Anal. Chem. 1989, 6 1 , 1889-1894
LITERATURE CITED Epstein, S.; Yapp, C. J.; Hall, J. H. Earth Planet. Sci. Lett. 1978,30, 24 1-25 1. Brenninkmeijer, C. A. M.; Geel, E.; Mook, W. G. €arfh Planet. Sci. Lett. 1982, 67,283-290. Wedeking, K. W.; Hayes, J. M. Isot. Geosci. 1983, 7 , 357-370. Doner, L. W.; Ajie, H. 0.;Sternberg, L. da S. L.; Milburn, J. M.; DeNiro, M. J.; Hlcks, K. B. J . Agrlc. FoodChem. 1987,35,610-612. Epstein. S.: Thompson, P.; Yapp, C. J. Science 1977, 798, 1207-1 215. Sternberg, L. S.; DeNiro, M. J. Science 1983,220, 947-949. Schimmelmann, A.; DeNiro, M. J. Geochim. Cosmochim. Acta 1988, 50, 1485-1496. Schimmelmann, A.; DeNiro, M. J.; Poulicek, M.; Voss-Foucart, M.-F.; Qoffinet, G.; Jeunianx, Ch. J . Archaeol. Sci. 1988, 73,553-566. Santrock, J.; Hayes, J. M. Anal. Chem. lB87, 59, 119-127. Hardcastie, K. G.; Friedman, I . Geophys. Res. Left. 1974, 7 , 165-167.
1889
(11) Rlttenberg, D.; Ponticorvo, L. I n t . J . Appi. Radlat. Isof. 1958, 208 -2 14. (12) Schimmelmann, A.; DeNiro, M. J. Anal. Chem. 1885,57.2644-2646. (13) Milburn, J. M.: DeNiro, M. J., unpublished work, University of California, Los Angeles, 1985. (14) Mook, W. G.; Grootes, P. M. Int. J . Mass Spectrom. Ion Phys. 1973, 72, 273-298. (15) Clayton, R. N.; Epstein, S. J . Geoi. 1958,66,352-373. (16) Thompson, P.; Gray, J. I n t . J . Appi. Radiat. Isot. 1977, 2 8 , 41 1-415.
RECEIVED for review February 21, 1989. Accepted May 24, 1989. This work was supported by National Science Foundation Grants EAR 85-04096, BNS 84-18280, and DMB 8405003. This is contribution number 4706 from the Division of Geological and Planetary Sciences at Caltech.
Isomer Discrimination of Disubstituted Benzene Derivatives through Gas-Phase Iron(1) Ion Reactions in a Fourier Transform Mass Spectrometer Asgeir Bjarnason’ and James W. Taylor* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 James A. Kinsinger,2 Robert B. Cody, and David A. Weil Nicolet Analytical Instruments, Madison, Wisconsin 5371 1
Gas-phase reactions of Fe’ with the Isomers of several dlsubstituted benzene derlvatives were studied in a Fourier transform mass spectrometer. The electron impact mass spectra of these compounds are generally too similar for routine isomer identification. The iron ion complexes and their fragments produce spectra that reveal lsomer differentiation. The method presented here fundamentally relies on the ability of Fe’ to form a brldge between the two substituents. Where such a brldglng reaction Is observed, an ion unique to that lsomer Is produced and identifies the Isomer. I n all the cases studled but one (xylene), the ortho Isomer can easily be ldenttfied, and In some cases ail three isomers can be dlfferentlated wlth thls method. Pressure variations within the normal operating range of the mass spectrometer were found not to interfere wlth the Isomer identification.
