Anal. Chem. 1994,66,861-866
Characterization of the Neutral Products Formed upon the Charge-Remote Fragmentation of Fatty Acid Ions Marcela M. Cordero and Chrys Wesdemlotls' Department of Chemistry, The Universw of Akron, Akron, Ohio 44325-3601 Collisionally activated dissociation (CAD) of the carboxylate anions, [M Hr,or diHthiated cations, [M - H + 2Lil+, from fatty acids results in charge-remote fragmentationsat the alkyl chain C-C bonds leading to structure-indicativehgmentions. The neutral molecules eliminated during these reactions are characterized in this study using neutralization-reionization massspectrometry (NRMS). Themajorneutrailtmesdetected are alkenes (or dienes, in the case of monounsaturated fatty acids), not alkanes or alkyl radicals. This is consistent with the initially proposed mechanism proceediig by a pericyclic l,&elimination of H2.
&hun@ 1
In mass spectrometry, structural information is obtained from the unimolecular dissociations of gaseous ions. These reactions are commonly initiated by the charge of the ion or, in radical ions, by the unpaired electron.' A notable exception is the charge-remote fragmentation of closed-shell, evenelectron ions, in which the decomposition occurs at a position remote to the charge and with no intervention by the chargea2v3 This class of fragmentations requires that the charge be well stabilized at its location, so that it exhibits no tendency for migration to the reactionsite. Among the positive and negative ions satisfying this prerequisite are ammonium cations, lithiated alcohols, dilithiated carboxylic acids, carboxylates, sulfates, and sulfonates. Charge-remote dissociations have been shown to be particularly valuable for elucidating the structures of fatty acids and other moleculeswith long hydrocarbon chains, which are generally difficult to characterize by non mass spectral methods.4 Upon FAB ionization, fatty acids can be desorbed asvery stablecarboxylateanions [M-HI-ordilithiatedcations [M - H 2Li]+.4.5 Subsequent collisionally activated dissociation (CAD)617causes charge-remote fragmentations which, with a saturated precursor, lead to a series of fragments by losses of CnH2,,+2units from the alkyl chain (Scheme 1). The resulting CAD spectrum contains a series of product ions regularly spaced by intervalsof 14mass units. Anunsaturation suppresses cleavages of the double bond and at the vinylic positions and shifts the masses of certain product ions. These unique features allow for the unequivocal diagnosis of the
fatty acid's connecti~ity.~ Charge-remotedecompositionshave been reported for many other compound types, including cationic and anionic surfactants, steroids, prostaglandins, phospholipids, and carbohydrates, as well as peptide^.^^^ The detailed mechanism of charge-remote fragmentations remains unclear. The original reaction pathway postulated by Jensen et al.5 was based on labeling data and involves a concerted, symmetry-allowed elimination with concomitant formation of alkenes and terminally unsaturated fatty acid ions (Scheme 1). A completely parallel mechanism operates during the gas-phase pyrolysis of carboxylic esters,* substantiating that a charge is not needed in such 1,4eliminations. Confirmatory evidence for the mechanism of Scheme 1 has been presented by Adams and Grossg and Wysocki et a1.10 by meansof MS/MS/MS experimentswhich indicate that the ionic products indeed are terminally unsaturated fatty acid ions. Although such ions could also arise via a symmetry-forbidden 1,Zelimination of alkanes, such a process would coproduce alkenes plus saturated fatty acid ions, which are not observed.3*5 The 1,4-H2 elimination mechanism is supported by molecular orbital calculations, which predict a lower critical energy for this pathway than for 1,Zeliminations or homolytic bond ruptures.11 Nevertheless, Wysocki and Ross have shown that alternative mechanisms, involving the loss of radicals instead of alkanes or alkenes, can simultaneously operate under specific conditions.12 Such pathways account for the elimination of CnHzn+l units upon CAD of precursor ions with very low initial internal energy, such as alkylpyridinium cations formed by chemical ionization. Most recently, Contado et al.
-
+
(1) McLaffcrty, F. W.; Turccck, F. Interpretation of Mass Spectra, 4th 4.; University Science Books: Mill Valley, CA, 1993. (2) Adams, J. Mass Spctrom. R N . 1990, 9, 141-186. (3) Gross, M. L. Int. J . Mass Spectrom. Ion Processes 1992,118/119,137-165.
