1206
Anal. Chem. 1881, 63,1205-1209
(2) Sunner, J.; Ikonorou, M. 0.:Kebarle, P. Int. J . Mss Spea?". Ion are first obtained from the semiempirical calculations with ROC. 1966, 82, 221-237. MOPAC. (By way of comparison, ref 10 reports A&(B) = (3) Lacey. M. P.; Keough, T. RaphjCo"un. Mass Spectrom. 1969, 3, -32.5 and AHHt(BH+)= 171.0 kcal/mol.) By using the slopes 46-50. (4) Sunner, J. A.; Kuiatunge, R.; Kebarle, P. AM/. Chem. 1966. 58, and intercepta from Table I1 in eq 3, the AM1 values are 1312-1316. corrected for systematic offsets. A&(B),. (AHAB)MOPAC (5) Vandlver. V. J.; Leasure, C. S.; Elceman, G. A. Int. J . kbss spscbwn. IOn Proc. 1965, 66, 223-238. - intercept}/slope= (-35.2 - 0.5293 kcal/mo1)/1.0267 = -34.8 (6) VoetS, R.; Francois, J.-P.: Martin, J. M. L.: Mullens, J.; Yperman, J.; kcal/mol. Similarly, A&(BH+),,, is corrected to 175.6 Van Poucke, L. C. J . Comput. Chem. 1969, IO, 449-467. kcal/mol. The gas-phase basicity is first calculated from the (7) Kass, S. R. J . Comput. chem. 1990, 1 1 , 94-104. (8) Halim, H.: Heinrich, N.: Koch, W.: Schmldt, J.; Frenking, J. J . Conput. adjusted heats of formation for the unprotonated and proChem. 1966, 7, 93-104. tonated species by using eq 7. GPB(B) = -AHH,(BH+),,. + (9) Stewart, J. J . comput. Chem. 1969. 10, 221-264. (10) Lias, Sharon 0.: Liebman, Joel F.; Levln, Rhoda D. J . phys. Chem. AHHt(B)mm, AHf(H+)+ TAS(1) = -175.55 -34.8 + 365.7 + Ref. Data 1964, 13, 695-808. -7.8 + -0.4 = 147.14 kcal/mol. The final term combines the 11) Grange, Andrew H.; OBrlen, Robert J.: Barofsky. Douglas F. Raphi entropy for the proton (-7.8 kcal/mol) and a second term (-0.4 Commw,. M a SpeCtrom. 1966, 2 . 163-166. 12) Dumdei, Bruce E.; Kenny, Donald V.: Shepson, Paul 6.: KIehdIenst, kcal/mol) for the "trivial" entropy loss. When evaluating the Tadeusz E.; b o , Chris M.: Cupltt, Larry T.: Claxton, Larry D. En-. GPB for a compound not in Table I, the magnitude of the SCI. Technd. 1966, 22, 1493-1498. trivial entropy loss can be estimated from the symmetry 13) DUmdei, Bruce E.; O'Brien, Robert J. FJehre 1964, 311, 248-250. 14) Shepson. P. 8.;Edney. E. 0.;Corse, E. W. J . php. Chem. 1964, 88, changes for the molecule (17,18).In the direct count method 4122-4126. . .__ . .- -. (23),the symmetry change for hydrogen peroxide protonation 15) Dewar. Michael J. S.; Zoebisch, Eve 0.;Heaty, Eamonn, F.; Stewart, James J. P. J . Am. Chem. Soc. 1965, 107. 3902-3909. is approximated by RT In (1/2) = -0.4, where the fraction 1/2 16) Chase, M. W., Jr.: Curnutt, J. L.; Downey, J. R., Jr.; McDonald, R. A.: is based on there being two paths to protonate the molecule Swerud. A. N.: Valenruela, E. A. J . phvs. Chem. Ref. Data 1962. i 1 , 695-940. but four paths for deprotonation. The final step is to correct (17) Moylan. chrlstopher. R.: Brauman, John I.Ann. Rev. phys. Chem. the GPB for systematic offsets, as was done for the enthalpies. 1963, 3 4 , 187-215. GPB(B),,,,, = (147.14 - 17.76 kcal/mo1}/0.9056 = 142.9 (18) Bailey, William F.; Monahan, Audrey, S. J . Chem. E&. 