INTRODUCTION Gas-phase reactions of metal ions with neutral molecules, most commonly small organic compounds, have received considerable interest in recent years. The study of these reactions became particularily feasible and appealing with the advent of Fourier transform mass spectrometers. Freiser and co-workers (1-8)produced metal ions by focusing a laser beam onto a piece of the metal in the Fourier transform mass spectrometer, and this method has been used by other workers although alternative methods for metal ion generation exist, Present address: Science Institute, University of Iceland, D u n h a a 3, IS-107 Reykjavik, Iceland. 5Present address: Industrial Labcxatories, Denver, CO.
such as electron impact on volatile metal carbonyls and heating metal salts (9,lO). As pointed out by Freiser (3),the laser ionization has several advantages over other methods for Fourier transform mass spectrometry (FT-MS). The method is convenient and “clean”, and because FT-MS is inherently a pulsed technique, the use of a pulsed ionization source is straightforward. Many of the volatile carbonyls are poisonous and the neutral carbonyl may be highly reactive with the sample and/or the ions present and may thus complicate analysis. A drawback to laser ionization is that it produces metal ions with an unknown distribution of electronic states such that an unknown, and possibly significant, fraction of the ions may be in excited states. A few researchers have commented on the selectivity of metal ions as chemical ionization agents (3-9, 11). Limited effort appears to have been made to demonstrate or harness the analytical potential of these types of reactions with the exception of the elegant work by Gross and co-workers (9), who found that Fe+ could be used to locate double bonds in olefins, and the work of Forbes et al. (7, 8) on pattern recognition methods for metal ion chemical ionization mass spectra. The study reported here is an effort to explore the method of Fe+ chemical ionization in the gas phase for the analytical purpose of distinguishing between isomers of disubstituted benzene derivatives. The criteria for their selection in this preliminary study were that (a) the electron impact (EI) spectra of the isomers were too similar for routine discrimination and (b) the compounds were volatile. (Methods for analyzing isomers of solid samples are under investigation now.) The E1 spectra of the chosen compounds show the same ions for all three isomers. Occasionally, some intensity var-
0003-2700/89/0361-1889$01.50/00 1989 American Chemical Society
1890
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
iations may be shown between the isomers in library spectra, but these intensity variations can be instrument dependent and are generally not well suited for routine isomer discrimination. Relying on small variations in peak intensities for isomer identification is difficult when using hyphenated techniques, such as gas chromatography/mass spectrometry, due to impurities that may be present from previous eluents. The method presented here usually relies on an abundance of a characteristic ion which is present in the spectrum of one isomer but shows very low intensity or is absent in the spectra of the other isomers. EXPERIMENTAL SECTION All measurements were done by use of a Nicolet FTMS-2000 Fourier transform mass spectrometer with a 3.0-T superconducting magnet and equipped with a Nicolet laser desorption interface. The laser was a Tachisto Model 215 C02 laser operating as an aperture-controlled stable resonator. Maximum output energy for this arrangement is on the order of 1J in a 40 ns wide (fwhm) pulse. Typical output energy used in these experiments was estimated to be ca. 0.01-0.05 J/puke. Metal ions were made by focusing the laser pulses onto the stainless steel tip of the direct insertion probe. As pointed out by Jacobson et al. (5), it is good practice to isolate the laser-produced iron ions by ejecting all other ions prior to monitoring the reactions in order to eliminate effects of other, uncharacterized species which are produced by the interaction of the laser pulse with the metal target. It is clear that an abundance of electrons of unknown energies is produced, and they may ionize neutral species present in the FT-MS cell and thereby interfere with the analysis. It was found that although negative ions are abundantly produced for most species containing highly electronegative atoms, positive ions were not produced by these laser-generated electrons except in the case of compounds containing iodine (e.g. iodobenzene). Nevertheless, a triple resonance experimental sequence was used that ejected all ions except iron prior to the variable delay (reaction time). This sequence also eliminated the other metal ions from the stainless steel target (mostly Cr, Mn, and Ni) and made the analysis of the spectra more straightforward. The triple resonance experimental sequence is part of the standard software provided by Nicolet, but briefly, the sequence of events in the