Optimum Sample Concentration for Resonance Raman Spectra

In resonance Raman spectroscopy, absorption of incident radiation and of scattered radiation increases with sample concentration. Scattering intensity at the focal point of the laser also increases with sample concentration. An optimum sample concentration is sought where the scattered radiation intensity emerging from the sample cell is at a maximum. Assuming a point-scattering model, Strekas et al. have derived the relationship: I = I0Jce−kc(re8 + lei I is the observed intensity, I0 the laser intensity on entry into the sample cell, J is the molar scattering coefficient, c is the molar concentration, es and ei are the decadic molar absorptivities at scattering wavelength and the incident wavelength, r and l are the pathlengths of the laser beam and the scattered radiation in the Raman cell, respectively, and k ∞ ln 10 = 2.303. This function has a maximum. Setting the differential with respect to concentration equal to zero gives the optimum concentration: copt = [k(re8 + lei)]−1 If r = l, Eq. (2) reduces to: copt = [kr(e8 + lei)]−1