The average benzene concentrations for the concentration steps in figure were used to construct a calibration curve shown in figure . The error bars equal three times the standard deviation (3σ) of the data at each step. The calibration curve demonstrates very good linearity over the measured concentration range. As an estimate for a detection limit, three times the standard deviation for the lowest measured concentration step (0.5 ppb) was 150 ppt. The largest standard deviation within one concentration step was for the first level, where one standard deviation (1σ) corresponded to a benzene concentration of 66 ppt. For the blank measurements, standard deviation was 30 ppt or less within one individual 30 min measurement set. Between the different 30 min blank measurement sets, the largest difference between two average values was 27 ppb, suggesting there is slight drifting in the background level over the measurement period of 4.5 h. The parameters representative of the measurement system are collected in table .