Automated simultaneous determination of total dissolved nitrogen and phosphorus in seawater by persulfate oxidation method

Deep UV-LED induced nitrate-to-nitrite conversion for total dissolved nitrogen determination in water samples through persulfate digestion and capillary electrophoresis

BACKGROUND

Capillary electrophoresis (CE) with capacitively coupled contactless conductivity detection (C4D) is widely used for water quality monitoring. However, there is currently no reported CE method for detecting total dissolved nitrogen (TDN), a crucial parameter for assessing water eutrophication. One challenge is the high sulfate concentration (100 mM) introduced during persulfate digestion, leading to overlap of nitrate (from TDN) and poor electrical stacking of nitrate in CE-C4D analysis.

RESULTS

We introduced an in-capillary UV-LED induced photoreaction to convert nitrate to nitrite, which can be baseline-separated from sulfate via the CE method, enabling accurate quantification of nitrate concentration derived from nitrite. A 2 nL post-persulfate digested sample solution within a fused silica capillary was exposed to UV-LED irradiation at the capillary tip. Subsequently, photoreduction-produced nitrite was electrophoretically separated from sulfate in an acidic buffer (pH = 3.7) within the same capillary, followed by contactless conductivity detection. The nitrate-to-nitrite conversion efficiency was influenced by irradiation wavelength, power, and duration, reaching a maximum efficiency of 77.4% when employing two 230 nm LEDs for 5 min. For more general applications, two 255 nm LEDs were used, providing a conversion efficiency of (66.4 ± 3.3)% (n = 11) for 5 min of irradiation. The proposed CE-C4D method exhibits a detection limit of 13 μM (0.18 mg N/L) and has been successfully employed for TDN determination in lake water samples.

SIGNIFICANCE

This innovative approach not only enhances the attractiveness of the CE-C4D method for the determination of water quality indicators but also highlights the potential for integrating deep-UV LEDs into environmental analysis.

Optofluidic Platform for Rapid On-Chip Analysis of Total Phosphorus in Surface Water Using Absorption Spectrometry

Optofluidic devices are of high interest for online monitoring and analyzing biochemical targets in water by integrating the complex on-chip pretreatment of target analytes and spectral analysis. Compared with the traditional bulk equipment, versatile optical detection and biochemical analysis are more easily integrated on an optofluidic chip, which promotes the development of on-chip real-time rapid detection and monitoring. Here, we report an optofluidic platform for online monitoring total phosphorous in water by absorption spectrometry, which naturally combines the merits of both the photo-Fenton effect and microfluidics to realize the rapid on-chip digestion of phosphate at room temperature and normal pressure. The functional cells for chromogenic reaction and optical absorption detection are, respectively, fabricated on the platform to analyze the content of total phosphorus in surface water. In the experiment, the on-chip digestion time of phosphate is dramatically declined to 8.6 sec, and thus, the detection time is greatly shortened to a few minutes. The detection range of total phosphorus is demonstrated as 0.005-1.00 mg L^-1, which satisfies the detection requirements of most environmental water samples. Its availability for measuring the total phosphorous in real water samples is also verified. Predictably, this platform is adapted to on-chip analysis of many other biochemical targets in water.

Single-cell genotyping of phytoplankton from ocean water by gel-based cell manipulation

A comprehensive understanding of phytoplankton diversity is valuable for assessing an environment of interest as phytoplankton are primary producers in various aquatic food webs. Microscopic analyses are useful for diversity assessment based on characteristic cell morphologies. However, phylogenetic classification based solely on morphology requires an extremely high level of expertise. The genetic approach is another option for evaluating phytoplankton diversity; however, it cannot reveal morphological information. To integrate these two approaches, we developed an original technology that is referred to as microcavity array (MCA)/gel-based cell manipulation (GCM). The model experiments using monocultures of various phytoplankton indicated that the efficiencies of cell recovery and isolation of single-cell plankton were dependent on cell size and shape. Cells with widths larger than the cavity width showed high level of recovery and isolation efficiency. Subsequent whole-genome amplification (WGA) of isolated single-cell plankton provided a sufficient amount (approximately 30 μg) of WGA products for genetic analyses. Furthermore, we showed that MCA/GCM could directly analyze phytoplankton in ocean water obtained from Suruga Bay, Japan, without any cumbersome pretreatment. These results indicate that MCA/GCM technology is a powerful tool for elucidating the phytoplankton diversity in marine environment. This article is protected by copyright. All rights reserved.