Chemical ionization mass spectrometry technique for detection of dimethylsulfoxide and ammonia

[1] A chemical ionization mass spectrometer (CIMS) was used to study reactions of protonated ethanol clusters (C₂H₅OH)_nH⁺ with dimethylsulfoxide (DMSO), dimethylsulfone (DMSO₂), ammonia (NH₃), and a series of nonmethane hydrocarbons (NMHCs) and volatile organic compounds (VOCs). The reactivity of the (C₂H₅OH)_nH⁺ cluster ions is a function of cluster size with reactivity decreasing as cluster size increases. Ethanol cluster ion distributions that formed at atmospheric pressure from 24 ppbv, 900 ppmv, and 1% ethanol/N₂ gas mixtures were studied. Small (C₂H₅OH)_nH⁺ clusters, those formed using the 24 ppbv ethanol/N₂ mixture, react at or near the collisional rate with DMSO, NH₃, acetone, and methyl vinyl ketone (MVK). The effective ion molecule rate coefficients are 1.8 × 10⁻⁹, 1.5 × 10⁻⁹, 1.0 × 10⁻⁹, and 1.6 × 10⁻⁹ cm³ molecule⁻¹ s⁻¹, respectively. Only DMSO and NH₃ react efficiently with the two larger (C₂H₅OH)_nH⁺ cluster ion distributions studied. The effective rate coefficients for DMSO and NH₃ with the 900 ppmv ethanol cluster ion distribution are 1.5 × 10⁻⁹ and 0.7 × 10⁻⁹ cm³ molecule⁻¹ s⁻¹, respectively. The effective rate coefficient for DMSO with the 1% ethanol/N₂ mixture is 0.35 × 10⁻⁹ cm³ molecule⁻¹ s⁻¹, while NH₃ reaches equilibrium with this cluster ion distribution. Experiments show that large (C₂H₅OH)_nH⁺ ion clusters must be used at relative humidities greater than 50% at 20°C to prevent formation of and subsequent interferences from H₃O⁺ ions. These results demonstrate that the (C₂H₅OH)_nH⁺ ion chemistry can selectively detect DMSO and NH₃ under most ambient atmospheric conditions with high sensitivity.