Article pubs.acs.org/ac
Real-Time Monitoring of In Situ Gas-Phase H/D Exchange Reactions of Cations by Atmospheric Pressure Helium Plasma Ionization Mass Spectrometry (HePI-MS) Athula B. Attygalle,* Rekha Gangam, and Julius Pavlov *
Center for Mass Spectrometry, Department of Chemistry, Chemical Biology and Biomedical Engineering, Stevens Institute of Technology, Hoboken, New Jersey 07030, United States S Supporting Information *
ABSTRACT: An enclosed atmospheric-pressure helium-plasma ionization (HePI-MS) source avoids, or minimizes, undesired back-exchange reactions usually encountered during deuterium incorporation experiments under ambient-pressure open-source conditions. A simple adaptation of an ESI source provides an economical way of conducting gas phase hydrogen/ deuterium (H/D) exchange reactions (HDX) in real time without the need for complicated hardware modifications. For example, the spectrum of [2H8]toluene recorded under exposed ambient conditions showed the base peak at m/z 96 due to fast leaching of ring hydrogens because of interactions with H2O vapor present in the open source. Such D/H exchanges are rapidly reversed if the deuterium-depleted [2H8]toluene is exposed to D2O vapor. In addition to the enumeration of labile protons, our procedure enables the identification of protonation sites in molecules unambiguously, by the number of H/D exchanges observed in real time. For example, molecules such as tetrahydrofuran and pyridine protonate at the heteroatom and consequently undergo only one H/D exchange, whereas ethylbenzene, which protonates at a ring position of the aromatic ring, undergoes six H/D exchanges. In addition, carbocations generated in situ by in-source fragmentation of precursor protonated species, such as benzyl alcohol, do not undergo any rapid H/D exchanges. Because radical cations, second-generation cations (ions formed by losing a small molecule from a precursor ion), or those formed by hydride abstraction do not undergo rapid H/D exchanges, our technique provides a way to distinguish these ions from protonated molecules.
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on Fourier transform ion cyclotron resonance mass spectrometers (FT-ICR-MS)19 or quadrupole ion traps22 because ions can be stored for relatively longer periods of time. On occasion, H/D exchange reactions with small molecules have been conducted by selected ion flow tube mass spectrometry (SIFTMS)23 and FTICR methods.24 In general, such investigations usually require sophisticated instrumentation. Most importantly, the undesirable deuterium back-exchange reactions to hydrogen are difficult to control in order to obtain meaningful results.25 Two landmark innovations, desorption electrospray ionization (DESI) and direct analysis in real time (DART), have paved the way for the rapid analysis of samples in their native states by mass spectrometry under ambient conditions.26 However, some authors27,28 have pointed out that the key buzzword “ambient” is often improperly used when these techniques are described in the literature, primarily because the exact boundaries between the techniques are somewhat blurred. To clearly distinguish atmospheric-pressure ionization techniques from ambient ionization approaches, Monge et al.29
he value of stable-isotope labeling techniques for structural and mechanistic studies by mass spectrometry was recognized decades ago.1 For structural investigations, in addition to incorporating stable-isotopes by chemical synthesis, hydrogen/deuterium (H/D) exchange reactions have been employed to enumerate the labile hydrogen atoms present in a molecule. Such exchanges were generally conducted in solution,2 or in the gas phase under chemical-ionization conditions, using ND3,3,4 D2O,5 or CH3OD6,7 as reagent gases. The most straightforward method to carry out an H/D exchange reaction is to dissolve the sample in D2O or CH3OD and analyze the reaction mixture by liquid chromatography− mass spectrometry (LC−MS) using D2O in the mobile phase,8−12 or add ND3 to the nebulizer, curtain, or collision gas13−16 under electrospray ionization conditions. Alternatively, D2O has been used as the sheath liquid17 or as one of the sprays in a dual-sprayer source18 to achieve H/D exchanges. All these procedures, however, are costly because of the high amount of deuteriated reagents consumed. Undoubtedly, gas-phase H/D exchange experiments between ions and deuteriated reagents are among the most useful techniques for probing reaction mechanisms and elucidating positional and conformational structures of both small19 and large molecules.20,21 Such investigations are usually conducted © 2013 American Chemical Society
