Abstract
In this study, we present reactions of with a series of analytes (A): acetone (C3H6O), methyl vinyl ketone (C4H6O), methyl ethyl ketone (C4H8O), and eight monoterpene isomers (C10H16) using a Selective Reagent Ionization Time-of-Flight Mass Spectrometer (SRI-ToF-MS). We studied the ion-molecule reactions at collision energies of 55 and 80 meV. The ketones, having a substantially lower proton affinity than NH3, produce only cluster ions (A) in detectable amounts at 55 meV. At 80 meV, no cluster ions were detected meaning that these adduct ions are formed by strongly temperature dependent association reactions. Bond energies of cluster ions and proton affinities for most monoterpenes are not known and were estimated by high level quantum chemical calculations. The calculations reveal monoterpene proton affinities, which range from slightly smaller to substantially higher than the proton affinity of NH3. Proton affinities and cluster bond energies allow to group the monoterpenes as a function of the enthalpy for the dissociation reaction . We find that this enthalpy can be used to predict the (A) cluster ion yield. The present study explains product ion formation involving ion chemistry. This is of importance for chemical ionization mass spectrometry (CIMS) utilizing as well as (H2O) as reagent ions to quantitatively detect atmospherically important organic compounds in real-time.
Highlights
In the 1990’s proton transfer reaction mass spectrometry (Hansel et al, 1995; Lindinger et al, 1998b) using H3O+ reagent ions became a widely used analytical instrument with applications in environmental science, medical applications, and food technology due to the large amount of volatile organic molecules, which can be quantitatively ionized
We found the best correlation coefficient (R2 = 0.79) to predict the yield of the cluster ion formation channel for monoterpenes when the fraction of measured adduct ions was correlated as a function of the reaction enthalpy Hr of reaction (12), which is the difference in proton affinities plus the bond energy BE: Hr = proton affinity (PA)(NH3) – PA(A) + BE(NH+4 -A)
We investigated the reactions of NH+4 with a series of organic analytes (A): acetone (C3H6O), methyl vinyl ketone (C4H6O), methyl ethyl ketone (C4H8O) and eight monoterpene isomers (C10H16)
Summary
In the 1990’s proton transfer reaction mass spectrometry (Hansel et al, 1995; Lindinger et al, 1998b) using H3O+ reagent ions became a widely used analytical instrument with applications in environmental science, medical applications, and food technology due to the large amount of volatile organic molecules, which can be quantitatively ionized. H3O+ undergoes proton transfer reactions with every analyte having a higher proton affinity (PA) than water [PA(H2O) = 165.0 kcal/mol (Hunter and Lias, 1998)]. Due to the higher proton affinity of ammonia PA(NH3) = 204.0 kcal/mol (Hunter and Lias, 1998), exothermic, .
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