Stable analysis of fluorescence is essential to ensure accurate evaluation of pesticide spray application efficiency when fluorescent tracers are used to quantify deposition and drift. The fluorescent intensity of five fluorescent tracers commonly used for the quantitative assessment of spray deposition and off-target loss was investigated with wash solutions over pH conditions from 6.86 to 10.4. The tracers selected in the tests were Acid Yellow 7, Eosin, Fluorescein, Pyranine, and Tinopal. The fluorescence of Pyranine was the most sensitive to the solution pH conditions, followed by Fluorescein and Tinopal. Acid Yellow 7 and Eosin had a nearly constant fluorescent intensity over the pH range from 6.86 to 10.40. The fluorescent strength of Fluorescein increased 1.3 times, Tinopal 1.25 times, and Pyranine 3.0 times as the pH value increased from 6.86 to 8.43, but it became nearly constant when pH value was greater than 8.43. However, Pyranine, Fluorescein, and Tinopal showed much stronger fluorescence than Acid Yellow 7 and Eosin with the same concentrations. A solution containing Fluorescein at pH 8.43 and higher demonstrated 124 times greater fluorescent intensity than the solution containing the same amount of Acid Yellow 7. The fluorescent strength should be examined not only with purified distilled water but also under various wash solution pH conditions during the selection of tracers for pesticide spray deposition and drift trials.
