with amperometric detection

A new method for the evaluation of the 'total antioxidant power' of olive oils, based on a flow injection analysis system with electrochemical detection, is described. It represents a attractive alternative to the mostly used Rancimat method since it is based on the chemical structure of antioxidants and does not require the manipulation of several parameters, such as temperature and oxygen pressure, to accelerate oil oxidation. The proposed procedure is simple, rapid, allows a throughput of 90 samples h-1 and provides a good precision: an RSD of 3.5% was obtained for caffeic acid at the concentration level of 5 mg L-1 (n = 12). A comparison of the proposed was obtained for caffeic acid at the concentration level of 5 mg L-1 (n = 12). A comparison of the proposed procedure with two other methods (Rancimat method and ABTS.+ decoloration assay) was performed to investigate the applicability and limitations of the proposed method.     

Enzymatic determination of choline in milk using a FIA system with potentiometric detection

The development of a FIA system for the determination of total choline content in several types of milk is described. The samples were submitted to hydrochloric acid digestion before injection into the system and passed through an enzymatic reactor containing choline oxidase immobilised on glass beads. This enzymatic reaction releases hydrogen peroxide which then reacts with a solution of iodide. The decrease in the concentration of iodide ion is quantified using an iodide ion selective tubular electrode based on a homogeneous crystalline membrane. Validation of the results obtained with this system was performed by comparison with results from a method described in the literature and applied to the determination of total choline in milks. The relative deviation was always < 5%. The repeatability of the method developed was assessed by calculation of the relative standard deviation (RSD) for 12 consecutive injections of one sample. The RSD obtained was < 0.6%.     

Optimization of a FIA system with amperometric detection by means of a desirability function: determination of sulfadiazine, sulfamethazine and sulfamerazine in milk

A sensitive and cheap FIA, with amperometric detection, analytical procedure is developed in this paper to determine sulfadiazine, sulfamethazine and sulfamerazine in milk. A multicriteria optimization based on the use of a desirability function is used for optimizing two analytical responses (peak height and its variability) since single-objective optimizations lead to conflicting experimental conditions. In the optimum conditions, the determination of the three sulfonamides in milk samples is carried out, the analytical procedure being validated according to Commission Decision 2002/657/EC. The decision limit at 0 and 100 microg L(-1) (which is the maximum residue limit in milk) are 13.9 and 110.2, 9.5 and 107.1 and 9.1 and 107.1 microg L(-1) for sulfadiazine, sulfamethazine and sulfamerazine, respectively. Whereas the values of capability of detection, CCbeta, obtained at 0 and 100 microg L(-1) were 26.9 and 119.8, 18.2 and 113.6, and 17.5 and 113.7 microg L(-1) for sulfadiazine, sulfamethazine and sulfamerazine, respectively. Recovery values between 67.4% and 119.1% are found for milk test samples of two brands of milk. The accuracy of the method is confirmed. The ruggedness of the procedure is evaluated by means of a Plackett-Burman design. The relative errors were lower than 2.5% (n=7).     

Optimization of a flow injection system with amperometric detection for arsenic determination.

A selective and sensitive analytical procedure for rapid arsenic determination by gas-diffusion flow injection analysis with amperometric detection was developed. The method is based on the arsenite reduction by NaBH(4). Derived arsine diffuses through a PTF membrane into the acceptor flow stream and is amperometrically determined on a platinum working electrode. The limit of detection (3 sigma) at room temperature was 5 microg/dm(3) of As(III). The relative standard deviation for a 1 mg/dm(3) As(III) standard was 1.96% for six repetitive injections. Arsenic(V) was determined after its prereduction with potassium iodide. Arsenic determination was not interferred with by 1 mg/dm(3) Sb(III), 5 mg/dm(3) Sn(II), 10 mg/dm(3) Se(IV), 1 mg/dm(3) As(V), 1 mg/dm(3) hydrasine, 1 mg/dm(3) Fe(II) or 0.5 mg/dm(3) Fe(III) solution. The throughput of this method was 60 analyses per hour. This method was successfully applied to arsenic determination in some power plant waste water samples.     

