Real-time differentiation of ethylbenzene and the xylenes using selected ion flow tube mass spectrometry

Rationale Monitoring of isomeric analytes using mass spectrometry usually requires a time-consuming chromatographic separation of the analytes before analysis. Selected ion flow tube mass spectrometry (SIFT-MS) can provide rapid direct analysis of ethylbenzene and xylene by utilizing the different reaction chemistry of the isomers. O-2(+) yields the same product ions from each isomer but in different ratios. OH- yields different product ions. Methods The reaction chemistry of C8H10 with the reagent ion OH- generated from a microwave discharge of moist air in a commercial SIFT-MS instrument was utilized in this study. The product ion from OH- yielded ions at different masses for each isomer. To gain an understanding of how the product ion from ethylbenzene of HO2- was generated, a theoretical study of the potential reaction surface was undertaken that accounted for the experimental observations. Results Measurements of OH- with ethylbenzene showed the product ion to be HO2- at m/z 33. The reaction of OH- with xylene yielded the major product ion at m/z 105, C8H9-. HO2- also underwent a slow secondary reaction with CO2 and O-2 present from air in the samples. These findings were supported by calculations of the potential energy surface for the reactions. Measurements made on a certified gas mixture of ethylbenzene and xylene in the concentration range up to 5000 ppbv gave a linear response for each analyte. Conclusions A fast, efficient method was developed for monitoring xylene and ethylbenzene in a mixture without the need for chromatographic separation before analysis using SIFT-MS.