Detection of xylene and toluene added to humidified air and human breath. Xylene (A) or toluene (B) vapour were added to either humidified bottled air (circles) or human breath (triangles) contained in Tedlar bags to produce a range of concentrations in duplicate. The standards were introduced into the sampling line of the SIFT-MS and after a delay of 10 seconds xylene and toluene levels were measured by reaction with the H3O+ precursor for a period of 10 seconds. These values were then plotted against the calculated standard concentrations. The best fit linear regression line is shown for both bottled air (solid line) and human breath (dashed line).