Received: November 9, 2013 Accepted: December 10, 2013 Published: December 10, 2013 928
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proposed a set of basic characteristics that should be present for a technique to be designated an “ambient ionization/sampling” procedure. The key criterion is the absence of an enclosure, which enables objects of unusual shape or size to be inserted directly into the source. One of the major disadvantages of not having an enclosure is that less control is provided over undesired reactions, particularly D/H back-exchanges, that could take place between analytes and other molecules present in the ambient gases. A method has been described to determine the number of labile hydrogen atoms present in molecules by hydrogen/deuterium (H/D) exchange reactions in the gas phase under DART conditions by saturating the open interface with D2O using an additional nebulizer.30 Recently, Kostyukevich et al. described a method to conduct H/D exchanges by placing a droplet of D2O between the ESI needle and the entrance orifice of the mass spectrometer, thereby saturating the atmosphere with deuteriated water vapor in the ESI region.31,32 In comparison with the solvent-spray methods, solvent-free methods offer better advantages because backexchange reactions from the protic solvents could be minimized. However, with ambient sources open to the atmosphere, significant amounts of deuteriated solvents (e.g., 2 mL of D2O) must be used to obtain useful results.30 Here, we describe a simple method to effect and monitor gas-phase H/D exchanges of protonated organic molecules in a helium plasma ionization (HePI)33 source. Furthermore, we show how such H/D exchanges can be applied to determine the preferred protonation sites of some aromatic compounds.
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EXPERIMENTAL SECTION Chemicals. To generate the plasma in the mass spectrometer, high purity helium (99.999%, Airgas, Radnor, PA) was used for all experiments. All the following chemicals were purchased from Sigma-Aldrich (St. Louis, MO) and used without further purification: n-hexane, cyclohexyl iodide, pyridine, tetrahydrofuran, benzene, toluene, 1,4-dimethylbenzene (p-xylene), ethylbenzene, styrene, bromobenzene, benzyl alcohol, aniline, nitrobenzene, benzaldehyde, acetophenone, 2nitrotoluene, 4-bromoaniline, hexamethylbenzene, 1,3,5-trimethylbenzene, naphthalene, and deuterium oxide (99.9 atom %). [Methyl-2H3]toluene, toluene-d8, and benzene-d6 were purchased from Cambridge Isotope Laboratories, Inc. (Tewkesbury, MA). Mass Spectrometry. The modification of a Z-spray ion source (with a cylindrical glass cover) of a Waters Micromass Quattro Ultima mass to function as a helium-plasma ionization (HePI) source has been described previously.34 High-purity helium at a flow rate of about 30 mL min−1 was passed through the sample introduction capillary needle held typically at +3.5 kV. Typically, the source temperature was kept at 100−150 °C, and the cone voltage was set to 5−10 V. For sample heating, nitrogen (50−150 L/min) was passed to the ion source though the heated “desolvation” gas line (Figure 1). Sample Introduction. Open Source. A cotton swab wetted with a sample of toluene, or [2H8]toluene (1−5 μL), was introduced to the open HePI source (i.e., with the glass cover removed) maintained at 150 °C, and mass spectra were recorded from m/z 20 to 120. Some samples (e.g., [2H8]toluene) were introduced by injecting 1−5 μL of neat liquid through a septum into the desolvation gas stream. Closed Source. To introduce liquid samples to a closed source (i.e., with the glass cover on), liquid samples (1−5 μL) were deposited on loosely packed silica gel (200 μm; 5 mg)
Figure 1. Sample placement and introduction of a cotton swab soaked with D2O to a closed HePI source (cone voltage 10 V).