A flow-injection system with amperometric detection for selective determination of antioxidants in foodstuffs and drinks

Amperometric detection in flow-injection systems intended for total determination of antioxidants (flavonoids, oxyaromatic acids, etc.) in various foodstuffs (berries, fruit, vegetables) and drinks (wine, tea, coffee, cognac, beer, etc.) was described and discussed. Advantages of this method were demonstrated. These measurements can underlie building of a bank of data on the content of antioxidants in foodstuffs and drinks.     

Novel determination system for urea in alcoholic beverages by using an FIA system with an acid urease column.

A novel determination method for urea using an acid urease column-FIA system was developed, and the system was applied to the determination of urea in rice wine. This novel FIA system was characterized by CO2 detection due to the property of acid urease and by a microfluidic gas-diffusion device with the use of an ultra-thin hollow fiber membrane. A biosensing system fabricated in this study was assembled with a double-plunger pump, a sample-injection valve, an immobilized acid urease column as a recognition element for the assay of urea, a gas-diffusion unit, and a flow-type spectrophotometer. The gas-diffusion unit consisted of a double-tubing structure in which the outer tubing was made of PTFE (i.d. 1.0 mm; o.d. 1.5 mm) and the inner tubing was of porous PTFE (i.d. 0.19 mm; o.d. 0.25 mm). Standard urea solutions (20 microl) were measured through monitoring variations in the absorbance of a coloring agent solution resulting from a pH shift due to carbon dioxide molecules being enzymatically generated. A wide and linear relationship was obtained between the concentration of urea (16 microM - 1.0 mM) and the change in absorbance. This FIA system has great advantages that the system did not suffer from ammonia and ethanol in samples. This system, armed with a microfluidic gas-diffusion device, was applicable to the determination of various substrates of many kinds of decarboxylase, amino-acid oxidase, and amino-acid oxygenase, producing CO2 and NH3 molecules.     

Enzymatic methods for the determination of alpha-glycerophosphate and alpha-glycerophosphate oxidase with an automated FIA system

A procedure for the enzymatic determination of alpha-glycerophosphate (alpha-GP) has been developed, using an automated in-house FIA system, with immobilized glycerol-3-phosphate oxidase (GPO) on non-porous glass beads, following optimization of the immobilization and analytical parameters. Fabricated single bead string reactors (SBSR) were used in connection with the FIA system, following optimization of its parameters. The half-life of GPO-SBSR regarding reduction of the enzyme activity was found to be 110 days for its use in 20 triplicate measurements daily and storage at 4 degrees C in the appropriate buffer. The regression equation of the calibration graph for the determination of alpha-GP was: A(max)=(10+/-2)x10(-4)+(22 134+/-12)x10(-4) (mmol l(-1)alpha-GP). The lower limit of quantitation was 0.74 mumol l(-1)alpha-GP and the RSD of the method 0.05% (r=0.9999). The same FIA system and procedure can be also used for the determination of the GPO activity, with the alpha-GP as substrate. The regression equation for this calibration graph was: A(max)=(23+/-18)x10(-4)+(190+/-1)x10(-4) (mug ml(-1) GPO), the lower limit of quantitation was 0.782x10(-3) mg ml(-1) (0.782 ppm) GPO and the RSD of the method 0.53% (r=0.9999). Serum samples obtained from hospitalized patients were deproteinized by gel filtration and analyzed under pseudo-first order conditions, at various concentrations of alpha-GP. A kinetic study of the reduction of alpha-GP in serum versus time is given and an observed reaction rate constant k(ob)=106.5x10(-4) min(-1) was determined.     

Effects of the detection window on the determination of organic copper speciation in estuarine waters

The complexation of copper by natural organic ligands in sea water was measured by cathodic stripping voltammetry with ligand (catechol and quinolin-8-ol) competition. Two methods to determine copper complexation in estuarine waters were compared, one based on a complete titration of the complexing capacity of the sample and the other on measurement of the labile and total dissolved metal concentrations only. Values for log α_CuL (ihe α-coefficient for complexation of Cu²⁺ by natural organic ligands) ranging from 3.2 to 7.7 and from 3.3 to 7.8 could be detected by varying long α_CuAL (the α-coefficient for complexation of Cu²⁺ by the added competing ligand) from 3.4 to 8.9 in samples from the Tamar estuary and from the Channel. The two methods gave comparable results and showed that the type of sites detected depends on the detection window of the technique. This effect is due to the sea and estuarine water samples containing a series of complexing ligands forming complexes of greatly varying strength, thus causing a rnage of complex stabilities to be measured as a function of the detection window of each technique. A comparison showed that lower values for α_CuL are obtained by anodic stripping voltammetry as a result of that technique having a lower detection window. A detailed study of the Tamar estuary revealed a decrease in log α_CuL from 10.8 to 8.3 with increasing salinity, demonstrating that major cations compete with copper for the complexing sites. The free Cu²⁺ concentrations were very low throughout the estuary (16.2 < pCu²⁺ < 18.2) even though the total measurements to establish potential toxic effects of copper in natural waters.     

