Continuous Flow-Double Purge and Trap Method for Preconcentrating Mercury in Large Volumes of Seawater for Stable Isotope Analysis.

Mercury (Hg) isotopes provide a useful tool to understand Hg sources and processes in the environment. The Hg isotopic composition of seawater remains poorly constrained due to the lack of an efficient method to process large volumes of low-Hg-concentration seawater samples. Here, we develop a continuous flow-double purge and trap device for the in situ preconcentration of Hg in seawater. This method yielded a good Hg recovery of 91.7 ± 3.3% (n = 4, 1SD) for spiked seawater samples and gave reasonably similar Hg isotope ratios of NIST 8610, indicating a limited matrix effect and limited Hg isotope fractionation during processing of seawater. NIST 8610 δ202Hg (-0.55 ± 0.09‰, n = 4, 1SD) and Δ199Hg (0.07 ± 0.02‰, n = 4, 1SD) were similar to previously published data. The method was successfully applied to seawater collected from the Xiamen Bay and the South China Sea. The seawater samples showed a Hg recovery of 91.6 ± 5.4% (n = 12, 1SD). Seawater Δ199Hg (-0.04 ± 0.05‰, n = 7, 1SD) in the Xiamen Bay was different from seawater Δ199Hg (0.05 ± 0.07‰, n = 5, 1SD) in the South China Sea, which implies distinct Hg sources to coastal and open ocean areas and highlights the robustness of our method in understanding the Hg isotopic composition of seawater.

High-sensitive determination of tetracycline antibiotics adsorbed on microplastics in mariculture water using pre-COF/monolith composite-based in-tube solid phase microextraction on-line coupled to HPLC-MS/MS.

Microplastics (MPs) act as carriers for organic pollutants (e.g. antibiotics) and microorganisms (e.g. bacteria) in waters, leading to the proliferation of antibiotic resistance genes. Moreover, the antibiotics adsorbed on MPs may exacerbate this process. For further research, it is necessary to understand the types and amounts of antibiotics adsorbed on MPs. However, due to the heavy work of MPs collection and sample pretreatment, there is a lack of analytical methods and relevant data. In this study, an in-tube solid phase microextraction (IT-SPME) on-line coupled to HPLC-MS/MS method based on amorphous precursor polymer of three-dimensional covalent organic frameworks/monolith-based composite adsorbent was developed, which could efficiently capture, enrich and analyze tetracycline (TCs) antibiotics. Under the optimal extraction parameters, the developed method was capable of detecting TCs at levels as low as 0.48-1.76 pg. This method was applied to analyze the TCs adsorbed on MPs of different particle sizes in mariculture water for the first time, requiring a minimum amount of MPs of only 1 mg. Furthermore, it was observed that there could be an antagonistic relationship between algal biofilm and TCs loaded on MPs. This approach could open up new possibilities for analyzing pollutants on MPs and support deeper research on MPs.

Spectrophotometric determination of nitrate in small volume of seawater samples using a simple resorcinol method.

A simple resorcinol method to determine nitrate (NO3-) in seawater using a microplate reader with a 48-well plate was established. The method involved the nitration of resorcinol in sulfuric acid to form a pink product that was detected at 505 nm. Reagent concentrations were optimized, and the effect of salinity on NO3- determination was investigated. The detection limit of this method was 0.8 µM, while the upper limit of the linear range was 100 µM. The recoveries ranged from 91.5 to 109.7% for spiked seawater samples with different salinities. The proposed method was compared with two reference methods, and the results revealed a good correlation. Compared to conventional methods that require the preparation of reactants, the proposed method used aqueous solutions as reagents for the reaction, which was simpler and more convenient. Compared to the methods that used organic solvents for the direct determination of NO3-, the proposed method was suitable for estuarine and coastal water samples with large salinity variations. All results indicated that the proposed method can satisfy the requirements of laboratory analysis and demonstrate high application potential for use in field determination.

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.

Development of an online analyzer for determination of total phosphorus in industrial circulating cooling water with UV photooxidation digestion and spectrophotometric detection.

