Speciation of inorganic arsenic in aqueous samples using a novel hydride generation microfluidic paper-based analytical device (µPAD).

The development of the first microfluidic paper-based analytical device (µPAD) for the speciation of inorganic arsenic in environmental aqueous samples as arsenite (As(III)) and arsenate (As(V)) which implements hydride generation on a paper platform is described. The newly developed µPAD has a 3D configuration and uses Au(III) chloride as the detection reagent. Sodium borohydride is used to generate arsine in the device's sample zone by reducing As(III) in the presence of hydrochloric acid or both As(III) and As(V) (total inorganic As) in the presence of sulfuric acid. Arsine then diffuses across a hydrophobic porous polytetrafluoroethylene membrane into the device's detection zone where it reduces Au(III) to Au nanoparticles. This results in a color change which can be related to the concentration of As(III) or total inorganic As (i.e., As(III) and As(V)) concentration. Under optimal conditions, the µPAD is characterized by a limit of detection of 0.43 mg L-1 for total inorganic As (As(III) + As(V)) and 0.41 mg L-1 for As(III) and a linear calibration range in both cases of 1.2-8.0 mg As L-1. The newly developed µPAD-based method was validated by applying it to groundwater and freshwater samples and comparing the results with those obtained by conventional atomic spectrometric techniques.

Use of a Polymer Inclusion Membrane and a Chelating Resin for the Flow-Based Sequential Determination of Copper(II) and Zinc(II) in Natural Waters and Soil Leachates.

A bi-parametric sequential injection method for the determination of copper(II) and zinc(II) when present together in aqueous samples was developed. This was achieved by using a non-specific colorimetric reagent (4-(2-pyridylazo)resorcinol, PAR) together with two ion-exchange polymeric materials to discriminate between the two metal ions. A polymer inclusion membrane (PIM) and a chelating resin (Chelex 100) were the chosen materials to retain zinc(II) and copper(II), respectively. The influence of the flow system parameters, such as composition of the reagent solutions, flow rates and standard/sample volume, on the method sensitivity were studied. The interference of several common metal ions was assessed, and no significant interferences were observed (<10% signal deviation). The limits of detection were 3.1 and 5.6 µg L-1 for copper(II) and zinc(II), respectively; the dynamic working range was from 10 to 40 µg L-1 for both analytes. The newly developed sequential injection analysis (SIA) system was applied to natural waters and soil leachates, and the results were in agreement with those obtained with the reference procedure.

Development of a passive sampler based on a polymer inclusion membrane for total ammonia monitoring in freshwaters.

A passive sampler for determining the time-weighted average total ammonia (i.e. molecular ammonia and the ammonium cation) concentration (C TWA) in freshwaters, which incorporated a polymer inclusion membrane (PIM) as a semi-permeable barrier separating the aqueous source solution from the receiving solution (i.e. 0.8 mol L(-1) HCl), was developed for the first time. The PIM was composed of dinonylnaphthalene sulfonic acid (DNNS) as a carrier, poly (vinyl chloride) (PVC) as a base polymer and 1-tetradecanol as a modifier. Its optimal composition was found to be 35 wt% commercial DNNS, 55 wt% PVC and 10 wt% 1-tetradecanol. The effect of environmental variables such as the water matrix, pH and temperature were also studied using synthetic freshwaters. The passive sampler was calibrated under laboratory conditions using synthetic freshwaters and exhibited a linear response within the concentration range 0.59-2.8 mg L(-1) NH4(+) (0.46-2.1 mg N L(-1)) at 20 °C. The performance of the sampler was further investigated under field conditions over 7 days. A strong correlation between spot sampling and passive sampling was achieved, thus providing a proof-of-concept for the passive sampler for reliably measuring the C(TWA) of total ammonia in freshwaters, which can be used as an indicator in tracking sources of faecal contamination in stormwater drains.

Microfluidic paper-based analytical device for the speciation of inorganic nitrogen species.