Determination of glycerol in alcoholic beverages using packed bed reactors with immobilized glycerol dehydrogenase and an amperometric FIA system

A flow injection analysis system incorporating amperometric detection and enzyme reactor for glycerol determination in alcoholic beverages is described. The reactor is based on the glycerol dehydrogenase system, and the enzyme was immobilized through chemical modification on several supporting materials such as aminopropyl and isothiocyanate controlled pore glass, aminopolystyrene resin and m-aminobenzyloxymethyl cellulose. NADH, the product of the enzymatic reaction, was monitored amperometrically with a three-electrode wall-jet type flow through cell, at + 0.5 V vs. Ag/AgCl. The method was evaluated in the presence or absence of potassium and the following linear dynamic ranges were found: 2 x 10(-5) -2 x 10(-4) mol l(-1) and 4 x 10(-5) -4 x 10(-4) mol l(-1), respectively. The interference effects of various compounds were also studied. The relative standard deviation was found to be better than 1.0% (n = 6). The reactors are stable for over a period of 3 months and after about 2500 injections. Under optimum working conditions the sampling frequency was 30 samples h(-1). The successive application of the method was confirmed by comparison with a reference method. The mean relative error is 2.2% and the recovery 95-102%.     

New automatized method with amperometric detection for the determination of azithromycin

A FIA-amperometric method for azithromycin determination was developed. A working glassy carbon electrode and a Ag/AgCl/NaCl (3 M) reference electrode were used. The determination is based on the electrochemical oxidation of the azithromycin at 0.9 V in Britton-Robinson buffer solution (pH 8.0). Due to the adsorption of the reaction products on the electrode surface, an effective cleaner cycle was implemented. By using the optimum chemical and FIA conditions, a concentration linear range of 1.0-10.0 mg L⁻¹ and a detection limit (LOD) of 0.76 mg L⁻¹ are obtained. The method was validated and satisfactorily applied to the determination of azithromycin in pharmaceutical formulations.     

Multienzymatic-rotating biosensor for total cholesterol determination in a FIA system

Fabrication of an amperometric-rotating biosensor for the enzymatic determination of cholesterol is reported. The assay utilizes a combination of three enzymes: cholesterol esterase (ChE), cholesterol oxidase (ChOx) and peroxidase (HRP); which were co-immobilizing on a rotatory disk. The method is developed by the use of a glassy carbon electrode as detector versus Ag/AgCl/3 M NaCl in conjunction with a soluble-redox mediator 4-tert-butylcatechol (TBC). ChE converts esterified cholesterol to free cholesterol, which is then oxidized by ChOx with hydrogen peroxide as product. TBC is converted to 4-tert-butylbenzoquinone (TBB) by hydrogen peroxide, catalyzed by HRP, and the glassy carbon electrode responds to the TBB concentration. The system has integrated a micro packed-column with immobilized ascorbate oxidase (AAOx) that works as prereactor to eliminate l-ascorbic acid (AA) interference. This method could be used to determine total cholesterol concentration in the range 1.2 μM-1 mM (r = 0.999). A fast response time of 2 min has been observed with this amperometric-rotating biosensor. Lifetime is up to 25 days of use. The calculated detection limits was 11.9 nM. Reproducibility assays were made using repetitive standards solutions (n = 5) and the percentage standard error was less than 4%.     