(4) Adams, J.; Gross, M.L. Anal. Chem. 1987,59, 1576-1582. ( 5 ) Jensen, N.; Tomer, K. B.; Gross, M. L. J . Am. Chem. Soc. 1985,107,18631868. (6) McLaffcrty, F. W.; Bente, P. F., 111; Kornfeld, R.;Tsai, S.-C.; Howc,I. J . Am. Chem. Soc. 1973.95,2120-2129. (7) Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass Spctrometry/Muss Spectrometry; VCH Publishers, Inc.: New York, 1988.
I
(8) Sun, K. K.; Haym, H.W.; Holman, R. T. Org. Mass Spectrom. 1970, 3, 1035-1042. (9) Adams, J.; Gross, M. L. J . Am. Chem. Soc. 1989,111,435440. (IO) Wysocki,V. H.; Ross, M. M.;Homing, S.R.;Cooks, R. G. Rupid Commun. Mass Spctrom. 1988, 2, 214-216. (11) Siegel, M.M.; Colthup, N. B. Appl. Spectrosc. 1988, 42, 1214-1221. ( 12) Wysocki,V. H.; Ross. M.M.Int. J. Mass Spctrom. Ion Processes 1991,101, 179-21 1.
o o o ~ ~ ~ o o ~ ~ ~ ~ o ~ ~ ~ a ~ i ~ o ~ . ~ o ~ o Anal)rticelChemistty, Vd. 66, No. 6, Wrch 15, 1004 861 Q 1094 Amarkan Chemical sockty
Table 1. Studied Fatty Acids
name undecanoicacid
structure A
O
H
L
O
H
formula (mass, Da)
CiiHnO2
undecylenic acid
palmitic acid
CisHa2Oz (256)
palmitoleic acid AOH
stearic acid
+
do ~leHSeO2 n (284)
confirmed that unsaturated fatty acid ions can undergo losses of radicals by cleavages at the allylic bonds.13 An unequivocal answer about the identity of the neutral products from charge-remote fragmentations can be obtained by direct examination of these neutrals. This is possible with neutralization-reionization mass spectrometry (NRMS).l6l7 With this technique, the neutral products eliminated upon CAD of precursor ions with kiloelectronvolt kinetic energies can be postionized and detected in neutral fragmentreionization (NfR) mass spectra.18 NfR spectra consist of the superimposed collision-induced dissociative ionization (CIDI) mass s p e ~ t r a lof~ the , ~ ~individual neutral losses. So far, CIDI and NfR spectra have been used successfully to determine the structures of the neutral losses and thus elucidate the decomposition mechanisms of aniline and methyl acetate cations21v22 and, more recently, of protonated peptide^.^^^^^.^^ The present investigation employs NRMS to achieve the first experimental characterization of the neutrals eliminated upon high-energy CAD of FAB-generated fatty acid ions. The fatty acids studied include the saturated undecanoic (Cll), myristic (c14), palmitic (c16), and stearic (CIS) acids as well as the monounsaturated undecylenic (Cl1) and palmitoleic (c16) acids. Their structures are summarized in Table 1.
EXPER I MENTAL SECTION The experiments were performed with a modified VG AutoSpec tandem mass spectrometer that has been described (13) Contado, M. J.; Adams, J.; Jensen, N.J.; Gross, M.L. J. Am. Soc. Muss Spctrom. 1991, 2, 180-183. (14) Wesdemiotis, C.; McLafferty, F. W. Chem. Reus 1987, 87, 485-500. (15) Terlouw, J. K.;Schwarz, H. Angew. Chem., Int. Ed. Engl. 1987,26,808-815. (16) Holmes, J. L. Mass Spectrom. Rev. 1989,8,513-539. (17) McLafferty, F. W. Science 1990, 247, 925-929. (18) Cordero, M. M.; Houser, J. J.; Wesdemiotis, C. Anu/.Chem. 1992,65, 1594-
1601.
