a method is disclosed for identifying the presence or concentration of a target species in a sample by transmitting a wave of ultrasonic vibration through the sample and calculating , from measured values for time of flight of the wave through the sample and its attenuation during that time , an identification parameter dependent on the type or concentration of the target species in the sample and independent of the distance travelled by the wave through the sample . the method is more convenient to use than existing identification techniques because of the removal of distance from the measurements and calculations necessary . the method is of particular use in identifying polymers , for example , in reclamation and recycling processes . an apparatus for performing the method is also disclosed .