Anal. Chem. 1987, 59, 1670-1674
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Meeting, St. Louis, MO, Sept 1 9 8 6 paper 195. (9) Deutsch, R. D.; Hieftje, G. M. Appl. Spectrosc. 1985, 39, 214. (IO) Deutsch, R. D.; Keilsohn, J. P.; Hieftje, G. M. Appl. Spectrosc. 1985, 39, 531. (11) Beenakker, C. I . M. Spectrochim. Acta, Part B 1978, 318, 483. (12) Layman, L. R.; Lichte, F. E. Anal. Chem. 1982, 54, 638. (13) Wilson, D. A.; Vickers, G. H.; Hieftje, G. M.; Zander, A. T. Spectrochim. Acta, Part 8 1987, 428, 29. (14) Hieftje, G. M. Anal. Chem. 1972, 44(6), 81A. ( 1 5 ) Wilson, D. A.; Vickers, G. H.; Hieftje, G. M., submitted for publication in J . Anal. Atom. Spectrosc. (16) Horiick, G.: Tan, S. H.; Vaughan, M. A.; Rose, C. A. Spectrochim. Acta, Pan B 1985, 408, 1555. (17) Vickers, G. H.; Wilson. D. A.; Hieftje, G. M.; Zander, A. T. Presented at the 34th Annual Conference on Mass Spectrometry and Allied Topics,
Cincinnati, OH, June 1986; paper MPD6. (18) Wilson, D. A. Ph.D. Dissertation, Indiana University, 1987. (19) Douglas. D. J.; French, J. B. Spectrochim. Acta, Part8 1986, 4 1 8 , 197. (20) Matousek, J. P.; Orr, B. J.; Selby, M. Talanta 1986, 33, 875. (21) Wilson, D. A.; Hieftje, G. M. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA, Feb 1985; paper 1167.
RECEIVED for review January 5,1987.Accepted March 3,1987. Supported in part by the National Science Foundation through Grant CHE 83-20053,the Office of Naval Research, The Upjohn Company, and American Cyanamid.
Collision-Activated Dissociation of Negative Ions in an Ion Trap Mass Spectrometer Scott A. McLuckey* and Gary L. Glish
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Paul E. Kelley Finnigan-MAT Corporation, 355 River Oaks Parkway, S u n Jose, California 95134
Polyatomic anlons derived from several dkdtrotduene isomers, 2,4,&trinitrotduene, and 1,3,5-trkdlro-l,3,5-trlazacyckhexane have been formed by us8 of negative ion chemical ioniratlon and trapped In an ion trap mass spectrometer. Mass spectrometry/mass spectrometry daughter Ion spectra were obtained for most of these ions. I n most cases, the parent anions were converted very eff lciently to daughter ions (>lo%), but for the (M He)- ions the efficiencies were relatively low. Conversion efMendes were d#rerved to differ by as much as 2 orders of magnitude between anions. The major cause for these ditferences is atlrlbutaMe to dmerences in the propensity for collisional electron detachment vs. that for collision-activated dissociation.
-
The performance characteristics of the ion trap or threedimensional quadrupole (1-4) have been dramatically improved in recent years with the development of the mass selective instability mode of mass analysis and the use of a light background gas (usually helium) at a pressure of =1 mtorr (4, 5). It has been found that collisions with the background gas tend to reduce large amplitude motions of the ions in the ion trap (6,7). The net effect of the background gas is to concentrate the ions in the center of the trap, thereby enhancing mass resolution and sensitivity. These developments have resulted in the recent commercial introduction of an ion trap as a detector for gas chromatography (8) and has generated interest in other possible uses of the ion trap as an analytical tool. Recently, for example, it has been demonstrated that two or more stages of mass spectrometry can be performed in a single ion trap (9) making available the technique of mass spectrometry/mass spectrometry (MS/MS) (10, 11). Most analytical applications of MS/MS use the technique of collision-activated dissociation (CAD) between stages of mass analysis in order to identify a mass-selected ion. CAD of positive ions has been demonstrated in an ion trap (9, 12) where an ac voltage (the frequency of which is tuned to excite only one m / z ratio) with an amplitude of a few hundred millivolts is applied to the end caps of the trap for a few tens of milliseconds. CAD can then result from 0003-2700/87/0359-1670$0 1S O / O
collisions between the translationally excited ions and the background gas. The two major objectives of this study were to form polyatomic anions in an ion trap and to determine the facility with which these ions undergo CAD using the conditions under which the ion trap is currently being operated for MS/MS. Although the widespread use of negative ions in analytical mass spectrometry is relatively recent, the high sensitivity and specificity obtained for the analysis of some compounds in the negative ion mode relative to the positive ion mode has been amply demonstrated (13). In an ion trap both positive and negative ions can be trapped simultaneously under conditions in which both ion polarities are formed (14). With trapping times on the order of tens to hundreds of milliseconds, it is conceivable that positive ion-negative ion recombination could reduce sensitivity for a compound ionized in an ion trap. The only negative ions reported to have been trapped in a three-dimensional quadrupole are I- (15) and CI(14). These ions were formed by photodissociation of thallium iodide and 50-eV electron impact of dichloromethane, respectively. I t has recently been demonstrated that positive ion chemical ionization could be readily performed in an ion trap using an appropriate pulse sequence (16, 17). We therefore chose to investigate the possibility of using negative ion chemical ionization (NICI) in an ion trap with molecules known to form very stable negative ions. We studied p-dinitrobenzene, 2,4-dinitrotoluene (DNT), 2,6-dinitrotoluene, 3,4-dinitrotoluene, 2,4,6-trinitrotoluene (TNT), and 1,3,5trinitro-1,3,5-triazacyclohexane(RDX). As a further test of the applicability of the ion trap to the study of negative ions we also studied the CAD of anions derived from these compounds.
