Reducing laser energy to suppress air interference for plasma amplification LIBS detection of nitrogen in an aerosol solution†

Abstract

Nitrogen (N) content is a significant indicator to evaluate eutrophication. The interference of nitrogen in air is the biggest hurdle for the direct detection of N in aerosol solution. In this work, the liquid was changed to liquid aerosol by plasma amplification LIBS with the aid of inert gas argon to reduce the interference of air. To suppress the interference of air, the effect of laser energy on the qualitative and quantitative analysis of N in aerosol solution by plasma amplification LIBS was studied. The laser energy threshold (E_th) of N I 746.831 nm reduced with the concentration of N in aerosol solution. Moreover, for the blank sample, the E_th of N I 746.831 nm was 81.77 mJ. The optimal laser energy was 88.00 mJ, and the highest relative blank deviation was observed at this value. The results show that by reducing the laser energy closer to this E_th, the limit of detection (LoD) and background equivalent concentration (BEC) were reduced from 2.79 and 337.60 ppm to 0.99 and 9.06 ppm, respectively. Furthermore, the R², the average relative error (RE_AV), and the root mean square error of cross validation (RMSECV) were improved from 0.9539 to 0.9844, 10.59 to 6.65%, and 11.45 to 7.48 ppm, respectively. The recoveries of the N element in real samples were analyzed by the standard addition method and the recoveries were in the range of 95.85–104.38%. The results indicate that the reduction of the laser energy can mitigate the interference of air in the detection of N in aerosol solution by plasma amplification LIBS.