Tonic Water

The active fluorescent ingredient within tonic water is quinine. This organic compound is extracted from the bark of the Cinchona tree and is responsible for the bitter taste of tonic water, as well as the liquid's anti-malarial properties. The brightness of light emitted by quinine, when exposed to ultraviolet light, is so reliable and stable that quinine is used as a fluorescent intensity standard, against which levels of fluorescents are measured and machinery calibrated.