The present invention provides an autofocus mechanism for a spectroscopic system (400) that is adapted to determine a property of a volume of interest. The volume of interest has an optical property that varies with time. The invention provides measurement means that are adapted to measure the fluctuations of the optical property of the volume of interest for determining the position (428) of the volume of interest. The spectroscopic system being further adapted to focus an excitation beam (418) into the determined volume of interest and for collecting return radiation (420) emanating from the volume of interest for spectroscopic analysis. Preferably, inelastically scattered radiation of an excitation beam (428) is separated from elastically scattered radiation for spectroscopic analysis. The elastically scattered radiation of the excitation beam is in turn exploited in order to measure the fluctuations of the optical property of the volume of interest. Making use of a control loop allows for maximization of the amplitude and/or intensity of the fluctuation inherently specifying the position of a volume of interest e.g. the center of a capillary vessel (450).