A compact flow injection analysis system for surface mapping of phosphate in marine waters

The design, construction and validation of a compact, portable flow injection analysis (FIA) instrument for underway analysis of phosphate in marine waters is described. This portable system employs gas pressure for reagent propulsion and computer controlled miniature solenoid valves for precise injection of multiple reagents into a flowing stream of filtered sample. A multi-reflection flow cell with a solid state LED photometer is used to detect filterable reactive phosphate (0.2 mum) as phosphomolybdenum blue. All the components are computer controlled using software developed using the Labviewtrade mark graphical programming language. The system has the capacity for sample throughput of up to 380 phosphate analyses per hour, but in the mode described here was operated at 225 analyses per hour. Under these conditions, the system exhibited a detection limit of 0.15 muM, reproducibility of 1.95 % RSD (n=9) and a linear response (r(2)=0.9992) when calibrated in the field with standards in the range 0.81-3.23 muM. The system was evaluated for the mapping of phosphate concentrations in Port Phillip Bay, south eastern Australia, and during the course of a 150 km cruise, 542 analyses were performed automatically. In general, good agreement was observed between analyses obtained using the portable FIA system and those obtained from manual sampling and laboratory analysis.     

A highly accurate method for determination of dissolved oxygen: Gravimetric Winkler method

A high-accuracy Winkler titration method has been developed for determination of dissolved oxygen concentration. Careful analysis of uncertainty sources relevant to the Winkler method was carried out and the method was optimized for minimizing all uncertainty sources as far as practical. The most important improvements were: gravimetric measurement of all solutions, pre-titration to minimize the effect of iodine volatilization, accurate amperometric end point detection and careful accounting for dissolved oxygen in the reagents. As a result, the developed method is possibly the most accurate method of determination of dissolved oxygen available. Depending on measurement conditions and on the dissolved oxygen concentration the combined standard uncertainties of the method are in the range of 0.012–0.018 mg dm⁻³ corresponding to the k = 2 expanded uncertainty in the range of 0.023–0.035 mg dm⁻³ (0.27–0.38%, relative). This development enables more accurate calibration of electrochemical and optical dissolved oxygen sensors for routine analysis than has been possible before.     

A multisyringe flow injection Winkler-based spectrophotometric analyzer for in-line monitoring of dissolved oxygen in seawater

An integrated analyzer based on the multisyringe flow injection analysis approach is proposed for the automated determination of dissolved oxygen in seawater. The entire Winkler method including precipitation of manganese(II) hydroxide, fixation of dissolved oxygen, dissolution of the oxidized manganese hydroxide precipitate, and generation of iodine and tri-iodide ion are in-line effected within the flow network. Spectrophotometric quantification of iodine and tri-iodide at the isosbestic wavelength of 466 nm renders enhanced method reliability. The calibration function is linear up to 19 mg L⁻¹ dissolved oxygen and an injection frequency of 17 per hour is achieved. The multisyringe system features a highly satisfying signal stability with repeatabilities of 2.2% RSD that make it suitable for continuous determination of dissolved oxygen in seawater. Compared to the manual starch-end-point titrimetric Winkler method and early reported automated systems, concentrations and consumption of reagents and sample are reduced up to hundredfold. The versatility of the multisyringe assembly was exploited in the implementation of an ancillary automatic batch-wise Winkler titrator using a single syringe of the module for accurate titration of the released iodine/tri-iodide with thiosulfate.     

A compleximetric method for the determination of dissolved oxygen in water

A new method is proposed for the determination of dissolved oxygen in water. The iron(III) formed from FES at pH 7.5 is titrated with EDTA solution in presence of salicylic acid indicator after adjustment of the pH to about 2.4 The method is slightly less precise than the Winkler method for pure waters but more accurate for polluted waters; it is simple and convenient for field use.     

A simple and rapid method for measuring dissolved oxygen in waters with gold microelectrode

Dissolved oxygen (DO) is an essential indicator in chemical, biological and biochemical processes and needs to be rapidly measured in many cases. In this work, a rapid, sensitive and simple electrochemical method, first derivative linear sweep voltammetry (FD-LSV), was established for the determination of DO. The peak height of FD-LSV was found to be influenced by scan rate and microelectrode surface area. An empirical formula was proposed to describe the relationship between the FD-LSV peak height and the influencing factors. Compared with other electrochemical methods for DO determination, the FD-LSV method needed lesser time and was more accurate. This method was successfully applied for the determination of the DO levels of various environmental water samples.     