experiment is as follows. Following the generation of the metal ions by the laser pulse, Fe+ is isolated by employing two ejection events that eject all other ions from the cell. The iron ions are stored in the cell during a programmable delay event and react with the sample. The ions generated in the primary reactions of Fe+ with the sample react again with the neutral sample molecules and the product ions of these secondary reactions react again and so on. A particular ion may be isolated by using another set of ejection pulses after an appropriate reaction time when its intensity is relatively high. The reactions of this ion with the neutral sample molecules can then be monitored by using a second variable delay event as a reaction time. Reaction paths can be elucidated in this manner, and exact mass measurements aid in the identification of each ion. The mass measurement accuracy achieved was on the order of 10 ppm. The pressure of the reagent gas was generally -3 X Torr but was varied for a few test compounds from 3 x IO4 to 1 x 106 Torr. The identification process described below could be employed over the entire pressure range. Reaction time was varied from 0.003 to 25.6 s. Normally, ten transients were collected and averaged to minimize effects of laser power fluctuations. The effect of the laser power output was of interest, especially on the relative intensity of the ions produced in the ion-molecule reactions. Varying the laser power resulted in only very slight changes in the relative intensity of the observed ions, except for M+ (the molecular ion), whose intensity increased with increased laser power. This, presumably is because of increased charge transfer reactions. At higher laser power the average kinetic energy of the iron ions produced is increased. This was supported by examining the total intensity of Fe+ at different trapping voltages as the laser power was increased. Larger trapping potentials were required to trap the majority of iron ions produced as the laser power was increased. These observations are in agreement with the results of Kang and Beauchamp (12) who studied the rela-
2,
2 - METHOXYPHENOL
Flgure 1. Mass spectra obtained after reactions of Fe+ with the (a) ortho, (b) meta. and (c) para isomers of methoxyphenol for 0.4 s. Pressure of the neutral methoxyphenol gas in the cell was 8 X lo-' Torr in each case. tionship between laser power and kinetic energy of her-generated metal ions. It was observed in our study that the molecular ion was produced not only in charge transfer reactions with kinetically excited Fe+ but also, to some extent, through charge transfer reactions with some of the product ions from the Fe+ reactions with the sample-presumably those produced in collisions with iron ions of higher kinetic energies. The use of relatively low laser energy in this study and low trapping voltages minimized effects of excess kinetic energy. The internal energy distribution of the laser desorbed iron ions was of concern. The reactions of a few samples were analyzed while maintaining a pressure of nitrogen collision gas at 10 times the pressure of the reagent gas with both present in the cell during the reaction period. No significant differences were observed between the mass spectra obtained with or without the nitrogen gas present. This suggests either that a very small fraction of the iron ions are in excited states or that the reactions of Fe+ in the different excited states are identical with those in the ground state. The compounds investigated in this study were the isomers of xylene, methoxyphenol (Figure l),fluoroacetophenone (Figure 2), fluoroanisole (Figure 3), and methylanisole. AU reagents were commercially obtained and used without further purification except for a few freeze-pump-thaw cycles to remove dissolved gases. RESULTS AND DISCUSSION The spectra obtained for the reactions of Fe+ with disubstituted benzene derivatives reveal that for analytical purposes these reactions can be separated into three classes. The first class exhibits complex reactions, and identification of each isomer can be readily accomplished. The second class exhibits very similar reactions of the meta and para isomers which are too similar for routine discrimination. The ortho isomer, however, can be easily distinguished from the other two, as its reactions are vastly different. The third class, which only
ANALYTICAL CHEMISTRY, VOL.
61, NO. 17, SEPTEMBER 1, 1989 Fe
3 - FLUOROACETOPHENONE
I.
OH
Ii
2-
1891
i
I
la
I
lb
\ /
_I
W E
0
0 MASS I N A M U FLUOROACETOPHENONE
4 -
o0 I
'y
t
b o
*
b o
- CH4
Id
IC
rniz 164
Figure 4. A proposed reaction mechanism for the reaction of Fe+ with
o -methoxyphenol.
0
MASS I N A M U
Figure 2. Mass spectra obtained after reactions (followingthe isolation) of the ionic species at m / z 146 with the (a)meta and (b) para isomers of fluoroacetophenone for 1.0 s (see text). >.- 2 - F L U O R O A N I S O L E
t-u
W
> H
t-
6 -JO
150 3
t.
HO
w
m
!
+
200
210
[Fe(M-HF)]+
.
d
' 4
+O
170 180 190 MASS I N A M U
160
- FLUOROANISOLE
-
'
z
.
