Absorption coefficient and purine photobleaching rate in colon mucosa during resonance Raman spectroscopy at 251 nm.

In contrast to spectroscopy at longer wavelengths, typical attributes of ultraviolet resonance Raman (UVRR) spectroscopy of biologic tissue are higher absorption coefficient, mu, and higher photobleaching rate, kappa. This study was aimed at measuring mu and kappa during UVRR spectroscopy of human colon tissue at 251 nm. mu was used to estimate the penetration depth of the excitation light; kappa was used to predict the rate of signal decrease that was due to photobleaching as a function of laser fluence and tissue thickness. The fitting of the equations through description of a three-state transition model to experimental data that consisted of a purine UVRR signal gave mu = 0.0169 ? 0.0023 mum(-1) and kappa = 0.572 ? 0.168 (mJ/mum(2))(-1). kappa remained independent of power P for P < 1 mW, but higher power values resulted in a higher photobleaching rate. As predicted by the model, signal decrease that was due to photobleaching was slower as sample thickness was increased.