EXPERIMENTAL SECTION All experiments were carried out with a Finnigan-MAT ITMS research ion trap (shown schematicallyin Figure 1)equipped with a DeTech Model 300 conversion dynode/electron multiplier detector (18). Helium was used as a background gas at a pressure of =1 mtorr. Samples were admitted via a solids probe and for most chemical ionization experiments water was admitted into the vacuum system to a pressure of =2 X lo" torr through a separate valve. NICI MS/MS spectra were obtained by using 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
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'oo7
Scan Acquisition Supplementary RF Voltage
(Computer)
RF Voltage I
I
1
I
100
50
150
Flgure 1. Schematic diagram of the Finnigan-MAT ITMS research ion trap configured for MS/MS operation.
200
250
m/z
I
Figure 3. NICI mass spectrum of TNT. Table I. Negative Ion H,O Chemical Ionization Mass Spectra of Three Dinitrotoluene (DNT) Isomers Listing All Ions >5% of the Base Peak
Supplementary RF Voltage
ON OFF
mlz
ion
182 181 165 152 137
M( M - H*)( M - OH')( M - NO')'
122
n Figure 2. Pulse sequence employed for MS/MS analysis (daughter ion scan)of ions formed via chemical ionization in the ion trap: (A) ionize and store CI reagent ions: (B) CI reagent ions react with neutral sample molecules and form sample ions: (C) select parent (remove ail m / z below parent m l z ) ; (D) select mass range for daughter spectrum: (E) CAD parent and store daughters: (F) scan resultant mass spectrum. the following pulse sequence (16) (shown in Figure 2): electrons from a filament above one of the end caps were gated into the trap through a hole in the end cap for 213 ms. The rf voltage applied to the ring electrode was adjusted to a suitably low level (=lo0 V p-p) such that during this period the relatively low mass reagent ions were stored efficiently. Next, the electrons were gated off and a 100-ms reaction period followed where the rf voltage applied to the ring electrode of the trap was set ( ~ 1 6 V 0 p-p) to trap both reagent ions and any higher mass ions which may be produced via ion/molecule reactions. MS/MS data were then acquired following NICI by scanning the rf voltage up to the parent ion of interest (thereby eliminating all m / z values lower than that of the parent ion) and returning the voltage to a low m / z cutoff of =40 and applying an ac voltage of 2200 mV to the end caps at a frequency corresponding to the frequency of motion of the parent ion in the direction between the end caps (9) (thereby translationally exciting the parent ion in the direction between the end caps) for a period of = l O ms. Following this reaction period, the rf voltage was then quickly scanned up at a rate of 5.5 X 103Da/s. As the rf voltage is increased,progressively higher masses lose their stable trajectories in the direction between the end caps. A hole is situated in the end cap opposite the filament where ions can pass out of the trap and be drawn into the conversion dynode or multiplier. NICI mass spectra were obtained by using the same scan function with the elimination of those steps only required for MS/MS. Between 10 and 20 scans were accumulated and averaged. For the detection of negative ions a conversion dynode voltage of +5 kV was used with -2 kV applied to the front of the multiplier. In this method only the ions of m / z less than that of the parent are eliminated from the trap prior to the ac excitation voltage applied to the end caps. Only the parent ion of interest, however, is translationally excited so that the ions which are observed at lower mass come primarily from the ion of interest. (Though not
46
(M - 2NO')NO,-
% base peak 2,4-DNT 3,4-DNT
2,6-DNT 46 32
27
27
100 28
32
50 9
100
100 5
26
27
26
7 11
employed here, a newer technique has been developed to allow for the selection of only one nominal m/z value (16).) Under the conditions used here ions undergo multiple collisions with the background gas. The energy of the collisions depends upon the amplitude of the rf voltage applied to the ring electrode, the amplitude of the ac voltage applied to the end caps, and the duration of the applied ac voltage. The effects of these parameters on the MS/MS spectra obtained with an ion trap have been discussed (19).
