Impact of Ambient Vapors on Spectra of 4-Nitroaniline Recorded under Atmospheric Solids Analysis Probe (ASAP) Mass Spectrometric Conditions.

Thermally desorbed 4-nitroaniline (4-NA), upon atmospheric pressure chemical ionization (APCI), generates gaseous ions for its protonated species. The APCI mass spectrum recorded under mild in-source ion-activating conditions from 4-NA showed a peak at / 139, whereas that acquired under high ion-activating conditions showed two additional peaks at / 122 (OH loss) and 92 (NO loss). The spectrum changed instantaneously when acetonitrile vapor was introduced to the source. In the new spectrum, both / 122 and 92 peaks were absent, while a new peak appeared at / 93. Ion-mobility separation carried out with the / 139 ion revealed that the initial ion represented the thermodynamically favored -protonated tautomer. The ion population changed to an ensemble dominated by the less-favored -protomer when acetonitrile vapor was introduced to the ion source. The -protomer, upon collisional activation, loses NO to generate an / 93 ion, which was confirmed to be the 4-dehydroanilinium ion. Ion mobility provided a practical way to monitor the changes secured by acetonitrile vapor because the two protomers showed different arrival times. Under spray-ionization conditions, the formation of the thermodynamically less favored protomer has been attributed to . Our study demonstrated that the less favored amino-protomer could be generated by introducing acetonitrile vapor under nonspray conditions. Apparently, under APCI conditions, protonated water vapor attaches to the nitro group to generate a proton-bound heterodimer, which upon activation dissociates to yield the -protomer. In contrast, protonated acetonitrile makes a tighter complex preferentially with the amino group, which upon activation breaks to the -protomer.