Simultaneous quantification of duloxetine and 1-naphthol in pharmaceutical and biological matrices using a sensitive and eco-friendly synchronous fluorescence technique

Background Duloxetine hydrochloride (DL) is a widely prescribed serotonin–norepinephrine reuptake inhibitor requiring stringent quality control due to the potential presence of 1-naphthol (NP), a genotoxic process-related impurity. DL and NP exhibit nearly identical native fluorescence spectra, posing a significant challenge for their simultaneous quantification by conventional fluorometric methods. Objective This study aimed to develop and validate a rapid, sensitive, and sustainable synchronous fluorescence spectroscopy (SFS) method with improved environmental profile for the simultaneous determination of DL and NP in pharmaceutical and biological matrices, without chromatographic separation or chemical derivatization. Method A dual-wavelength SFS approach was optimized using a fixed wavelength difference (Δλ = 125 nm), enabling selective detection at 356–362 nm for DL and 359–366 nm for NP in an 80% methanol–Britton–Robinson buffer system (pH 3.0). The method was fully validated by ICH Q2(R1) guidelines and rigorously evaluated for sustainability using four complementary greenness metrics: AGREE, ComplexMoGAPI, Analytical Eco-Scale, and Blue Applicability Grade Index (BAGI). Results Linear calibration curves (r² ≥ 0.9996) were obtained over 20–500 ng/mL for both analytes. The limits of detection were 6.19 ng/mL (DL) and 6.54 ng/mL (NP), with intra- and inter-day precision (%RSD < 1.5%) and mean recoveries of 100.26% (DL) and 99.89% (NP). The method showed excellent specificity in Cymbalta® capsules and spiked human plasma. Comprehensive greenness assessment confirmed high sustainability: AGREE score = 0.79, ComplexMoGAPI = 72.5/100, Analytical Eco-Scale = 81, and BAGI = 72.5/100, collectively classifying the method as “excellent green analysis” with strong practical applicability. Conclusion The proposed SFS method represents a chromatography-free, derivatization-free, and validated platform with enhanced sustainability features that successfully balance analytical performance, environmental responsibility, and laboratory feasibility. It offers a sustainable, cost-effective, and regulatory-compliant solution for routine quality control and bioanalytical monitoring of DL and its genotoxic impurity NP.