For all xylene isomers, any interference was below quantification and the equivalent benzene concentration remained zero to within the standard deviation of the measurement. For toluene, the average equivalent benzene concentration over the 20 min measurement was 0.166 ppb. Estimating the toluene concentration in the sample from its vapor pressure and scaling the interference accordingly, we calculate that 1 ppm of toluene would result in an error corresponding to 6 ppt in the measured benzene concentration. For ethylbenzene, a higher interference signal corresponding to 16.8 ppb benzene concentration was measured. However, unlike with toluene, the spectrum measured from the ethylbenzene sample matched the benzene reference spectrum very well and we believe that the interference was due to benzene impurity in the ethylbenzene sample (99.8 % pure, ACROS organics), since benzene is one of the most likely impurities. The peak corresponding to benzene was detected in the mass spectrum as well, but we cannot say with certainty what is the contribution of possible fragmentation and actual benzene impurity to the mass peak. A more pure ethylbenzene sample was not available to us at this time. If we assumed that the interference was actually entirely from ethylbenzene, we estimate that 100 ppb of ethylbenzene would result in a 0.13 ppb error in the measured benzene concentration, when the ethylbenzene concentration was approximated from its vapor pressure.