(19) Burgers, P. C.; Holmes, J. L.; Mommers, A. A,; Szulejko, J, E.;Terlouw, J. K. Org. Mass Spectrom. 1984, 19,442447. (20) Clair, R.; Holmes, J. L.; Mommers, A. A.; Burgers, P. C. Org. MussSpecfrom. 1985,20,207-212. (21) (a) Burgers, P. C.; Holmes, J. L.; Mommers, A. A,; Terlouw, J. K. Chem. Phys. Lett. 1983, 102, 1-3. (b) Denim, P. 0.;Wesdemiotis, C.; McLafferty, F. W. J . Am. Chem. SOC.1983,105,7454-7456. (22) (a) Burgers, P.C.; Holmes, J. L.; Hop, C. E. C. A,; Terlouw, J. K. Org. Muss Spectrom. 1986, 21, 549-555. (b) Wcademiotia, C.; Feng, R.; Danis, P. 0.; William& E.R.; McLafferty, F. W. Org. Mass Spectrom. 1986,21,689-695. (c) Holmes, J. L.;Hop, C. E. C. A.;Terlouw, J. K. Org. MassSpectrom. 1986, 21,776-178. (23) Cordero, M. M.;Wdemiotis, C. In Blologlcal Mass Spectrometry;Caprioli, R.,Gross, M.L., Matsuo, T., Seyama, Y.,Eds.;John Wiley & Sons,Ltd.: Chichester, UK, 1994; Chapter 2.5, pp 119-126. (24) Cordero, M. M.; Wdemiotii, C. Org. MussSpectrom. 1993,28,1041-1046.
862
in detail.25 In this EBE instrument, the first two sectors serve as MS-1 and the second electric sector serves as MS-2. One collisioncell (Cls-1) is located in the field-free region following the ion source and two (Cls-2, Cls-3) in the field-free region between MS-1 and MS-2. The last two collision cells are separated by a deflector electrode that can be floated up to f 5 kV. The fatty acids were ionized by FAB using a cesium ion gun operated at -20-keV. [M - HI- or [M - H 2Li]+ precursor ions were mass-selected by MS-1 and subjected to dissociating collisions with He in Cls-2. The ionic fragments generated were mass-analyzed by MS-2 and recorded in CAD spectra. For the detection of the complementary neutral fragments, all ions exiting Cls-2 were electrostatically deflected and the remaining neutral beam was reionized to cations by collisionswith 0 2 in Cls-3. Mass analysis of the newly formed cations through MS-2 lead to the respective -NrR+ or +NrR+ He/O2 spectra. The acronym NfR stands for neutral fragment-reionization and the superscripts indicate the charges of the precursor ion and the ultimate product ions, respectively. The +NfR+He/O2 spectrum of cetyltrimethylammonium, the only non-fatty acid ion studied, was obtained similarly by generating this cation via FAB ionization of cetyltrimethylammonium chloride. Reference mass spectra for alkanes and alkenes were obtained by neutralization-reionization (NR) of the corresponding molecular cations. For example, the +NR+Xe/O2 spectrum of 1-heptene (M) resulted after charge exchange neutralization of M'+ with Xe and reionization of M with 0 2 . Reference mass spectra of alkyl radicals were acquired by reionizing the radical mixtures eliminated upon CAD of cationized amines.26 For example, +NfR+He/02 of the radical cation of 1-dodecylamine yielded a spectrum containing the convoluted CIDI spectra of CnH2n+1 radicals, n = 2-11. Absolute abundances, Le., NR (or NfR) yields, werecalculated by dividing the total ion current in the NR (or NfR) spectra by the intensity of the precursor ion. The volatile compounds were ionized by electron impact (EI) at 70 eV, and all precursor ions were accelerated to 8 kV before mass selection by MS-1. He, Xe, and 0 2 were the dissociation, neutralization, and reionization targets, respectively. The pressure of each gas was adjusted to achieve a 30%reduction of the precursor ion intensity. Triethanolamine served as matrix for [M - HI- desorption and a saturated solution of LiI in 3-nitrobenzyl alcohol as matrix for [M - H + 2Li]+ production. The fatty acids were added directly to the matrix. With cetyltrimethylammonium chloride, a droplet of a -25% water solution was applied directly on the FAB probe tip and allowed to evaporate before introduction of the probe into the instrument. The spectra shown are multiscan summations combining 20-500 scans, and the reproducibility of relative abundances is better than f20%. The -.+NfR+and +NR+spectra display only the m / z range that contains discernible products. Approximately 5-10 pmol of total sample was required to obtain spectra with adequate signal-to-noise ratio; in some
Analyticel Chemlstty, Vol. 66,No. 6,Merch 15, 1994
M. J.; Cordero, M.M.;Wksdemiotis, C.; Bott, P. A. Int. J. Mass (26) Audier, H.E.: Milliet, A.;Sozzi, 0.;Denhez, J. P. Org. MussSpctrom. 1984, 19, 79-8 1, and references therein. (25) Polce,
Spectrom. Ion Processes 1992, 113, 35-38.
