Non-resonance micro-Raman spectroscopic studies on crystalline domoic acid and its aqueous solutions

Domoic acid (DA) is a neurotoxin naturally present in the marine ecosystem. Since DA's toxicity has been explained by its molecular structure and particularly because of its ethylenic double bond, spectroscopic investigation of this molecule is of importance. We carried out Raman spectroscopy on crystalline DA and on DA in aqueous solutions (28,000-25 ng DA/mL) and assigned Raman modes in comparison with the Raman spectra of its substructures. Noise-free, clear Raman signal from the solutions containing low concentrations of DA were obtained by applying the drop coating deposition Raman (DCDR) technique. Raman spectra reveal that crystalline DA exists in the zwitterionic form. The Raman spectra of the DA aqueous solutions were analysed in the light of their pH whereas the variation in the spectra was attributed to the hydration, the degree of protonation and crystallinity of the solid film. We show that DCDR can be applied for the rapid detection of domoic acid down to 25 ng DA/mL (0.025 ppm).