An apparatus for determining a quantitative property of a sample substance by means of magnetic resonance is disclosed. The apparatus comprises a conveyor for conveying sample containers containing the sample substance through a measuring station. The measuring station comprises a magnet system for generating a constant magnetic field of high homogeneity. The measuring station, further, comprises a probe head adapted for letting sample containers be conveyed therethrough and for generating a high frequency magnetic field. A magnetic resonance measuring unit determines the quantitative property of the sample substance contained in the probe head. The probe head excites and detects, resp., the magnetic resonance essentially only within that section of the sample container which contains the sample substance. The probe head comprises a split-ring resonator which, as seen in a conveying direction of the conveyor, has a passage cross-section for letting run the sample containers therethrough. The high frequency magnetic field is generated essentially only in an area of the passage cross-section through which the sample containers run during conveying.