A system and method for in vitro analysis of therapeutic agents comprising a reservoir adapted to hold a therapeutic agent, a first flow cell having a first cell chamber adapted to receive at least a first sample of the therapeutic agent, a second flow cell having a second cell chamber adapted to receive at least a second sample of the therapeutic agent, the first flow cell having a first path length (be′) and the second flow cell having a second path length (be″), the first path length being substantially equal to a sensitivity factor (f)×be″, a membrane chamber having a biological cell membrane therein adapted to receive at least a third sample of the therapeutic agent, the membrane chamber being further adapted to detect the membrane potential of the biological cell membrane; and spectroscopic detection means for detecting the spectral characteristics of the first and second therapeutic agent samples.