A spray tower apparatus was developed to emit a constant volume of a solution onto a moving belt of chromatographic paper. A metering and fluid mixing device operating on the principle of continual dilution enables an exponentially varying spectrum of chemical dosages to be sprayed onto the paper strip. The degree of deposit and concentration per unit area on the paper can be altered by nozzle setting and adjustment of the linear speed of the paper strip. The excess spray droplets are removed from the spray chamber by vacuum control.