The present invention enables to determine the property of a fluid, such as for the purposes of in vivo blood analysis. First the position of a volume of interest through which the fluid flows is determined by means of an optical detection step by making use of an objective. Preferably the optical detection step is an imaging step. Next the objective is moved to bring the focal point of the objective to the volume of interest. In this position an optical spectroscopic step is performed. This has the advantage that the measurement beam for performing the optical spectroscopy travels along the optical axis for optimum efficiency.