Anal. Chem. 1988, 60,2084-2089
2084
Isomer Identification by Metastable Decay Rates of Laser-Produced Ions Gary R. Kinsel a n d M u r r a y V. Johnston*
Department of Chemistry and Biochemistry, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309-0215 A new method for Isomer ldentlflcatlon is presented. Metastable Ions are produced by resonance enhanced multlphoton lonlratlon In a llnear tlmeof-flight mass spectrometer. From the shapes and lntensltles of metastable tails in the mass spectra of 0-, m-,and pthioroanlllne, parent ion decay rates for formation of m / z 91, 92, 100, and 102 are calculated. These values are different for each compound. I n contrast to slmple measurements of relatlve ion abundances, decay rate measurements provide additional structural and mechanistic Information Including whether or not the respective parent Ions are rearranging to a common form prior to fragmentation. Potentlal analytlcal appllcatlons of this technlque are dlscussed.
Distinguishing among different isomeric configurations of an organic molecule by mass spectrometry can be difficult. The 70-eV electron impact (EI) mass spectra of ortho-, meta-, and para-substituted benzenes are in many cases virtually identical. Knowledge of the exact isomeric configuration can be of great importance since different isomeric configurations may have widely varying toxicological properties and environmental impacts. Various mass spectrometric techniques have been developed to distinguish among the different isomers of a molecule. In E1 mass spectra, differences in either fragment ion abundances (1) or the kinetic energy released to the fragment ion (2-4) can be used to identify a particular isomer. Each can be augmented by incorporating photodissociation (5-7) or collision induced dissociation (8, 9) steps into the procedure. These measurements can be extremely sensitive to the internal energy of the precursor ion. Both of these approaches will fail if small differences in the fragmentation dynamics of different isomers are masked by a broad internal energy distribution of the ensemble of parent ions produced ( 1 0 , l l )or if parent ions of different configurations isomerize to a common structure or set of structures prior to fragmentation. We present evidence here that resonance enhanced multiphoton ionization (REMPI) combined with a linear time-of-flight mass spectrometer (TOF-MS) can also be used for isomer identification. In contrast to the previously mentioned techniques, REMPI creates a much narrower distribution of parent ion internal energies that can provide the required sensitivity to detect small changes in the fragmentation energetics of different isomers. Originally, it was believed that REMPI mass spectra could not be used for isomer identification since specific isomer pairs exhibited identical REMPI mass spectra (12,13). Recent work by our group, however, has shown that CTH80 and C8H,,O configurational isomers can indeed be distinguished on the basis of their REMPI mass spectra (14). These compounds can also be distinguished by 70-eV E1 mass spectra and by daughter ion collision induced dissociation spectra. In another report, three isomers of iodobutane were found to give distinguishable REMPI mass spectra even though their 70-eV E1 mass spectra are identical (15). In this case, differences * Author to whom correspondence should be sent
between REMPI and E1 fragmentation were attributed to the lack of isomerization during the REMPI process. In this paper, we show that 0-,m-, and p-chloroaniline can be distinguished on the basis of parent ion decay rates for metastable ions produced by REMPI even though these compounds cannot be distinguished on the basis of their 70-eV E1 mass spectra. Decay rates are extracted by deconvoluting the overlapping tails of metastably broadened mass peaks from a linear TOF-MS. As in conventional mass spectrometry where kinetic energy release measurements provide additional structural information over that obtained from fragment ion abundances alone, metastable decay rate measurements in REMPI-TOF-MS also provide additional structural and mechanistic information including whether or not the respective parent ions are rearranging to some uniform precursor configuration prior to fragmentation. In this paper, we will consider a special case of REMPI where the parent ion is produced by a resonant two photon ionization. Ionization occurs by the sequential absorption of two photons through a resonant intermediate electronic state. The parent ion is created with an unknown amount of internal energy that depends on the amount of energy removed with the ejected electron. The uncertainty in the internal energy of the parent ion can be confined to a “small’! outer boundary equal to the difference between the energy of the two photons and the ionization energy (IE) of the molecule, plus the molecule’s initial thermal internal energy. This outer boundary is usually much narrower than those encountered in E1 ionization or in collision induced dissociation and typically ranges from ca. 0 to 1.5 eV when using radiation in the near-ultraviolet regime to ionize simple aromatic molecules. The actual internal energy distribution of the parent ion ensemble may be much smaller than the outer bound. Photoelectron spectroscopy studies have shown that in many cases parent ion ensembles are created with very narrow internal energy distributions (15-19). This result is a consequence of selection rules governing Franck-Condon allowed transitions in resonant two photon ionization. The narrow internal energy distributions encountered in REMPI cause the observed decay rates for primary fragmentation processes (i.e. those produced by a direct one photon dissociation of the parent ion) to be extremely sensitive to small differences in the fragmentation energetics of similar molecular systems. This aspect has recently been demonstrated quite dramatically by Kuhlewind and co-workers in their investigation of inter- and intramolecular isotope effects on the unimolecular fragmentation of benzene and its deuteriated analogues (20). There are several possible reasons for primary daughter ions of different isomeric parent ions to be formed at different rates during the REMPI process. These are illustrated with the aid of hypothetical unimolecular decay rate vs internal energy ( k vs E ) curves for parent ion fragmentation (Figure 1). One possibility is that the parent ion of each isomer is created with a different internal energy content (Figure 1A). This difference could arise from different ionization energies or heats of formation of the precursor compounds or by selection rules governing which vibrational states of the parent ion are populated by resonant two photon ionization. Even if the
