Development of a micro-distillation microfluidic paper-based analytical device as a screening tool for total ammonia monitoring in freshwaters
Graphical abstract
Introduction
Ammonia is one of the most widely produced chemicals in the world, owing to its numerous uses in industry and agriculture; the worldwide production of ammonia was estimated to be 140 million metric tonnes in 2016 alone [1]. As a pollutant, it typically enters water bodies as agricultural runoff and through discharge of poorly treated sewage effluent [2]. This often has far reaching implications for the health of aquatic environments, fuelling the production of algal blooms leading to the eutrophication of waterways [3]. Ammonia has been shown to be toxic to many forms of aquatic life at quite low concentrations [4], and consequently trigger values for ammonia nitrogen in freshwater systems have been established in the range of 0.32–2.3 mg N L−1, depending on the level of protection and freshwater pH [5].
The ability to readily and accurately determine ammonia concentrations is therefore important for assessing water quality and implementing appropriate management practices. Existing ammonia monitoring techniques include the use of the ammonia ion-selective electrode, and the spectrophotometric Berthelot-phenate method [6]. Flow injection analysis systems employing optical detectors have also been proposed, often using membrane separation of ammonia gas generated from the sample before its spectrophotometric detection using an acid-base indicator [4] or fluorimetric detection [7]. These gas-diffusion techniques are usually restricted to the laboratory, preventing rapid, on-site sample analysis. Samples containing surface active agents may also cause problems because they can promote membrane wetting and leakage. Ammonia test kits and strips are also commercially available but are prone to both inaccuracy due to measurement errors in the field and interference by suspended particulates and natural organic matter.
Microfluidic paper-based analytical devices (μPADs) offer a viable alternative to the aforementioned techniques for field monitoring of ammonia in environmental waters. Originally developed for clinical point-of-care assays [8], μPADs have since been employed in a range of different areas of chemical analysis, including environmental monitoring [9,10]. They have several advantages over traditional monitoring techniques in that they are low cost, portable and have high sample throughput.
Determination of higher concentrations of ammonia in soil solution and wastewater samples using μPADs with membrane-based [11] or membraneless [12] ammonia separation has been reported, but there are no accounts of robust μPADs suitable for measurement of total ammonia at the concentrations typically found in freshwaters (i.e., <10 mg N L−1).
This paper describes a μPAD with a sensitivity which makes it suitable for the determination of total ammonia in freshwaters. Enhanced sensitivity was achieved by the use of an appropriately shaped μ-distillation chamber which allowed the free diffusion of ammonia gas across the two layers of the μPAD and facilitated enhanced preconcentration of the analyte onto the detection zone. Two different working ranges were also achieved by using different acid-base indicators (i.e., nitrazine yellow or bromothymol blue).
Section snippets
Reagents and solutions
All reagents were of analytical grade and all solutions were prepared in deionized water (≥18 MΩ cm, Millipore Synergy 185) unless stated otherwise.
Ammonia stock solution (100 mg N L−1) was prepared fortnightly from ammonium chloride (Ajax Finechem), and standards were then prepared daily by dilution from this stock solution in the range of 0.5–10 mg N L−1. Ammonium chloride salt was dried at 105 °C, and periodically weighed until constant mass was observed prior to stock solution preparation.
Comparison between μPADs utilizing a gas-permeable membrane and a μ-distillation chamber
The efficiency of ammonia gas transport across a gas-permeable hydrophobic membrane or headspace is deemed to be a major factor governing sensitivity in gas-diffusion flow injection analysis systems designed for ammonia detection [[20], [21], [22]]. Ammonia transport efficiencies across PTFE membranes in flow analysis systems have been reported to be as low as < 30% [20], and as high as 62% [23].
Colorimetric ammonia analysis via gas-diffusion has also been reported in a μPAD format and is
Conclusions
The development and validation of portable, easy to use and inexpensive μPADs based on μ-distillation for the screening of total ammonia in environmental freshwaters have been described.
A 2.4-fold increase in analytical signal at 7.8 mg N L−1 was achieved by eliminating the gas-permeable membrane barrier between the two paper layers and replacing it with a μ-distillation chamber. The inclusion of supporting ‘pillars’ in the μ-distillation chamber (Fig. 2B) has increased the robustness and
Acknowledgements
The authors are grateful to Purnendu (Sandy) Dasgupta from the University of Texas (Arlington) for valuable suggestions regarding this study.
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