Evaluation of on-line preconcentration and flow-injection amperometry for phosphate determination in fresh and marine waters

Dissolved reactive phosphorus (DRP) was determined as orthophosphate (PO₄-P) in fresh and saline water samples by flow-injection (FI) amperometry, without and with in-valve column preconcentration. Detection is based on reduction of the product formed from the reaction of DRP with acidic molybdate at a glassy carbon working electrode (GCE) at 220 mV versus the Ag/AgCl reference electrode. A 0.1 M potassium chloride solution was used as both supporting electrolyte and eluent in the preconcentration system. For the FI configuration without preconcentration, a detection limit of 3.4 μg P l⁻¹ and sample throughput of 70 samples h⁻¹ were achieved. The relative standard deviations for 50 and 500 μg P l⁻¹ orthophosphate standards were 5.2 and 5.9%, respectively. By incorporating an ion exchange preconcentration column, a detection limit of 0.18 μg P l⁻¹ was obtained for a 2-min preconcentration time (R.S.D.s for 0.1 and 1 μg P l⁻¹ standards were 22 and 1.0%, respectively). Potential interference from silicate, sulfide, organic phosphates and sodium chloride were investigated. Both the systems were applied to the analysis of certified reference materials and water samples.     

A high throughput chemiluminescence method for determination of chemical oxygen demand in waters

A novel and high throughput chemiluminescence (CL) method for determination of chemical oxygen demand (COD) in water sample was originally developed based on potassium permanganate-glutaraldehyde CL system. With this method, dissolved organic matter in water samples was digested by excess acid potassium permanganate, the reacted mixture solutions containing surplus KMnO₄ were added in wells of a 96-well plate, followed by injection of glutaraldehyde in the wells, and CL was then produced along with the reaction of the added glutaraldehyde with the surplus KMnO₄ and detected by a photomultiplier tube (PMT). The difference (ΔI) between the CL intensity for distilled water and that for sample water was proportional to the COD value of water sample. The calibration graph was linear in the range of 0.16-19.24 mg L⁻¹ with a detection limit of 0.1 mg L⁻¹. A complete analysis could be performed in 40 min including digestion and detection, giving a very high throughput of 3 × 96 samples in about 60 min. Compared with the conventional methods, this method is simple and sensitive and consumes very limited and cheap reagents. Owing to its rapid, automatic, high throughput and low cost characteristics, the presented CL method has been applied successfully to the determination of COD in real water samples (n = 32) with satisfactory results.     

Flow injection system with in-line Winkler's procedure using 16-way valve and spectrofluorimetric determination of dissolved oxygen in environmental waters

Flow injection spectrofluorimetry with in-line Winklers procedure was developed for the dissolved oxygen (DO) determination. 2-Thionaphthol reacted with iodine produced by Winkler’s method to form fluorescence inactive disulfide compound. To automate the process completely, a 5-channel flow system with a newly designed 16-way valve was assembled. The system consisted of a dispersion coil (DC), a precipitate formation coil (PFC), a precipitate dissolving coil (PDC), and extraction coil (EC). A calibration can be constructed by using a standard iodine solution for dissolved oxygen. The calibration graph was linear over the range 1.2×10⁻⁴∼6.0×10⁻⁴ mol l⁻¹ iodine (1.96∼9.80 mg O l⁻¹)). The relative standard deviation (n=6) was below 0.3% for the 4×10⁻⁴ mol l⁻¹ iodine (6.27 mg O l⁻¹) determination. The sample throughput was 12/h.     

Automatic method for the determination of ortho-phosphate and total dissolved phosphorus in the marine environment

Summary An automatic method for the determination of ortho-phosphate in seawater is described. It is based on the original molybdenum blue method and is very sensitive because of the very small dilution of samples by reagents. An all-glass reaction manifold is proposed, the use of plastic material should be avoided as far as possible. The described method may also be used in a slightly modified version for the determination of total dissolved phosphorus, when samples are first digested with peroxidisulfate. Orthophosphate and total dissolved phosphorus can be determined exactly in the concentration range from 0.01–5 g at P/l with a sampling frequency of 30 h^–1.

