R. Forteza

Flow techniques in water analysis

In the present work the main flow techniques for the analysis and monitoring of several parameters of interest in the quality control of different types of waters are reviewed. Firstly, a review involving the advantages and disadvantages of flow techniques, from those currently out-dated, such as segmented flow analysis (SFA), to the most modern techniques, such as flow injection analysis (FIA), sequential injection analysis (SIA) and multi-commutation techniques (MCFA), is carried out. On the other hand, a new technique, the multi-syringe flow analysis (MSFA) is hereby described for the first time as both a fast and robust alternative. Its possibilities, limitations and potential advantages when using this technique either on its own or coupled to SIA, which carries out a previous sample handling, are outlined.

Gas diffusion techniques coupled sequential injection analysis for selective determination of ammonium

The design of an analytical system based on the coupling of gas-diffusion separation and sequential injection analysis is described and applied to ammonium determination. The sample and an alkaline solution are sequentially aspirated using an automatic burette and mixed by flow reversal while they are being propelled to a gas-diffusion unit. There, the ammonia formed diffuses through an hydrophobic membrane into an acid-base indicator solution. The change in the absorbance of the acid-base indicator solution used as acceptor stream is measured and referred to ammonium content in the sample. The system requiring simple and commonly available instrumentation is automated and has been applied to the determination of ammonium in environmental samples.

Speciation analysis of inorganic arsenic by a multisyringe flow injection system with hydride generation-atomic fluorescence spectrometric detection

In this study, a new technique by hydride generation-atomic fluorescence spectrometry (HG-AFS) for determination and speciation of inorganic arsenic using multisyringe flow injection analysis (MSFIA) is reported. The hydride (arsine) was generated by injecting precise known volumes of sample, a reducing sodium tetrahydroborate solution (0.2%), hydrochloric acid (6M) and a pre-reducing solution (potassium iodide 10% and ascorbic acid 0.2%) to the system using a multisyringe burette coupled with one multi-port selection valve. This solution is used to pre-reduce As(V) to As(III), when the task is to speciate As(III) and As(V). As(V) is determined by the difference between total inorganic arsenic and As(III). The reagents are dispensed into a gas-liquid separation cell. An argon flow delivers the arsine into the flame of an atomic fluorescence spectrometer. A hydrogen flow has been used to support the flame. Nitrogen has been employed as a drier gas (Fig. 1). Several variables such as sample and reagents volumes, flow rates and reagent concentrations were investigated in detail. A linear calibration graph was obtained for arsenic determination between 0.1 and 3 microg l(-1). The detection limit of the proposed technique (3sigma(b)/S) was 0.05 microg l(-1). The relative standard deviation (R.S.D.) of As at 1 microg l(-1) was 4.4 % (n=15). A sample throughput of 10 samples per hour was achieved. This technique was validated by means of reference solid and water materials with good agreement with the certified values. Satisfactory results for speciation of As(III) and As(V) by means of the developed technique were obtained.

Preconcentration and determination of inorganic arsenic using a multisyringe flow injection system and hydride generation-atomic fluorescence spectrometry

A new multisyringe flow injection system for inorganic arsenic determination at trace levels by hydride generation-atomic fluorescence spectrometry (HGAFS) is presented. Preconcentration on a solid-phase was carried out using a column packed with an anion-exchange resin (Amberlite IRA-410). The reagents are dispensed to the system using a multisyringe burette coupled with two multi-port selection valves. Different parameters were changing in order to make the system as effective as possible. An analytical curve was obtained for arsenic determination between 50 and 2000ngl(-1). This new approach improved five times the sensitivity over a MSFIA-HGAFS technique developed previously by the authors. Detection limit of the proposed technique was (3sigma(b)/S) of 30ngl(-1). The relative standard deviation (R.S.D.) of As at 1mugl(-1) was 4.8% (n=7). A sample throughput of 10h(-1) has been achieved. The proposed method has been applied to different reference solid and water materials with satisfactory results.

Monitoring of environmental parameters by sequential injection analysis

The use of sequential injection analysis (SIA), as applied to the control of environmental parameters, is reviewed. Its advantages and drawbacks are compared with those of flow injection analysis (FIA). FIA is simpler and easier to handle, has a higher sample throughput, and may be controlled manually, but SIA is more robust and versatile and better adapted for multiparametric and stopped flow techniques.