10
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t
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60
27
40 97
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20
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-
Figwr 1. CAD spectra of [M HI- from (a) undecanolc and (b) palmitic acids. The nominal C,,H2,,+2losses are marked by and glve rise to fragments with centroids at mlz 71 and QQ 14x; (a) x = 0-5 for undecanoic and (b) 0-10 for palmltlc acid.
+
cases it was necessary to suspend data acquisition in order to load more sample before continuing with more scans. All compounds used are available from Sigma and/or Aldrich and were introduced into the mass spectrometerwithout further purification.
20
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Flguro 2. -F(rR+ spectra of [M HI- from (a) undecanolc and (b) palmitic acids. The marked mlz values correspond to the alkenyl bn series C,Ji2,-,+.
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RESULTS AND DISCUSSION Neutral losses must be postionized before they can be detected and characterized. Most suitable for this purpose is ionization into cations; conversion into anions is much less useful owing to its 10-1000 times poorer effi~iency.~'Therefore, all neutral fragment-reionization experiments described employ reionization to positive ions. [M- Hr Anions from SaturatedFatty Acids. The majority of the fragment ions present in the CAD spectra of the saturated [M - HI- anions formally arise by eliminations of CnH2n+2 unit^.^-^ This is illustrated for undecanoic and palmitic acids in Figure 1, in which the nominal CnHzn+2 losses are marked by asterisks. The only fragments not belonging to this series are the products of H2O loss (at m/z 167 and 237, respectively) and the ions at m / z 58 and 86 which correspond toeliminationsofCnH2n+l. Theabundances of the latter two fragments are small in respect to the total fragment ion currents, confirming that the cleavages of CnH2,,+lradicals are of minor importance in high-energy CAD of FAB-generated ions. Analogous results are obtained for myristic and stearic acids. Obviously, all saturated [M -HIanions mainly cleave CnH2,+2 moieties, which could consist of alkanes, alkyl radicals plus 'H, or alkenes plus H2. After reionization into cations, the neutral CAD fragments from [M - HI- lead to the -NfR+ spectra shown in Figures 2 and 3. Expectedly, as the size of the fatty acid increases, the abundances of the higher mass ions also increase. Nonetheless, the -NfR+ spectra are dominated by relatively small ions. According to the CAD spectra, the largest CnH/,,+2 entity eliminated from [M - HI- is CgHlg (114 Da) for undecanoic acid, CllH24 (156 Da) for myristic acid, C13H28 (184 Da) for palmitic acid, and ClsH32 (212 Da) for stearic acid; yet the largest ions observed in the -NfR+ spectra are at m/z 91-97. The extensive fragmentation of the neutral losses is mainly due to the fact that collisional ionization deposits high average internal energies (hard process), paralleling E1 (vide infra).28 (27) Wesdemiotis, C.; Fcng, R. Org. Mass Sprctrom. 1988.23.416-418. (28) Danis, P. 0.;Fcng, R.; McLaffcrty, F. W. Anal. Chem. 1986, 58, 355-358.
