ANALYTICAL CHEMISTRY, VOL. 51, NO 4, APRIL 1979
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Electron Impact Excitation of Ions from Organics: An Alternative to Collision Induced Dissociation R. B. Cody and B. S. Freiser' Department of Chemistry, Purdue University, West La fayette, Indiana
A technique has been developed for electron Impact excitation of ions from organics (EIEIO). The technique uses trapped ion cyclotron resonance (ICR) spectroscopy and together with the ICR photochemical techniques and unusual capabilities for studying ion-molecule reactions, will aid in the evolution of ICR as an analytical instrument. I t is demonstrated on a variety of substituted benzene radical cations. Ions are generated and subsequently excited In a continuous electron beam while being trapped in the source region. The spectra obtained by EIEIO are shown to be analogous to those obtained by collision induced dissociation and yield characteristic structural information.
Obtaining the fragmentation pattern generated from ionization of a neutral sample is typically the primary objective in conventional mass spectrometry. A great deal of additional information, however, is available by further probing the structures of individual ions in the mass spectrum. Such information is of obvious importance in fundamental studies of unimolecular ion and ion-molecule reactions and in separation and identification of complex mixtures ( 1 , 2 ) . T o achieve these ends, a variety of innovative methods have been developed including collision induced dissociation (CID) ( 1 4 ) and photodissociation (PDS) (7-13). These techniques are similar in t h a t both probe the ion structure by monitoring unimolecular dissociation following excitation, with the major difference being the mode of excitation. In CID, also referred to as collisional activation (CA),the unimolecular dissociation of a n ion is monitored following excitation of the ion by collision with a target gas (Process 1).
M
+
AB+ [AB+]* A+ B (1) In PDS, unimolecular dissociation is monitored following light absorption by the ion (process 2). hv
+
-
+
-
+
AB+ [AB+]* A+ B (2) Interestingly, photodissociation pathways need not mimic those observed by CID (6). I n this paper we report an extension of a new method utilizing ICR (14, 15) in which unimolecular dissociation is monitored following excitation by electron impact (process 3) in analogy t o CID and PDS. e-
AB' [AB+]* A+ B (3) Until recently quantitative as well as qualitative studies of electron impact dissociative excitation of ions were limited and N2+(16-18). We wish to diatomics such as Hz+,Dz+,02+ t o report that ICR-EIEIO techniques appear to hold not only considerable promise for routine investigation of these fundamental processes for polyatomic ions, but also provide ICR with important CID-like capabilities. +
EXPERIMENTAL The ion cyclotron resonance spectrometer used in the present study is a modified V-5900 series manufactured by Varian Associates (19, 20). The ICR cell is one which has been used extensively for studies of ion photoexcitation processes as described 0003-2700/79/035 1-0547$01 .OO/O
47907
in detail (7-11) and required no modification. Parent ions from the substituted benzenes (in the pressure range from 1-10 x Torr) were formed and subsequently excited in a continuous electron beam while being trapped in the source region. Ion formation was accomplished by switching the electron energy to a value lying a few electron volts above the ionization potential for 15 ms. For the remainder of the trapping period, the electron energy remained below the ionization potential at a value which could be varied readily. All of the electron energies given have been corrected for the space charge potential of the electron beam which has the effect of reducing the electron energy. This correction can be substantial at high emission currents and low electron energies (14). These effects also prevented data from being obtained below about 2 eV because of instability in the current. Each of the chemicals used was a commercial sample of high purity with the exception of the labeled phenetole, and was used as supplied except for freeze-pump-thaw cycles to remove noncondensable gases. Mass spectrometry revealed no detectable impurities.
