Window upgrade kit 1042485-002 is designed for use with MultiGas 203X/603x Fourier Transform Infrared (FTIR) gas analyzers having 180 or 200mL gas cell volumes previously fitted a window material other than zinc selenide (ZnSe). This kit includes two Zinc Selenide (ZnSe) optical windows enabling broad spectroscopic analysis across mid-IR wavelengths while supporting a wide variety of sample types due to the materials low reactivity. Replacement window kit 133525-G13 also includes four replacement 149-0026 Kalrez oring seals as well as two appropriate depth window retainers and associated mounting hardware. (If replacing existing ZnSe windows, alternative windows kit 133525-G13 should be ordered.)