Comparison of excitation wavelengths in surface-enhanced Raman spectroscopy coupled to thin-layer chromatography for tadalafil and vardenafil detection as adulterants in herbal healthcare products

Phosphodiesterase-5 inhibitors (PDE5Is) are frequently adulterated into dietary supplements and herbal healthcare products, in which sildenafil, vardenafil and tadalafil are commonly found. In our previous study, a thin layer chromatography coupled to surface-enhanced Raman spectroscopy (TLC-SERS) method was developed for the detection of sildenafil in herbal products. In the current study, TLC-SERS was further explored for the detection of vardenafil and tadalafil in herbal products. The performances of TLC-SERS using two excitation wavelengths, 633 nm and 785 nm, were also compared for the analysis of these compounds. With the chosen wavelengths of 633 nm for vardenafil and 785 nm for tadalafil, other factors influencing the selectivity and sensitivity of the TLC-SERS method were also optimized. The methods at the chosen analytical conditions were validated and applied to twenty-four real market samples. Two samples were found positive for PDE5Is, and both were adulterated with multiple compounds: one positive for vardenafil and tadalafil and one positive for sildenafil and tadalafil. Parallel analysis by an LC-MS/MS method resulted in the same findings, validating the feasibility of the developed TLC-SERS method.