1976, 55, 489-493. kcal/mol. (19) Kessee, R. 0.;Castleman, A. W. Jr. J . phys. Chem. Ret. Deta 1986, Gas-phase basicities are important in many chemical ap15, 1011-1061. (20) Nlcol. Gordon; SunMn, Jan: Kebarle, P. Int. J . Mess Specbwn. Ion plications. These include studies of gas-phase proton-transfer procesSes 1966, 84, 135-155. reactions, evaluation of solvation and hydration effects, and (21) Houriet, R.; Schwarr, H.; Schleyer, P.v.R. Now. J . Chim. 1961, 5, APIMS. Experimentally determined GPBs are not available 505. (22) Hkaoka, K.; Taklmoto, H.; Morise, K. J . Am. chem. Soc. 1966, 108, for many compounds of interest in environmental or atmos5683-5689. pheric chemistry. The ability of semiempirical calculations (23) Bishop, D. M.; Laidler, K. J. J . Chem. mys. 1965, 42, 1688. to predict GPB theoretically to within a few kilocalories of experimental uncertainty should have important implications RECEIVED for review September 10,1990. Accepted March for analytical chemists in a variety of applications. 19, 1991. This work was supported by the Environmental Protection Agency under Grant 811876 with partial support LITERATURE CITED from the National Science Foundation under Grant ATM(1) Sunner, Jan: Gordon, Nlcol; Kebarle, Paul. Ana/. Chem. 1966, 60, 1300- 1307. 8615163.
+
Selective Adduct Formation by Dimethyl Ether Chemical Ionization in a Quadrupole Ion Trap Mass Spectrometer and a Conventional Ion Source Jennifer BrodbeJt,* Chien-Chung Liou, and Tracy Donovan Department of Chemistry, University of Texas a t Austin, Austin, Texas 78712-1167
Dlnwthyl ethw b evaluated as a selective chemkal IoniraUon refor mono- and Mwbattuted oxyaromatk compounds.
INTRODUCTION
The popularity of chemical ionization (I, 2) for routine analytical applications has stimulated interest in the characterization of novel reagent gases for selective and and In a q u a d r u ~ eIontrap mass sP@drometer cornsensitive analysis (3). This growing interest has largely been pard, and adduct Ions are characterized by USlW collkhpromoted by the increasing understanding of fundamental activated dl8s0ciation. M h y l ether mrate both gas-phase ion chemistry, including aspects of the thermofuMlonal QrmP and m b n a i d d l v w UpOn reaction with chemistry, mechanism, and kinetics of i o n / m o l d e reactions. the aromatic ComPOundS in the WadruPOle Ion trap. me In many cases, isomers, including enantiomers (4), can be POMMal WmlVltY attributed to substituent effects. W S distinguished on the basis of differences in their reactive 8eiectlvity is not observed when a conventional chemical behavior. has largely been a failing of electron ionization ionkatkn m c e k used to fonn the adduck. Ovwal, adduct mass spectrometry. For example, in some cases, the electron knr are of greater abundance in the conventional lon source ionization mass spectra of the ortho, meta, and para isomera than In the low-pressure envlronment of the quadrupole ion of bisubstituted aromatic compounds are not distinguishable (5). Moreover, the use of ion/molecule reactions to ionize a trap mass spectrometer.
