The sulfuric acid spray method of detection is destructive and the solutes cannot be recovered after detection. After development an drying the plate is sprayed with a 50% v/v aqueous solution of sulfuric acid in a fume hood. The hood will be exposed to extremely corrosive materials and, thus, all fittings should be made of glass or acid resistant plastic. After spraying, and while still in the fume hood, the plate is heated to about 239˚C for about 10 minutes on a hot plate or in an oven. The solutes are partially oxidized leaving behind a charred deposit of black carbon that is easy to distinguish. This method will detect most involatile organic compounds. A suitable spray cupboard or hood for sulfuric acid spray is shown in figure 18.
This method of detection is a simple modification of the sulfuric acid method, but an oxidizing agent is added to the sulphuric acid to facilitate the reaction and consequently, is also destructive. The plate is sprayed with a 5% w/v potassium dichromate solution in 40% v/v aqueous sulfuric acid solution. The plate is then heated for about 10 minutes at about 200oC to ensure charring is complete. This method will also detect most involatile organic compounds and it is thought to be more sensitive than the simple sulfuric acid spray.