inside a one-side sealed glass capillary (1.5 cm; 1.5 mm i.d.) and with the aid of a small wad of “museum putty,” the sample tube was attached to a premarked position on the inner side of the cylindrical glass enclosure of the ion source (Figure 1). Spectra were acquired for about 50 s from a sample of a mixture of toluene and benzyl alcohol (1:1 v/v) placed inside the closed source, and a cotton swab soaked in D2O (20−30 μL) was inserted into the source enclosure through one of the holes in the front side of the source. The distance between the sample tube and the swab was about 3 in. The heater for the desolvation gas line (set at 150 °C) was employed to deliver a stream of hot nitrogen to the source region to facilitate the evaporation of D2O from the cotton swab. Similar experiments were conducted with benzene, [2H3]toluene, [2H8]toluene, n-hexane, cyclohexyl iodide, pyridine, tetrahydrofuran, toluene, 1,4-dimethylbenzene (p-xylene), ethylbenzene, styrene, bromobenzene, benzyl alcohol, aniline, nitrobenzene, benzaldehyde, acetophenone, 2-nitrotoluene, 4bromoaniline, hexamethylbenzene, 1,3,5-trimethylbenzene, and naphthalene.
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RESULTS AND DISCUSSION Toluenium ions have been the focus of intensive investigations for many decades.35−37 Traditionally, toluenium ions have been characterized by NMR spectroscopic methods (sometimes in superacid media)38 and gas-phase IR techniques.36 Mass 929
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Figure 2. Mass spectra recorded from [2H8]toluene (nominal mass of 100), under (a) ambient HePI conditions (open source), and (b) atmospheric pressure HePI conditions (enclosed source). (c) Chronogram recorded by injecting a sample of [2H8]toluene into the nitrogen “desolvation gas” of an open HePI source. Profile spectra (m/z 20−120) were acquired for three minutes. Insets I, II, and III show spectra generated by coadding acquisitions recorded during 0−5, 6−10, and 11−15 s after the sample introduction, respectively.
Figure 3. Mass spectra recorded two minutes after exposing (a) [2H8]toluene (nominal mass = 100) and (c) [methyl-2H3]toluene (nominal mass = 95) to atmospheric HePI conditions (closed source). After recording the spectra by the multiple-channel analysis (MCA) acquisition program for 30 s, a cotton swab soaked in D2O was introduced and spectra were recorded for 2 min further (enclosed source) (b and d, respectively).
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Figure 4. Ion abundance chronograms recorded for m/z 78, 79, 80, 81, 82, 83, 84, 85, and 86 recorded by exposing the vapor emanating from a sample of benzene (nominal mass 78) to an enclosed HePI source under atmospheric pressure conditions. After 45 s of data acquisition (m/z 20 to 120), a cotton swab soaked in D2O was inserted to the source and removed after a period of 160 s. Insets I, II, and III show coadded spectra between 15 and 45, 100−135, and 290−320 s, respectively.