0
io
-
210
do
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510
610
810
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-
Flgurr 3. -NP+ spectra of [M HI- from (a) myristic and (b) stearic acids. The marked m/z values correspond to the alkenyl ion series C,,H2,r+. x300.00
x
I
60 '
m s ; \ { h do' ' 1bO 110
w 2b'
'
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-
Flguro 4. CAD spectra of [M HI- from (a) undecylenic and (b) palmitolelc acids. The nominal C,,H~urc~ and Cfia ksses are marked by and#, re8pecthrely. These losseoproducefragmentswlth cenlro#s at mlz (a) 71 and QQ 14x (x = 0-4) for undecylenic acid and (b) 14x (#, x = 0-3) plus 167 4- 14y (*, y = 0-5) for 71 and QQ palmitoielc acM.20
+
+
Neutral fragment-reionization produces significant ions at m/z27,41,55, and 69 (Figures 2 and 3), which belong to the alkenyl ion series CnH2-1+. The alkyl ion series CnH2n++1+ ( m / z29,43,57, etc.) is markedly weaker. In E1 mass spectra, a more abundant alkenyl than alkyl series diagnoses the presence of alkenes.' Thus, it seems that the main neutral losses upon CAD of fatty acid carboxylates are mixtures of alkenes (plus Hz), in agreement with the postulate of Scheme 1. In order to substantiate this preliminary conclusion, it must be verified that the collisional ionization of alkanes, alkyls, and alkenes also leads to characteristic differences in the abundances of the alkyl vs alkenyl series (vide infra). [M -HI-Anions from Unsaturated Fatty Acids. The CAD spectra of [M - HI- from monounsaturated fatty acids have been observed to present a discontinuity or "gap" in their fragment ion abundance pattern? This is shown in Figure 4 for undecylenic and palmitoleic acids, which contain a terminal and an internal olefinic bond, respectively. For both, the CAD products arising from the unfavorable cleavages of the vinylic bonds and the double bond itself have low abundances,while Anelytlcal Chemktty, Voi. 66, No. 6,M r c h 15, 1994
063
100% 804
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i
Flgure 5. -Np+ spectra of [M palmitolelc acids. 100'-
- HI-
AI
from (a) undecylenic and (b)
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their larger kinetic energies, which warrant less scattering losses and a significantly better transmission e f f i c i e n ~ y . ~ ~ J ~ * ~ ~ Although [M - HI- anions yield a larger amount of small neutral fragments upon CAD than [M - H + 2Li]+ cations, these fragments are preferentially discriminated against and cannot visibly change the -,+NfR+spectra. The difference in the proportions of the larger neutral losses from [M - HI-and [M - H 2Li]+ does also not affect the appearance of the -,+NfR+ spectra since hydrocarbon species of the same homologous series yield comparable product ions irrespective of exact size (vide infra).' Thus, the major reasons for the similarity between the -,+NfR+spectra of [M - HI- and [M - H 2Li]+ are that these spectra (i) do not depend on the abundance of small neutral losses and (ii) primarily contain the large neutral losses which yield common types of fragment ions. The bias against neutral fragments of low mass parallels the discrimination against low-mass ions in kiloelectronvolt CAD, which has been well d ~ c u m e n t e d . ~The . ~ ~effect on neutrals is more eve re,^^,^^ as they cannot be directed or focused. Such mass discrimination also accounts for the inability todetect any H2 (or H') coproduced upon dissociation. The resemblance of the -.+NfR+spectra of fatty acid [M - HI-anions and [M - H 2Li]+ cations indicates that both precursors produce the same kind of neutral fragments. This independence on the nature of the charge site is consistent with the charge-remote mechanism of Scheme 1. Comparison to Reference Spectra of Alkanes, Alkenes, and Alkyls. To ascertain whether the principal neutral losses from charge-remote fragmentations are alkanes, alkenes, or alkyl radicals, it is necessary to establish reference mass spectra for these species by collision-induceddissociativeionization. With alkanes and alkenes, this can be achieved by neutralizationreionization of the corresponding molecular cations. Figures 8 and 9 show the +NR+ spectra for the alkanes heptane and dodecane and the alkenes 1-heptene and 1-dodecene, respectively. For comparison, the corresponding E1 mass spectra are included as insets.32 The +NR+spectra of tridecane and 1-tridecene were also measured and are virtually identical to those of Figures 8b and 9b. Nominal ClzH24/C13Hza are among the largest neutral fragments possible from the studied
+
- +
Figure 8. CAD spectra of [M H 2Li]+ from (a) palmitic and (b) palmitolelc acids. The nominal C,442&2and losses are marked by and #, respecthrely. These lossesproduce fragments wlth centroids at mlz (a) 85 14x (x = 0-12) for palmitic acM and (b) 85 14x (#, x = 0-5) plus 181 14y (*, y = 0-5) for paimitoleic acid.