Spectrophotometric catalytic determination of Fe(III) in estuarine waters using a flow-batch system

A flow-batch system was developed for the determination of Fe(III) in estuarine waters with high variability in salinity. The method is based on the catalytic effect of iron(III) on the oxidation rate of N,N-dimethyl-p-phenylenediammonium dichloride (DmPD) by hydrogen peroxide and the formed product is spectrophotometrically monitored at 554 nm. A controlled addition of sodium chloride to every assayed sample is accomplished for in-line individual salinity matching.The proposed system processes about 30 samples h⁻¹ and yields reproducible results. Relative standard deviations were estimated as <1.5% after 10 injections of typical samples (10.0-50.0 μg l⁻¹ Fe; ca. 0.5 mol l⁻¹ Cl). Synthetic samples (15.0 μg l⁻¹ Fe; 0.25-1.0 mol l⁻¹ NaCl) were efficiently processed, and no significant differences in results were found at a probability level of 99.7%. The method works for the full range of salinities. Only 120 μg DmPD are consumed per determination. The analytical curve is linear up to about 60 μg l⁻¹ Fe (r>0.999; n=5) and the detection limit is 5 μg l⁻¹ Fe. Results are in agreement with graphite furnace atomic absorption spectrometry.     

Flow injection analysis system based on amperometric thin-film transducers for free chlorine detection in swimming pool waters

This work reports on the performance of a user-friendly flow injection analysis (FIA) system for the monitoring of free chlorine. A methacrylate flow cell integrating a gold thin-film microelectrode, together with an on-chip gold counter electrode, both fabricated by microfabrication technology, provided robustness, low output impedance, rapid response and low cost to the proposed flow system. An external Ag/AgCl reference electrode placed downstream the chip completes the electrochemical cell. Amperometric detection of chlorine was carried out at a set potential of +350 mV, without oxygen interference. The proposed flow system responded linearly to chlorine concentrations in a range from 0.2 to 5 mg l⁻¹, with a sensitivity of 0.23 μA l mg⁻¹, the estimated limit of detection being 0.02 mg l⁻¹. In addition, the system response was kept stable for at least 10 days (±3σ criterion), by keeping the flow system in an inert atmosphere when not in use. Fifteen samples of swimming pool waters were analyzed and no matrix effects were detected. Also, results were in good agreement with those obtained by a standard method. The excellent analytical performance of the system together with its good working stability would also enable its application for the detection of chlorine in other matrices such as tap water or chlorine stock solutions.     

A mediator-free screen-printed amperometric biosensor for screening of organophosphorus pesticides with flow-injection analysis (FIA) system

A mediator-free amperometric biosensor for screening organophosphorus pesticides (OPs) in flow-injection analysis (FIA) system based on anticholinesterase activity of OPs to immobilized acetylcholinesterase enzyme (AChE) has been developed. The enzyme biosensor is prepared by entrapping AChE in Al₂O₃ sol-gel matrix screen-printed on an integrated 3-electrode plastic chip. This strategy is found not only increase the stability of the embedded AChE, but also effectively catalyze the oxidative reaction of thiocholine, making the Al₂O₃-AChE biosensor detects the substrate at 0.25 V (versus Ag/AgCl), hundreds mini-volt lower than other reported mediator-free ones. The Al₂O₃-AChE biosensor is thus coupled to FIA system to build up a simple and low-cost FIA-EC system for screening OPs in real samples. A wide linear inhibition response for dichlorvos, typical OP, is observed in the range of 0.1-80 μM, corresponding to 7.91-84.94% inhibition for AChE. The detection limit for dichlorvos is achieved at 10 nM in the simulated seawater for 15 min inhibiting time, which allows the biosensor quantitatively detects the ecotoxicological effect of the real samples from the seaports in eastern China, where the OPs pollution is confirmed by GC-MS.     

HPLC with amperometric detection for the determination of pesticides of high polarity

A relatively simple procedure is described for the selective and highly sensitive determination of amitrole (3-amino-1,2,4-triazole) by ionpair HPLC with amperometric detection. The separation was achieved on a polar CN-column using hexane, propanol, water (62:36:2) with LiClO₄ (2.5 g·l^–1) and trichloroacetic acid (2 g·l^–1) as electrolytes. The determination range lies between 500 pg and 500 ng and the limit of detection was found to be 200 pg (s.d.: 11%; N=9). The minimum detectability for the alternative UV-monitoring was 2.5 ng. The developed method may be helpful in production and quality control and for the selective determination of amitrole in commercial formulations.