Evidently, this methodology provides a simple way to enumerate the exchangeable protons by conducting on-the-fly H/D exchange reactions without the need of large quantities of deuteriated reagents or hardware modifications (see the Supporting Information for a real-time video presentation). Davies et al. had observed similar D/H back exchanges while analyzing deuteriated indole derivatives under high-temperature APCI-LC-MS conditions when water was present in the mobile phase.44 The mechanism by which water vapor interacts with toluenium ions has been described by Chiavarino et al.43 Furthermore, the leaching-out of deuterons from [2H8]toluene could be prevented, or greatly retarded, by maintaining an atmosphere enriched in D2O vapor. In fact, even the initially depleted number of deuterons of [2H8]toluene could be restored by introducing D2O vapor to the enclosed chamber (Figure 3, panels a and b). Analogously, the ring hydrogens of the protonated [methyl-2H3]toluene could be substituted with deuterons by exposing sample vapor to D2O vapor under similar conditions (Figure 3, panels c and d). To investigate the rapid H/D exchange procedure in greater detail, we placed a sample of benzene in the enclosed source and recorded its spectrum (Figure 4). Initially, the spectrum showed the base peak at m/z 79 for protonated benzene (Figure 4, inset I). After the introduction of D2O vapor from a cotton swab, the spectrum changed rapidly. New peaks were immediately observed between m/z 81 to 86, which indicated
spectrometry provides a more convenient method to investigate toluenium and other protonated aromatic compounds. Under positive-ion generating vacuum-CI,39,40 APCI,40 or DART41 conditions, it is well-known that toluene undergoes facile protonation to produce an ion of m/z 93.42,43 Spectra recorded under HePI conditions from toluene also show the base peak at m/z 93 for the toluenium cation (Figure S1 of the Supporting Information). However, we noted that the spectrum recorded from [2H8]toluene (nominal mass of 100), under open-source ambient HePI conditions, showed the base peak at m/z 96 and not at m/z 101 (Figure 2a). In contrast, the spectrum from the same sample acquired under atmospheric-pressure HePI conditions (enclosed source) showed a cluster of peaks spreading up to m/z 102 (Figure 2b). From our results, it was apparent that ring deuterons undergo very rapid exchanges with active hydrogens from water vapor present in ambient gases. To support this hypothesis, a sample of [2H8]toluene was introduced through the nitrogen “desolvation gas” supply line to the HePI source kept open to the atmosphere. Initially, the spectrum looked very similar to that recorded under enclosed-source conditions (Figure 2c, inset I). However, within a few seconds, the appearance of the spectrum changed rapidly and became very similar to that previously recorded under open source conditions (Figure 2c, inset III). From the results, it was apparent that the CD3 deuterium atoms are resilient to rapid D/H exchanges. 931
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Figure 5. Ion abundance chronograms recorded for m/z 91, 93, 94, 95, 96, 97, and 98 by exposing the vapor emanating from a sample of toluene and benzyl alcohol in a glass microcapillary attached to the inside wall of the enclosed HePI source. After 20 s of data acquisition (m/z 20−120), a cotton swab soaked in D2O was inserted to the source and removed at 170 s. Insets I, II, and II show coadded spectra between 15 and 35, 160−180, and 255−275 s, respectively.
tetrahydrofuran and m/z 109 for protonated anisole, respectively, both underwent one H/D exchange under these in-source conditions (Figure SI3 of the Supporting Information). Subsequently, we exposed a 1:1 (v/v) mixture of benzyl alcohol and toluene to D2O vapor inside the enclosed HePI source and noted that the intensity of the m/z 91 ion for the benzyl cation remained unchanged, whereas the intensity of the m/z 93 peak for protonated toluene decreased rapidly (Figure 5, inset II). The signal for the m/z 94 ion showed a small initial increase followed by a gradual decrease to reach a constant value (Figure 5). The intensities of other ions such as m/z 95, 96, 97, and 98 exhibited gradual increases to reach plateau levels, indicating the attainment of an overall steady state of H/D exchange. This steady state, which was reached in about two minutes, was disrupted when the D2O swab was removed from the enclosure. Subsequently, the spectrum changed rapidly to that of the
that the H/D exchange with the ring hydrogen atoms is a rapid process. The peak intensities stabilized in about 20 s, indicating a steady H/D exchange state had been reached (Figure 4, inset II). At 160 s, the cotton swab was removed; the peak intensities of m/z 81−86 decreased quickly, due either to back exchange of the ring deuterons with hydrogen from H2O vapor or to a fresh influx of protonated [d0]benzene (Figure 4, inset III). Further experimental data were obtained by recording a spectrum from a sample of benzyl alcohol. Protonated benzyl alcohol is a short-lived species that loses water rapidly to produce the benzyl cation (m/z 91; Figure SI2a of the Supporting Information). Our results indicate that the insource-generated benzyl cation (m/z 91) does not undergo any rapid H/D exchanges. Analogously, the m/z 85 and 83 carbocations generated from n-hexane and cyclohexyl iodide, respectively, did not undergo any H/D exchanges (Figure S2c and S2e of the Supporting Information), as expected.34 On the other hand, the m/z 73 and 145 ions for protonated 932
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Figure 6. HePI mass spectra recorded from (a) ethylbenzene and (b) styrene samples placed in a closed source. After 20 s of data acquisition, a cotton swab soaked in D2O was introduced and spectra were recorded for an additional 2 min (c and d).