RESULTS The phenomenon of EIEIO is particularly evident in appearance potential measurements a t high emission currents as illustrated by Figure 1,which shows the intensity of C&+ generated from cyanobenzene as a function of electron energy at a trapping time of 100 ms. At low emission currents, the fragment ion C6H4+,arising from loss of HCN from the parent ion, appears a t about 3.5 eV above the ionization potential of the parent neutral. This appearance potential measurement is consistent with the estimated thermodynamic threshold (14). At higher emission currents, however, CfiH4+begins to appear with the parent cation a t the ionization potential which, significantly, is not affected by emission current, eliminating the possibility of an artifact arising from electron energy distribution broadening. T h e striking behavior of C&+ is attributed to EIEIO reaction 4.
As expected this reaction becomes even more apparent both as the emission current is increased (Figure l),and as the trapping time is increased as illustrated in Figure 2 which shows the temporal variation of CfiH4+ and CsH5CN+obtained at an electron excitation energy of 7.5 eV and at an emission current of 5 FA. I t is evident from the above results that mass spectra a t a particular electron energy will also be affected by emission current and trapping conditions. Figures 3 and 4 compare the conventional ICR single resonance drift spectra of trifluorotoluene, n-propylbenzene, and isopropylbenzene to their corresponding EIEIO spectra obtained at trapping times of 100 ms. Again the appearance of fragment ions in the EIEIO spectra are apparent, arising from reactions 5-7 (see Table I),
e-
C6H5CF3+
products
(5)
products
(6)
e-
CfiH5C3H,+
e-
C6H,CH(CH3)2+ 0 1979 American Chemical Society
products
(7)
548
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
- I
I
Figure 1. Appearance potential measurements on cyanobenzene at different electron emission currents. As emission current is increased, the fragment ion C6H,+ ( m l z 76) begins to appear with the parent ion ( m l z 103) at the ionization potential. This behavior is attributed to EIEIO reaction 4 in text M/z IO5
i
I I
Z
W
t
Figure 4. (a) Single resonance spectrum of n-propylbenzene at low emission currents. (b) Single resonance spectrum of n-propylbenzene under EIEIO conditions. (c) Single resonance spectrum of isopropylbenzene under EIEIO conditions
w
I t is of interest to compare the EIEIO spectra obtained by ICR to the corresponding CID spectra. A direct comparison is made on phenetole radical cation in Figure 5. The labeled compounds used are involved in another study in our laboratory and were chosen for this experiment because of availability rather than for any mechanistic purpose. T h e major fragments arising from EIEIO of C6H,0C2D,+(m/z 127) a t 72 ms trapping time shown in Figure 5b include loss of C2D4 and loss of C3D40to form C6H@D+ (m/z 95) and C5H5D+ (m/z 67), respectively. As shown in Figure 5c, t h e corresponding ions also appear as the major components in the CID spectrum of C6H50CD2CH3+ (m/z 124). Several other ions also appear in the CID spectrum, however, which are not observed in the EIEIO spectrum including, for example, loss of the ethoxy moiety to form C&+ ( m / z 77). In Figure 6a the EIEIO spectrum of C6H50CzD5+is repeated a t a n increased trapping time of 432 ms. The two major fragment ions, C6H50D+and C a 5 D +are again evident as well as the lower mass species m / z 39 and 40, but the ion corresponding to ethoxy loss is still not observable. Also evident in an examination of Figure 6a is that the CSH,D+ intensity has increased relative to that of C&@D+. While Figure 6b is a repetition of the CID spectrum from Figure 5c of C6Hs0CD2CH3+for comparison, a straightforward explanation for the increased C5H5D+intensity can be derived from a comparison of the EIEIO spectrum in Figure 6a to the CID spectrum of C6Hs0H+ shown in Figure 6c. T h e CID spectrum of C&150H+ shows loss of CO and C3H30to produce C5H6+ and C3H3+,respectively, as the predominant fragmentation pathways (21). The corresponding products for the labeled phenol ion, C6H50D+,would be C5H5D+and both C3H3+and C3HzD+all of which are observed in Figure 6a. Thus, it is apparent t h a t C6H50D+having excess internal energy is being produced and subsequently fragmenting, and
LT l
0
l
20
l
I
I
I
/
l
60 (msec)
80
40
!
