The invention provides an infrared detection system for conformation and secondary structure analysis, notably for the direct non-invasive qualitative secondary structure analysis of a single selected protein within a complex mixture, as e.g. a body fluid, by vibrational spectroscopic methods utilizing a quantum- cascade laser. For the analysis it is not required that the selected substance be isolated, concentrated, or pretreated by a special preparative procedure. A difference-spectrum between the unbound and antibody-bound protein of interest is performed by which the much larger background absorbance is cancelled. The presented quantum- cascade laser set-up provides sufficient S/N and stability to subtract the several orders of magnitude larger background absorbance.