An autofocus mechanism for a spectroscopic system determines a time varying optical property of a volume of interest. The mechanism measures the fluctuations of the optical property of the volume of interest for determining the position of the volume of interest. The spectroscopic system focuses an excitation beam into the determined volume of interest and collects return radiation emanating from the volume of interest for spectroscopic analysis. Preferably, inelastically scattered radiation of an excitation beam is separated from elastically scattered radiation for spectroscopic analysis. The elastically scattered radiation of the excitation beam is measured for fluctuations of the optical property of the volume of interest. A control loop maximizes the amplitude and/or intensity of the fluctuations and specifies the position of a volume of interest e.g. the center of a capillary vessel.