In postchromatographic reactions, the reagents are applied to the layer through the gas phase (volatile reagents only), or as a solution using an impregnation technique. Gas phase methods are fast and convenient but limited by the number of useful reagents, for example, iodine, ammonia, hydrogen chloride, etc.,which are applied by inserting the layer into a tank containing a saturated atmosphere of the reactive vapors. Spraying, dipping, or contact with a transfer pad (over pressure derivatization) is used for reagents in solution. Spray techniques using simple atomizers have a long history in TLC [48,49]. Reagent application by this method,however, is quite difficult to perform well. The homogeneity of the reagent distribution over the layer depends on many factors, such as droplet size, distance between the spray device and the layer,direction of spraying, and discharge rate of the reagent. A number of pneumatic atomizer devices are available but manual reagent application is not easy to control and requires a ventilation device(spray chamber) or fume hood to minimize exposure to hazardous vapors. A better option for quantitative methods is an auto mated spray device, such as the ChromJet 20 DS, Fig. 14.