Quantitative analysis of the Tricholoma ustale-derived toxin, ustalic acid, in mushroom and food samples by LC–MS/MS

•Ustalic acid is the toxic component in poisonous mushroom, Tricholoma ustale.•The quantitative analysis of the ustalic acid content in food samples was conducted.•The quantitation limits were 10ng/g (shiitake mushroom) and 0.40ng/g (miso soup).•This method was applied to leftover samples from a food poisoning case.•This is the first report to determine ustalic acid in the leftovers of the case.