Buoyed by this lead, we searched the underivatized extract LC-MS trace for ions corresponding to [C 10 H 10 O 3 +H] + , and discovered a weak peak in the underivatized extract at this m/z, marked with an arrow in Figure . UV absorbance chromatograms corresponding to this weak MS peak had the characteristic complex UV spectrum of a diyne (Figure ). We hypothesized that this molecule was a previously unreported analog of repandiol, missing either one epoxide group or instead missing one primary alcohol. To distinguish between these possibilities, we conducted deuterium exchange LC-MS, clearly showed a mass shift of +2 Da (Figure ), thereby leading us to hypothesize that the unknown dietary internal alkyne we had detected was repanol (11).