Recognition and resolution of isomeric alkyl anilines by mass spectrometry

Mario Benassi 0 3 Yuri E. Corilo 0 3 Diana Uria 0 1 Rodinei Augusti 0 2 Marcos N. Eberlin 0 3 0 Published online October 17, 2008 Received June 27, 2008 Revised October 3, 2008 Accepted October 3, 2008 1 Federal University of Minas Gerais , Belo Horizonte, Brazil 2 Waters Corporation, Simonsway, United Kingdom 3 ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, State University of Campinas , Campinas, Brazil Two MS techniques have been used to recognize and resolve a representative isomeric pair of N-alkyl and ring-alkyl substituted anilines. The first technique (1) uses MS/MS to perform ion/molecule reactions of structurally-diagnostic fragment ions (SDFI) whereas the second (2) uses traveling wave ion mobility spectrometry (TWIMS) of the pair of protonated molecules followed by on-line collision-induced dissociation (CID), that is, MS/TWIMS-CID/MS. Isomeric C7H7N ions of m/z 106 (1= from 4-butylaniline and 2 from N-butylaniline) are formed as abundant fragments by 70 eV EI of the anilines, and found to function as suitable SDFI. Ions 1= and 2 display nearly identical unimolecular dissociation chemistry, but contrasting bimolecular reactivity with ethyl vinyl ether, isoprene, acrolein, and 2-methyl-1,3-dioxolane. Ion 2 forms adducts to a large extent whereas 1= is nearly inert towards all reactants tested. The intact protonated anilines are readily resolved and recognized by MS/TWIMS-CID/MS in a SYNAPT mass spectrometer (Waters Corporation, Manchester, UK). The protonated N-butyl aniline (the more compact isomer) displays shorter drift time and higher lability towards CID than its 4-butyl isomer. The general application of SDFI 1= and 2 and other homologous and analogous ions and MS/TWIMS-CID/MS for absolute recognition and resolution of isomeric families of N-alkyl and ring-alkyl mono-substituted anilines and analogues is discussed. (J Am Soc Mass Spectrom 2009, 20, 269 -277) 2009 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry - contain and retain the structural information of its specific isomeric precursor; (3) be stable gaseous species with relatively high barriers for structural rearrangements, and (4) display contrasting dissociations or bimolecular reactivities, or both, induced and properly rationalized in terms of their distinct structures. Although the task is by no means easy, we have so far succeeded in finding several proper SDFI for different families of constitutional isomers. For instance, pyridinium ions function as SDFI for ortho, meta and para-substituted pyridines [2, 3], pyrimidinium ions for mono-substituted pyrimidines [4], hydroxy- and amino-benzoyl cations for isomeric acylanilines and acylphenols, nitrobenzoyl cations for acyl nitrobenzenes [5], pyrrolyl cations for isomeric mono-substituted pyrroles [6], and naphthyl acylium ions for isomeric - and -acyl naphtalenes [7]. Alkyl substituted anilines represent a family of isomers that has challenged most MS approaches for isomer recognition and resolution [8]. Their EI-MS, CI-MS(/MS) and ESI-MS(/MS) [9] are usually very similar and fail to distinguish these important constitutional isomers. Herein, we show that isomeric C7H7N ions of m/z 106, usually formed to a large extent by 70 eV EI, function as SDFI, and therefore Iexactly the same mass. Mass spectrometry measomers have distinct structures and properties, but sures primarily mass; hence when dealing with intact gaseous species, MS is inherently blind for distinguishing isomers. Many MS strategies based on the correlation between structure and physicochemical properties, such as those exploring uni- or bimolecular reactivity, have therefore been developed to recognize and resolve gaseous isomeric ions [1]. Isomers sometimes display, however, very similar gaseous reactivities and still end up being indistinguishable by these approaches. Isomers have also been treated in case by case approaches and, up to now, no general MS strategy for isomer recognition and resolution is available. We have recently proposed a MS/MS strategy for wide-ranging, absolute (noncomparative) recognition and resolution of constitutional isomers based on structurally diagnostic fragment ions (SDFI) [2]. The main SDFI requisites are that they must: (1) be a fragment ion shared by most (ideally by all) members of the isomeric family; (2) allow the recognition and resolution of N-alkyl versus ring-alkyl mono-substituted anilines. Although protonated anilines seem to form no suitable SDFI upon CID, we also demonstrate that the intact protonated molecules can be distinguished by MS/ TWIMS-CID/MS. The generalization of these two techniques to homologue and analogue isomers is also discussed. The experiments with the SDFI were conducted in an Extrel (Pittsburgh, PA) pentaquadrupole mass spectrometer, which is described in detail elsewhere [10]. For the MS/MS experiments, ions were generated by 70 eV electron ionization and selected by the first quadrupole (Q1). Ion/molecule reactions [11] with the neutral reactants ethyl vinyl ether, isoprene, acrolein, or 2-methyl-1,3-dioxolane, or CID with argon as the target gas were performed in q2. Product ion mass spectra were recorded by scanning Q5 while operating Q3 and q4 in the rf-only mode. For MS3 experiments, CID with argon of the selected (Q3) ions was performed in q4. The collision energies were typically near 1 eV for ion/molecule reactions and 15 eV for CID. For the MS/TWIMS-CID/MS experiments, a Waters SYNAPT high definition mass spectrometer (HDMS; Waters Corporation, Manchester, UK) equipped with an ESI source was employed. More details about this hybrid mass spectrometer and its operation parameters are presented elsewhere [12]. In brief, the instrument has a hybrid quadrupole ion mobility orthogonal acceleration time-of-flight (oaTOF) geometry. Ions are transported to the mobility separation section through the quadrupole analyzer, which is operated in the resolving mode. The mobility section comprises three consecutive, gas-filled, traveling wave (TW) RF ion guides. Ions are accumulated in the TW trap and periodically released into the TW mobility cell, where they separate according to their mobility through the action of a continuous train of DC pulses. The ions exiting the TW mobility cell are then transferred to the oa-TOF analyzer for MS analysis. Ion drift times are recorded by synchronization of the oa-TOF MS acquisition with the release of ions from the TW trap to the TW mobility cell. Ions were released from the TW trap for 100 ms in every 13 ms. Mass spectra were acquired in the positive ion mode, and the capillary voltage was 4 kV. The ion source block and nitrogen desolvation gas temperatures were set to 100 and 350 C, respectively. The TW trap and transfer cells were operated at pressures of 10 2 mbar of argon and the TW mobility cell at ca. 1 mbar of nitrogen. TWIMS was done at 300 m/s and the DC pulse height was around 15 V for best separation. Resu (...truncated)