A high-resolution analyser for the measurement of ammonium in oligotrophic seawater

Abstract

In this work, we describe a high-resolution fluorometric shipboard analyser and an improved method to determine NH 4 + in oligotrophic seawater. The limit of detection is <5 nM, calculated with 95% confidence level using the weighted regression line applied to the standard addition method using real samples prepared with low nutrient seawater from the Atlantic. The results are summarised and cross-compared with spiked artificial seawater (ASW) and spiked Milli-Q water samples. The analyser has a precision of ±1-4% with a high performance over a wide range from 5 nM to 25 μM. The methodology of NH 4 + analysis is based on the fluorescent product formed between o-pthaldialdehyde and NH 4 + in the presence of sulfite. Due to the high resolution of the developed system, we were able to study in depth the sensitivity of the method to salinity, amines, amino acids and potential interferences from particles/algae. The method was found to be sensitive to salinity variations, reducing the signal by up to 85% at 5 nM; this effect decreased at higher concentrations of ammonium. It was noted that the interference from amines at low concentrations was negligible; however, at either high amino acid or high amine concentrations, the signal was depressed. To test for the effect of particles on the system, the system was tested with samples containing phytoplankton (Dunaliella primolecta) cells at different concentrations prepared with ASW to simulate the effect of a phytoplankton bloom. This experiment assessed the potential impact of both particles and other potential fluorescence interferences from cells and/or ammonium leaching from cells. This experiment showed that a phytoplankton bloom could potentially have an impact of up to 12% on the signal of interest. Thus, we propose that this method is suitable for oligotrophic environments rather than coastal and eutrophic environments. The reagent was found to be stable for 17 days and standards of 1 μM were stable for 6 days under laboratory conditions. The developed analyser was successfully demonstrated in the North Atlantic Ocean, in an area of oligotrophic, low NH 4 + oceanic waters.

Chromium Monitoring in Water by Colorimetry Using Optimised 1,5-Diphenylcarbazide Method

“…Biofouling is significantly minimised in optical detection systems as they do not require direct contact between the sensor and the sample. Optical detection methods are widely used in microfluidic analysis [46,47,48,49]. Colorimetric methods can be implemented using simple and low-cost detection systems based on light emitting diodes (LEDs) as light source and photodiode detectors, making them suitable for use in portable microfluidic detection systems [50].…”

Development and Deployment of a Microfluidic Platform for Water Quality Monitoring

“…Mowlem and co-workers at the National Oceanography Centre, Southampton, UK, have developed in-situ sensors for a range of chemical parameters [23][24][25][26][27] including nitrite, nitrate, ammonia, phosphate and iron. The first of this series of microfluidic chemical analysis systems [23] was used to detect nitrate and nitrite with a limit of detection (LOD) of 0.025 µM for nitrate (0.0016 mg L -1 as NO 3 − ) and 0.02 µM for nitrite (0.00092 mg L -1 as NO 2 − ).…”

“…The device was deployed in an estuarine environment (Southampton Water, UK) to monitor nitrate and nitrite concentrations in waters of varying salinity and was able to track changes in the nitrate-salinity relationship of estuarine waters due to increased river flow after a period of high rainfall. In subsequent work, nanomolar detection limits were achieved for iron [24], ammonium [25], phosphate and nitrate [26]. Like the CHEMINI system, these analysers are designed primarily for oceanographic applications, and the level of engineering required to achieve the high analytical performances required in this environment means that the cost of such systems is likely to be prohibitive in terms of deploying large numbers of devices for routine monitoring applications.…”

A novel ammonium‐free seawater preparation method for determination of trace quantities of ammonium in seawater

“…In the surface seawater of the South China Sea (SCS), ammonium is present in dozens of nanomolar concentrations (Zhu et al 2013). In order to measure such low levels of ammonium in the oceanic waters, different sensitive methods have been developed and applied (Amornthammarong and Zhang 2008;Plant et al 2009;Bey et al 2011;Zhu et al 2014;Hashihama et al 2015;Kodama et al 2015). However, there are several discrepancies in the application of these methods, since they require measurements of nanomolar concentrations of ammonium (Plant et al 2009).…”

“…However, practically low ammonium seawater (LASW), prepared and used as "blank" is considered essentially "ammonium-free", although this assumption cannot be applied to trace/nanomolar ammonium analysis . The most widely used and well applied method for LASW involves the collection of low nutrient seawater (LNSW) from the oligotrophic ocean, where nutrients are at present in nanomolar quantities (Pai et al 2001;Bey et al 2011). However, this LASW is not easy to collect and also gets easily contaminated during its transportation and storage.…”