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 performance
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.