"*v Of 'Orma under chemkal lonizatlon conditions In a conventional ion source me
ad
0003-2700/91/0363-1205$02.50/0
Q 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 13. JULY 1, 1991
compound of interest often results in concentration of the total ion current among a few structurally relevant product ions rather than the broad distribution of ion current among the many fragment ions typically produced by conventional electron ionization (5). An increasing number of studies have focused on the examination of alternative chemical ionization agents, including both organic (6-14) and metallic (15-18) reagents. The growing interest in characterizing new chemical ionization agents stems not only in part from its analytical utility but also due to the increasing availability and rapidly developing capabilities of two ion-trappinginstruments, Fourier transform ion cyclotron resonance (FTICR) (19)and quadrupole ion trap mass spectrometers (20). These mass spectrometers allow pressure and time control of ion/molecule reactions and offer the ability to do multistage activation (21) and reaction experiments, which allow detailed study of mechanisms and product distributions of ion/molecule reactions. In ion traps (22-251, ion/molecule reactions are conducted typically at much lower pressures than in conventional chemical ionization sources, such as those used with quadrupole and sector instruments. Thus, collisional stabilization of product ions and adducts occurs to a lesser extent in the ion traps, and this may greatly affect the selectivity and sensitivity of various CI reagents for analytical applications. A comparison of the number of collisions an ion undergoes in a conventional chemical ionization source and in a quadrupole ion trap illustrates the difference in collisional stabilization attained in each environment. In a conventional ion source with a reagent gas pressure of 1Torr, an analyte ion will experience approximately 500 collisions with the neutral reagent gas (26) during its residence in the source. Because of the high number of collisions, the ionic population substantially attains equilibrium conditions, and a variety of loosely bound electrostatic complexes may be stabilized sufficiently for subsequent detection. Alternatively, in the quadrupole ion trap environment in which the typical reagent gas pressure is 1.0 X 10" Torr and the helium buffer gas pressure is 1 mTorr, an analyte ion will experience approximately loo0 collisions with helium (26) and only 10 collisions with the neutral reagent gas during a 300-ms period. Overall, the removal of excess internal energy is less efficient in a quadrupole ion trap than in a conventional higher pressure chemical ionization source because collisions with the reagent gas molecules, which are typically molecules with several vibrational modes and higher molecular weights than helium, are much more effective in removing internal energy from an ion than collisions with helium. A detailed investigation of product formation from ion/ molecule reactions of dimethyl ether has been undertaken to explore the reactions with simple aromatic compounds, including both mono- and bisubstituted oxyaromatics. The primary objectives were (1)to determine the analytical utility of this low molecular weight, commercially available gas for functional group selectivity and isomer distinction of simple aromatic compounds and (2) to compare adduct formation from chemical ionization in the recently popularized yet incompletely characterized quadrupole ion trap mass spectrometer to that of a more conventional ion source.
EXPERIMENTAL SECTION Ion/molecule reactions were examined with a Finnigan quadrupole ion trap mass spectrometer (ITMS) (27). A typical ion/molecule reaction sequence was initiated with a short electron ionization pulse, after which a selected reagent ion was isolated by using the appropriate application of dc and radiofrequency voltages. The dc voltages used to isolate an ion were typically -100 V, and the rf voltage was about 600 V at 1.1 MHz. The chosen reagent ion was then allowed to undergo ion/molecule reactions with the neutral analyte for a period of 10-1OOO ms. The
-b
50 40
30 20 10 n "
I
0
,
. 200
400
600
800
Time (ms) Flgure 1.
time in an
Relative abundance of dimethyl ether ions as a function of ITMS instrument.
Table I. Reactions with Dimethyl Ether Ions in an ITMS
Instrument
compd
anisole phenol
benzaldehyde acetophenone
(M + 1)+
(M+ 13)'
(M+ 15)+
+ + + +
+ +
no
no no
no
+ +
