Abstract

In previous papers we have demonstrated two different, two-color resonance-enhanced multiphoton ionization (REMPI) schemes for the simultaneous measurement of trace amounts (ppbV to pptV) of nitrogen monoxide (NO) and nitrogen dioxide (NO₂). The goal of this study is to provide a laser ionization–mass spectrometric scheme capable of measuring ppmV to ppthV concentrations of NO and NO₂ within vehicle exhaust containing up to ppthV of aromatic hydrocarbons and a time frame of seconds. Two ionization schemes are used here to measure NO and NO₂ in simulated automobile exhaust with three different sources. REMPI Scheme 1 uses broad-bandwidth light and an effusive source to measure NO (limit of detection (LOD) 300 ppmV), NO₂ (LOD 100 ppmV), and aromatic hydrocarbons (via photoionization) along with fragments (via electron impact). REMPI Scheme 2 uses narrow-bandwidth light and a medium pressure laser ionization (MPLI) source to measure NO (LOD 60 ppmV), NO₂ (LOD 3 ppmV), and fragments (via electron impact). The LOD is determined using 10-second sampling times. A newly developed delayed-ion extraction technique for MPLI is then applied to REMPI Scheme 2, dramatically reducing the electron impact signal, so that only NO and NO₂ are observed. We conclude that Scheme 2 with delayed-electron extraction is best suited for measuring <i>in situ</i> NO and NO₂ within engine exhaust.

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