Evaluation of flow injection methods for ammonium determination in wastewater samples

A comparative study of flow injection methods for the determination of ammonium ion in wastewater samples is presented. It includes methods based on the Berthelot reaction and methods based on gas diffusion. Detection is either spectrophotometric or conductimetric. Analytical characteristics of each method are described in terms of reproducibility, sensitivity and selectivity. The performance of several gas permeable membranes to be used in gas-diffusion systems is discussed. The stopped flow approach is also investigated in order to improve the detection limits. Calibration graphs are obtained for each system, with and without that preconcentration step. Finally, a number of wastewater samples are analyzed using the described methods and the results are compared.

Preconcentration and atomic absorption determination of iron by sequential injection analysis

A sequential injection analysis (SIA) assembly for the atomic absorption determination of Fe(III) in natural waters is proposed. Iron is preconcentrated on a microcolumn packed with a chelating resin (Chelex 100) that is inserted in the manifold. The sample is passed through the column and the iron retained by the resin is subsequently eluted with 2 M HNO(3). The proposed SIA system affords automatic preconcentration, elution, detection of Fe(III), data acquisition and treatment. When 9 ml of iron solution containing 0.4 or 1 mg l(-1) was passed through the resin, the retention efficiency was 93.1 +/- 0.6 and 7.4 +/- 3.0% respectively, and when 27 ml of iron solution of 0.2 mg l(-1) was preconcentrated, the retention was 8.4 +/- 2.9%. The detection limits thus achieved is 12 mug l(-1) when 9 ml of sample are preconcentrated and 6 mug l(-1) for 27 ml.

Fast sequential injection determination of benzo[A]pyrene using variable angle fluorescence with on-line solid-phase extraction

A methodology for the analysis of drinking water for one of the most potent carcinogenic agents known; benzo[a]pyrene (BaP), in the presence of other interfering PAHs is presented. The methodology described is based on the sequential injection analysis of the sample on to a microcolumn (containing 5 mg of C18) where extraction and preconcentration of BaP takes place, followed by elution of BaP with 1 ml of 1,4-dioxane and subsequent detection by using variable angle fluorescence. The advantages of the method include the small amount of stationary phase employed together with the possibility of re-using the phase in order to carry out a large number of injections without the need for column re-packing. Also noteworthy is the small volume of 1,4-dioxane used to elute the BaP retained on the column and the small sample volumes required (9-10 ml) for achieving detection limits at the ng l-1 level. Thus, a methodology for BaP determination is obtained which complies with the requirements of the 98/83/EC Directive which fixes a maximum admissible concentration for this pollutant in waters for public consumption of 10 ng l-1. The variable angle spectra obtained are further processed by means of the multiple linear regression technique. The detection limit for BaP is 2.5 ng l-1, and the linear range is between 7.5 and 280 ng l-1.

Speciation of nitrogen in wastewater by flow injection

A rapid method for the sequential determination of nitrite, nitrate and total nitrogen is proposed. Nitrite was determined directly by using the Griess reaction, which was also used to quantify nitrate after reduction to nitrite with hydrazine. For total nitrogen determination, the nitrogen-containing compounds (organic substances and nitrite and ammonium ions) were oxidized photochemically using a UV lamp and converted into nitrate, which was then reduced to nitrite and determined spectrophotometrically. Under the optimized conditions, up to 220 µmol l–1 N-NO2– and 240 µmol l–1 N-NO3– can be determined, the detection limits being 2 µmol l–1 N-NO2– and 8 µmol l–1 N-NO3–. The relative standard deviation for nitrite and nitrate are 1.5 and 2.3%, respectively. The photo-oxidation method for total nitrogen determination has a linear range of 30–1000 µmol l–1 N, with a relative standard deviation of 3%. The proposed method was applied to the determination of nitrate, nitrite and total nitrogen in wastewaters.

Total nitrogen determination by flow injection using on-line microwave-assisted digestion

Abstract A method for the determination of total nitrogen in wastewaters involving flow injection and on-line microwave-assisted digestion is described. The method is based on the oxidation of nitrogen-containing compounds to nitrate by means of peroxodisulfate using a microwave oven as a radiation source. Nitrate is determined by the Griess-Ilosvay reaction, after reduction to nitrite by hydrazine under alkaline conditions, using sulfanilamide and N -(1-naphthyl) ethylene diamine. A thorough study of the factors affecting the digestion process has been carried out using several nitrogen-containing compounds as test substances. Under the optimized conditions, the described method allows quantitative digestion of all the model compounds for concentrations up to 20 mg l −1 N. The method enables the analysis of 10 samples in triplicate per hour, the relative standard deviation (RSD) of 10 replicates being ca. 3%. The detection limit (3 × SD of blank) is 0.21 mg l −1 N. The system has been applied to the determination of total nitrogen in wastewaters and the results have been compared with those obtained by Kjeldahl digestion.