initial species of protonated toluene (Figure 5, inset III). Moreover, the recorded spectra also showed that the rapid H/ D exchange does not take place with the hydrogens of the CH3 group within the duration of our tandem mass spectrometric experiments. Investigations on the toluenium ion in solution35 and in the gas phase36,42 have indicated that the protonation can take place at any ring position, although ortho and para positions are favored.43 However, ab initio calculations show that the formation of the para-protonated isomer in only slightly thermodynamically more favorable than the other site isomers.42,43 Moreover, the H/D exchange experiments in the ion source can provide unambiguous information about the site of protonation. For example, the protonation of ethylbenzene appears to occur at a ring position by a mechanism similar to that proposed for the protonation of toluene (Figure 6a) because the intensity of m/z 107 peak rapidly decreased and a cluster of new peaks spreading up to m/z 113 appeared in the spectrum, after D2O was introduced. In contrast, protonated styrene, although it is also a monosubstituted benzene derivative, underwent only one H/ D exchange, without incorporation of further D atoms, confirming that the protonation occurred at the side chain and not at a ring position. The protonation efficiency of gaseous molecules depends on their gas-phase proton affinities (or relative gas-phase basicities, GB). When a molecule bears several sites competing for protonation, the attachment occurs preferentially and selectively at the most “basic” site.45 Styrene has a much higher proton affinity (GB) than water, thus it is not surprising that only a single H/D exchange at the side chain takes place to afford the α-phenylethyl cation.45
Experiments with pyridine revealed that the protonation takes place exclusively on the nitrogen atom because only one deuterium atom could be introduced by the H/D exchange procedure (Figure SI4 of the Supporting Information). Under similar conditions, protonated benzene acquired up to seven deuterons (Figure 4). Moreover, protonated pyridine was more defiant to the gas-phase H/D exchange reaction because even after several minutes only a little increase of the intensity of the m/z 81 peak was observed (Figure SI4 of the Supporting Information). Furthermore, under HePI conditions, nitrobenzene underwent facile protonation (Figure SI5 of the Supporting Information). Although it is a monosubstituted benzene derivative, exchange results support that in nitrobenzene the protonation takes place at the substituent group and not at the ipso or any other ring position. Evidently, protonation occurs on the nitro group because it is relatively more basic than the benzene ring.45 Further results showed that protonated onitrotoluene, although it is a toluene derivative, does not behave similar to protonated toluene. Protonated toluene undergoes up to six H/D exchanges (Figure 5), whereas protonated o-nitrotoluene undergoes only one (Figure SI5d of the Supporting Information). In addition, ions such as protonated tetrahydrofuran undergo only one exchange, which indicates that the protonation occurs on the oxygen atom (Figure SI3 of the Supporting Information). The benzyl cation, on the other hand, does not undergo any H/D exchanges because its conjugate base (a neutral C7H6 species) is of such exceedingly high GB that ambient H2O or D2O molecules are unable to effect a proton abstraction (Figure 5). 933
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Figure 7. HePI mass spectra recorded from (a) bromobenzene and (c) p-bromoaniline samples placed in a closed source. After 20 s of data acquisition, a cotton swab soaked in D2O was introduced and spectra were recorded for further 2 min (b and d).
neutral compound does not possess any ring hydrogens. Mesitylene on the other hand underwent several H/D exchanges. However, the exchange reaction was sluggish, compared to that of toluene, because the steric effects of the methyl groups hinder the ion molecule interaction with the gaseous D2O molecules. Our results show that protonated naphthalene undergoes H/D exchange to a much smaller extent than benzenium ions do, presumably due to the fact that the charge might not be equally delocalized over the two fused rings.