A simple, effective and automatic analyzer was developed for rapid determination of total phosphorus and the presence of phosphonates in industrial circulating cooling water monitored. The key components of the analyzer were an ultraviolet (UV) photooxidation digester and a spectrophotometer. The total phosphorus was first converted to orthophosphate using the UV digester, and then its content was measured using the vanadomolybdophosphoric acid method. A bus topology and distributed control system were used. Industrial standard communication Modbus protocol was used for communication between the main controller and a computer. The online analyzer automatically processed the determination steps such as sample injection, reagent mixing, online digestion, signal detection, data acquiring and analysis. The detection linear range was 0.2-50 mgP L-1, and detection limit of 0.04 mgP L-1. The relative standard deviations of the method for 1 mgP L-1 and 20 mgP L-1 phosphate samples were 2.6% and 2.1% (n = 11), respectively. The recoveries of several real samples at various concentrations ranged from 91.5% to 103.8%. The total phosphorus of a certified reference material (GSBZ50033-95) was 1.50 ±â€¯0.04 mgP L-1 (n = 3), consistent with the certified value (1.51 ±â€¯0.02 mgP L-1). The analyzer had been used successfully in a circulating cooling water workshop; its integrated software-hardware not only demonstrated to be versatile but also suitable for monitoring total phosphorus in industrial or other water bodies.

Spectrophotometric flow injection determination of dissolved titanium in seawater exploiting in-line nitrilotriacetic acid resin preconcentration and a long path length liquid waveguide capillary cell.

A sensitive spectrophotometric method for the determination of dissolved titanium (Ti) in seawater is developed. It involves in-line preconcentration and a long path length liquid waveguide capillary cell (LWCC). Nitrilotriacetic acid (NTA) resin is used to preconcentrate Ti from ∼25 mL seawater sample at pH 1.7, and elution is accomplished with 0.8 mol L-1 hydrochloride acid. The eluted Ti solution is buffered to pH 6.0 with 1.0 mol L-1 ammonium acetate and mixed with 1.5 mmol L-1 Tiron solution. The mixture is then injected into LWCC and measured by spectrophotometry at 420 nm. Before the preconcentration step, the sample is treated with 7 mmol L-1 ascorbic acid to reduce Fe(III) to Fe(II), in order to eliminate the Fe interference. The method is not interfered by Fe(III) and Cu(II) present in seawater samples at concentrations 50-fold higher in relation to Ti, and by Cd(II), Pb(II), Cr(VI), Mn(II), Al(III), Zn(II), and Ni(II) at concentrations 100-fold higher in relation to Ti. It is time efficient (7.5 minutes per sample), sensitive (0.10 nmol L-1 detection limit), precise (1.40% measurement RSD at 1.00 nmol L-1 Ti) and is characterized by a linear range of 0.50-5.00 nmol L-1 Ti. The method was applied to analysis of natural water samples collected from the Jiulongjiang Estuary, Fujian, China.

An automatic reserve flow injection method using vanadium (III) reduction for simultaneous determination of nitrite and nitrate in estuarine and coastal waters.

An automatic reserve flow injection method using vanadium (III) as a reducing agent for the simultaneous determination of nitrite and nitrate in estuarine and coastal waters was reported for the first time. Vanadium (III) chloride was used as an environmentally friendly substitute to toxic cadmium for reducing nitrate to nitrite. The experimental parameters were optimized based on a univariate experimental design. The salinity effect of estuarine and coastal waters was carefully investigated. Under the optimized conditions, the detection limit of the proposed method was 0.06 µmol L-1 and 0.13 µmol L-1, and the linearity was up to 20 µmol L-1 and 80 µmol L-1 for nitrite and nitrate detection, respectively. The relative standard deviations were below 1.5% (n = 7). The recovery of spiked estuarine and coastal water samples varied from 100.0 ±â€¯2.5% to 107.5 ±â€¯2.5% for nitrite and 90.7 ±â€¯0.3% to 98.0 ±â€¯1.0% for nitrate. The sample throughput was approximately 15 h-1. The analytical results obtained with the proposed method displayed good agreement with the classic copperized cadmium column reduction method. The automatic reserve flow injection method has been successfully applied to analyze the nitrite and nitrate in estuarine and coastal water samples.

High-frequency underway analysis of ammonium in coastal waters using an integrated syringe-pump-based environmental-water analyzer (iSEA).