~
110 ' 1 MASS I N A M U FLUOROANISOLE 120
i-
[Fe(M-HF)]+
>
H
-I
l
150
160
190 170 180 MASS I N A M U
C6H400Fe+ mlz 164
+ CH4
(1)
A possible mechanism for this reaction is presented in Figure 4. Fe+ attaches to either oxygen and the proximity of the
Q 10
E
-
Fe+ + o-C6H4(OH)(OCH3)
second oxygen atom results in bridge forming with elimination of methane. The meta isomer shows two dominant primary reactions (eq 2 and 3). Mechanisms for these reactions are presented
W HO Lo W
the sample usually differentiate immediately between the ortho isomer and the other two. In those cases where differentiation of the meta and para isomers is possible, the differences sometimes appear in the primary reactions in some cases in secondary or higher order reactions. In the discussion below, we present several speculative reaction mechanism schemes in an attempt to explain why isomer differentiation is achieved with this technique. It should be emphasized that the reaction schemes are not directly supported by experimental results, but they are believed to be reasonable in view of the reaction products and the fact that isomer specificity is observed. Methoxyphenol. It should be noted that library E1 mass spectra for the isomers of methoxyphenol vary from one source to another and identification should not be attempted without obtaining mass spectra for the pure compounds first. According to the Wiley/NBS Mass Spectral Database (3rd ed.) the E1 mass spectrum of the ortho isomer of methoxyphenol can be used for identification due to the high abundance of the ionic species at m / z 109. The meta and the para E1 spectra are too similar for routine identification. All three isomers of methoxyphenol could be differentiated through primary Fe+ reactions. The main reaction for the ortho isomer proceeds according to
200
Fe(C6H5(0H))+ + CH20
210
Flgure 3. Mass spectra obtained after the reactions of Fe' with the (a) ortho, (b) meta, and (c) para isomers of fluoroanisole for 0.4 s.
Pressure of the neutral fluoroanisole gas in the cell was 3 X lo-' Torr in each case. xylene is representative of those compounds studied, showed very simple reactions and very similar reaction rates for all three isomers so that none of the isomers could be identified. It appears that electronegative substituents and, in particular, those containing oxygen facilitate the greatest variety in reactions with Fe+. Further, the primary reactions of Fe+ with
Fe'
+
I:
(2)
M6H4(0H)(OCH3)
m/z150 C6H4(0)(0CH2)Fe+ + HZ (3) m/z 178
in Figure 5. Reaction 2 is analogous to the main reaction of Fe+ with anisole (13). After attachment to the oxygen of the methoxy group, the Fe+ can insert into the bonds on either side of the oxygen. If it inserts on the phenyl side, elimination of formaldehyde (CH20) follows and the ion labeled 2c is formed. After attachment to the oxygen of the hydroxyl group, the bridge to the carbon of the methoxy group can form with
L
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
1892
OH
1
/F:-H
OH
?
OH I
Fe+
I
OCH3 OCH3
2c
Be
H'
OH
t
/
- H P
OCH3 2e
mz
Figure 6. A proposed reaction mechanism for the reaction of Fe+ with
178
Flgure 5. A proposed reaction mechanism for the reaction of Fe+ with rn-methoxyphenol.
the elimination of hydrogen (H,) forming the ion labeled 2e. The reactions of Fe+ with the para isomer yield mainly two reaction products of interest, according to reactions 4 and 5.
&
CFe'
+ p
OCH3
miz 162
€
Fe(C6H5(OH))+ + CHPO (4)
m/z 150 Fe(CsHd(0CHd' + Hz0 WZ 162
(5)
A suggested mechanism for reaction 5 is presented in Figure 6. The elimination of water is one of the main reactions of Fe+ with phenol (13) and is only observed here for the para isomer. The elimination of formaldehyde, which is analogous to the reaction of Fe+ with anisole, is the other main reaction here, as was the case with the meta isomer. This is in accordance with the expected behavior. Greater distance between the two functional groups reduces the possibility of Fe+ interacting with both a t the same time. This means that the reactions of the para isomer would be expected to most resemble the reactions of the monosubstituted derivatives. The ortho isomer is the other extreme. The proximity of the two functional groups gives rise to an entirely different behavior, because of the possibility of Fe+ forming a bridge between the two groups. None of the major reactions of the monosubstituted species are observed for the ortho isomer. The meta isomer reactions are in-between the other two. The elimination of formaldehyde is observed as in the case of anisole, but none of the reactions analogous to those of phenol are observed. A reaction path involving bridging between the two functional groups is observed, unique to the meta isomer. To summarize, the ortho isomer reacts to form the ion at mlz 164 but no ions at m f z 150,162, or 178. The meta isomer reacts to form ions a t mlz 150 and 178, but the ion at mlz 162 can only be seen in negligible amounts. The para isomer predominantly reacts to form two ions, at m f z 150 and 162. Figure 1shows the pertinent portion of the spectra for each isomer. The spectra in Figure 1 were all obtained after a reaction time of 0.4 s at a pressure of 8 X lo4 Torr. All spectra show, in addition to the ions discussed above, an ion at m f z 180, which is Fe(M)+,where M stands for the neutral methoxyphenol molecule. From these observations it is clear that isomer differentiation can be accomplished via Fe+ reactions in gas phase.
