Abstract: An apparatus provided with a measuring head for determining with the aid of X-ray fluorescence or X-ray absorption measuring techniques the surface weight of a measuring subject comprising coatings or surface-deposits or the concentration of basic substances in the subject to be measured, comprising liquid, slurries, powder or solid samples. The apparatus includes means for moving the measuring head (1) over the subject (5) to be measured and also over a reference sample (4), such as a sample or an empty sample-container, or vice versa. The measuring aperture provided in the measuring head (1) is covered with a protective film (7) for the purpose of preventing the ingress of particles from the subject (5) into the aperture. Means (3) are also provided for checking the dirt-coating on the protective film (7) regularly and for sending an order to a film-carrying device (8, 10, 11) to advance film when the dirt-coating is found unacceptable.