Raman-based quantification of N-phenyl-1-naphthylamine: a model study toward lubricant monitoring

This study explores the use of Raman Spectroscopy (RS) as a rapid and nondestructive analytical method for detecting and quantifying the antioxidant N-phenyl-1-naphthylamine (NPN) in lubricants. Solutions containing different concentrations of NPN were prepared in several solvents, and their Raman spectra were processed using baseline correction, smoothing, and normalization. Among the tested solvents, n-decyl acetate provided the strongest and cleanest Raman response with minimal spectral interference. An empirical linear relationship between Raman peak intensity and NPN concentration was established, yielding a correlation coefficient of r = 0.95 and it should be noted that below 0.2 wt% NPN concentration, the Raman signal was no longer distinguishable from the fluorescence background under the current instrumental configuration. Therefore, 0.2 wt% represents the practical detection limit rather than a statistically derived limit of detection (LOD). The limitation arises primarily from fluorescence interference and detector sensitivity constraints. Density functional theory (DFT) calculations were performed to assign vibrational modes and showed strong agreement with experimental spectra. Temperature-dependent experiments further demonstrated that Raman peak positions of NPN remain stable across a wide range of thermal conditions, while intensity variations can be captured through an empirical time–temperature correlation model. The findings indicate that RS offers a reliable, fast, and cost-effective approach for in-situ monitoring of antioxidant content as a proof-of-concept for lubricant monitoring.