+
+
+
the signals resulting from allylic bond ruptures are enhanced, thus causing the aforementioned "gap". Introduction of a double bond in the alkyl chain should also affect the neutral losses, making them on average more unsaturated. Indeed, this is mirrored by the -NfR+ spectra of undecylenic and palmitoleic acids, which are substantially distinct from those of undecanoic and palmitic acid, viz. Figures 5 vs 2. The spectra for undecylenic and palmitoleic acids are dominated by more unsaturated ions, such as m / z 39, 54, 67, and 81, which are characteristic for dienes.1J9 [M- H 2Li]+ Cations. The CAD products of the [M -H 2Li]+ cations from palmitic and palmitoleic acids (Figure 6) are completely analogous to those observed from the respective [M - HI- anions (Figures l b and 4b). Confirming earlier documented trends: the relative abundances of the low-mass fragments are higher for the cationic precursors. One would, therefore, expect that the neutral mixtures generated upon CAD of [M - H 2Li]+ contain a greater proportion of large neutrals than the mixtures eliminated from [M - HI-. However, the corresponding +,-NfR+spectra are strikingly similar, viz. Figure 7a vs 2b and 7b vs 5b. Parallel results are obtained with all other fatty acids studied. This apparent discrepancy, Le., the observation of different abundance patterns in the CAD spectra of [M - HI-and [M -H 2Li]+ but not in the corresponding -,+NfR+spectra, is caused in part by mass discrimination effects. Generally, the heavier neutral fragments30dominate NfR spectra due to
+
+
+
+
+
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~~
(30) Based on the neutral fragments dominating the +NfR+spectra of protonated
(29) An ion of m / r 182 is also present in the CAD spectrum of Figure 4b. This fragment corresponds to the allylic cleavage of C4H9-.13 The abundance of this ion is small in respect to the total CAD fragment ion current. Because of this and the relatively light mass of C4H9' (see next section), this radical docs not provide any noticeable contribution to the respective -N,R+ spectrum ([C4Hs+] is minuscule in Figure Sb).
804 AneWCal Chembtry, Vol. 66, No. 6, M r c h 15, 1994
a 'heavy" neutral loss contains Z'/J of the mass of the precursor ion. Smaller fragments do not contribute appreciably to the spcctmm, even if their reionization efficiencies are similar to those of the larger fragments. (31) Li, X.; Harrison, A. G. J . Am. Chem. SOC.1993, 115,6327-6332. (32) McLafferty, F. W.; Stauffer, D. B. Wi/ey/NBS Registry of Mass Spectral Dora; Wiley: New York, 1989.
m,
b X10
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2 5 50 I5
125
175
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Figure 8. +NR+ spectra of Ma+from (a) heptane and (b) dodecane. The marked mlz values correspond to the alkyl Ion serles Cfi2,+1+. Inset are the respecttve E1 spectra.
, - ,
,
130
,
,
,
,
,
,
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140 150 1kO
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Figure 10. (a)CADand (b)+N,R+spectraof M*+from ldodecylamlne. The +N,R+ spectrum of Ionized 1-trldecylamine contains very slmilar peak patterns. Nominal Cfi2hl losses are marked In the CAD spectrum, and the alkyl Ion serles is indicated In the +Np+ spectrum.
The alkyl ions at m / z 71 and 85 are minuscule in the +NR+ spectra of alkenes (Figure 9); they are also negligibly small in the -,+NfR+ spectra of the fatty acid ions, particularly in regard to other fragments within the correspondingpeak groups (Figures 2,3, and 7a). Hence, intact alkanes arenot eliminated during the charge-remote fragmentation of fatty acid [M HI- anions or [M - H + 2Li]+ cations. The reference spectra of heptene and dodecene (Figure 9) Figure 0. +NR+ spectra of M*+from (a) 1-hepteneand (b) ldodecene. include the alkenyl ion series CnH2,1+ ( m / z 27, 41, 55, 69, The marked m/z values correspond to the alkenyl serles Cfi261+ or the alkene series C,&'+. The E1spectra of l h p t e n eand l-dodecene 83,97) as well as the alkene ion series CnHZn*+( m / z 28,42, are shown as Insets. 