RESULTS AND DISCUSSION NICI Mass Spectra. Negative reagent ions OH- and 0'were readily formed under the conditions described above with (OH-)/(O-') = 3 and with no other reagent ions greater than 5% of the intensity of OH-. Figure 3 shows the maSS spectrum observed for T N T obtained with water as the reagent gas. The base peak in the spectrum (m/z 226) corresponds to (M - Ha)formed primarily by proton abstraction by OH-. The gas phase acidity of water is 16.9 eV and that of T N T is 14.2 eV (20). A relatively intense peak is also observed a t m / z 227 indicating that the molecular anion is also formed. Electron transfer from the reagent ions and electron capture can both give this ion. The electron affinities of OH' and 0 are 1.8 and 1.5 eV, respectively. The electron affinity of T N T has not been measured but is expected to be a few tenths of an electronvolt less than the 2.6-eV electron affinity of trinitrobenzene. When the water inlet was closed the total ion abundance observed decreased to roughly half of that observed in Figure 3 and the abundance of (M - H*)decreased relative to most of the other ions in the spectrum including M'-. The intense ions a t m / z 210, 197, 181, 167, 151, and 139 can be attributed to fragmentations of M'- (see below). Table I lists the relative abundances of the major ions in the mass spectra of 2,4-DNT, 3,4-DNT, and 2,6-DNT. The isomers are readily distinguishable based on the relative abundances of M'-, (M - H')-, (M - OH')-, and (M - NO')-. The 2,4-DNT isomer is most clearly distinguishable from the other two isomers in that (M - He)- is the base peak in the mass spectrum. Also, (M - OH')- and (M - NO')- are more
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
210 'OOW3
Table 11. Relative Daughter Ion Abundances Observed in the Ion Trap MS/MS Spectra of the Major High Mass Ions in the Negative Chemical Ionization Mass Spectrum of RDX"
I
1 (M-OH.1-
$
129268-
(M + NOz)197 I
1 3 7 (M-W)'i 46 7
50
'
; I , ' ' , ,
1 I ' 1 ', 1 ' U1 , . '
100
1
I,
200
(M - NOz)-
102-
(M - "02, NOz)-
(CHZNCHZNNOz)-
(15%) (58%) (8%) (95%) m / z % base m / z % base m / z % base m / z % base 2(h :+)-.
d , l I ' l ' IIi , , I ' ,, l I,,h ' l ~
150
176-
~l
46 ' , -
250
100
102
100
85 58
100
58
31
17
46
100
46
61
*Parent ion to daughter ion conversion efficiencies are listed Darentheticallv.
m/n
Figure 4. MS/MS daughter ion scan of the molecular anion of
TNT.
intense than Ma-. For the 3,4 and 2,6 isomers the (M - NO')ions are most abundant but differences are observed in the relative abundances of the respective M'- and (M - H*)-ions and a (M - OH')- ion is essentially absent for the 3,4 isomer. The (M - OH')- ion is due to an ortho effect and is observed for some anions with a ring substituent containing labile protons ortho to a nitro group (21). The NICI mass spectrum of RDX (molecular weight 222 amu) was acquired. As has been observed elsewhere with negative chemical ionization (22,23) of RDX, a peak at m / z 268 is observed corresponding to (M + NOz)-. No ions near to the molecular weight are apparent. By far the base peak in the spectrum corresponds to NOz- with significant peaks a t m / z 176, 129, and 102 corresponding to (M - NOz)-, (M - HN02, NOz)-, and (CHzNCH2NNO2)-,respectively. NICI MS/MS Spectra. MS/MS spectra wete obtained for most of the ions listed in the mass spectra discussed above. In the present mode of operation for obtaining MS/MS data using an ion trap, ions are subjected to many low-energy collisions (12) prior to the second mass analysis step. In many respects the CAD experiment in the ion trap is similar to that in a triple quadrupole instrument where electronvolt energy collisions occur in a rf-only central quadrupole. A major difference, however, which could cause significant differences in the appearance of MS/MS spectra obtained by using the ion trap and a beam-type instrument, for example, is that sample still present as background in the trap allows for the possibility of charge transfer from a daughter ion to a sample molecule. This cannot occur in beam-type instruments since ionization, mass analysis stages, and collisional excitation are physically separated. A mechanism that competes with CAD for negative ions is electron detachment. Electron detachment is particularly important for anions with neutral counterparts with low electron affinities. The compounds studied here, however, have high electron affinities (e.g., the electron affinity of 2,4-DNT is 21.4 eV (20)) which minimizes electron detachment but may tend to enhance