Abstract: In the present work, the combination of both the Langmuir adsorption and the spectral correction (MPASC) technique are described. It provides a very helpful experimental method for the study of aggregation of a dye on biomacromolecules. The helix, winding and folding of DNA form double electrostatic films and their surface will selectively attract charged organic compounds in only monolayer untill kinetic equilibrium approached. The physico-chemical constants such as equilibrium constant and binding stoichiometry of the aggregate may be determined. The adsorption of polychrome blue (PCB) on nucleic acids has been investigated at pH 7.24 and it obeys Langmuir monolayer adsorption. Results showed that the adsorption ratio of PCB to DNA-PCB is 0.5 :1 and that to RNA-PCB 0.3 :1, their adsorption constants are KDNA-PCB=5.42×104 and KRNA-PCB=2.82×104（30 ℃） respectively, and their real adsorptivity ε(DNA-PCB, 625 nm)=5.65×103 andε(RNA-PCB, 625 nm)=3.85×103 (mol－1•L)•cm. By varying the operation condition, we observed that the adsorption ratio of PCB to DNA decreased with increasing ionic strength and temperature. This indicates that the interaction is an adsorption but not a complexation. This interaction was applied to the quantitative determination of DNA. In the presence of EDTA, many ions or compounds such as protein, alkali earths, Mn(II), Cu(II),Fe(II), Zn(II), Pb(II), Ni(II), Co(II) and Cd(II) have no interference to the direct determination of nucleic acids.

Key words: MPASC technique, Double electrostatic films, Langmuir aggregation