Accurate methods and related robust analytical instruments for sensitive shipboard determination of ammonium in coastal waters are highly desirable for both oceanographers and environmental scientists. 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 ammonium in coastal areas. The chemistry is based on a modified indophenol method using o-phenylphenol. The effects of reagent concentrations and sample temperatures were evaluated. The detection limit was 0.15 µM with a 3-cm Z-flow cell, and the linearity was as high as 200 µM. The relative standard deviations at different concentrations (2, 10, and 20 µM) were 2.2%, 0.33%, 0.32% (n = 11). For n = 288 and without any stoppage during repeated analysis for 24 h, the relative standard deviation was 0.85%. The sample throughput was 12 h-1. The effects of salinity and five organic nitrogen compounds were evaluated and showed no interference using the proposed protocol for ammonium analysis. Between results obtained by reference and the present methods, there were no significant differences in the measurements of reference materials and different aqueous samples (n = 51). The analyzers worked well in the transect of 420 km during 7 cruises. A total of 716 analyses were performed automatically on board, demonstrating the capability of iSEA in automated real-time mapping of ammonium distribution in a shipboard laboratory.

Low-Cost Automatic Sensor for in Situ Colorimetric Detection of Phosphate and Nitrite in Agricultural Water.

This study proposed a low-cost sensor for in situ automatic monitoring of phosphate and nitrite in agricultural water environments, involving a series of "Fish-Bite" reservoirs, multiple reagent capsules, and a colorimetric sensor. The Fish-Bite reservoir is an alternative to the pumps, valves, and filters that are widely used for water sample collection and also offers a closed cell for chromogenic reactions afterward. Up to two capsules can be embedded in each reservoir to support chromogenic reactions that use two different reagents in sequence. From the results of calibration tests in the laboratory, the limit of detection was found to be approximately 0.01 mg/L for both phosphate and nitrite, with a linear range of 0.01-1.00 mg/L for phosphate and 0.01-0.20 mg/L for nitrite. Furthermore, an in situ experiment was successfully carried out in an irrigation canal beside farmland to demonstrate the practicability and robustness of the device. The averaged concentrations of phosphate and nitrite were 0.0113 mg/L and 0.0383 mg/L, respectively. The relative deviations were 20.2% and 11.7%, respectively, referred to results obtained by using the standard spectrophotometric methods. With the advantages of being robust, fast, and low cost, this in situ device is promising for the formation of agricultural sensor networks.

Simultaneous determination of total dissolved nitrogen and total dissolved phosphorus in natural waters with an on-line UV and thermal digestion.

A flow injection method combined with an on-line UV and thermal digestion for simultaneous determination of total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) in natural waters was established in this study. A novel flow manifold made the proposed system compact and automatic. The conversion rates of various nitrogen and phosphorus compounds to their nitrate and phosphate forms with different digestion models and different concentrations were well investigated using the flow injection technique. The reagent concentrations for colorimetric analysis were optimized based on a univariate experimental design. The detection limits were 0.8 µmol L-1 and 0.2 µmol L-1, and linear analytical ranges were up to 300 µmol L-1 and 25 µmol L-1 for TDN and TDP, respectively. The sample throughput was ~ 5 h-1. The recovery of spiked natural water samples varied from 86.8% to 102.6% for TDN and 88.0% to 102.0% for TDP. The present approach was successfully applied for the determination of TDN and TDP in natural water samples and was found to have good agreement with reference methods. The outcomes of present study indicated that the proposed method is suitable for routine analysis as well as for potential on-line monitoring.

Development of an Integrated Syringe-Pump-Based Environmental-Water Analyzer ( iSEA) and Application of It for Fully Automated Real-Time Determination of Ammonium in Fresh Water.

The development of a multipurpose integrated syringe-pump-based environmental-water analyzer ( iSEA) and its application for spectrophotometric determination of ammonium is presented. The iSEA consists of a mini-syringe pump equipped with a selection valve and laboratory-programmed software written by LabVIEW. The chemistry is based on a modified indophenol method using o-phenylphenol. The effect of reagent concentrations and sample temperatures was evaluated. This fully automated analyzer had a detection limit of 0.12 µM with sample throughput of 12 h-1. Relative standard deviations at different concentrations (0-20 µM) were 0.23-3.36% ( n = 3-11) and 1.0% ( n = 144, in 24 h of continuous measurement, ∼5 µM). Calibration curves were linear ( R2 = 0.9998) over the range of 0-20 and 0-70 µM for the detection at 700 and 600 nm, respectively. The iSEA was applied in continuous real-time monitoring of ammonium variations in a river for 24 h and 14 days. A total of 1802 samples were measured, and only 0.4% was outlier data (≥3 sigma residuals). Measurements of reference materials and different aqueous samples ( n = 26) showed no significant difference between results obtained by reference and present methods. The system is compact (18 cm × 22 cm × 24 cm), portable (4.8 kg), and robust (high-resolution real-time monitoring in harsh environments) and consumes a small amount of chemicals (20-30 µL/run) and sample/standards (2.9 mL/run).