0003-2700/88/0360-2084$01.50/0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
2085
Table I. Energetic Data for REMPI of 0 - , m-, p-Chloroaniliie at 266 nm o-chloro- m-chloro- p-chloroaniline aniline aniline
A
w
b-
e
a
11
'2
IE AE (M-Cl) AE - IE (M - C1) AE (M - HNC) AE - IE (M - HNC) max parent ion ink energy after 2 photon ionization (2hv-IE) max parent ion int energy distribution after photon absorption (hv to 3hv-IE) a
INTERNAL ENERGY
Figure 1. Hypothetlcal k vs E curves for isomeric parent ions I, and I,. The horizontal arrows represent absorption of a photon by the parent ion to reach a higher internal energy. (A) Initial internal energies of I, and I2 are different. (B) Threshold energies for fragmentation of I1 and I, are different. (C) k vs E curves for I1 and I, are different. fragmentation dynamics are identical for parent ions produced from different precursor molecules, the decay rates will be different since the internal energies are different. This suggests that parent ions that isomerize to a common structure prior to fragmentation may also be distinguishableby REMPI although an example of such a situation has not been documented. A second possibility is that the appearance energies (AE's) for the primary fragmentation processes change as a function of molecular structure (Figure 1B). Even if the initial internal energy of the parent ions and the shape of the k vs E curves for the isomers are identical, a small change in the AE for fragment ion formation can result in a significant change in the observed rate. The largest changes will be observed in the threshold region where metastable ions are produced. A final possibility is that the k vs E curve for each isomer is slightly different (Figure IC). An example of this situation would be "ortho effect" rearrangements involving neighboring groups on an aromatic ring. These reactions are more probable when the substituents are located ortho to each other rather than para or meta. As with the second possibility, small differences in the k vs E curves will most likely be accentuated in the threshold region where metastable ions are produced. It is not possible to determine conclusively which of these conditions apply to the chloroaniline isomers and it is quite possible that all contribute to the observed differences in the metastable decay rates. However, if any of the above three conditions are met for a particular set of isomeric compounds, then REMPI-TOF-MS performed at a wavelength that produces metastable decompositions for the primary fragment ions should produce these ions at different rates and therefore allow the isomers to be distinguished.
EXPERIMENTAL SECTION Our experimentalapparatus has already been discussed in detail (21)and will only be briefly reviewed here. The fourth harmonic of a NdYAG laser at 266 nm is focused with a 15 cm focal length cylindrical lens into the source region of a linear TOF-MS. Laser-produced ions are accelerated out of the source region using
8.09O
7.9b
12.25' 4.16' 1.4
12.04' 3.95' 1.23 4.66-5.89
4.7-6.1
8.W
12.50' 4.32' 12.300 4.12' 1.14
4.66-5.80
Reference 22. Reference 23.
a continuous drawout voltage and separate in space and time in a 1-m field-free drift tube. The ion current is detected by a dual microchannel plate detector (R. M. Jordan Co.) and sent to a LeCroy 100-MHztransient digithr/signal averager. The acquired data may be manipulated with the 3500 SA operating system of the LeCroy or sent to a PC-XT for calculation of the metastable decay rates. Since the total residence time of m l z 100 ions in the source and acceleration regions of our mass analyzer is on the order of 1.5 ps, total parent ion decay rates on the order of 5 X lo4 to 1 X lo7 s-l can be readily measured for this mlz region. All samples were introduced into the mass spectrometer via a room-temperature molecular leak inlet. 2-Chloroaniline (>99.5% pure), 3-chloroaniline(ca. 99% pure), and 4-chloroaniline(>99% pure) were purchased from Fluka and used without further purification.
RESULTS AND DISCUSSION REMPI Fragmentation of the Chloroaniline Isomers. Pertinent energetic information for REMPI fragmentation of the chloroanilineisomers is given in Table I. In each case, it is possible to undergo a resonantly enhanced two photon ionization a t 266 nm. Previous work in our group with pchloroaniline has shown that the parent ion is indeed formed predominantly by this mechanism (21).The upper limit for the internal energy of a parent ion created by two photon ionization is given by 2hv - IE if the initial thermal internal energy is neglected. For p-chloroaniline, the outer boundaries of the internal energy distribution for an ensemble of parent ions produced by 266-nm radiation are 0-1.14 eV. Similar values for 0- and m-chloroaniline are given in Table I. The actual internal energy distributions may be significantly smaller than these ranges. Photoelectron spectroscopy studies performed on both chlorobenzene and aniline have shown that under similar ionization conditions these parent ions are created with a relatively narrow distribution of internal energies (16,17).Although we are not able to confirm that this is also the case for the chloroaniline isomers, it is likely that the internal energy distributions for the chloroaniline parent ions are much smaller than the outer boundaries. Parent ion dissociation proceeds by the absorption of additional photons. We have already shown for p-chloroaniline that m/z 91 and 92 (loss of C1 and HC1 from both m/z 127 and 1291,100and 102 (loss of HNC from either m / z 127 or 129) are the only ions formed by a one-photon dissociation of the respective parent ions at 266 nm (21). Reaction pathways are .CI or -HCI CgHgN+'+ CgHgN''
