A gas analysis apparatus for analysis of sample gas wherein the apparatus has a mechanism for cleaning optical elements such as reflectors and windows in the apparatus is provided. The gas analysis apparatus includes a light source for emitting light, a body having an isolated cavity in which the light emitted by the light source propagates, a detector for analyzing light scattered by the sample gas in the isolated cavity, and a spraying system connected to the body to spray a sublimable substance. The isolated cavity has one or more optical elements each with a surface. The spray of the sublimable substance is directed onto such a surface for in situ removal of contaminants. The body has a sample inlet port for introducing the sample gas into the cavity and an outlet port for venting gas from the cavity.