p -methoxyphenol.
Fluoroacetophenone. This compound is another example where all three isomers can be identified through Fe+ reactions. The ortho isomer can easily be identified through the primary iron ion reactions, but the other two react in a similar manner. A closer examination of the reactions of the meta and para isomers, in particular the secondary reactions, does reveal differences that can be used for identification. The reactions of Fe+ with the ortho isomer proceed according to eq 6 and 7. Suggested mechanisms for these reactions are Fe'
+
o-C&d(F)(C(O)CH3)
c
-
Fe(CsHSF)'
+
CHpCO
(6)
m/z 152 Fe(C6H4(F)CH3)' + CO
(7)
m/z 166
presented in Figure 7. The elimination of CO is observed in the reactions of acetophenone (13)but not the elimination of CH,CO, indicating that the presence of the fluorine facilitates that reaction. The ion a t mlz 166 reacts further to eliminate FeF and form the stable ion (C7H7)+at m / z 91. It is not clear whether the ion at m / z 91 is the tropilium ion (ionization potential (IP) = 6.24 eV (14))or the benzyl cation (IP = 7.20 eV (14)). In either case it has a lower ionization potential than FeF (FeI, for example, has an IP of 7.8 eV (14)) and thus retains the charge upon fragmentation. The ion at mlz 152 reacts further to form FFe(M)+,where M stands for the neutral fluoroacetophenone molecule. These secondary reactions suggest that the structures labeled 4e and 4h may rearrange so that the iron inserts into the C-F bond. The main reactions of the meta and para isomers are shown in reactions 8 and 9. Double resonance studies reveal that Fe(CsH4(F)CH3)' + CO Fe'
+
mOrP-c6H4(F)C(O)CH3
(8)
m/z 166
Fe(CsH3CH3)'
m/z
+
[CO + H F ] (9)
146
the ionic species a t m / z 146 is not produced from the ionic product of reaction 8. Instead, it appears that the metal ion interacts with both functional groups simultaneously to eliminate CO and HF. A similar phenomenon has been observed in a study of the chemistry of Co+ with 1,4-disubstituted butanes by Tsarbopoulos and Allison (15). Despite the long chain, abstraction from both functional groups through
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
r
,Fe
F
0
I
1893
/t
&"""'. Fe+ -ifCH3 F
Fe
F
0 Il-Fe
t
I
F
5a
5b
I t
c
4b
\ / FI
'i'
4c
0
I
F
tft
FeRCCH3
4d
1
. CH2CO
bF{O "CH3
4g
1.