56,70, 84,98), both of which are characteristic for olefins.' These series are present in the neutral fragment-reionization spectra of the saturated fatty acid ions (viz. Figures 2,3, and fatty acid ions, whereas C7H14 represents a considerably 7a). The more intense alkenyl ion series is clearly discernible smaller loss. in the -,+NfR+spectra, while the alkene ion series appears as Neutralization of the investigatedalkane and alkene cations a nonresolved shoulder. Undoubtedly, the neutral mixtures with Xe is end other mi^.^^ In this case, the intermediate arising upon CAD of saturated fatty acid ions contain alkenes, neutrals are formed in their ground electronic states but can corroborating the mechanism of Scheme 1. The marked have some vibrational excitation, depending on the Francksimilarity of the +NR+ spectrum of dodecene (or tridecene) Condon factor of the charge exchange process.14-16*34 Such with Figures 2 , 3 and 7a confirms the mentioned domination excitation is usually insufficient for unimolecular decay, as of -l+NfR+ spectra by the large neutral losses. The poorer has been demonstrated for the neutralization of methane, resolution in the-.+NfR+ spectra, as compared to the reference ethane, and butene ions by both experiment and t h e ~ r y . ~ ~ ~ ~ ~ +NR+ spectra of alkenes, is mainly due to the reverse activation Consequently, the fragments observed in the +NR+ spectra energy of charge-remote fragmentations." The amount of of Figures 8 and 9 mainly result upon reionization. The this energy released into translational modes upon dissociation extensive dissociation observed in these +NR+spectra indicates causes spreads in the kinetic energies of the eliminated alkenes that collision-induced dissociative ionization is a hard ionwhich in turn broaden the signals of the resulting -*+NfR+ ization method.28 Compared to EI, +NR+ substantially spectra. decreases the abundances of M'+ and increases those of highly The types of ions arising upon the collisional ionization of unsaturated fragments, which require large critical energies.33 alkyl radicals can be assessed from reference +NfR+spectra Thus, CIDI imparts higher average internal energies than of amine radical cations. Ionized amines are known to cleave electron impact. This could explain why higher mass fragment CnHln+l moieties upon CAD,26as illustrated for l-dodecyions, which are less energy demanding, are less intense or even lamine in Figure loa. CIDI of such losses leads to the absent from the +NR+ spectra. appearance of a notable alkyl ion series, including m / z 29,43, It is important to notice that the alkyl ion series CnHzn+l+ 57,7 1, and 85 (Figures lob). The ions of m / z 7 1 and 85 have ( m / z 29, 43, 57, 71, 85) is more abundant for alkanes than substantial relative abundances in Figure 10b but are minute for alkenes under both E1 and +NR+ conditions. The in the +NfR+ spectra of the investigatedfatty acid ions (Figures differences are most pronounced for the heavier cations of Consequently, radicals do not constitute a 3, and 7a). 2, m / z 57 and, especially, m / z 7 1 and 85 which for alkanes have significant component of the neutral losses from charge-remote sizable abundances within the peak groups C5H2-I1+ ( m l z fragmentations of multikiioelectronvolt carboxylate or dil62-71) and CaH4_13+( m l z 76-85), respectively (Figure 8). ithiated precursor ions. (33) Lias, S.G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Lcvin, R . D.; According to Figures 8b and 1Ob, the alkyl ion seriesextends Mallard, W. G. J . Phys. Chem. Ref. Data 1988, 17, Suppl. 1. to m / z 85 in both the+NR+spectrum of dodecane and +NfR+ (34) Gellene, G.I.; Porter, R. F. Acc. Chem. Res. 1983, 16, 200-207. spectrum of 1-dodecylamine. Larger ions are barely dis( 3 5 ) Hop, C. E. C. A.;Holmes. J. L.; Wong, M. W.; Radom, L. Chem. Phys. Lett. 1989, 1-59, 580-586. cernable in the former reference spectrum but the latter (36)Feng, R.; Wcsdemiotis, C.; Zhang, M.-Y.; Marchetti, M.; McLafferty, F. W. displays peaks beyond m / z 85. These heavier products must J . Am. Chem. Sw. 1989,111, 1986-1991.
