Monitoring of nitrite, nitrate, chloride and sulfate in environmental samples using electrophoresis microchips coupled with contactless conductivity detection

• Glass ME-C4D chips were used to monitor anionic species environmental samples.
• Electrophoretic separations were achieved within 60 s with excellent repeatability.
• Inorganic anions were detected in real samples with accuracy between 91 and 105%.
• Nitrification steps were sucessfully monitored in aquatic environments.

This report describes the development of an analytical methodology on microchip electrophoresis (ME) devices coupled with capacitively coupled contactless conductivity detection (C4D) to monitor inorganic anions in environmental samples. The buffer composition as well as detection operating parameters were optimized to achieve the best separation selectivity and detector sensitivity, respectively. Electrophoretic separations of Cl−, NO3−, SO42− and NO2− were successfully performed within 60 s using a running buffer composed of 30 mmol L−1 latic acid and 15 mmol L−1l-histidine (His). The best detectability levels were found applying a sinusoidal wave with 1100-kHz-frequency and 60-Vpp amplitude. Quantitative analyzes of inorganic anions were carried out in the presence of Cr2O72− ion as internal standard (IS), which ensured great repeatability in terms of migration times (<1%) and peak areas (6.2–7.6%) for thirty consecutive injections. The analytical performance revealed a linear behavior for concentration ranges between 0–120 μmol L−1 (Cl−, NO2− and NO3−) and 0–60 μmol L−1 (SO42−) and limits of detection (LODs) varying from 2.0 to 4.9 μmol L−1. The concentration levels of anionic species were determined in aquarium, river and biofertilizer samples with recovery values between 91% and 105%. The nitrification steps associated with conversion of ammonium to nitrite followed by the conversion of nitrite to nitrate were successfully monitored in a simulated environment without fishes during a period of twelve weeks. Lastly, the monitoring of anionic species was carried out during eight weeks in an aquarium environment containing ten fishes from Danio rerio (Ciprynidae). The recorded data revealed the absence of nitrite and a gradual increase on the ammonium and nitrate concentration levels during eight weeks, thus suggesting the direct conversion of ammonium to nitrate. Based on the data herein reported, the proposed analytical methodology can be used for routine environmental analysis.

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