Determination of Usnic Acid in Lichen Toxic to Elk by Liquid Chromatography with Ultraviolet and Tandem Mass Spectrometry Detection

Usnic acid is unambiguously confirmed by tandem mass spectrometry (MS/MS) in tumbleweed shield lichen, Xanthoparmelia chlorochroa. The lichen contains 2% usnic acid by liquid chromatography with UV quantification at 282 nm. The UV linear range for usnic acid quantification is from its 4 ng limit of detection to 2 μg injected. UV signal saturation is recognized by distortion of the usnic acid UV spectrum. Positive ion electrospray−tandem mass spectrometry offers no similar means to recognize quantification data recorded above the linear range of electrospray. Electrospray ionization capacity and matrix effects limit the reliability of tandem mass spectrometry quantification. The combination of UV quantification and MS confirmation provides a reliable analytical method for measuring usnic acid levels in plant material. Keywords: Elk deaths; liquid chromatography-tandem mass spectrometry; LC-MS/MS; tumbleweed shield lichen; Usnea barbata; usnic acid; Xanthoparmelia chlorochroa