Device and methods for detecting/quantifying a fluorescent taggant in a liquid sample. Generally, the liquid samples are fuels having low concentrations (measured in ppb) of a fluorescent taggant. The detection/quantification generates a predicted concentration of the fluorescent tagging compound using a process selected from the group of a multivariate process, a background subtraction process, or a combination of both. The invention addresses the detection of an adulteration of gasoline and diesel fuels.