The spectrum recorded from bromobenzene under closedsource conditions showed a cluster of peaks in the m/z 156− 160 region for both protonated molecules (m/z 157 and 159 for C6H679Br+ and C6H681Br+, respectively) and molecular ● radical cations (m/z 156 and 158 for C6H579Br+ and ●
C6H581Br+ , respectively). When D2O vapor was introduced to the enclosed source, the molecular cations did not undergo any D/H exchanges, whereas the protonated molecules experienced exchanges up to six deuterons (Figure 7b). Consequently, some of the peaks in the low-resolution spectrum shown in Figure 7b now represent unresolved isobaric molecular ions and isotopologs. For example, the ● peak at m/z 158 now represents a mixture of C6H581Br+ , 13 C1C5H679Br+, and C6H5D79Br+ (Figure 7b). The overall result confirmed that the protonation had taken place at a ring position of bromobenzene. In contrast, p-bromoaniline was defiant to electron loss and showed peaks only for the protonated species (m/z 172−175, Figure 6d), which underwent three H/D exchanges when exposed to D2O vapor, confirming that the protonation had taken place at the amino group and not at any of the ring positions. This dramatic regioselectivity demonstrated unambiguously that the amino group, the most basic moiety in the molecule, is the protonation site. Analogous results were obtained from hexamethylbenzene, mesitylene, xylene, and naphthalene (Figure S6 of the Supporting Information). For example, protonated hexamethylbenzene underwent only one H/D exchange because the
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CONCLUSIONS Back-and-forth exchange of H and D atoms in gas-phase ions has been an area of intense research for many decades.45−47 The results presented here demonstrate that an enclosed atmospheric-pressure helium-plasma ionization source provides a way not only to enumerate labile protons but also to carry out H/D exchanges with more regulation and control. The exchange reaction that takes place at room temperature, and atmospheric pressure is rapid and specific; its progress can be monitored in real time. Protonated molecules undergo rapid H/D exchanges. However, radical cations or second-generation cations do not undergo rapid H/D exchanges. In other words, this method can also be used to determine unambiguously whether the origin of a carbocation is from a protonation or from a hydride abstraction reaction. Such determinations are important for structure elucidation experiments. For example, a saturated hydrocarbon undergoes hydride abstraction to yield a 934
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(17) Lam, W.; Ramanathan, R. J. Am. Soc. Mass Spectrom. 2002, 13, 345−353. (18) Wolff, J.-C.; Laures, A. M-F. Rapid Commun. Mass Spectrom. 2006, 20, 3769−3779. (19) Kleingeld, J. C.; Nibbering, N. M. M. Tetrahedron 1983, 39, 4193−4199. (20) Wang, F.; Li, W.; Emmett, M. R.; Hendrickson, C. L.; Marshall, A. G.; Zhang, Y. L.; Zhang, Z. J. Biochemistry 1998, 37, 15289−15299. (21) Ramanathan, R.; Gross, M. L.; Zielinski, W. L.; Layloff, T. P. Anal. Chem. 1997, 69, 5142−5145. (22) Schaaff, T. G.; Stephenson, J. L., Jr.; McLuckey, S. A. J. Am. Soc. Mass Spectrom. 2000, 11, 167−171. (23) Lifshitz, C. Int. J. Mass Spectrom. 2004, 234, 63−70. (24) Freitas, M. A.; Shi, S. D.