A spectroscopic apparatus for determining a concentration and/or spatial gradient of an analyte of a bodily fluid that provides determination of a position of a capillary vessel within a biological sample in order to focus spectroscopic excitation radiation to a volume that is in close proximity to the capillary vessel but does not overlap with the capillary vessel. The provided apparatus exploits the permeability of the vessel wall with respect to an analyte that is subject to analyte concentration determination. By means of biological transport processes, the concentration of an analyte of a bodily fluid located in the capillary vessel influences the concentration in the surrounding of the capillary vessel. Spectroscopic analysis of a volume outside the capillary vessel can therefore serve for a precise and reliable analyte concentration determination inside the capillary vessel.