For the NDMA measurement, ascorbic acid solution (50 L,
0.1 M) was added to the tap water sample (5 mL). The sample
solution was passed through a well-washed anion-exchange and
reverse-phase mixed-mode SPE column (Oasis MAX, 30 mg sorbent
in 1 mL volume tube, Waters, MA, USA). The Oasis MAX cartridge
had been preconditioned with 4 mL ultrapure water and 4 mL acetonitrile
(HPLC grade, Wako), followed by 4 mL ultrapure water.
After introduction of the sample, the first milliliter that passed
through the SPE cartridge was discarded, and the second milliliter
was collected in an autosampler vial (2 mL clear glass; septum
cap comprising Teflon-lined silicone rubber; Shimadzu). Ion chromatography
was performed after 0.45 m filtration only, and TOC
measurements were performed directly.