A. Cladera

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.

Design of a very versatile software program for automating analytical methods

The present article proposes the design of a hardware-independent system for automating various analytical methods. System design is based on the division of the software into four layers, defining a communications interface among them. The definition of these layers allows the isolation of the highest, corresponding to the analytical application, from the lowest, the hardware, composed of the various parts of the block diagram. Although this system was initially designed for automating flow techniques (flow-injection analysis, FIA, and sequential-injection analysis, SIA), its conception allows the development of much more general applications, depending on the users imagination. The system is very open, in the sense that it is not necessary to adapt the program as system possibilities are expanded. Thus, if new instrumentation (modules in the block diagram) is incorporated, one need only develop independently a minimum of software to comply with the previously defined interface. This system has been developed under the 32-bit Windows 95 environment, making the exchange of information with other applications very simple and allowing the various possibilities offered by the program to be executed simultaneously (for example, data acquisition and processing).

Potentiometric Stripping Analysis: A Review

Abstract A bibliographic review (150 references) on potentiometric stripping analysis (PSA) is performed. Theoretical, instrumental, analytical applications and advantages, and inferences of other modern PSA techniques are considered, like derivative PSA, constant-current PSA, multichannel potentiometric monitoring stripping analysis, differential PSA, constant-current enhanced PSA, derivative adsorptive PSA, kinetic PSA and reductive PSA. Implementation of PSA in flow systems is also considered, namely continuous-flow and flow-injection systems.

A robust multisyringe system for process flow analysis. Part I. On-line dilution and single point titration of protolytes

A robust multichannel manifold involving a multisyringe piston pump used for performing process flow analysis is proposed. The simultaneous sequential determination of two analytes with a sampling frequency of 97 injections per hour is demonstrated. A robust and rapid continuous flow titration system, including a predilution step, was developed. A quantitative dilution, avoiding the need for diluting the standards too, is achieved in two steps by first splitting the concentrated sample stream and further dilution in a mixing chamber. A wide range of dilution grades (from 80 to 1150) can be easily implemented by software instructions, without the necessity of mechanical reconfiguration of the manifold. The resulting system gives medium or low dispersion peak signals with a sampling frequency of over 75 injections per hour for off-line diluted samples, and more than 33 injections per hour for concentrated samples. The proposed assemblage allows the automation of the single point flow injection titration procedure for the assay of acids and bases in concentrated process solutions. Recoveries of 97.5–103% were found with a maximum RSD value of 5%.

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.

Sequential injection spectrophotometric analysis of nitrite in natural waters using an on-line solid-phase extraction and preconcentration method

An automatic sequential injection analysis (SIA) set-up for the isolation, preconcentration and spectrophotometric determination of nitrite in waters based on the Shinn reaction was designed and evaluated. The system performs the on-line azo dye formation and its subsequent extraction on a solid phase (monofunctional C18), which is held inside a glass column incorporated into the system. A large sample volume (maximum, 10 ml) is sequentially segmented with sulfanilamide and N-(1-naphthyl)ethylenediamine dihydrochloride by using an iterative method. The azo dye collected is eluted with a small volume of 80% methanol and conducted to a diode-array spectrophotometer for quantitative analysis. It has been proved that the retention efficiency is maintained for up to 45 sample injections of 10 ml in spite of the high chromogenic reagent acidity. Nitrite has been determined within the 13.4–160 ng ml−1 and 0.83–20 ng ml −1 ranges for 1 and 10 ml of sample, respectively. For these volumes, the detection limits are 5.9 and 0.32 ng ml −1, the enhancement factors 17 and 170 and the sample throughput 15 and 3 h−1, respectively. A maximum RSD of 4.0% was achieved in all determinations. It is an advantage of this approach that it is possible to use the same mass calibration graph for any sample volume.

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.

Multisyringe flow system: determination of sulfur dioxide in wines

A multisyringe flow system for the spectrophotometric determination of sulfur dioxide in wines is described. The methodology is based on the well-known reaction among SO2, formaldehyde and pararosaniline. The proposed manifold also includes a gas diffusion unit in order to prevent the colour interference of red wines in the spectrophotometric measurement. The proposed method was successfully applied to the determination of free SO2 (2–75 mg l−1) and total SO2 (10–250 mg l−1) in wines, just by changing a few operating parameters in the controlling software. A sampling-rate of 25–30 samples per hour was achieved with good repeatability for 10 consecutive injections of wine samples (RSD < 3.2%). The results obtained from 10 wine samples for each determination were statistically comparable to those obtained by the recommended procedure.

A new instrumental implementation of sequential injection analysis

The performance of a new system for sequential injection analysis equipped with a liquid driver constituted by an ordinary burette for conventional automatic titrations actuated by a stepper motor was assessed. The proposed system circumvents the high cost of conventional liquid drivers and the problems typically posed by highly absorbing chromogenic reagents, which provide a varying background in this technique.

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.