Spex SamplePrep XRF window film serves as the sample window over our X-Cell sample cups, allowing for uniform transmission of x-rays while effectively containing the liquid sample. Kapton polyimide film is chemically imperious to almost anything except strong alkali solutions. It is often considered the closest thing to a bombproof XRF window film due to its incredible strength and heat resistance to over 300°C (572° F). The phosphate-based surface coating improves its handling characteristics.