Optimization of a salinity-interference-free indophenol method for the determination of ammonium in natural waters using o-phenylphenol.

The indophenol blue (IPB) method based on Berthelot's reaction is one of the most widely used methods for the determination of ammonium in natural waters. This study comprehensively optimized the kinetics of the IPB reaction under different reagent concentrations, temperature and salinity. The normally used toxic and odorous phenol was replaced by the less toxic, stable flaky crystalline compound, o-phenylphenol. With the application of nitroprusside as the catalyst, the reaction can be finished within 20min at room temperature and the formed color compound is stable for 24h. Under the optimized conditions, the method shows high reproducibility (relative standard deviations of 0.64-1.71%, n = 11), highly linear calibration up to 100µM (R2 = 0.9995, n = 165, 17 days) and a low detection limit of 0.2µM. This method was successfully applied to measure ammonium in estuarine and coastal surface water (n = 63). The results showed insignificant differences with the results obtained using both the standard AutoAnalyzer method and a fluorometric o-phthaldialdehyde method at the 95% confidence level. Compared with previous studies, this method shows the advantages of relatively fast reaction, low toxicity and easy reagent preparation. It is salinity-interference-free and robust (no temperature control is required, reagents can be stored up to 10 days), and suitable for routine analysis under harsh field conditions.

A catalytic spectrophotometric method for determination of nanomolar manganese in seawater using reverse flow injection analysis and a long path length liquid waveguide capillary cell.

A sensitive and precise method for determination of nanomolar manganese in seawater was developed, using reverse flow injection analysis, a long path length liquid waveguide capillary cell, and spectrophotometric detection. The reaction was based on manganese catalyzed oxidation of leucomalachite green with sodium periodate. Various experimental parameters were investigated and optimized. Foreign trace metal ions of iron, copper, zinc, nickel and aluminum did not cause obvious interference with manganese detection. Low manganese seawater was prepared and used as the blank and standards' matrix, to eliminate the seawater matrix effect. The method detection limit was 0.20nmolL-1, and the quantification range was 0.50-10.00nmolL-1, which should be sensitive enough and suitable for open ocean seawater analysis. The seawater certified reference material NASS-6 was used to test the accuracy, and good agreement was obtained. The proposed method was applied to analyze seawater samples collected at the SEATS station in the South China Sea. The vertical profile of the total dissolvable manganese is reported and discussed.

In-field determination of trace dissolved manganese in estuarine and coastal waters with automatic on-line preconcentration and flame atomic fluorescence spectrometry.

An automatic on-line preconcentration and detection system for analysis of trace dissolved manganese (Mn) in estuarine and coastal waters was established, using preconcentration with IDA chelating resin and detection with flame atomic fluorescence spectrometer (FAFS). The rinse (pre-eluent) solution was optimized, for removing the interference ions while retaining the target element. It was found that the interference ions affected the chelating efficiency of Mn, causing variation of the detection blank and sensitivity. This effect varied when sample volume presented as preconcentration time changed. The influence at preconcentration times of 120 s, 30 s and 10 s were carefully investigated and reported. Ten folds of the foreign trace metals Zn, Cu, Ni, Fe, and Al did not show obvious interference on Mn preconcentration and detection. The method detection limit was 0.9 nmol L-1 (n = 7, preconcentration time 120 s). The linear detection range could be adjusted with designed preconcentration time. In addition to high precision and accuracy, the proposed analytical system had the advantages of high integration, and required normal site preparation, low energy supply and simple auxiliary equipment, which was appropriate for in-field operation. Compared with other common in-field applied molecular spectrometry instruments, the inherent high selectivity and multi-element applicability of FAFS highlighted the superiority and potential of the proposed analytical system. It was successfully applied to in-field vehicle-board determination of dissolved Mn in coastal waters around Xiamen, Fujian, China, and it was also used to analyze natural water samples collected from the Jiulongjiang Estuary, Fujian, China.

Sequential determination of multi-nutrient elements in natural water samples with a reverse flow injection system.