product ion spectrum was recorded by using the mass-selective instability mode to eject ions from the trap onto an electron multiplier. Alternatively, a particular product ion was isolated and then collisionallyactivated prior to mass analyais. Collisional activation involved the application of an ac voltage of 0.1-0.7 V(p-p) at the axial frequency of motion of the ion of interest (typically 130 kHz). Typical reagent gas pressures were nominally 1X Torr, and helium buffer gas pressure was 1.0 mTorr. Aromatic compounds were introduced via a leak valve to 8 x lo-' Torr. Typical ion/molecule reactions times were 50 ms, activation times were 4 ms, and activation voltages were about 0.5 V(p-p). For the comparative study, a Finnigan TSQ-70 triple-stage quadrupole mass spectrometer (TQMS) (28)was utilized with reagent gas pressure set at nominally 1Torr. The collision energy was 20 V, and the collision gas (argon)pressure was 2 mTorr in the second quadrupole. Multiple collisions occur under these conditions. RESULTS AND DISCUSSION Reactions with Dimethyl Ether Ions in a Quadrupole Ion Trap. The population of dimethyl ether ions in the ITMS is shown as a function of reaction time in Figwe 1. At reaction times less than 300 ms, m / z 45 (CH2=OCH3)+ and m/z 47 (CH30CH3)H+are dominant, whereas at longer times methylated dimethyl ether, m / z 61, and the proton-bound dimer, m / z 93, increase. The proton-bound dimer likely is a result of the ion/molecule reaction of protonated dimethyl ether, mlz 47+, with neutral dimethyl ether. The majority of the methylated ion (mlz 61) is produced from the reaction of m / z 45 with dimethyl ether, but a small amount (about 10%) is produced from reactions of mlz 47 ions. When these reagent ions are permitted to react with the neutral aromatic compounds introduced into the trap, three types of products are formed (M + H)+, (M + 13)+,and (M + CH3)+(see Table I). The relative ion currents measured for a 50-ms reaction period indicate that the abundance of the protonated ion is typically twice that of the adduct ion. All the aromatic compounds protonate readily, but those with an ether or alcohol functional group preferentially form (M + 13)+,presumably via [M + (CH-HJ - CH30H]+,while those with a carbonyl functionality instead form (M+ 15)+, likely via [M + (CH2=OCH3)- CH,O]+. No monofunctional aromatic compound produced both (M+ 13)+and (M+ 15)+ adducts.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 13, JULY 1, 1091
Table 11. Adduct Formation for Bisubstituted Aromatic Compounds in an ITMS Inrtrument (M + 1)+
compd
p-hydroxyacetophenone
o-methoxyacetophenone m-methoxyacetophenone p-methoxyacetophenone o-methoxyphenol m-methoxyphenol p-methoxyphenol o-hydroxybenzaldehyde m-hydroxybenzaldehyde p-hydroxybenzaldehyde o-anisaldehydeO m-aniaaldehyde p-anisaldehyde o-vanillin vanillin
REACTION OF 4 5 t WITH ANISOLE 45
(M + 13)+ (M + 15)'
+ + + + + + + + + + + + + + + + +
o-hydroxyacetophenone m-hydroxyacetophenone
+
no no no
+ + + + +
no
no no no no no
+
no
40
+ + + + + no +
+
+
00
"
100
'
I
"
.
(
,
I40
120
60 i
K
The ortho, meta, and para isomers of the bifunctional aromatic compounds reactad via either formation of (M 13)+ or (M + 15)+. The only exception was m-methoxyacetophenone, which generated both types of adducts (see Table 11). The abundance of these adducts relative to the protonated ions is typically 2&30%. For those compounds with multiple functional groups, (M + 15)' is produced for the meta and para isomers if at least one of the functionalities is a carbonyl group, while (M 13)+ is formed if the two functional groups are in ortho positions or if neither functionality is a carbonyl group. One final comparison is made for two trisubstituted isomers, o-vanillin and vanillin. Vanillin (3-methoxy-4-hydroxybenzaldehyde) has two substituents in ortho positions and two metal para interactions between the hydroxy and methoxy groups with the carbonyl substituent. Thus, either formation of (M + 15)+ or (M 13)+ could be expected on the basis of the trends established for the bisubstituted aromatics. Reactions of vanillin with dimethyl ether ions result in (M + H)+ and (M 15)+ only, and (M 13)+ is not observed. However, for the o-vanillin (3-methoxy-2-hydroxybenzaldehyde), (M + 13)+ and (M + H)+ are observed, but not (M + 15)+. These results again suggest that the isomeric relationship of the carbonyl functional group to the other Substituents is the most important determinant of the selectivity of adduct formation. Even a favorable ortho interaction of two other functional groups cannot compensate for the carbonyl effect. This positional selectivity is likely due to the deactivating and activating directional propertiea of the substituents toward electrophilic aromatic addition. The carbonyl substituents are meta-directing, deactivating substituents, whereas the hydroxy substituent is a strong orthojpara activator and the methoxy substituent is a moderately strong ortho/ para director. The ortho isomers (o-hydroxybenzaldehydeand ohydroxyacetophenone)have two directors that both enhance the same aromatic position (adjacent to the hydroxy group) for methyne addition. Alternatively, the favored formation of the methyne addition product (M + 13)+ by all the ortho-substituted isomers could be due to an ortho effect, suggesting that the close spatial proximity of the substituents permits a functional group interaction that enhances the elimination of methanol after addition of the methoxymethyl cation to the aromatic species. For the para-substituted isomers, the functional groups are spatially separated, and the favored product is directed by the absence or presence of a carbonyl group. Suitable model compounds are not currently available for structural comparisons.
