Real-Time Underway Mapping of Nutrient Concentrations of Surface Seawater Using an Autonomous Flow Analyzer.

High-frequency field nutrient analyzers offer a promising technology to solve time-consuming and laborious sampling problems in dynamic and complex river-estuarine-coastal ecosystems. However, few studies on the simultaneous underway analysis of five key nutrients (ammonium, nitrite, nitrate, phosphate, and silicate) in seawaters are available because of the limitations of the technique. In this study, a state-of-the-art autonomous portable analyzer for the shipboard analysis of nutrients in the environment of varied salinities and concentration ranges was reported. The analyzer consisted of compact hardware that was well suited for shipboard deployment with minimal maintenance. Moreover, a novel LabVIEW-based software program was developed, containing additional functions such as automated calibration curve generation, autodilution of high-concentration samples, and a user-friendly interface for multiparameter analysis using a single instrument. After the optimization of chemical reactions and work flow chart, the analyzer exhibited low limits of detection, a large linear range with automated dilution, and relative standard deviations of less than 2% (n = 11). Compared to other flow-based techniques, this analyzer is more portable and consumes less reagent with an autonomous data processing function and applicability within a broad salinity range (0-35). The analyzer was successfully applied for real-time analysis in the Jiulong River Estuary-Xiamen Bay with excellent on-site accuracy and applicability. The relationship between high spatial resolution nutrient concentrations and salinities showed very different patterns in estuarine and coastal areas, indicating the benefit of using an underway automated analyzer for chemical mapping in a dynamic environment.

An automated analyzer for the simultaneous determination of silicate and phosphate in seawater.

Accurate and automated determination methods for silicate and phosphate are in high demand to improve understanding of biogeochemistry and ecology in dynamic and complex estuarine-coastal ecosystems. Here, a portable automated analyzer is reported for the simultaneous determination of silicate and phosphate in water samples with varying salinity. After comprehensive optimization of the chemical reactions and flow manifold, the system demonstrated limits of detection of 0.09 and 0.05 µmol L-1 for silicate and phosphate, respectively, exhibiting linearity at concentrations up to 400 and 200 µmol L-1 with automated dilution, achieving relative standard deviations (n = 11) of 0.27% (20 µmol L-1 silicate), 0.51% (5 µmol L-1 phosphate) and 0.80% (1 µmol L-1 phosphate). Compared with similar automated flow analyzers, the system exhibited advantages, such as low consumption of reagents (10-20 µL/sample), portability (4.8 kg), rapid start-up (5 min), reliability (automated analysis of quality control sample) and applicability within a broad salinity range (from 0 to 35) allowing analysis of dynamic estuarine and seawater samples. The system was successfully applied to a routine monitoring program by a national marine station and is potentially suitable for installation on different observation platforms for on-line and real-time underway analysis of nutrients in coastal areas.

Development of a versatile smartphone-based environmental analyzer (vSEA) and its application in on-site nutrient detection.

The citizen-science-based environmental survey can benefit from the smartphone technology used in chemical and biological sensing of a wide range of analytes. Quantification by smartphone-based colorimetric assays is being increasingly reported, however, most of the quantification uses empirical formula or complex exhaustive methods. In this study, a versatile and robust algorithm is proposed to overcome these limitations. A model is established to simulate and analyze the conversion process from the camera's spectral information into RGB (Red, Green, Blue) color information. Moreover, the feasibility of the algorithm for the quantification of different analytes is also explored. Based on this algorithm, a versatile smartphone-based environmental analyzer (vSEA) is built and its reliability, versatility, and analytical performance are comprehensively optimized. The good linearity (R2 ≥ 0.9954) and precision (relative standard deviations < 5.3%) indicates that the vSEA is accurate enough to quantify the nutrients in most natural waters. Furthermore, the vSEA is used for the field measurement of five important nutrients, and the results show no significant difference compared to conventional methods. The vSEA offers a simpler and easier method for the on-site measurement of nutrients in natural water bodies, which can aid in the emergency monitoring of aqueous ecosystems and the performance of citizen-science-based research.

Towards citizen science. On-site detection of nitrite and ammonium using a smartphone and social media software.