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Automatische Methode zur Bestimmung von ortho-Phosphat und gelöstem Gesamt-Phosphor in Meerwasser

Zusammenfassung Für die Bestimmung von ortho-Phosphat in Seewasser wird eine automatische Methode beschrieben, die auf der üblichen Molybdänblaumethode basiert und sehr empfindlich ist, weil die Proben mit einer sehr geringen Menge Reagenz verdünnt werden. Für die Reaktionseinheit wird ein Allglassystem vorgeschlagen, der Einsatz von Plastikmaterial sollte weitgehend vermieden werden. Die beschriebene Methode ist in leicht modifizierter Form auch für die Bestimmung von gelöstem Gesamt-Phosphor geeignet, wenn die Proben zuvor mit Peroxodisulfat aufgeschlossen werden. Die Methode liefert bei einer Probenfrequenz von 30 h^–1 für ortho-Phosphat und gelösten Gesamt-Phosphor genaue Ergebnisse im Bereich von 0,01–5 g at P/1.

     

The rapid determination of chemical oxygen demand in waste waters and effluents by flow injection analysis

Chemical oxygen demand (COD) and the limitations of the standard method of determination are discussed. A fully automated method based on a commercial     

An enzymatic flow injection method for the determination of oxygen

An enzymatic flow injection method for the determination of dissolved oxygen is described. Oxygen from the sample is reduced quantitatively to hydrogen peroxide in a packed-bed reactor containing immobilized glucose oxidase β-d-Glucose is used as a cosubstrate. The effluent is mixed with a stream containing chromogens and fed into a second reactor containing immobilized peroxidase. The coloured product formed is monitored spectrophotometrically. The response is linear from the detection limit (2–5 μM O₂ to air-saturated samples (0.3 mM O₂) when the peak areas are plotted versus the O₂ content in the samples (50 μl). The maximum speed for 1% carry-over is 60 samples per hour and the results are available 25 s after the start of sampling. Broadening of the peak is caused by adsorption of the coloured product in the peroxidase reactor.     

Rapid determination of dissolved organic phosphorus in soil leachates and runoff waters by flow injection analysis with on-line photo-oxidation

A rapid method suitable for the determination of dissolved organic phosphorus (DOP) in soil leachates and runoff waters is presented. The flow injection (FI) manifold contains an in-line PTFE reaction coil wrapped around a low power UV lamp and is based on the spectrophotometric determination of dissolved reactive phosphorus (DRP) and mineralised DOP at 690 nm after reduction of phosphomolybdate to molybdenum blue with tin(II) chloride. The linear range was 0-1.5 mg 1(-1) PO(4)-P, with a detection limit (3 s) of 7 mug 1(-1) and a sample throughput of 40 h(-1). Tolerance to potential matrix interferences in soil pore waters, particularly Al(III), Si(IV), Fe(II) and Fe(III), was achieved using a combination of on-line sample pre-treatment by a strong acid ion exchange column, low photoreactor pH and acid induced control of the kinetics of the molybdenum blue reaction. The results obtained with this manifold were in good agreement with those obtained by a batch spectrophotometric reference method.     

A flow injection-ETAAS system for the on-line determination of total and dissolved silica in waters

In this work total (Si-tot) and ‘soluble’ or reactive (Si-sol) concentrations of silicon in natural and tap waters were sequentially determined by electrothermal atomic absorption spectrometry (ETAAS). First, samples were on-line diluted based on the merging-zone principle in order to allow the determination of Si-tot within the 300–1000 μgSi l⁻¹ range. After the dilution process, a sub-sample was collected in the capillary of a sampling arm assembly (SAA). Thereafter, samples were subject to a precipitation/dissolution process in order to allow the determination of Si-sol within the 280–850 μgSi l⁻¹. Si-sol was precipitated with ammonium chloride and collected on the walls of a knotted coil. The precipitate was dissolved with ammonium molybdate in an acidic medium (HNO₃) and a sub-sample was then collected in the SAA. In both cases, 10 μl volumes of the sub-sample were injected into the atomizer with the previous introduction of 20 ng of Eu as chemical modifier (10 μl) by the spectrometer autosampler. The recovery values obtained with natural waters spiked samples were over 46% and the agreement between observed and certified samples values was good. The proportion of Si-sol in comparison with the Si-tot was high (85–95%) in most natural waters. The precision of the method was 2.4–3.5 and 4.5–6.2% (n=10) for the determination of Si-tot and Si-sol, respectively.     

