Instrument and method for detecting pesticides and other analytes. A sample to be tested is mounted on a cassette and inserted into a housing which is substantially impervious to light, and light from a source within the housing is directed toward the sample to induce fluorescent emission from analyte on the sample. Fluorescent emissions from the sample are monitored with a detector within the housing to detect emissions having a spectral content characteristic of the analyte to be detected. Data from the detector is processed, and information based on the processed data is displayed. In some embodiments, the instrument is calibrated with data from a reference sample of known concentration or density. The detector measures the analyte in units of mass per unit area, and in some embodiments the mass per unit area is converted to units of concentration or density of the analyte in the sample.