An X-ray source (101) is configured to controllably irradiate a sample (105) with incident X-rays through a sample window (104) that has a two-dimensional area. A detector (102) is configured to detect fluorescent radiation coming from the irradiated sample. A carrier (301, 401, 402, 501, 601) is essentially transparent to X-rays and disposed to spatially coincide with a substantial part of the two-dimensional area of the sample window (104). Marker material (303, 403, 602), which is responsive to X-rays by emitting fluorescent radiation, is mechanically supported by said carrier and essentially evenly distributed across at least that part of the carrier that spatially coincides with said two-dimensional area of the sample window.