A microfluidic paper-based analytical device (µPAD) utilizing gas-diffusion separation and solid-phase reduction was developed for the first time for the determination of both ammonium and nitrate, which are the dominant inorganic nitrogen species in environmental waters. The device consists of 3 filter paper layers accommodating the sample, reagent and detection zones. The reagent zone is separated from the detection zone by a semipermeable hydrophobic membrane and acts as a solid-phase reactor where nitrate is reduced to ammonia by Devarda's alloy microparticles, integrated into a µPAD for the first time. The detection zone incorporates the acid-base indicators bromothymol blue (BTB) or nitrazine yellow (NY) and changes colour in two steps. Initially the colour change is caused by ammonia generated by the reaction of ammonium and sodium hydroxide in the sample zone. This colour change is followed by a subsequent colour change as a result of the ammonia produced by the reduction of nitrate by the Devarda's alloy microparticles. The corresponding reflectance value changes are used for the quantification of the two inorganic nitrogen species in the ranges 6.5-100.0 or 2.1-15.0 mg N L-1 for ammonium and 18.2-100.0 or 4.2-15.0 mg N L-1 for nitrate when BTB or NY are used, respectively. Under optimal conditions the limits of quantification of ammonium and nitrate in the case of BTB were determined as 6.5 and 18.2 mg N L-1, respectively, while the corresponding values in the case of NY were found to be 2.1 and 4.2 mg N L-1. The newly developed µPAD was stable for 62 days when stored in a freezer and 1 day at ambient temperature. It was validated with a certified reference material and successfully applied to the determination of ammonium and nitrate in spiked environmental water samples and soil extracts.

A cross-linked polymer inclusion membrane for enhanced gold recovery from electronic waste.

A cross-linked polymer inclusion membrane (CL-PIM) incorporating the extractant trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate (Cyphos® IL 104) was developed for the first time for the enhanced Au(III) recovery from aqua regia digests of electronic waste (discarded mobile phones). Cellulose triacetate (CTA), poly(vinyl chloride) (PVC) and poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) were examined as base polymers. The suitability of poly(ethylene glycol) dimethylacrylate (PEGDMA), poly(ethylene glycol) divinyl ether (PEGDVE) and N-ethylmaleimide (NEM) as cross-linking agents, and the possibility of using triarylsulfonium hexafluorophosphate (TASHFP) and 2,2-dimethoxy-2-phenylacetophenone (DMPA) as initiators were investigated. It was demonstrated that the CL-PIMs composed of Cyphos® IL 104 (30 wt%), PVDF-HFP, PEGDMA (base polymer to cross-linking agent ratio 6:4) and DMPA (1 wt%) or TASHFP (2 wt%) transported Au(III) from 2.5 mol L-1 hydrochloric acid solutions twice as fast as their non-CL-PIM counterpart, showing excellent stability over five successive transport experiments. However, in aqua regia feed solutions (6 mol L-1 acidity) only the CL-PIM containing TASHFP was able to achieve complete Au(III) recovery. AFM studies revealed that the PVDF-HFP-based CL-PIMs had a much higher surface contact area when compared to their non-CL counterpart, and this is proposed to be the reason for their superior transport performance. The CL-PIM that showed good transport efficiency in aqua regia was also applied to aqua regia digests of electronic waste from two mobile phones, and Au(III) was selectively recovered in less than 24 h, while other metals present in significantly higher concentrations were not transported.

Use of an ether-derived 3-hydroxy-4-pyridinone chelator as a new chromogenic reagent in the development of a microfluidic paper-based analytical device for Fe(III) determination in natural waters.