co
F
F
4e
I
4f
m:z 152
4h
mlz 166
Figure 7. A proposed reaction mechanism for the reaction of Fe+ with
o -fluoroacetophenone. the interaction of a single cobalt ion was observed. As with the reactions of the ortho isomer, the elimination of CO is observed analogous with the reactions of acetophenone (13). Reaction 6 is not observed for either the meta or the para isomers and is therefore unique to the ortho isomer. A t longer reaction times, when secondary reactions have occurred, some differences in the spectra of the meta and para isomers become apparent. An example of this is illustrated in Figure 2. The ion at m / z 146 was isolated through the use of double ejection pulses. It was then allowed to react with the neutral fluoroacetophenone for 1 s at a pressure of 3 X lo-' Torr prior to ion detection. Parts a and b of Figure 2 show the mass spectra for the meta and the para isomers, respectively, obtained after the reactions of the ion at m / z 146 (Fe(C,H,)+). Characteristic is the formation of an ion at m / z 236, which is present in high abundance in the meta isomer spectrum but in low abundance in the para spectrum. Exact mass measurements support that the formula for this ion is Fe(C7H&+, but the reaction mechanism is not obvious. Further differences between the meta and the para mass spectra are observed in the abundance of the ionic species at m/z 374, which has the formula Fe(C7H&M+, where M stands for the neutral fluoroacetophenone molecule. Because this ion is produced from Fe(C,Hs)+ at m / z 236, it is observed in high abundance in the mass spectrum of the meta isomer but in minor abundance in the para isomer mass spectrum. The meta and para isomers can be differentiated based on the abundance of the ion species at m / z 236 produced from the isolated ion at m / z 146. Fluoroanisole. The E1 spectra of the isomers of fluoroanisole are indistinguishable and, thus, cannot be used for isomer identification. These isomers represent the second class
5c
5d
mir 167
Figure 8. A proposed reaction mechanism for the reaction of Fe+ with
o-fluoroanisole. of compounds with respect to their reactions with Fe+. The ortho isomer can easily be identified, but, the other two react in a very similar manner making isomer identification difficult. Figure 3 shows the mass spectra obtained after a reaction time of 0.4 s for the ortho, meta, and para isomers. Although the differences between the ortho spectrum and the others are obvious, the most characteristic ions at shorter reaction times are at m / z 167 and 201. Both of these ions are in high abundance in the ortho spectrum but absent or in minor abundance in the spectra of the other isomers. The reaction producing these ions is believed to be Fe+
-
+ C6H4FOCH3
+ CH3'
Fe(C6H4FO)+ m / z 167
(10)
The ion at m/z 167 then reacts again with the neutral molecule to produce an ion at mlz 201 (FFe(M)+),which suggests that the iron may be bonded to the fluorine in the former ion. A possible mechanism for reaction 10 is shown in Figure 8. Dehydrofluorination, analogous to reactions of Fe+ with fluorobenzene (13) is observed for the meta and para isomers to produce (Fe(M - HF)+) at m / z 162 as is elimination of formaldehyde, analogous to the anisole reactions (13) to produce (Fe(M - CH20)+)at m/z 152. In the case of the ortho isomer, the latter reaction is observed, but the former only as a minor side reaction. Identifying the ortho isomer from the other two isomers is the abundance of the ions at m/z 167 and 201. Some variations in the abundance of several ion species can be seen in the spectra of the meta and the para isomers. These differences are not readily recognizable for routine isomer identification. A similar behavior was observed in the case of methylanisole. The ortho isomer reacted uniquely with Fe+ to form a complex ion with the elimination of methane. A suggested mechanism for this reaction is shown in Figure 9. As in the case of fluoroanisole, the iron ion is believed to form a bridge between the functional groups in the case of the ortho isomer, but, the increased distance between these groups in the meta and the para isomers does not allow the formation of such a bridge. Thus, the ortho isomer reacts uniquely and can be identified through the Fe+ reactions, whereas the meta and the para isomers cannot be distinguished. Xylene. The reactions of Fe+ with the xylenes were very simple. Production of Fe(M)+was observed followed by the generation of Fe(M)2+,where M stands for the neutral xylene molecule. A t higher laser power M+ and (M - CH3)+could also be detected. No significant differences between the re-
1894
ANALYTICAL CHEMISTRY, VOL. 61, NO. 17, SEPTEMBER 1, 1989
v Ea
H
6c
6b
miz 162
Flgwe 9. A proposed reaction mechanism for the reaction of Fe' with o -methylanisole.
actions of the isomers could be established. These reactions are analogous to the reactions of toluene (13). Of the compounds studied, only xylene belonged to class three, where none of the isomers could be differentiated. Methoxyphenol and fluoroacetophenone were in class one, where all three isomers could be differentiated. The other compounds, methylanisole and fluoroanisole, belong to class two, where one isomer can readily be differentiated from the other two.
