Continuous Flow Injection Analysis: Quenched continuous fluorescence of 5-hydroxy tryptamine for the determination of paracetamol using a blue-purple laser diode fluorimeter and a photodiode as a detector at 0- 90° angle

A flow injection–fluorescence quenching method was developed for the determination of paracetamol using serotonin hydrochloride as a native fluorophore. To the best of our knowledge, no prior FIA system has used serotonin hydrochloride as a quenching fluorophore for paracetamol. Continuous fluorescence was excited by a 405 nm laser diode in a custom-built laser diode fluorimeter equipped with a 90° photodetector aligned for emission measurement at 525 nm. The system demonstrated a stable signal response under dynamic flow conditions, enabling rapid and reproducible analysis. The calibration curve was linear over the range of 0.1–45 µmol L⁻¹, with a correlation coefficient of r = 0.9980. The detection limit was 50 nmol L-1 (0.4913 ng/sample), and precision studies yielded RSD% values below 0.5% for target concentrations of 1 and 5 µmol L-1 (n = 6). Analytical results from the developed method showed no significant deviation from the labelled dose (500 mg/tablet) as confirmed by a one-sample t-test. However, a paired t-test comparing the developed method with a conventional Spectrofluorometric technique produced a significant difference, highlighting the improved performance of the FIA approach. The technique combines minimal reagent use, high sampling throughput (60 samples/hour), and nanomolar detection, making it valuable for routine pharmaceutical analysis at trace levels.