Citizen scientists-based water quality surveys are becoming popular because of their wide applications in environmental monitoring and public education. At present, many similar studies are reported on collecting samples for later laboratory analysis. For environmentally toxic analytes such as ammonium and nitrite, on-site detection is a promising choice. However, this approach is limited by the availability of suitable methods and instruments. Here, a simple on-site detection method for ammonium and nitrite is reported. The chemistry of this method is based on the classic Griess reaction and modified indophenol blue reaction. Digital image colorimetry is carried out using a smartphone with a custom-made WeChat mini-program or free built-in applications (APPs). Using a simple and low-cost analytical kit, the detection limit of 0.27 µmol/L and 0.84 µmol/L is achieved for nitrite and ammonium, respectively, which are comparable to those achieved with a benchtop spectrophotometer. Relative standard deviations (n = 7) for low and high concentrations of nitrite are 3.6% and 4.3% and for ammonium are 5.6% and 2.6%, respectively. Identical results with a relative error of less than 10% are obtained using different smartphones (n = 3), color extracting software (n = 6), and with multiple individual users (n = 5). These results show the robustness and applicability of the proposed method. The on-site application is carried out in an in-campus wastewater treatment plant and at a local river. A total of 40 samples are analyzed and the analytical results are compared with that obtained by a standard method and a spectrophotometer, followed by a paired t-test at a 95% confidence level. This proposed on-site analytical kit has the advantages of simplicity and portability and has the potential to be popular and useful for citizen science-based environmental monitoring.

Automated Determination of Dissolved Reactive Phosphorus at Nanomolar to Micromolar Levels in Natural Waters Using a Portable Flow Analyzer.

Automated in-field methods for measuring dissolved reactive phosphorus (DRP) over a large concentration range are in high demand for the purpose of better understanding the biogeochemistry of phosphorus in the river-estuary-coast continuum to the open ocean. Here, an automated portable and robust analyzer was described for the determination of nanomolar to micromolar levels of DRP in natural waters. The quantification of DRP was based on classic phosphomolybdenum blue (PMB) chemistry. All the components of the analyzer were computer-controlled using LabVIEW-based laboratory-programmed software. When equipped with a 3 cm Z-type flow cell, the system demonstrated linearity with concentrations up to 12 µmol L-1, a sampling rate of 20 h-1, a limit of detection of 0.11 µmol L-1, and relative standard deviations (RSDs) of 0.4-4.6% (n = 11-576). When a solid-phase extraction cartridge was combined with the analyzer, the PMB formed from the sample was automatically concentrated on the hydrophilic-lipophilic balanced sorbent. The concentrated PMB compound was eluted with NaOH solution and measured in the spectrophotometric system. Under optimal conditions, the nanomolar-level mode afforded a sampling rate of 8 h-1, a limit of detection of 1.7 nmol L-1, and RSDs of 3.0-5.7% (n = 11-120). The system exhibited advantages that included a wide linear range, high sensitivity and reproducibility, low reagent consumption, and insignificant interference from salinity, silicate, arsenate, and other P-containing compounds. The system was successfully applied for discrete sample analysis, fixed site online monitoring, and the real-time underway measurement of DRP in riverine-estuarine-coastal waters.

Simultaneous underway analysis of nitrate and nitrite in estuarine and coastal waters using an automated integrated syringe-pump-based environmental-water analyzer.

Methods for determining nitrate and nitrite have been comprehensively developed. However, there are few studies of simultaneous shipboard high-frequency monitoring of these two nutrients in estuarine and coastal area. In this study, a multipurpose integrated syringe-pump-based environmental-water analyzer (iSEA) was combined with an on-line filtration system for underway analysis of nitrate and nitrite in saline samples. Vanadium chloride was used instead of a toxic cadmium column to reduce nitrate to nitrite, which was measured on the basis of the classic Griess reaction. This fully automated analyzer had a limit of detection of 0.02 µmol L-1 for nitrite and 0.14 µmol L-1 for nitrate. The sample throughput was 12 h-1 for simultaneous measurement of nitrite and nitrate. With automated dilution, the calibration curve for nitrate was linear up to a concentration of 400 µmol L-1 (R2 > 0.999). The relative standard deviation of 24-h measurement (n = 288) of nitrite is 0.92% and that of nitrate is 1.4%. Both the reference solutions and samples of different salinities (range of 0-35) were measured (n = 85). According to the statistical t-test (P = 0.95), the results were insignificantly different from the results obtained using the reference method. After several cruise tests, the analyzer showed excellent spatial resolution for underway analysis of nitrite and nitrate in estuarine and coastal waters.