Real-Time Determination of Aromatics in Automobile Exhaust by Single-Photon Ionization Ion Trap Mass Spectrometry

Instrumentation was constructed for the determination of oxygenates and aromatics by single-photon ionization ion trap mass spectrometry (SPI-ITMS) with a direct-sampling interface for application to real-time automotive exhaust measurements. Vacuum ultraviolet light at 118 nm was produced by focusing 355-nm pulses from a Nd:YAG laser into a gas cell containing xenon and passing them through ports in the ring electrode of a modified commercial ion trap mass spectrometer. The direct-sampling system consisted of a heated capillary interface with a pulsed valve. Good ionization efficiency was observed for analytes with ionization potentials (IPs) below 10.1 eV, with most molecules yielding molecular ions as the most abundant ionization product. SPI provides significant selectivity against interferences with IPs greater than 10.5 eV. Tandem mass spectrometry was employed to distinguish between analytes that produced isobaric base peaks via SPI. Detection limits between 0.03 and 0.2 ppm were obtained for seven compounds by SPI, with precision of 50% at 1 ppm, a time resolution of 5 s, and a linear dynamic range of 2−3 orders of magnitude. Accuracy was verified by the simultaneous determination of six compounds at ∼1 ppm in a gas mixture standard. Real-time analysis of automobile exhaust by SPI-ITMS demonstrated the potential of the system for emissions measurements.