Portable analytical system integrating purge and trap and microplasma optical emission spectrometry for online discriminative detection of NOx.

Pollution caused by nitrogen oxides (NOx, mainly NO and NO2) has attracted considerable attention due to its negative implications on the environment and human health. Herein, a novel, portable, and battery-operated system integrating a purge and trap (P&T) system with a miniaturized point discharge optical emission spectrometer (μPD-OES) was developed for monitoring atmospheric NOx. Among NOx, NO2 can be selectively and directly absorbed by sodium hydroxide (NaOH) solution and converted to nitrite (NO2-). In contrast to NO2, NO cannot be absorbed by NaOH and must be oxidized to NO2 by KMnO4 prior to its absorption. When NO2 and NOx samplers were exposed simultaneously, NO can be calculated as the difference. The concentration of NOx in the air was determined by analyzing the NO2-. The obtained NO2- solution was purged with an argon carrier gas at a rate of 400 mL min-1 for 5 min to remove all the volatile organic compounds contained in the solution before reacting with cyclamate to generate volatile cyclohexene, which was efficiently isolated and preconcentrated by P&T. Subsequently, the cyclohexene was desorbed from the P&T unit and swept into the μPD-OES for its detection under optimized conditions. The system demonstrated a detection limit of 0.02 ppbv (0.04 μg/m3) for NO2, surpassing conventional methods. Precision expressed as the relative standard deviation (RSD, n = 11) was better than 3.5 %. The system was validated by analyzing two NO2 standard gases and fourteen ambient air samples, and the obtained analytical results demonstrated its promising potential for rapid, sensitive, and precise NOx evaluation and monitoring.