Deirdre Cogan

Integrated flow analysis platform for the direct detection of nitrate in water using a simplified chromotropic acid method

This work describes the first use of a direct nitrate analyser using chromotropic acid. A simplified chromotropic acid method eliminating several steps previously associated with this method is employed in the platform. In a sulphuric acid medium, chromotropic acid reacts with nitrate ions and produces a characteristic yellow colour associated with an absorbance band in the visible region (λmax = 430 nm). The modified method allows for nitrate determination over the linear range 0.9–80 mg L−1 nitrate with a limit of detection of 0.73 μg L−1 nitrate. Validation was achieved by analysing water samples from various sources including groundwater, trade effluent and drinking water by the modified method and by ion chromatography. The method was implemented on a flow analysis platform incorporating a paired emitter–detector diode (PEDD) as the optical detector. An excellent correlation coefficient of 0.993 was obtained between the modified method and ion chromatography. The modified chromotropic acid method represents a rapid, simple, low cost technique for the direct determination of nitrate in water.

The development of an autonomous sensing platform for the monitoring of ammonia in water using a simplified Berthelot method

This study demonstrates that by combining a modified version of the Berthelot method with microfluidic technologies and LED based optical detection systems, a low cost monitoring system for detection of ammonia in fresh water and wastewater can be developed. The assay developed is a variation on the Berthelot method, eliminating several steps previously associated with the method to create a nontoxic and simple colorimetric assay. The previous Berthelot method required the addition of three reagents, mixed sequentially with the sample, which complicates the microfluidic system design. With the modified method, comparable results were attained using a single reagent addition step at a 1 : 1 v/v reagent to sample ratio, which significantly simplifies the fluidic handling requirement for integration into an autonomous sensing platform. The intense colour generated in the presence of ammonia is detected at a wavelength of 660 nm. The method allows for ammonia determination up to 12 mg L−1 NH4+ with a limit of detection of 0.015 mg L−1 NH4+. Validation was achieved by analysing split water samples by the modified method and by ion chromatography, resulting in an excellent correlation coefficient of 0.9954. The method was then implemented into a fully integrated sensing platform consisting of a sample inlet with filter, storage units for the Berthelot reagent and standards for self-calibration, pumping system which controls the transport and mixing of the sample, a microfluidic mixing and detection chip, and waste storage. The optical detection system consists of a LED light source with a photodiode detector, which enables sensitive detection of the coloured complex formed. The robustness and low cost of the microfluidic platform coupled with integrated wireless communications makes it an ideal platform for in situ environmental monitoring. This is the first demonstration of a fully functional microfluidic platform employing this modified version of the Berthelot method.

Development of a low cost microfluidic sensor for the direct determination of nitrate using chromotropic acid in natural waters

Progress towards the development of a miniaturised microfluidic instrument for the direct measurement of nitrate in natural waters and wastewater using chromotropic acid is presented. For the first time, the chromotropic method for nitrate analysis has been transferred to a microfluidic chip configuration that can withstand the extremely acidic nature of the reagent within a field deployable platform. This simple method employs one reagent mixed in a 1 : 1 ratio with the sample to produce a yellow colour absorbing strongly at 430 nm. A stopped flow approach is used which, together with the very rapid kinetics and simple reagent stream, enables an uncomplicated microfluidic design and field deployable platform with a sample throughput of 9 samples per h, limits of detection of 0.70 mg L−1 NO3− and 0.31 mg L−1 NO3− for seawater samples, with a dynamic linear range from 0–80 mg L−1 NO3− and long-term reagent stability of up to 6 months. Validation was achieved by analysing split water samples by the analyser and ion chromatography, resulting in an excellent correlation co-efficient of 0.9969. The fully integrated sensing platform consists of a sample inlet with filter, storage units for chromotropic reagent and standards for self-calibration, pumping system which controls the transport and mixing of the sample, a microfluidic mixing and detection chip, and waste storage, all contained within a ruggedized, waterproof housing. The optical detection system consists of a LED light source with a photodiode detector, which enables sensitive detection of the coloured complex formed. The low cost of the platform coupled with integrated wireless communication makes it an ideal platform for in situ environmental monitoring.