An integrated system was developed for automatic and sequential determination of NO2-, NO3-, PO43-, Fe2+, Fe3+ and Mn2+ in natural waters based on reverse flow injection analysis combined with spectrophotometric detection. The system operation was controlled by a single chip microcomputer and laboratory-programmed software written in LabVIEW. The experimental parameters for each nutrient element analysis were optimized based on a univariate experimental design, and interferences from common ions were evaluated. The upper limits of the linear range (along with detection limit, µmolL-1) of the proposed method was 20 (0.03), 200 (0.7), 12 (0.3), 5 (0.03), 5 (0.03), 9 (0.2) µmolL-1, for NO2-, NO3-, PO43-, Fe2+, Fe3+ and Mn2+, respectively. The relative standard deviations were below 5% (n=9-13) and the recoveries varied from 88.0±1.0% to 104.5±1.0% for spiked water samples. The sample throughput was about 20h-1. This system has been successfully applied for the determination of multi-nutrient elements in different kinds of water samples and showed good agreement with reference methods (slope 1.0260±0.0043, R2=0.9991, n=50).

Mercury isotope fractionation during transfer from post-desulfurized seawater to air.

Samples of dissolved gaseous mercury (DGM) in the post-desulfurized seawater discharged from a coal-fired power plant together with samples of gaseous elemental mercury (GEM) over the post-desulfurized seawater surface were collected and analyzed to study the mercury isotope fractionation during transfer from post-desulfurized seawater to air. Experimental results showed that when DGM in the seawater was converted to GEM in the air, the δ202Hg and Δ199Hg values were changed, ranging from -2.98 to -0.04‰ and from -0.31 to 0.64‰, respectively. Aeration played a key role in accelerating the transformation of DGM to GEM, and resulted in light mercury isotopes being more likely to be enriched in the GEM. The ratio Δ199Hg/Δ201Hg was 1.626 in all samples, suggesting that mercury mass independent fractionation occurred owing to the nuclear volume effect during the transformation. In addition, mass independent fractionation of mercury even isotopes was found in the GEM above the post-desulfurized seawater surface in the aeration pool.

Mercury isotope signatures of seawater discharged from a coal-fired power plant equipped with a seawater flue gas desulfurization system.

Seawater flue gas desulfurization (SFGD) systems are commonly used to remove acidic SO2 from the flue gas with alkaline seawater in many coastal coal-fired power plants in China. However, large amount of mercury (Hg) originated from coal is also transferred into seawater during the desulfurization (De-SO2) process. This research investigated Hg isotopes in seawater discharged from a coastal plant equipped with a SFGD system for the first time. Suspended particles of inorganic minerals, carbon residuals and sulfides are enriched in heavy Hg isotopes during the De-SO2 process. δ(202)Hg of particulate mercury (PHg) gradually decreased from -0.30‰ to -1.53‰ in study sea area as the distance from the point of discharge increased. The results revealed that physical mixing of contaminated De-SO2 seawater and uncontaminated fresh seawater caused a change in isotopic composition of PHg isotopes in the discharging area; and suggested that both De-SO2 seawater and local background contributed to PHg. The impacted sea area predicted with isotopic tracing technique was much larger than that resulted from a simple comparison of pollutant concentration. It was the first attempt to apply mercury isotopic composition signatures with two-component mixing model to trace the mercury pollution and its influence in seawater. The results could be beneficial to the coal-fired plants with SFGD systems to assess and control Hg pollution in sea area.

Automated determination of nitrate plus nitrite in aqueous samples with flow injection analysis using vanadium (III) chloride as reductant.

Determination of nitrate in aqueous samples is an important analytical objective for environmental monitoring and assessment. Here we report the first automatic flow injection analysis (FIA) of nitrate (plus nitrite) using VCl3 as reductant instead of the well-known but toxic cadmium column for reducing nitrate to nitrite. The reduced nitrate plus the nitrite originally present in the sample react with the Griess reagent (sulfanilamide and N-1-naphthylethylenediamine dihydrochloride) under acidic condition. The resulting pink azo dye can be detected at 540 nm. The Griess reagent and VCl3 are used as a single mixed reagent solution to simplify the system. The various parameters of the FIA procedure including reagent composition, temperature, volume of the injection loop, and flow rate were carefully investigated and optimized via univariate experimental design. Under the optimized conditions, the linear range and detection limit of this method are 0-100 µM (R(2)=0.9995) and 0.1 µM, respectively. The targeted analytical range can be easily extended to higher concentrations by selecting alternative detection wavelengths or increasing flow rate. The FIA system provides a sample throughput of 20 h(-1), which is much higher than that of previously reported manual methods based on the same chemistry. National reference solutions and different kinds of aqueous samples were analyzed with our method as well as the cadmium column reduction method. The results from our method agree well with both the certified value and the results from the cadmium column reduction method (no significant difference with P=0.95). The spiked recovery varies from 89% to 108% for samples with different matrices, showing insignificant matrix interference in this method.