M. Inês G.S. Almeida

A membraneless gas-diffusion unit-multisyringe flow injection spectrophotometric method for ammonium determination in untreated environmental samples.

A new design of a membraneless gas-diffusion (MGD) unit coupled to a multisyringe flow injection system is proposed. The spectrophotometric determination of ammonium using an acid-base indicator was chosen to show the feasibility of this approach. Hence, in alkaline medium, ammonium ions are transformed into ammonia (donor channel) which diffuses through the headspace into the acceptor stream (bromothymol blue solution), causing a pH change and subsequently a colour change. The exploitation of the enhanced potentialities of this re-designed MGD device was the main purpose of the present work. Hence, several strategies concerning flow management were studied seeking to characterize and improve the analytical features of the methodology and moreover, untreated environmental samples were analysed without previous filtration. Consequently, stopped flow in acceptor channel with continuous flow in donor channel was chosen for the application to wastewater and spiked river water samples. A linear concentration range between 10.0 and 50.0 mg L(-1) of NH(4)(+), a limit of detection of 2.20 mg L(-1) and a determination frequency of 11h(-1) were obtained.

Multi-syringe flow injection system with in-line microwave digestion for the determination of phosphorus.

A multi-syringe system for spectrophotometric determination of total phosphorus involving in-line digestion is proposed. Sample and digestion solution were dispensed and directed towards a digestion vessel located inside a domestic microwave oven (MWO) where sample digestion took place. Afterwards, the digested sample was merged with the necessary reagents for the colorimetric determination based on the molybdenum blue method. Several digestion conditions were studied regarding composition of digestion solution, digestion time and power set on the MWO. The system was applied to waste water samples and results shown a good agreement with the reference method. Repeatable results (R.S.D.<2.41%) and determination frequency of 12h(-1) were obtained.

Polymer inclusion membranes (PIMs) in chemical analysis - A review

This review highlights the increasing interest in polymer inclusion membranes (PIMs) in analytical chemistry as they are adapted to new and novel applications. PIMs are polymer-based liquid membranes and were first introduced 50 years ago as the sensing membranes in ion-selective electrodes and optodes. More recently however, PIMs have been used for other applications in analytical chemistry such as for sample separation, sample pre-concentration, electro-driven extraction, and passive sampling, and have also been incorporated into on-line and automated analysis systems. The present review provides a general overview of the analytical chemistry applications of PIMs reported in the literature to date and illustrates their versatility for solving challenging chemical analysis problems.

Multi-syringe flow injection system for the determination of available phosphorus in soil samples

Considering the importance of monitoring the levels of nutrients present in soils and their availability to plants, an automatic methodology is proposed based on multi-syringe flow injection analysis (MSFIA) for the spectrophotometric determination of available phosphorus in soil extracts. This fully computerized flow technique allowed the development of a flow network where sample and reagents were intercalated and sent further towards the detection system. The colorimetric determination was based on the molybdenum blue method with ascorbic acid as reducing reagent and the Egner–Riehm method was applied to extract phosphorus from soil samples. A linear calibration curve was obtained between 0.75 and 15.0 mg L−1. A determination frequency of 15 h−1 was achieved, with good repeatability for 12 consecutive injections of soil extracts (RSD<1.7%). The results obtained from 12 soil samples were statistically comparable to those attained by the usual batch method.

Developments of microfluidic paper-based analytical devices (μPADs) for water analysis: A Review

Water pollution is a serious environmental problem affecting millions of people, and the demand for frequent water quality monitoring is increasing. The need for analytical platforms that combine high sensitivity, selectivity and accuracy with low cost, portability and user friendliness remains a challenge. Microfluidic paper-based analytical devices (μPADs) are recognised as a powerful analytical platform that can satisfy these requirements. The aim of this review is to provide a detailed overview of the μPADs that have been developed for the determination of important water quality parameters, such as nutrients, metals and organic contaminants, in a range of waters. A description of the fabrication and detection methods selected for these applications is provided, and the performance of the μPADs with respect to their precision and accuracy is critically assessed. The potential of these devices for real-life applications is also critically examined, particularly if they can determine the concentrations specified in water quality guidelines or the maximum recommended concentrations for various waters, as well as if they are suitable for field applications.

MULTISYRINGE FLOW INJECTION POTENTIALITIES FOR HYPHENATION WITH DIFFERENT TYPES OF SEPARATION TECHNIQUES

The advantages of combining flow techniques with separation techniques are noteworthy. Flow-based methods are excellent tools for automating analytical procedures owing to their speed of analysis, reagent and sample saving ability, versatility, and inexpensive equipment. However, flow-based methods alone do not allow the separation of several analytes in a mixture. On the other hand, separation techniques (viz., chromatographic separations, capillary electrophoresis), widely used in analytical research due to high selectivity, require more expensive instrumentation. Thus, the combination of both techniques achieved the required selectivity with a more versatile, automated and, in some cases, low-cost methodology. Multisyringe flow injection evidences interesting features with the utilization of the aforementioned combination of methods. Its potential for hyphenation with different separation techniques have been recently explored and are depicted in the present paper.

