Flow-injection determination of 9,10-phenanthrenequinone with catalytic photometric detection

A catalytic photometric method with a flow-injection system is described for the determination of 9,10-phenanthrenequinone. It is based on the catalytic effect of 9,10-phenanthrenequinone on the redox reaction of 1,2-dinitrobenzene with formaldehyde under alkaline conditions. 9,10-Phenanthrenequinone at the 5.0 x 10(-8)-5.0 x 10(-6)M level can be determined at a rate of 20 samples/hr. The detection limit is 1.0 x 10(-8)M (40 pg in a 10-microl injection).     

Flow-injection determination of secondary amines in non-aqueous solution with fluorescence detection

The procedure is based on derivatization with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole to form a fluorescent adduct. Interference from primary amines is overcome by on-line chemical masking with an equimolar mixture (58 mM) of o-phthalaldehyde and 2-mercaptoethanol with a tolerance of 20 mM 1-ethylpropylamine for the determination of dibutylamine (0.3 mM). The r.s.d. for dihexylamine (0.34 mM) is 1.3% (n=5) and the limit of detection (unweighted least-squares method) is 0.06 mM.     

Flow-injection determination of sugars with immobilized enzyme reactions and chemiluminescence detection

Glucose is determined by reaction with gluocose oxidase to produce hydrogen peroxide which is quantified via a chemiluminescence reaction with luminol. Sucrose, maltose, lactose and fructose are determined by enzymatic conversion to glucose (using invertase, amyloglucosidase, lactase. and glucose isomerase, respectively) and subsequent determination of the glucose, All enzymes are immobilized on controlled-pore glass and contained in flow-through reactors. For glucose, sucrose, and maltose the linear log-log working range 0.2 μM-1 mM, with a detection limit of 0.1 μM; for lactose and fructose the linear working range is 3 μM-1 mM with a detection limit of 1 μM. Assay time is 2 min.     

Flow-injection methods for the determination of uracil derivatives with voltammetric detection

Flow-injection methods are described for the determination of 18 uracil derivatives and related compounds, by means of differential-pulse amperometry (d.p.a.) or differential-pulse cathodic stripping voltammetry (d.p.c.s.v.). The carrier stream is a borax/KNO₃/HNO₃ (of NaOH) solution containing 0.001% (v/v) Triton X-100. This surfactant displaces the oxygen reduction peak to such negative potentials that deaeration is unnecessary for detection of compounds having peak potentials in the range 180–70 mV (vs. Ag/AgCI) at pH 7.6. At the hanging mercury drop electrode, the uracil derivative is deposited from the flowing sample at a fixed potential more positive than the relevant peak potential and stripped under stopped-flow or slow-flow conditions. In the amperometric mode, a constant potential also more positive than the relevant peak potential is applied to the dropping mercury electrode and the resulting peak is measured under flow conditions. Linear calibration graphs were found for most of the compounds at 10^?6–10^?7 M by d.p.a, and about one order of magnitude lower by d.p.c.s.v.. The limit of determination for 5-iodouracil was 5×10^?9 M (ca. 1.2 ng ml^?1). Separation is needed for applications to blood or urine. Simple deproteination followed by high-performance liquid chromatography with a reversed-phase column proved satisfactory. Separations of various uracil derivatives, and of 5-fluorouracil, uric acid and 5-fluorodeoxyuridine, are described; spectrophotometric and amperometric detectors were used sequentially to check performance.     

Flow-injection catalytic determination of molybdenum with blamperometric detection in a microprocessor-controlled system

The flow-injection determination of molybdenum(VI) is based on its catalytic effect on the oxidation of iodide by hydrogen peroxide. The triiodide ion formed in this reaction is detected amperometrically in a flow-through cell containing two platinum wire electrodes polarized at 100 mV. After optimization of the measuring conditions, the detection limit is 1.2 μg l^?1 Mo(VI) and the linear range extends to 1 mg l^?1. Interference of various metal ions and their removal is described. The procedure was tested on the determination of molybdenum(VI) in soil extracts.     

Flow-injection determination of ascorbic acid and cysteine simultaneously with spectrofluorometric detection.

