Hi,
These two papers describe some methods you can use.
1. Day, E. S., Capili, A. D., Borysenko, C. W., et al.; Determining the affinity and stoichiometry of interactions between unmodified proteins in solution using Biacore. Analytical Biochemistry (440) 1: 96-107; 2013.
2. Oda, M. and Azuma, T.; Reevaluation of stoichiometry and affinity/avidity in interactions between anti-hapten antibodies and. Molecular Immunology (37) 18: 1111-1122; 2000.
Alternative methods to investigate the stoichiometry of an interaction are gel filtration (when the binding component add sufficient mass to be separated on the column), iso thermal calorimetry, analytical ultracentrifugation and dynamic light scattering.
Kind regards
Arnoud