9"
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Analytical Chemistty, Vol. 66, No. 6, March 15, lGG4
865
-
originate from the decomposition C12H27N0+(dodecylamine) H,N+ ( m / z 18) C12H23' (167 Da), which liberates the smallest ionic and largest neutral CAD fragment. C12H23', formally an alkenyl radical, generates after ionization a small but remarkable C12H23+cation ( m / z 167) and most probably also contributes to several other unsaturated high-mass fragments in the +NfR+spectrum of Figure lob. It is noteworthy that theunsaturated radical C12H23. yields a recovery peak and high-mass fragments. If such radicals were eliminated to a large extent from the unsaturated fatty acids studied, then similar high-mass ions ought to be present in the corresponding --+NfR+spectra (Figure 5 ) . This is not the case, in keeping with predominance of the mechanism of Scheme 1 also for the unsaturated fatty acid ions. The absolute abundances (Le., +NR+ or +NfR+yields) in the referencespectraof ionized tridecane ( m / z 184), tridecene ( m / z 182), and dodecylamine ( m / z 185) are similar (- l P 3 , f50%), consistent with comparable collisional ionization efficiencies for alkanes, alkenes, and alkyls from precursors of similar ~ i z e . 3 ~Thus, alkane and alkyl losses would be detectable, if produced in appreciableamounts together with the observed alkenes. Their absence from the -l+NfR+spectra of fatty acid ions provides convincing evidence that they are not important neutral losses in charge-remotefragmentations at kiloelectronvolt energies. Competition with Charge-Initiated Processes. The very small abundances of Li+ and Liz+ in the CAD and +NfR+ spectra of [M - H 2Li]+ (Figures 6 and 7) confirm that decompositions at the charge site are of minor importancefor these precursor ions. The high stabilization of the positive charge at the dilithiated carboxylatecenter obviously hinders its participation in dissociations. Less stabilized cations can undergo competitive, not charge-remote fragmentations; an example is the cetylammonium cation, (CH3)3N+(CHz)lsCH3. Its high-energy CAD spectrum (Figure 1la) includes products from charge-remotefragmentationsbut also several alkyl cations (namely, m/z 43, 57, 71, and 85). The latter can only be formed if the amino substituent is lostn3* The +NfR+ spectrum of cetylammonium (Figure 11b) corroborates the presence of two types of dissociations. The alkene losses from charge-remote fragmentationsgive rise to the alkenyl/alkene ion pattern also observed for fatty acids (see, e.g., Figures 2b and 7a). On the other hand, the charge-
+
+
~
~~~~
(37) The relative *NR+ effcimcia of n-C12H$+, I-C1zHu'+, n-C13Ha.+, and I-C13H& are 0.7/0.6/0.8/1.0; the absolute value for I-C13Hr'+ is 8 X 10-4 (+SO%). The *NfR* efficiency of n-CIIHuNH$+ (precursor of alkyls) also is 8 X 10-4. At the transmittan- wed, 5 4 0 % of the mass-selected precursor ion beam b converted to neutraWuJ Hence, the collirional ionization efficiencia are of the order of 1 0 - V (38) The obrved alkyl ion, am formally generated by 1- of (CH3)sN followed by low ofCJi1, Thewidepeakat m/z 57-58 may a h indudesome(CHJ)~N* ( m / z 59) comrponding to nominal 1- of C1a3,'; (CHI),"+ may also arise by conrscutive cleavaga from the larger fragments. (39) Such product ions dominate the reference +NR+ spectrum of (CH3),N'+: BeranovB, $.; Wademiotis, C. Inf. J. Muss Specfrom. Ion Rocemcs, in p r a .
-
000
Am!~U~aIchemistry, Vd. 66, No. 6,March 15, 1994
n
40
20
27
0
20
20
$0
(0
50
do
7'0
8'0
do
1bO
li0
HjZ
Flgwo 11. (a) CAD and (b) +Np+spectra of the cetyhmmonium cation. The nomlnal losses are marked by ' and the atkyl cations by A. The denote ions with centrolds at mlz 100 14x, x = 0-12.
+
initiated loss of (CH3)3N is documented by the additional products (CH3)3N'+ ( m / z 5 9 ) and (CHs)zN+=CH2 ( m / z 58).39
CONCLUSIONS NRMS methods allow for the identification of the neutral products from unimolecular decay and can thus provide unique insight about enigmatic reaction mechanisms. The experiments of this study show that the charge-remote fragmentation of saturated hydrocarbon chains at kiloelectronvolt energies leads to elimination of alkenes, in agreement with the originally proposed 1,4-H2 elimination mechanism (Scheme l),2-s Alkanes and alkyl radicals are not eliminated to any measurableextent. Carboxylate anions or dilithiated cations from fatty acids, which contain ideally stabilized charge sites, primarily undergo charge-remotedissociations. In contrast, quaternaryammonium cations also decompose by competitive, charge-initiated processes, presumably because trimethylamine represents a good leaving group. Collisional cationization of alkanes and alkenes deposits higher internal energies than electron impact with the result that +NR+ or +*-NfR+spectra are dominated by low-mass products. Nevertheless, CIDI supplies types of characteristic fragment ion series similar to EI, thus permitting spectral interpretation based on well-established criteria from E1 studies.' ACKNOWLEDGMENT The National Institutes of Health are thanked for generous financial support. We acknowledge partial support of this work by the Ohio Board of Regents and the University of Akron. We thank Michael J. Polce, &rka Beranovit, and Fred A. Wiedmann for experimental assistance and helpful discussions. Received for review September 15, 1993. Accepted January 3, 1994.' Abstract published in Aduonce ACS Absfrocfs, February 1 , 1994.
