Improvement of On-Site Sensor for Simultaneous Determination of Phosphate, Silicic Acid, Nitrate plus Nitrite in Seawater

Accurate, on-site determinations of macronutrients (phosphate (PO₄^3-), nitrate (NO₃^-), and silicic acid (H₄SiO₄)) in seawater in real time are essential to obtain information on their distribution, flux, and role in marine biogeochemical cycles. The development of robust sensors for long-term on-site analysis of macronutrients in seawater is a great challenge. Here, we present improvements of a commercial automated sensor for nutrients (including PO₄^3-, H₄SiO₄, and NO₂^- plus NO₃^-), suitable for a variety of aquatic environments. The sensor uses the phosphomolybdate blue method for PO₄^3-, the silicomolybdate blue method for H₄SiO₄ and the Griess reagent method for NO₂^-, modified with vanadium chloride as reducing agent for the determination of NO₃^-. Here, we report the optimization of analytical conditions, including reaction time for PO₄^3- analysis, complexation time for H₄SiO₄ analysis, and analyte to reagent ratio for NO₃^- analysis. The instrument showed wide linear ranges, from 0.2 to 100 μM PO₄^3-, between 0.2 and 100 μM H₄SiO₄, from 0.5 to 100 μM NO₃^-, and between 0.4 and 100 μM NO₂^-, with detection limits of 0.18 μM, 0.15 μM, 0.45 μM, and 0.35 μM for PO₄^3-, H₄SiO₄, NO₃^-, and NO₂^-, respectively. The analyzer showed good precision with a relative standard deviation of 8.9% for PO₄^3-, 4.8% for H₄SiO₄, and 7.4% for NO₂^- plus NO₃^- during routine analysis of certified reference materials (KANSO, Japan). The analyzer performed well in the field during a 46-day deployment on a pontoon in the Kiel Fjord (located in the southwestern Baltic Sea), with a water supply from a depth of 1 m. The system successfully collected 443, 440, and 409 on-site data points for PO₄^3-, Σ(NO₃^- + NO₂^-), and H₄SiO₄, respectively. Time series data agreed well with data obtained from the analysis of discretely collected samples using standard reference laboratory procedures and showed clear correlations with key hydrographic parameters throughout the deployment period.

Development of a sequential injection analysis device and its application for the determination of Mn(II) in water

A sequential injection analysis device has been developed and applied for the automated determination of Mn(II) in environmental water samples. Differential pulse cathodic stripping voltammetry is selected as the electrochemical detection method. The device consists of several electronic equipment. The electrochemical flow cell was designed for replacing the traditional three-electrode system and introducing high reproducibility. An electrochemical analyzer saddled with laboratory-programmed software written by Embarcadero Delphi 10.2. For higher current response, various determination parameters such as the flow rate, the medium pH, the deposition potential and the thickness of gasket in the electrochemical flow cell have been optimized. Under the optimal conditions, the detection limit (3σ/slope) of 0.63 μg L^-1 and a calibration curve (R² = 0.9987) of current response and Mn(II) concentration from 2.5 μg L^-1 to 200 μg L^-1 could be achieved. The device was successfully applied to the determination of trace Mn(II) in environmental water samples, and in continuous real-time monitoring of Mn(II) variations in tap water for 14 days. The results are consistent with the reference method and the average recovery is found to be 95.2%-101.4%. The device shows high sensitivity and reproducibility in the determination of Mn(II), and presents a great potential for on-site and real-time detection of metal ions where rapid, low-cost and low-volume analysis is required.

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).

Automatic in-syringe dispersive liquid-liquid microextraction of ⁹⁹Tc from biological samples and hospital residues prior to liquid scintillation counting

A new approach exploiting in-syringe dispersive liquid-liquid microextraction (DLLME) for (99)Tc extraction and preconcentration from biological samples, i.e., urine and saliva, and liquid residues from treated patients is presented. (99)Tc is a beta emitter with a long half-life (2.111 × 10(5) years) and mobility in the different environmental compartments. One of the sources of this radionuclide is through the use of its father (99m)Tc in medical diagnosis. For the first time a critical comparison between extractants and disperser solvents for (99)Tc DLLME is presented, e.g., tributyl phosphate (TBP), trioctylmethylammonium chloride (Aliquat®336), triisooctylamine (TiOA), as extractants in apolar solvents such as xylene and dodecane, and disperser solvents such as acetone, acetonitrile, ethanol, methanol, 1-propanol, and 2-propanol. The system was optimized by experimental design, and 22.5% of Aliquat®336 in acetone was selected as extractant and disperser, respectively. Off-line detection was performed using a liquid scintillation counter. The present method has a (99)Tc minimum detectable activity (MDA) of 0.075 Bq with a high extraction/preconcentration frequency (8 h(-1)). Urine, saliva, and hospital residues were satisfactorily analyzed with recoveries of 82-119%. Thus, the proposed system is an automatic powerful tool to monitor the entry of (99)Tc into the environment. Graphical Abstract (99m)Tc is widely used in Nuclear Medicine for diagnosis. Its daugther (99)Tc is automatically monitored in biological samples from treated patients by in-syringe dispersive liquid-liquid microextraction.