An apparatus for analyzing a substance , comprising an excitation transmission device in the form of a laser light source for generating one or more electromagnetic excitation beams (SA) having at least one excitation wavelength, wherein the excitation transmission device radiates the one or more electromagnetic excitation beams (SA) into a volume which is located in the substance below a first region of the surface of the substance, an optical medium which is in direct contact with the first region of the surface of the substance , a detection device for detecting a response signal (SR), wherein the apparatus comprises a device for emitting an electromagnetic measurement beam which is arranged such that the emitted measurement beam penetrates into the optical medium and is totally reflected at the interface (GF) between the substance and the optical medium , wherein the detection device and the device for emitting the electromagnetic measurement beam are aligned with respect to one another such that the detection device is configured to detect the reflected measurement beam which forms a time-dependent response signal (RS), wherein the amplitude of the response signal (RS) is influenced by the wavelength of the excitation beam and the intensity modulation thereof, and comprising a device for analyzing the substance on the basis of the detected response signal (SR), the excitation transmission device is mechanically fixedly connected directly - or indirectly by means of an adjustment apparatus - to the optical medium , or the device for emitting a measurement beam and/or the device for detecting the reflected measurement beam and/or the excitation transmission device is mechanically fixedly connected directly to the optical medium and/or is coupled thereto by means of an optical waveguide . an excitation transmission device in the form of a laser light source for generating one or more electromagnetic excitation beams (SA) having at least one excitation wavelength, wherein the excitation transmission device radiates the one or more electromagnetic excitation beams (SA) into a volume which is located in the substance below a first region of the surface of the substance, an optical medium which is in direct contact with the first region of the surface of the substance , a detection device for detecting a response signal (SR), wherein the apparatus comprises a device for emitting an electromagnetic measurement beam which is arranged such that the emitted measurement beam penetrates into the optical medium and is totally reflected at the interface (GF) between the substance and the optical medium , wherein the detection device and the device for emitting the electromagnetic measurement beam are aligned with respect to one another such that the detection device is configured to detect the reflected measurement beam which forms a time-dependent response signal (RS), wherein the amplitude of the response signal (RS) is influenced by the wavelength of the excitation beam and the intensity modulation thereof, and comprising a device for analyzing the substance on the basis of the detected response signal (SR), the excitation transmission device is mechanically fixedly connected directly - or indirectly by means of an adjustment apparatus - to the optical medium , or the device for emitting a measurement beam and/or the device for detecting the reflected measurement beam and/or the excitation transmission device is mechanically fixedly connected directly to the optical medium and/or is coupled thereto by means of an optical waveguide . <EOS>