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.

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.

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.

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 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.

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).