The present invention provides an FTIR system comprising a first light source emitting light of a first wavelength, a sample volume with an adjacent sensor surface, a detector for detecting light reflected at said sensor surface. The sensor surface is illuminated by said first light source fulfilling the condition of total internal reflection and generating an evanescent field with a decay length within the sample volume. The system further comprises means for changing the decay length of the evanescent field and means for correlating the detected signals with the change of the decay length of the evanescent field.