Focal molography is a new method for label-free molecular interaction analysis in crude samples. In contrast to refractometric optical sensors, focal molography is insensitive to nonspecific molecular interactions. This unique property is achieved with a special 2D nanopattern of molecular binding sites on a chip, termed mologram. A mologram is designed such that molecules bound to it diffract light constructively into a focal spot. The intensity of the focused light is measured to quantify the amount of bound molecules. In biological samples, highly abundant off-target molecules readily adsorb to the surface of the sensor. Yet, this process is completely random and the off-target molecules do not bind to the ordered binding sites of the mologram. Thus, their scattering is uniform in all spatial directions and therefore they hardly contribute to the measured light intensity in the narrow solid angle of the focal spot.