A continuous approach for the determination of Cr(VI) in sediment and soil based on the coupling of microwave-assisted water extraction, preconcentration, derivatization and photometric detection

A dynamic system for the continuous leaching of Cr(VI) from sediment and soil based on both microwave assistance and iterative change of the flow direction of the extractant through the sample cell has been developed. The microwave-assisted extractor has been coupled to a photometric detector through a flow injection interface in order to develop a fully automated method. The Cr(VI) extracted was monitored after derivatization with 1,5-diphenylcarbazide. Two approaches are proposed which differ in the inclusion of a preconcentration minicolumn packed with a strong anion exchange resin. A 0.04 M ammonium buffer solution was used as extractant and 0.2 g of sample—river sediment spiked with 50 and 5 μg g⁻¹ for the method without preconcentration (method A) and with preconcentration (method B)—was subjected to 8-14 min of 300 W microwave-assisted extraction. The within-laboratory reproducibility and repeatability were 2.6 and 1.9 for method A, and 4.0 and 2.6 for method B. The proposed methods have been compared with the reference EPA method 3060/7196.     

A compact portable flow analysis system for the rapid determination of total phosphorus in estuarine and marine waters

The development and evaluation of a portable flow analysis system for the in situ determination of total phosphorus is described. The system has been designed with rapid underway monitoring in mind. The system employs an ultra-violet photo-reactor and thermal heating for peroxodisulfate digestion of total phosphorus to orthophosphate, followed by spectrophotometric detection with a multi-reflective flow cell and low-power light emitting diode using the molybdenum blue method. Reagents are stored under gas pressure and delivered using software controlled miniature solenoid valves. The fully automated system has a throughput of 115 measurements per hour, a detection limit of 1 μg P L⁻¹, and gives a linear response over the calibration range of 0-200 μg P L⁻¹ (r² = 0.9998), with a precision of 4.6% RSD at 100 μg P L⁻¹ (n = 10). Field validation of the instrument and method was performed in Port Philip and Western Port Bays in Victoria, SE Australia, where 2499 analyses were performed over a 25 h period, over a cruise path of 285 km. Good agreement was observed between determinations of samples taken manually and analysed in the laboratory and those measured in situ with the flow analysis system.     

A method for the determination of technetium in environmental waters

A method is described which can be used to determine technetium-99 levels in a range of water types. Ruthenium isotopes which may interfere in the analysis are removed from the sample by precipitation before concentration of pertechnetate onto an ion-exchange column. Other nuclides can be removed from the column using NaOH before elution of the technetium using NaSCN. The technetium in the NaSCN eluent can then be extracted into butan-2-one which can be evaporated onto a planchette. Technetium-99m is used as a yield tracer and after this has decayed away to negligible levles the amount of technetium on the planchette can be determined by measuring the rate of beta radiation emission from the final concentrate.     

Using sequential injection analysis for fast determination of phosphate in coastal waters

A sequential injection analysis system (SIA) is described which is suited for the fast determination of filterable molybdate reactive phosphate (FRP, 0.2 μm) in coastal waters. It processes up to 270 samples per hour with a detection limit (3σ) of 0.05 μM and is used for surface mapping of phosphate in areas with steep concentration gradients like the Wadden Sea. The determination is based on the reaction of phosphate with acidic molybdate to phosphomolybdate, which builds non-fluorescent ion pairs with rhodamine 6G. The remaining rhodamine fluorescence is detected at 550 nm with an excitation at 470 nm. Syringe pump, valve and detector were controlled by a self made python programme, which was optimised for high speed SIA measurements in monitoring applications.     

Use of flow injection analysis based on iodometry for automation of dissolved oxygen (Winkler method) and chemical oxygen demand (dichromate method) determinations

Summary Two analytical methods for water analysis were miniaturized and partly automated by the flow injection analytical approach. In the standard Winkler method for dissolved oxygen determination, the final step, the titration of liberated iodine with Na₂S₂O₃, was replaced by amperometric detection in a flowing stream. The concentration of dissolved oxygen was determined by the standard addition method using KIO₃ as a standard reagent. The dichromate method for chemical oxygen demand determinations was also miniaturized. Oxidation of organic substances by K₂Cr₂O₇ in H₂SO₄ (1+1) medium was carried out under standard conditions and the excess of dichromate ions was determined electrochemically via measurement of released I₂ after addition of KI in a continuous stream. The chemical oxygen demand of a sample was calculated on the basis of the current without standard addition.

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Anwendung der iodometrischen Fließinjektionsanalyse zur Automation der Bestimmung von gelöstem Sauerstoff (Winkler-Methode) und chemischem Sauerstoffbedarf (Dichromat-Methode)