In the interference measurements, the shorter sampling cycle of 1 min was used. We first evaluated the interference from humidity. The benzene concentration in the sample flow was set to a constant level of 3 ppb. The air supply used to dilute the benzene was first sent through a water evaporator to control the water concentration in the sample flow. Benzene response was measured with four water concentration levels ranging from 0 to 2.5 % in mole fraction. The water concentration in the sample flow was verified with a humidity meter (ChipCap 2-SIP, Telaire). Each humidity level was measured for 20 min. A small decrease in the measured benzene concentration as a function of increasing humidity was observed. The benzene concentration decreased by approximately 0.14 ppb when the water concentration increased by 1 % in mole fraction. The interference from toluene, ethylbenzene and the xylene isomers was also evaluated. The samples were generated by passing a flow of compressed air, which was first purified with an active carbon filter, over the air space of liquid sample of each of the interferents. The resulting flow was assumed to contain a concentration in the order of the vapor pressure of the interferent. The flow was then diluted to a ratio of 1/1000 with compressed air. The diluted flow was sampled with the benzene measurement setup for 20 min for each interferent. The interference was evaluated as the apparent benzene concentration reported by the instrument. The presence of the interferents in the diluted flow was verified with a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF 1000, Ionicon). However, the instrument was not calibrated to the specific compounds under analysis and hence we could not verify the concentrations. The results of the interference measurements should be taken as or-der of magnitude estimates.