This article reports on the development and validation of a disposable microfluidic paper-based analytical device (µPAD) for on-hand, in-situ, and cheap Fe(III) determination in natural waters complying with World Health Organization guidelines. The developed µPAD used 3-hydroxy-4-pyridinone (3,4-HPO) as a colour reagent due to its considerably lower toxicity than traditionally used iron analytical reagents. It was selected among a group of hydrophilic 3,4-HPO chelators containing ether-derived chains in their structure which were prepared using green methods. The relatively high water solubility of these chelators improved the detection limit and applicability as µPAD reagents. Under optimal conditions, the µPAD is characterised by a quantification range between 0.25 and 2.0 mg/L, a detection limit of 55 µg/L and 15 min of analysis time. The signal stability extends up to 4 h and the device is stable for at least one month. The reagent consumption is below 0.2 mg per analysis and the µPAD method was validated by analysis certified reference materials and by comparison with atomic absorption results (RD < 10%). The newly developed µPAD was successfully applied to the determination of iron in river, well and tap waters with no need of any prior sample pre-treatment.

Development of a micro-distillation microfluidic paper-based analytical device as a screening tool for total ammonia monitoring in freshwaters.

An easy-to-use, portable 3D microfluidic paper-based analytical device (µPAD) for the determination of total ammonia (i.e., ammonia + ammonium) in freshwaters is described. It consists of two layers of paper patterned with hydrophilic circular zones, one impregnated with sodium hydroxide (sample zone) and another (detection zone) with an acid-base indicator (nitrazine yellow (NY) or bromothymol blue (BTB)), separated by a µ-distillation chamber. Ammonium ions present in the water sample are converted into ammonia gas by reaction with sodium hydroxide in the sample zone. Ammonia then diffuses through a headspace and reacts with an acid-base indicator in the detection zone, the reflectance of which can be related to the total ammonia concentration. The analytical signal at 7.8 mg L-1 offered by the µ-distillation chamber-based µPAD is more than double of that obtained using a gas-permeable membrane. The proposed µPAD is characterised by a limit of detection of 0.32 or 0.47 mg N L-1 and working concentration ranges of 0.5-3.0 mg N L-1 or 2.0-10 mg N L-1 when using NY or BTB indicators, respectively. This is the first µPAD whose working range covers almost the entire trigger value range (0.32-2.3 mg N L-1) for ammonia nitrogen in freshwater systems which makes it suitable as a field screening tool for ammonia in freshwaters. An inter- and intra-device repeatability of 7.6% and 9.0%, respectively, has been achieved for the NY-based µPAD (3 mg N L-1) and 13 and 2.5% (8 mg N L-1) for the BTB-based µPAD. The NY-based µPAD is stable under vacuum for at least 12 days at room temperature, and 56 days if stored in a freezer (≤-20 °C).

Automatic determination of arsenate in drinking water by flow analysis with dual membrane-based separation.

The sequential application of a polymer inclusion membrane (PIM), composed of poly(vinylidenefluoride-co-hexafluoropropylene) and the anionic extractant Aliquat 336, and a microporous polytetrafluoroethylene (PTFE) gas-permeable membrane was utilized for the first time to develop a flow analysis (FA) system, for the automatic determination of trace levels of arsenate (As(V)) in drinking water as arsine. The system incorporated a flow-through extraction cell for separation and preconcentration of arsenate and a gas-diffusion cell for the separation of arsine prior to its spectrophotometric determination based on the discoloration of a potassium permanganate solution. Under optimal conditions the FA system is characterized by a limit of detection of 3.0 µg L-1 As(V) and repeatability of 1.8% (n = 5, 25 µg L-1 As(V)) and 2.8% (n = 5, 50 µg L-1 As(V)). The newly developed FA method was successfully applied to the determination of arsenate in drinking water samples in the µg L-1 concentration range.

Direct introduction of slurry samples in multi-syringe flow injection analysis: determination of potassium in plant samples.

