Flame detection (ionization or chemiluminescence) is used for the detection of substances of low volatility, which lack a chromophore for optical detection [91,92]. In this case, the separation is performed on specially prepared thin, quartz rods with a surface coating of silica or alumina attached by sintering. The rod scan be impregnated with inorganic salts (silver nitrate, boric acid,copper sulfate, etc.) for additional selectivity. The rods are developed in the normal way, usually held in a support frame, which also serves as the scan stage after the rods have been removed from the developing chamber and dried. Ten rods, typically, can be held in the support frame and automatically scanned in order. The rods are moved at a controlled speed through a hydrogen flame and the signal processed in a similar manner to the flame ionization detector and flame photometric detector (selective response to sulfur and phosphorous compounds) used in gas chromatography, Fig. 24.