CONCLUSIONS This study on several disubstituted benzene derivatives reveals that the iron-ion reactions can be diagnostic for isomer identification for a number of compounds where the electron impact spectra at 70 eV of these compounds are not. In all but one case (the xylenes) the ortho isomer can easily be identified through these iron-ion/molecule reactions, but in some cases the spectra for the meta and para isomers are too similar for routine discrimination and careful attention and reliance on peak intensity variations are required for identification. The isomer discrimination method presented here fundamentally relies on the ability of Fe+ to form a bridge between the two substituents. Where such a bridging reaction is observed, an ion unique to that isomer is produced and identifies the isomer. Where there is no possibility of such bridging reactions, identification of the isomer must rely on intensity variations between the spectra of the isomers. Although transition-metal ions have the ability to form bonds to more than one functional group and are therefore suitable for this
application, various other ionic species, organic as well as inorganic, singly as well as multiply charged, may prove to be equally or better suited for this purpose. The size, shape, and composition of the reactant ion would be determined by the nature of the isomer system to be identified. On the basis of the results presented here, predictions can be made for what other isomer systems the method may be useful. For example, in the case of dihydroxybenzene one would expect an iron bridge to form between the two oxygens, with the elimination of H2in the case of the ortho isomer. Such a bridge would not be expected either in the case of the meta or the para isomers due to the increased distance between the functional groups and, thus, only the ortho isomer could be identified by using this method. A similar study of the reactions of other transition-metal ions (in some cases doubly charged ions as well as singly charged) is under way to determine if other metals may provide better discrimination in some cases and if a matrix of a several metals may be the best choice for general applications. Also in progress is a systematic study of the reactions of a wide range of isomer systems to reveal the generality of this method.
ACKNOWLEDGMENT A.B. and J.W.T. thank Nicolet Analytical Instruments, and especially Mr. Mark Johnston, for access to the FTMS-2000.
LITERATURE CITED Burnier, R. C.; Cariin, T. C.; Reents, W. D.; Cody, R. 6.; Lengel, R. K.; Freiser, 8. S.J. Am. Chem. SOC. 1979, 101, 7127. Cody, R. 6.; Burnier, R. C.; Reents, W. D.; Carlin, T. J.; McCrery. D. A,: Lenael. R. K.; Freiser, B. S. I n t . J. Mass Smctrom. Ion Phw. 1980, 33, 37. Freiser, B. S. Anal. Chim. Acta 1985, 178, 137. Freiser, B. S. Talanta 1985, 3 2 , 697. Jacobson, D. B.: Byrd. G. D.; Frelser, B. S. Inwg. Chem. 1984, 2 3 , 553.
Buinier, R. C.; Byrd, G. D.; Freiser, 8. S. Anal. Chem. 1880, 5 2 , 1641. Forbes, R. A.; Tews. E. C.; Freiser, B. S.;Wise, M. B.; Perone, S. P. Anal. Chem. 1988, 5 8 , 684. Forbes, R. A.; Tews, E. C.; Huang, Y.; Freiser, B. S.;Perone, S. P. Anal. Chem. 1987, 59, 1937. Peake, D. A.; Cross, M. L. Anal. Chem. 1985, 5 7 , 115. Armentrout, P. 6.; Hcdges, R. V.; Beauchamp, J. L. J. Am. Chem. SOC. 1981, 103, 784. Lombarski, M.; Allison, J. I n t . J. M a s Specbom. Ion Phys. 1983, 49,281. Kang, H.; Beauchamp, J. L. J. Phys. Chem. 1985, 89, 3384. Bjarnason, A.; Taylor, J. W., submitted for publication in J. Am. Chem. SOC. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin. R. D.; Mallard. W. G. J. Phys. Chem. Ref. Data 1888, 17. Suppl. 1. Tsarbopoulos, A.; Allison, J. Organometallics 1984, 3 , 86.
RECEIVED for review December 21, 1988. Accepted June 7, 1989. The authors gratefully acknowledge the support from the Wisconsin Alumni Research Foundation, the Center for X-Ray Lithography, the Olin-Hunt Corporation, The Upjohn Company, and the National Science Foundation through Grant CHE-8508731.