-H.; Hendrickson, C. L.; Marshall, A. G. J. Am. Chem. Soc. 1998, 120, 10187−10193. (25) Katta, V.; Chait, B. T. Rapid Commun. Mass Spectrom. 1991, 5, 214−217. (26) Weston, D. J. Analyst 2010, 135, 661−668. (27) Weston, D.; Ray, A. D.; Bristow, A. W. T. Rapid Commun. Mass Spectrom. 2011, 25, 821−825. (28) Huang, M. Z.; Yuan, C.-H.; Cheng, S.-C.; Cho, Y.-T.; Shiea, J. Annu. Rev. Anal. Chem. 2010, 3, 43−65. (29) Monge, M. E.; Harris, G. A.; Dwivedi, P.; Fernández, F. M. Chem. Rev. 2013, 113, 2269−2308. (30) Vail, T. M.; Jones, P. R.; Sparkman, O. D. J. Anal. Toxicol. 2007, 31, 304−312. (31) Kostyukevich, Y.; Kononikhin, A.; Popov, I.; Nikolaev, E. Anal. Chem. 2013, 85, 5330−5334. (32) Kostyukevich, Y.; Kononikhin, A.; Popov, I.; Kharybin, O.; Perminova, I.; Konstantinov, A.; Nikolaev, E. Anal. Chem. 2013, 85, 11007−11013. (33) Yang, Z.; Pavlov, J.; Attygalle, A. B. J. Mass Spectrom. 2012, 47, 845−852. (34) Yang, Z.; Attygalle, A. B. J. Am. Soc. Mass Spectrom. 2011, 22, 1395−1402. (35) Fărcaşiu, D.; Melchior, M. T.; Craine, L. Angew. Chem., Int. Ed. Engl. 1977, 16, 315−317. (36) Douberly, G. E.; Ricks, A. M.; Schleyer, P. v. R.; Duncan, M. A. J. Phys. Chem. A 2008, 112, 4869−4874. (37) Fărcaşiu, D. Acc. Chem. Res. 1982, 15, 46−51. (38) Olah, G. A.; Schlosberg, R. H.; Porter, R. D.; Mo, Y. K.; Kelly, D. P.; Mateescu, G. D. J. Am. Chem. Soc. 1972, 94, 2034−2043. (39) Munson, M. S. B.; Field, F. H. J. Am. Chem. Soc. 1967, 89, 1047−1052. (40) Kuck, D. Mass Spectrom. Rev. 1990, 9, 583−630. (41) Cody, R. B.; Laramée, J. A.; Durst, H. D. Anal. Chem. 2005, 77, 2297−2302. (42) Mormann, M.; Salpin, J.-Y.; Kuck, D. Int. J. Mass Spectrom. 2006, 249, 340−352. (43) Chiavarino, B.; Maria Elisa Crestoni, M. E.; Di Rienzo, B.; Fornarini, S.; Lanucara, F. Phys. Chem. Chem. Phys. 2008, 10, 5507− 5509. (44) Davies, N. W.; Smith, J. A.; Molesworth, P. P.; Ross, J. J. Rapid Commun. Mass Spectrom. 2010, 24, 1105−1110. (45) Kuck, D. Protonated Aromatics and Arenium Ions. In Encyclopedia of Mass Spectrometry; Nibbering, N. M. M., Ed.; Elsevier: Amsterdam, 2005; Vol. 4, pp 229−242. (46) Kuck, D. Int. J. Mass Spectrom. 2002, 213, 101−144. (47) Kuck, D.; Ingemann, S.; de Koning, L. J.; Grützmacher, H.-F.; Nibbering, N. M. M. Angew. Chem., Int. Ed. Engl. 1985, 24, 693−695.
carbocation, which is isobaric to that produced by protonation of a monounsaturated hydrocarbon bearing the same number of carbon atoms. Such an isobaric ions can be distinguished from one another by an H/D exchange reaction. A composite MS peak due to the two ions present in a mixture will split because only the protonated alkene will participate in the H/D exchange reaction, resulting in a unit positive mass shift. Moreover, it is known that gas-phase protonation of aromatics often happens at different locations of the carbon framework.45 The method described here enables locating the protonation site in a simple and convenient manner.
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ASSOCIATED CONTENT
S Supporting Information *
Additional figures as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Bristol-Myers Squibb Pharmaceutical Company (New Brunswick, NJ) for the donation of the Waters Quattro Ultima mass spectrometer used for this study. We thank one of the reviewers for the very thoughtful suggestions made, which definitely improved the quality of this paper.
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