130
Fjgure 2. Temporal variation of ions involved in the process C6H,CN+ -%C6H,+ -I-HCN obtained at an electron excitation energy of 7.5 eV
and at an emission current of 5 MA 1
'J
2
L
-
I
I V Z ' V
V L L '
L---L-
Y,
I
Figure 3. (a) Single resonance spectrum of trifluorotoluene at 100-ms trapping time, 14-eV and 3.5eV electron energy ion formation and base voltages, respectively, and 3-pA emission. (b) Same conditions as (a) at 7.2-eV base voltage
whereas at the same electron energies, only the parent cations are observed in the conventional single resonance spectra.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
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3 CR -
c 5pA
IC
CR-EIEIO ~-
5O p A
J L
Figure 5. (a) Single resonance spectrum of labeled phenetole at low emission current. (b) Single resonance spectrum of labeled phenetole under EIEIO conditions. (c) CID spectrum of labeled phenetole
Table I. Comparison of Photodissociation, Electron Impact Excitation, and Collision Induced or Metastable Dissociation of Benzene Radical Cations CID/ ion PDS EIEIO metastable [toluene ]’ -H -H, ‘ZH3 -H [n-propylbenzene]’ -C2H, -C,H,, C,H, -C,H,, C,H, [isopropylbenzene]’ -CH, -CH3, C,H,, -CH,, C,H, C.H-5--,
[p-diethylbenzene]’ -H -H, C,H, -H, CF,, -H, F, CF, [trifluorotoluene]’
-C,H, -H, F , CF,
[p-fluorotrifluorotoluene ]’ [p-chlorotrifluorotoluene 1’ [ benzonitrile ]’ [benzaldehyde]+
-H, F, CF,
-H, F, CF,
-c1
-c1
F
-H, F, CF, -C1
-HCN Xa
)
C I D -C&OH
Figure 6 . (a) ICR-EIEIO spectrum of labeled phenetole. (b) CID spectrum of labeled phenetole. (c) CID spectrum of phenol. See text for explanation
The energy of the exciting electrons, as expected, has a profound effect on both the product ion yields, i.e., the EIEIO cross section, and on the product distribution in cases where more than one ion can be observed. Figures 7 and 8 demonstrate these features. Figure 7, reported in an earlier study ( 1 4 ) , compares the relative dissociation cross section for process 4 as a function of excitation energy to both the photoelectron spectrum of cyanobenzene reported by Rabalais and Colton (22) and to the photodissociation spectrum of C6H5CN+reported by Orlowski et al. (10). The energy axis of the photoelectron spectrum is adjusted such that the first vertical ionization potential of cyanobenzene is zero on the photodissociation and electron impact dissociation scales. The photodissociation spectrum (Figure 7b) consists of a band at high energy obtained by monitoring process 8 CGHSCN’
[phenetole ]’
4
2
3
+
+ hv
-
CGH4+
X
=
(8)
and a band at low energy attributed to the two photon process (9).
a
+ HCN
2
data not available.
it is surmised t h a t the phenol ion produced by EIEIO from phenetole ion is itself undergoing EIEIO fragmentation. EIEIO spectra were obtained from a variety of substituted benzenes and the data are summarized in Table I. Also included in Table I are the corresponding results obtained by photodissociation and collision induced dissociation (or metastable) experiments where available for comparison.
hu
hv
C G H ~ C NF? + [C,H,CN+]*
c~H4+ + HCN
(9)
Comparison of the photodissociation spectra to the photoelectron spectrum indicates that the high energy band arises P* transition and the lower energy absorption from a P P transition (10). proceeds through a P The cross section for electron impact dissociation of C6H5CN+(Figure 7a) is observed to rise from a threshold of
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
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