+
,
60
40
$
+
I21
k,
no no no no
Anisaldehyde is methoxybenzaldehyde.
+
( M+13)+
61
no
+ + + + +
1207
i-
30 20
io 0 0
100
200
300
400
500
600
TIME (ms) Flgure 3. Distribution of ion current from reactions of dimethyl ether ions and anisole in the ITMS instrument as a function of time.
With the selected-ion isolation capabilities of the ITMS, the reactive ions at m/z 45, 47, and 61 were individually isolated from the dimethyl ether ion population and allowed to react with each aromatic compound, and the resulting products were characterized by using collisional activation. The reactive species at m/z 45 is responsible for both the appearance of (M + 13)+ and (M + 15)+, whereas m / z 47 induces only proton transfer, (M + H)+. The ion at m / z 61 induces only a slight amount of protonation of the aromatic substrates. One example is shown in Figure 2. The reaction with anisole results in (M + 13)' at m / z 121. Dissociation of m / z 121 yields fragments at m / z 91 via loss of formaldehyde, 78,77, and 65. The reactions of dimethyl ether with anisole were studied as a function of time in the ITMS to examine the variation of the ion population with time (Figure 3). At very short reactions times (500 ms), usually the sum of the abundances of the adduct ions is less than the protonated ion, and in fact is often only a relatively minor contribution (Le. 25% of ion current). This reduction in absolute adduct formation is likely due to the much lower reagent gas pressure used in the ion trap, typically 5 orders of magnitude lower than in the TSQ source. Thus, not only are reactive ion/molecule collisions
0
6
13 10
10 17 5
much less frequent, but also stabilizing ion/molecule collisions are far less frequent. These combined factors mean that the probability of a reaction encounter occurring is 5 orders of magnitude lower in the ion trap and that a product ion formed with sufficient energy to decompose may do so before being stabilized by many deactivating ionlneutral collisions. However, by increasing the reaction time, there is a greater probability of a reactive collision occurring, so the abundance of adduct ions can be enhanced by using longer storage times. The relative distribution of ions also changes as a function of time in the ITMS; this is due in part to changes in the reagent ion population as a function of time and the increasing probability that reactions between product ions and neutral sample or reagent molecules will occur, resulting in both subsequent reactions and/or neutralization of the initial anal@ ion population. Finally, the mechanism of ion storage for an ion trap depends on the constant application of a varying radio frequency voltage, so that ions are continuously being accelerated and decelerated throughout their residence in the trap. This kinetic energy, although partly diminished by collisions with helium buffer gas, may influence the way in which ions react relative to their reactions in an essentially thermal equilibrium of a high-pressure source. The ITMS has capabilities for mass selection of reagent ions, and this affords an additional degree of ionization selectivity not accessible with conventional CI sources. Thus, it is possible to determine the identities of the reagent ions responsible for production of each type of adduct ion, so mechanisms of adduct formation are more easily determined. Also, ionization conditions can be "tuned" to promote a particular ion/molecule reaction. Such specificity can only be approximated by adjusting reagent gas pressures in the conventional CI sources. Recently, it has been demonstrated that reagent ion selection experiments can be performed by using a triple-quadrupole mass spectrometer (29). For these experiments, the desired reagent ion from the source is mass-selected by using the first quadrupole, then allowed to ionize neutral sample molecules introduced in the second quadrupole. The third quadrupole is used for mass analysis of the resulting product ions. However, product ion structures cannot be characterized by collision-activated dissociation experiments because additional quadrupole stages would be necessary to perform the parent ion selection, activation, and mass-analysis operations. The pressure difference between the two types of mass spectrometers likely accounts for the absence of a substituent positional selectivity of the dimethyl ether reactions in the TQMS. Regardless of the identity of the functional group, both (M + 13)' and (M + 15)+adducts were often observed in the higher pressure CI source. Presumably, this is because the efficiency of collisional stabilization of less energetically favorable adducts (Le. the methyne addition product) is greater in the TQMS than in the low-pressure trap. Product ions formed in the quadrupole ion trap, therefore, are ones that are energetically favorable without collisional removal of excess internal energy. Comparison of the collision-activated dissociation spectra of adduct ions formed in the TSQ instrument and the ITMS for each of the aromatic compounds and each reagent gas
ANALYTICAL CHEMISTRY, VOL.
