Sensitive Detection of Nitrite and Nitrate in Seawater by 222 nm UV-Irradiated Photochemical Conversion to Peroxynitrite and Ion Chromatography-Luminol Chemiluminescence System

Sensitive detection methods for nitrite (NO₂^-) and nitrate (NO₃^-) ions are essential to understand the nitrogen cycle and for environmental protection and public health. Herein, we report a detection method that combines ion-chromatographic separation of NO₂^- and NO₃^-, on-line photochemical conversion of these ions to peroxynitrite (ONOO^-) by irradiation with a 222 nm excimer lamp, and chemiluminescence from the reaction between luminol and ONOO^-. The detection limits for NO₂^- and NO₃^- were 0.01 and 0.03 μM, respectively, with linear ranges of 0.010-2.0 and 0.10-3.0 μM, respectively, at an injection volume of 1 μL. The results obtained by the proposed method for seawater analysis corresponded with those of a reference method (AutoAnalyzer based on the Griess reaction). As luminol chemiluminescence can measure ONOO^- at picomolar concentrations, our method is expected to be able to detect NO₂^- and NO₃^- at picomolar concentrations owing to the high conversion ratio to ONOO^- (>60%), assuming that contamination and background chemiluminescence issues can be resolved. This method has the potential to emerge as an innovative technology for NO₂^- and NO₃^- detection in various samples.

Ion-selective electrodes based on laser-induced graphene as an alternative method for nitrite monitoring

Nitrite is an important food additive for cured meats; however, high nitrite levels pose adverse health effects to humans. Hence, monitoring nitrite concentration is critical to comply with limits imposed by regulatory agencies. Laser-induced graphene (LIG) has proven to be a scalable manufacturing alternative to produce high-performance electrochemical transducers for sensors. Herein, we expand upon initial LIG studies by fabricating hydrophilic and hydrophobic LIG that are subsequently converted into ion-selective sensors to monitor nitrite in food samples with comparable performance to the standard photometric method (Griess method). The hydrophobic LIG resulted in an ion-selective electrode with improved potential stability due partly to a decrease in the water layer between the electrode and the nitrite poly(vinyl) chloride-based ion-selective membrane. These resultant nitrite ion-selective sensors displayed Nernstian response behavior with a sensitivity of 59.5 mV dec-1, a detection limit of 0.3 ± 0.1 mg L-1 (mean ± standard deviation), and a broad linear sensing range from 10-5 to 10-1 M, which was significantly larger than currently published nitrite methods. Nitrite levels were determined directly in food extract samples of sausage, ham, and bacon for 5 min. These sensor metrics are significant as regulatory agencies limit nitrite levels up to 200 mg L-1 in finished products to reduce the potential formation of nitrosamine (carcinogenic compound). These results demonstrate the versatility of LIG as a platform for ion-selective-LIG sensors and simple, efficient, and scalable electrochemical sensing in general while demonstrating a promising alternative to monitor nitrite levels in food products ensuring regulatory compliance.

Removal of the Harmful Nitrate Anions from Potable Water Using Different Methods and Materials, including Zero-Valent Iron

Drinking water containing nitrate ions at a higher concentration level of more than 10 mg/L, according to the World Health Organization (WHO), poses a considerable peril to humans. This danger lies in its reduction of nitrite ions. These ions cause methemoglobinemia during the oxidation of hemoglobin into methemoglobin. Many protocols can be applied to the remediation of nitrate ions from hydra solutions such as Zn metal and amino sulfonic acid. Furthermore, the electrochemical process is a potent protocol that is useful for this purpose. Designing varying parameters, such as the type of cathodic electrode (Sn, Al, Fe, Cu), the type of electrolyte, and its concentration, temperature, pH, and current density, can give the best conditions to eliminate the nitrate as a pollutant. Moreover, the use of accessible, functional, and inexpensive adsorbents such as granular ferric hydroxide, modified zeolite, rice chaff, chitosan, perlite, red mud, and activated carbon are considered a possible approach for nitrate removal. Additionally, biological denitrification is considered one of the most promising methodologies attributable to its outstanding performance. Among these powerful methods and materials exist zero-valent iron (ZVI), which is used effectively in the deletion process of nitrate ions. Non-precious synthesis pathways are utilized to reduce the Fe²⁺ or Fe³⁺ ions by borohydride to obtain ZVI. The structural and morphological characteristics of ZVI are elucidated using UV–Vis spectroscopy, zeta potential, XRD, FE-SEM, and TEM. The adsorptive properties are estimated through batch experiments, which are achieved to control the feasibility of ZVI as an adsorbent under the effects of Fe⁰ dose, concentration of NO₃⁻ ions, and pH. The obtained literature findings recommend that ZVI is an appropriate applicant adsorbent for the remediation of nitrate ions.

A novel ion chromatography cycling-column-switching system for the determination of low-level chlorate and nitrite in high salt matrices

A novel ion chromatography cycling-column-switching system was developed for the determination of chlorate and nitrite in high salt matrices. The simple system included a pump, two valves, a single eluent, and a conductivity detector. Both online pre-concentration and matrix elimination were achieved by this method. The target anions were eluted from the concentrator column to the analytical columns circularly. Chloride matrix was then eliminated completely. The method was applied to the determination of low-level chlorate and nitrite in the chloride matrix. Our experimental results demonstrated that this system is of advantages such as high sensitivity, facile automation and simple sample pretreatment, which might be a promising approach for environmental researches and food control.

Evaluation of an LC-MS/MS assay for 15N-nitrite for cellular studies of L-arginine action

The utility of an LC-MS/MS assay for nitrite determination in studying L-arginine (ARG) cellular action was examined in vitro. EA.hy926 human endothelial cells or cellular fractions (membrane and cytosol) were exposed to 0-500 μM of (15)N(4)-ARG for 2 h. (14)N-nitrite and (15)N-nitrite in the cell lysate and in the incubation medium were derivatized with 2,3-diaminonaphthalene (DAN) to form (14)N- and (15)N-naphthotriazole (i.e., (14)N-NAT and (15)N-NAT). Peak responses of (14)N-NAT and (15)N-NAT were analyzed by LC-MS/MS with 1H-naphth[2,3-d]imidazole as an internal standard. The calibration curves of DAN-derivatized (14)N-NAT and (15)N-NAT from (14)N-nitrite and (15)N-nitrite were linear. Intra- and inter-day variability of the quantification was below 14.2% in quality control samples. Following incubation of EA.hy926 cells with (15)N(4)-ARG, saturable increases of (15)N-nitrite accumulation with increasing (15)N(4)-ARG exposure were observed clearly. This increase however could not be detected by the classical fluorescence method, nor were changes in (14)N-nitrite level observed. When cellular fractions were exposed to (15)N(4)-ARG, (15)N-nitrite formation was only observed in the membrane fragments. The sensitive and selective LC-MS/MS method reported here can be applied to quantify accumulated nitrite levels in human endothelial cells. The selectivity of this stable-isotope labeled LC-MS/MS method offers an advantage over other traditional methods for elucidating cellular ARG action when its stable isotope is employed as a substrate.