The present work explores the slurry sampling approach for automatic, flow-based plant analysis. For this purpose, pinch valves were introduced into a multi-syringe flow injection analysis manifold to provide the repeatable aspiration of a few microliters of plant suspension before the material was further processed through the flow system. For validation of the proposed approach, the determination of potassium by flame emission spectrometry was implemented. Several parameters were studied: the concentration of plant particles in the sample suspension and the utilization of matrix modifiers. Microwave digestion was also implemented; no significant difference was found when certified reference material was analyzed with or without the in-line digestion step. The system was successfully applied to 13 samples within a concentration range of 2.5 to 100 mg g(-1). A determination frequency of 28 h(-1) was achieved and the precision was better than 4.0% (n = 12).

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.

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.

A short work-flow to effectively source faecal pollution in recreational waters - A case study.

Microbial pollution of recreational waters poses a significant public health risk which, unless mitigated, will continue to increase with population growth. Water managers must implement strategies to accurately discriminate and source human from animal faecal contamination in complex urbanised environments. Our case-study used a new combination of chemical (i.e. ammonia) and microbial (i.e. Escherichia coli, Bacteroides spp.) faecal monitoring tools in a targeted multi-tiered approach to quickly identify pollution hot-spots and track high-risk subterranean stormwater drains in real-time. We successfully located three point sources of human faecal pollution (both episodic and constant pollution streams) within 11 catchments in a total monitoring time of four months. Alternative approaches for obtaining such fine-scale accuracy are typically labour intensive and require expensive equipment.

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.

Development of a microfluidic paper-based analytical device for the determination of salivary aldehydes.

A low cost, disposable and easy to use microfluidic paper-based analytical device (µPAD) was developed for simple and non-invasive determination of total aldehydes in saliva with a potential to be used in epidemiological studies to assess oral cancer risk. The µPAD is based on the colour reaction between aldehydes (e.g. acetaldehyde, formaldehyde), 3-methyl-2-benzothiazolinone hydrazone (MBTH) and iron(III) to form an intense blue coloured formazan dye. The newly developed µPAD has a 3D design with two overlapping paper layers. The first layer comprises 15 circular detection zones (8 mm in diameter), each impregnated with 8 µL of MBTH, while the second layer contains 15 reagent zones (4 mm in diameter). Two µL of iron(III) chloride are added to each one of the second layer zones after the addition of sample to the detection zones in the first layer. All hydrophilic zones of the µPAD are defined by wax printing using a commercial wax printer. Due to the 2-step nature of the analytical reaction, the two paper layers are separated by a cellulose acetate interleaving sheet to allow for the reaction between the aldehydes in the saliva sample with MBTH to proceed first with the formation of an azine, followed by a blue coloured reaction between the azine and the oxidized by iron(III) form of MBTH, produced after the removal of the interleaving sheet. After obtaining a high resolution image of the detection side zone of the device using a flatbed scanner, the intensity of the blue colour within each detection zone is measured with Image J software. Under optimal conditions, the µPAD is characterised by a working range of 20.4-114.0 µM, limit of detection of 6.1 µM, and repeatability, expressed as RSD, of less than 12.7% (n = 5). There is no statistically significant difference at the 95% confidence level between the results obtained by the µPAD and the reference method (Student's t-test: 0.090 < 0.38). The optimized µPAD is stable for more than 41 days when stored in a freezer (≤-20 °C).

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.

Development of a passive sampler for Zinc(II) in urban pond waters using a polymer inclusion membrane.

The use of a polymer inclusion membrane (PIM) in a novel passive sampler to measure the time-weighted average concentration of Zn(II) in urban waters is described. The passive sampler consists of a compartment containing an acidic receiving solution, which is separated from the external source solution by a PIM consisting of 40 wt% di-2-(ethylhexyl) phosphoric acid as the extractant, and 60 wt% poly-(vinyl chloride) as the base polymer. Two laboratory passive sampling techniques were tested. One involved immersion of the passive sampler into a source solution ("dip-in" approach) for a predetermined period of time while in the other one the source solution was flown past the membrane of the sampler ("flow-through" approach). The latter approach was found to be more suitable for the calibration of the passive sampler under laboratory conditions. A successful application using the "dip-in" sampling approach in urban waters has been conducted for proof of concept.

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.