Metrological comparison of fiber-optic and conventional spectrophotometry for visible spectra of liquid dye solutions

This study evaluates the consistency between optical fiber Spectroscopy and conventional spectrophotometry for three representative dyes — Fluorescein, Methylene Blue, and Nile Red — measured in both polystyrene and quartz cuvettes. For the non-fluorescent dyes, Methylene Blue and Nile Red, optical fiber and spectrophotometry produce fully consistent spectra across all concentrations and in both materials, with scaling factors close to unity and negligible offsets, confirming that non-emissive analytes are reliably quantified by both techniques. Fluorescein, however, exhibits concentration-dependent deviations caused by its strong fluorescence, with opposite trends in quartz and polystyrene due to their different optical properties and their distinct ability to re-collect emitted light. Despite these effects, the distortions remain systematic and attributable to well-defined physical mechanisms. These findings demonstrate that optical fiber is robust for non-fluorescent compounds and can be applied confidently to solid or gel matrices, whereas fluorescent analytes require calibration strategies that account for matrix-dependent emission contributions — an essential consideration for absorption studies and drug-release monitoring in heterogeneous materials.