A simple and sensitive spectrofluorometric method was developed for the simultaneous determination of ascorbic acid and cysteine by a flow-injection system. This method is based on the reduction of Tl(III) with ascorbic acid or cysteine in acidic media, producing fluorescence reagent, TlCl3(2-) (lambdaex = 227 nm, lambdaem = 419 nm). The injected sample solution was divided into two separate streams. The first stream was treated with Tl(III) at pH 3.0 and then passed through a 270 cm reaction coil to the flow cell of the spectrofluorometer, where the fluorescence intensity was measured. This signal is related to ascorbic acid and cysteine concentration. The second part of the injected sample solution was treated with Tl(III) in HCl solution and then passed through a 50 cm reaction coil to the flow cell and the fluorescence intensity was measured. This signal is related only to cysteine. Thus, the ascorbic acid content was determined directly by the difference according to the calibration curve. Ascorbic acid and cysteine can be determined in the range of 1 x 10(-6) to 5.0 x 10(-5) M, at a rate of 16 samples per hour. The limits of detection (S/N = 3) were 8 x 10(-7) M for ascorbic acid and 7 x 10(-7) M for cysteine. The influence of potential interfering substances was studied. The proposed method was successfully applied to the simultaneous determination of both analytes in real samples.     

Flow-injection determination of lead by hydride generation and conductometric detection

Plumbane produced from the lead analyte in a flow-injection manifold by reaction with sodium borohydride is passed through a porous poly(tetrafluoroethylene) membrane in a gas-diffusion cell. The hydride reacts with bromine in the acceptor stream resulting in ionization which is detected by conductivity measurement. Direct mixing of the carrier with a reagent stream yields a limit of detection of approximately 1 mg/L. An improved detection limit of about 200 g/L can be achieved by the incorporation of an auxiliary stream containing persulphate as oxidizing agent. The application of the method to the determination of lead in road dust and soil samples is demonstrated.     

Flow-injection determination of lead by hydride generation and conductometric detection

Plumbane produced from the lead analyte in a flow-injection manifold by reaction with sodium borohydride is passed through a porous poly(tetrafluoroethylene) membrane in a gas-diffusion cell. The hydride reacts with bromine in the acceptor stream resulting in ionization which is detected by conductivity measurement. Direct mixing of the carrier with a reagent stream yields a limit of detection of approximately 1 mg/L. An improved detection limit of about 200 microg/L can be achieved by the incorporation of an auxiliary stream containing persulphate as oxidizing agent. The application of the method to the determination of lead in road dust and soil samples is demonstrated.     

Flow-injection determination of trace amounts of dopamine by chemiluminescence detection

A flow-injection analysis (FIA) for the determination of dopamine has been developed. The method is based on the inhibition effect of dopamine on the iron(II)-induced chemiluminescence (CL) of 10,10'-dimethyl-9,9'-biacridinium dinitrate (lucigenin). The presence of a non-ionic surfactant, polyoxyethylene (23) lauryl ether (Brij 35), caused an increase in the inhibition effect. The present method allows the determination of dopamine over the range 1x10(-8)-2x10(-7) mol dm(-3). The relative standard deviation was 0.7% for eight determinations of 6x10(-8) mol dm(-3) dopamine. The detection limit (S/N=3) was 2x10(-9) mol dm(-3) with the sampling rate of 40 samples h(-1). The effect of other catecholamines and compounds of similar structure on the lucigenin CL reaction was studied: quinone, hydroquinone, norepinephrine, pyrocatechol and l-dopa suppressed the CL intensity.     

Flow-injection system with ion-exchange separation and potentiometric detection for the determination of three halides

The use of ion exchangers in flow-injection systems is reviewed briefly. In the method described, halides are separated on a short column of a strongly basic ion-exchange resin (Dowex 1-X8) placed in the flow-injection conduit, with a laboratory-made tubular silver/silver halide ion-selective electrode as potentiometric sensor. The response capabilities of the different halide-selective electrodes to a wide concentration range (20-5000 mg dm^?3) of single and mixed halide solutions with and without the incorporated ion-exchange column are compared. By careful selection of suitable concentrations of the potassium nitrate carrier/eluent stream to satisfy the requirements of both the ion-exchange column and the halide-selective electrode, it is possible to separate and determine chloride, bromide and iodide in mixed halide solutions with a detection limit of 5 mg dm^?3. The bromide-selective electrode is the most satisfactory detector.     

