Stock furan and d4-furan solutions were prepared in 20 ml headspace vials by adding 1 ll of native furan or d4-furan in cold methanol weekly. Working solutions were prepared daily with 100 ll of refrigerated stock solutions and 10 ml of high perfor- mance liquid chromatography grade water. The final concentration of furan and d4-furan solutions was approximately 1.0 lg/ml for the stock solution and 0.1 lg/ml for the working solution. All stock and working solution vials were sealed with silicone–PTFE septa and aluminium seals and were stored at 18 and 4 °C until analysis, respectively.