TY - JOUR
T1 - Which hydrogen atom of toluene protonates PAH molecules in (+)-mode APPI MS analysis?
AU - Ahmed, Arif
AU - Ghosh, Manik Kumer
AU - Choi, Myung Chul
AU - Choi, Cheol Ho
AU - Kim, Sunghwan
PY - 2013/3
Y1 - 2013/3
N2 - A previous study (Ahmed, A. et al.; Anal. Chem. 84, 1146-1151(2012) reported that toluene used as a solvent was the proton source for polyaromatic hydrocarbon compounds (PAHs) that were subjected to (+)-mode atmospheric-pressure photoionization. In the current study, the exact position of the hydrogen atom in the toluene molecule (either a methyl hydrogen or an aromatic ring hydrogen) involved in the formation of protonated PAH ions was investigated. Experimental analyses of benzene and anisole demonstrated that although the aromatic hydrogen atom of toluene did not contribute to the formation of protonated anthracene, it did contribute to the formation of protonated acridine. Thermochemical data and quantum mechanical calculations showed that the protonation of anthracene by an aromatic ring hydrogen atom of toluene is endothermic, while protonation by a methyl hydrogen atom is exothermic. However, protonation of acridine by either an aromatic ring hydrogen or a methyl hydrogen atom of toluene is exothermic. The different behavior of acridine and anthracene was attributed to differences in gas-phase basicity. It was concluded that both types of hydrogen in toluene can be used for protonation of PAH compounds, but a methyl hydrogen atom is preferred, especially for non-basic compounds.
AB - A previous study (Ahmed, A. et al.; Anal. Chem. 84, 1146-1151(2012) reported that toluene used as a solvent was the proton source for polyaromatic hydrocarbon compounds (PAHs) that were subjected to (+)-mode atmospheric-pressure photoionization. In the current study, the exact position of the hydrogen atom in the toluene molecule (either a methyl hydrogen or an aromatic ring hydrogen) involved in the formation of protonated PAH ions was investigated. Experimental analyses of benzene and anisole demonstrated that although the aromatic hydrogen atom of toluene did not contribute to the formation of protonated anthracene, it did contribute to the formation of protonated acridine. Thermochemical data and quantum mechanical calculations showed that the protonation of anthracene by an aromatic ring hydrogen atom of toluene is endothermic, while protonation by a methyl hydrogen atom is exothermic. However, protonation of acridine by either an aromatic ring hydrogen or a methyl hydrogen atom of toluene is exothermic. The different behavior of acridine and anthracene was attributed to differences in gas-phase basicity. It was concluded that both types of hydrogen in toluene can be used for protonation of PAH compounds, but a methyl hydrogen atom is preferred, especially for non-basic compounds.
KW - Atmospheric pressure photo ionization
KW - Mechanism
KW - Quantum mechanical calculation
UR - http://www.scopus.com/inward/record.url?scp=84875882242&partnerID=8YFLogxK
U2 - 10.1007/s13361-012-0520-x
DO - 10.1007/s13361-012-0520-x
M3 - Article
AN - SCOPUS:84875882242
SN - 1044-0305
VL - 24
SP - 316
EP - 319
JO - Journal of the American Society for Mass Spectrometry
JF - Journal of the American Society for Mass Spectrometry
IS - 3
ER -