A method and apparatus for determining the concentration of a specific analyte in a sample of biological material are disclosed. The sample is placed in a sample container (10) which provides at least two radiation paths (14) with different lengths through the sample container (10), and is sequentially irradiated with electromagnetic radiation, e.g. X-rays. The amount of radiation penetrating the sample is detected, and absorbance is determined based on the detected radiation. During irradiation, the sample container (10) is moved in relation to the radiation source (1) and detector (5) so that absorbance measurements at different path-lengths are acquired. A regression line from the absorbance values and path lengths is determined, such that a slope of the regression line is obtained, and based on this slope, the concentration of the specific analyte is determined.