To better understand these differences, we constructed polyplex phase diagrams for LPEI and BPEI as a function of both N/P ratio and NaCl concentration. These phase diagrams help visualize the chemical conditions that favor polyplex formation. Using multivariate curve resolution-alternating leastsquares (MCR-ALS) analysis (see SI for details), we decomposed the experimental spectra of LPEI and BPEI polyplexes to extract two basis spectra, one representing polymerbound DNA and the other representing unbound DNA (Figure and). The extracted basis spectra closely match the experimental spectra of bound and unbound DNA (Figure and e), confirming the accuracy of the decomposition. Using these basis set spectra, we determined the fraction of bound and unbound DNA under different NaCl concentration conditions and polymer N/P ratios (Figure ). We accomplished this by modeling each experimental spectrum as a linear combination of the bound and unbound DNA basis spectra. From the extracted concentration profiles, we then calculated the probability of DNA binding to LPEI and BPEI (P b ) (Figure and f, Figure ):