APPLICATION OF PHOTOACOUSTIC PHASE-ANGLE SPECTROSCOPY (PHI-AS) TO EUMELANINS AND PHEOMELANINS

A new technique, based on the measurement of the phase angle of the ph otoacoustic signal, was used for investigating the absorption spectra of melanins in the dry state. The main advantages of such a method are the insensitivity to scattering of the light and the applicability to nontransparent or highly adsorbing and thermally thick samples. The s pectra obtained on different kinds of natural and synthetic melanins s how much more details than the corresponding ones obtained spectrophot ometrically on aqueous suspensions of the pigment. Some relevant featu res of the spectra still cannot be interpreted, yet we were able to sh ow the presence in eumelanins of dopachrome at 475 nm and melanochrome at 540 nm, and in pheomelanins of compounds previously isolated from red chicken feathers. Moreover, the well known trend of the optical ab sorption, i.e., its decrease with increasing wavelength, was unambiguo usly confirmed.