(M
+
+
H)+
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para isomers preferentially form (M 1 5 ) ' . In a conventional ion source, a greater abundance and variety of adducts is formed with dimethyl ether, but the structural selectivity afforded by using dimethyl ether in the ITMS is not o b ~ e d . In most cases the loosely bound (M + R)+ adducts produced in the conventional chemical ionization source are not observed in the quadrupole ion trap, presumably due to the reduced collisional stabilization in the ion trap environment. Finally, selective ion/molecule reactions can be used to derivatize a molecule to alter its dissociative behavior in order to obtain more structurally informative CAD spectra. Registry No. CH30CH3, 115-10-6; anisole, 100-66-3; phenol, 108-95-2;benzaldehyde, 100-52-7;acetophenone, 98-86-2;ohydroxyacetophenone, 118-93-4; m-hydroxyacetophenone, 12171-1;p-hydroxyacetophenone,99-93-4; o-methoxyacetophenone, 579-74-8; m-methoxyacetophenone, 586-37-8; p-methoxyacetophenone, 100-06-1; o-methoxyphenol,90-05-1;m-methoxyphenol, 150-19-6; p-methoxyphenol, 150-76-5; o-hydroxybenzaldehyde, 90-02-8;m-hydroxybenzaldehyde, 100-83-4;p-hydroxybenzaldehyde, 123-08-0; o-anisaldehyde, 135-02-4; m-anisaldehyde, 591-31-1; p-anisaldehyde, 123-11-5; o-vanillin, 148-53-8; vanillin, 121-33-5.
LITERATURE CITED Figurr 4. Comparison of CAD spectra for (M + 1)' and (M + 15)' adducts from reactions of dimethyl ether and acetophenone in the TQMS instrument.
indicates that in most cases the dissociative behavior of the resulting product ions is remarkably similar. That is, the mass-elected adducts at (M + HI+, (M + 13)+,and (M + 15)' dissociate to fragment ions of the same mass-to-charges with virtually the same relative abundances in both instruments. This supports the suggestion that the adduct ions formed in either environment dissociate through some common intermediates, although their initial internal energies may differ or they may isomerize after activation. Analytical Utility of Selective Adduct Formation. One potential advantage of using altemative ion/molecule reactions rather than proton transfer as an ionization technique is that product ions may be formed that yield more structurally informative collision-activated dissociation spectra than the CAD spectra of the simple protonated ions. Thus, ion/ molecule reactions may be used to derivatize a molecule of interest to alter its dissociative behavior. An example of this is shown in Figure 4. After collisional activation, protonated acetophenone fragments to m/z 106 (about 5% of the ion current) via loss of methyl or to m / z 43 (about 95% of the ion current) via loss of benzene. However, when dimethyl ether is used as the ionizing agent, a methylated adduct, (acetophenone + CHS)+,is formed. Upon collisional activation, this methylated species dissociatesvia loss of HzO, loss of CO, loss of formaldehyde, and formation of structurally indicative aromatic ions at m/z 91,79,77, and 65. A diagnostic scheme to monitor acetophenone would be more informative using the dimethyl ether reagent gas to produce the (M + 15)+ adduct ion rather than the simple protonated ion.
CONCLUSIONS Dimethyl ether demonstrates functional group and positional selectivity in the quadrupole ion trap. The methoxymethyl ether cation, mf z 45, either induces formation of (M + 13)' or (M + 15)' from each aromatic compound depending on the functionalgroup. Bifunctional aromatics generally form (M + 13)' if they are ortho substituted, whereas the meta and
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RECEIVED for review December 3,1990. Accepted March 12, 1991. This work has been supported by the Welch Foundation (Grant F-1155),the National Science Foundation (Postdoctoral Starter Grant), and the Society of Analytical Chemists of Pittsburgh.