Interfacing multisyringe flow injection analysis to flame atomic emission spectrometry: an intelligent system for automatic sample dilution and determination of potassium

Interfacing automatic flow systems to atomic emission/absorption spectrometry based on flame atomisation techniques may present some difficulties, especially when there are flow interruptions inherent to the technique of automation applied. Therefore, in the present work, different configurations for interfacing multisyringe flow injection analysis (MSFIA) systems to flame-based detectors (flame emission spectrometer in this case) are proposed and compared. To illustrate the feasibility of the selected approach, an intelligent flow system was implemented for in-line dilution of samples prior to determination of potassium content. By selecting the injection volume and the number of dilution channels activated, a dynamic concentration range (1–2000 mg L−1) was established, allowing direct introduction of soil extracts (1–20 mg L−1), parenteral solutions (20–600 mg L−1) and table wines (200–2000 mg L−1). The sampling frequency (three replicates per sample) was about 14 h−1.

Determination of salivary cotinine through solid phase extraction using a bead-injection lab-on-valve approach hyphenated to hydrophilic interaction liquid chromatography.

Cotinine, the first metabolite of nicotine, is often used as a biomarker in the monitoring of environmental tobacco smoke (ETS) exposure due to its long half-life. This paper reports on the development of an at-line automatic micro-solid phase extraction (μSPE) method for the determination of salivary cotinine followed by its analysis via hydrophilic interaction liquid chromatography (HILIC). The SPE methodology is based on the bead injection (BI) concept in a mesofluidic lab-on-valve (LOV) flow system to automatically perform all SPE steps. Three commercially available reversed-phase sorbents were tested, namely, Oasis HLB, Lichrolut EN and Focus, and the spherically shaped sorbents (i.e., Oasis HLB and Focus) provided better packing within the SPE column and hence higher column efficiency. An HILIC column was chosen based on its potential for achieving higher sensitivity and better retention of polar compounds such as cotinine. The method uses an isocratic program with acetonitrile:100mM ammonium acetate buffer, pH 5.8 in 95:5 v/v ratio as the mobile phase at a flow rate of 1.0 mL min(-1). Using this approach, the linear calibration range was from 10 to 1000 ng which corresponded to 5-500 μg L(-1). The corresponding μSPE-BI-LOV system was proven to be reliable in the handing and analysis of viscous biological samples such as saliva, achieving a sampling rate of 6h(-1) and a limit of detection and quantification of 1.5 and 3μgL(-1), respectively.

Flow injection spectrophotometric determination of V(V) involving on-line separation using a poly(vinylidene fluoride-co-hexafluoropropylene)-based polymer inclusion membrane

A poly(vinylidene fluoride-co-hexafluoropropylene)-based polymer inclusion membrane (PIM) using Cyphos® IL 101 (i.e. trihexyl(tetradecyl)phosphonium chloride) as the carrier and 2-nitrophenyl octyl ether as a plasticizer in a mass ratio of 55/35/10 was employed for the on-line extractive separation of V(V) prior to its spectrophotometric determination in a flow injection analysis (FIA) system using xylenol orange as the colorimetric reagent. The selectivity of the membrane allowed the determination of V(V) in sulfate solutions in the presence of a variety of cations and anions. The interference of molybdenum(VI) was eliminated by off-line extraction using the same PIM. A univariate sequential optimization of the newly developed FIA system was conducted and under optimal conditions the system is characterized by a linear concentration range of 0.5-8.0mgL-1, detection limit of 0.08mgL-1 and sample throughput of 4h-1. The relative standard deviation at the 3mgL-1 level of V(V) was 2.9% based on 8 replicate determinations. The membrane was stable, which was reflected by the standard deviation value for determinations over three consecutive days (24 determinations of 3mgL-1 V(V)) of 3.6%. The newly developed FIA system was applied to the determination of V(V) in water and dietary supplements samples and a good agreement with inductively coupled plasma optical emission spectrometry was observed.

Gas-diffusion-based passive sampler for ammonia monitoring in marine waters

A novel passive sampler based on gas-diffusion across a hydrophobic membrane is described for the determination of the time-weighted average concentration of dissolved molecular ammonia in high ionic strength aquatic environments, such as sea, coastal and estuarine waters, for a period of 3 days. The passive sampler developed is cheap, easy-to-use, reusable, and has a dynamic concentration range of 2.0-12µM, which covers the water quality guideline trigger value of 11.4µM (160µgL-1 NH3-N) for high conservation value waters, making this a powerful new tool for water quality managers involved in long-term ammonia monitoring. The gas-diffusion-based passive sampler was calibrated under laboratory conditions and deployed in a tank of seawater in the laboratory and at an estuarine site for proof of concept, and a good agreement between passive and spot sampling was achieved in both cases.