Flow-injection determination of l-tyrosine in serum with an immobilized tyrosinase reactor and fluorescence detection

Tyrosinase is immobilized on controlled-pore glass beads and packed into a stainless-steel column (5 cm × 4 mm i.d.). Serum is deproteinized with tungstate and sulphuric acid. The carrier stream is 0.3 M phosphate buffer (pH 7.2), and is mixed with 5 M potassium hydroxide after the enzyme reactor. The fluorescent dihydroxyindole formed is detected at 490 nm (excitation at 375 nm). The calibration graph is linear for 1 × 10^?7 ?1 × 10^?4 M tyrosine; the detection limit is 2 × 10^?8 M.     

Flow-injection determination of nitrite and nitrate with biamperometric detection at two platinum wire electrodes

The flow-injection determination of nitrite is based on oxidation of iodide by nitrite. The triiodide formed is detected amperometrically in a flow-through cell with two teflonized graphite or platinum wire electrodes polarized with a voltage of 100 mV. More sensitive and faster response was observed with the platinum wire electrodes. The same detector is used for determination of nitrate after reduction to nitrite in a reductor column containing copperized cadmium. Detection limits under optimized conditions are 6 μg l^?1 for both nitrite- and nitrate-nitrogen. Effects of oxygen and interfering metal ions are discussed.     

Flow-injection spectrophotometric determination of salbutamol with 4-aminoantipyrine

A flow-injection method is proposed for the determination of salbutamol. The method involves the condensation of salbutamol with 4-aminoantipyrine in the presence of hexacyanoferrate (III) in alkaline medium, producing a coloured quinoneimide that was detected absorptiometrically at 500 nm.

The values of four variables (two reactor lengths and two reagent concentrations) were optimised by means of the sequential simplex method and their influence studied in univariant way.

The method was validated and compared with the HPLC method established in the United States Pharmacopeia (USP). Linearity was demonstrated in the range 0–74.1 mg/L of salbutamol sulfate (r² = 0.9999). Commercial samples of pharmaceuticals containing salbutamol sulfate (tablets and oral solutions) were analysed and the results obtained with the proposed method agreed with the USP method in less than 1.6%, with precision similar to the HPLC method (1%–2% R.S.D.). The sampling frequency was 75 samples/hour.     

Flow-injection extraction-spectrophotometric determination of perchlorate with brilliant green

Perchlorate (0-2.5 μg ml^?1) is determined spectrophotometrically at 640 nm after extraction into benzene of its ion-associate with Brilliant Green in a flow-injection manifold with a membrane separator. The injection rate is 20 h^?1. The detection limit is 36 ng ml^?1, based on 250-μl injections. The system is applied to the determination of perchlorate in potassium chlorate after prior selective destruction of chlorate.     

Flow-injection determination of iodide with a silver electrode

A procedure is developed for the flow-injection determination of iodide using a modified silver electrode; it extends the analytical range of iodide by an order of magnitude as compared to potentiometric determination using an iodide-selective electrode with a polycrystalline membrane. The detection limit is 7 μg/L. The procedure was used to determine iodide in natural mineral waters and model solutions. The throughput was 20–25 samples/h.     

Flow-injection determination of malate with immobilized malate dehydrogenase

Malate dehydrogenase (MDH) is immobilized chemically on controlled-pore glass and used on-line in a glass minicolumn (25×2.5 mm i.d.). Malate solution passes through the minicolumn of immobilized MDH and the NADH formed is monitored spectrophotometrically from 9 × 10-^?4 down to 7 × 10^?6 M (36 ng in 40 μl) at 50 samples h^?1.     

Flow-injection spectrofluorimetric determination of ethylenethiourea

A flow-injection configuration is proposed for the fluorimetric determination of ethylenethiourea. The procedure is based on the inhibitory effect of ethylenethiourea on the oxidation of thiamine to thiochrome by mercury(II). A linear calibration graph was obtained between 0.1 and 2.0 μg mL^–1, with a sampling rate of 40 samples per hour and a relative standard deviation of about 1.11%. The usefulness of the method was tested for the determination of ethylenethiourea residues in water, milk, potatoes, pear, grape and apple. Received: 26 January 1998 / Revised: 6 April 1998 / Accepted: 9 April 1998