the possibility of electron transfer from a daughter ion to a parent molecule. For most of the anions studied here CAD was observed to occur with relatively high efficiency; i.e., the number of parent ions observed prior to excitation could be largely accounted for by the number of daughter ions detected. For example, Figure 4 shows the MS/MS spectrum of the molecular anion of TNT. The major high mass daughter ions observed have also been reported from a beam-type MS/MS instrument (23) and correspond to losses of H'(mlz 226), OH' ( m / z 210), NO' (mlz 197), and NOz' ( m / z 181). (Note that ions at m / z 228 and m / z 229 were also present in the NICI mass spectrum and were not removed prior to excitation of m / z 227. The absolute abundances of the ions at mlz 228 and mlz 229 were
unaffected by the MS/MS experiment.) The total number of counts observed for the daughter ions in this spectrum is roughly 75% of the number of counts measured for the T N T anion prior to excitation (-80% of the initial parent ion signal can be accounted for when residual parent ions are also included). This indicates, assuming that the efficiency of detection does not vary greatly with mass over the range of 40-230 Da, that the parent ions primarily undergo CAD upon translational excitation (as opposed to other ion loss mechanisms such as electron detachment, scattering, etc.), and that the daughter ions are trapped with high efficiency. Similar high efficiencies, where efficiency is defined as (totaldaughter ion signal)/(parent ion signal before CAD) X 100, have been observed for positive ion MS/MS in the ion trap (9). Efficiencies greater than 20% were also observed for the CAD of the (M - OH')- and (M - NO')- anions. Under the same ion signal conditions, however, less than 1% of the (M - W)in the mass spectrum appeared as daughter ions in the MS/MS experiment with m / z 226 as the parent ion despite the fact that the parent ion signal was attenuated to 20% of its initial value. Similar observations were also noted for the other nitroaromatic anions; viz., high efficiencies were noted for the M'- ions whereas efficiencies for the (M - H')- anions were somewhat lower. An effort was made to increase the efficiency by increasing the amplitude of the ac voltage applied to the end caps and increasing the low mass cutoff of the MS/MS experiment. With an increase of the low mass cutoff, the frequency of motion of the parent ion in the direction between the end caps is increased which has been shown for positive ions to increase the amount of energy which is deposited into the ion (19). This was ineffective, however, in increasing the efficiency for the (M - H.1- ions. Figure 5 compares the MS/MS spectra of the molecular anions of 2,4-DNT, 2,6-DNT, and 3,4-DNT. These isomeric ions are readily distinguishable by MS/MS just as they are by the negative chemical ionization mass spectra. The 2,4DNT anion, for example, shows an abundant OH' loss whereas the 2,6-DNT anion shows relatively less and the 3,4 isomer shows essentially none. The MS/MS efficiencies for these ions under these conditions were measured to be from 50 to 70%. The efficiencies observed for the (M - H')- ions, however, were 15-25%. Table I1 summarizes the MS/MS spectra of the major high mass ions observed in the negative ion mass spectrum of RDX using water as the reagent gas and gives the percentage of parent ions converted to detected daughter ions. Wide variation is observed in the efficiency with which the different parent ions are converted to daughter ions, reminiscent of the difference in CAD efficiencies observed for the M'- and (M - Ha)- anions of the nitroaromatics. The anion at m / z 176, for example, which corresponds to (M - NO2', CHZNNOz)-, readily fragments to give virtually only the anion at mlz 102 (95% efficiency). However, this ion, (CH2NCH2NN02)-, does
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987 152
3 , 4 -DNT ousN01183
165 152
'0°%1
2,4-DNT
Figure 5. MS/MS daughter ion spectra of the molecular anlons of (a) 3,4-DNT, (b) 2,6-DNT, and (c) 2,4-DNT.
not produce daughter ions as readily under the same conditions (8% efficiency). Another noteworthy observation is that the MS/MS data obtained by using the ion trap are very similar to the corresponding data reported for high collision energy MS/MS (23) in some cases but are markedly different in others. For example, the MS/MS data for the (M - NOz, CHzNNOz)-anion (mlz 176) is identical for the ion trap and for MS/MS data acquired with a reverse geometry sector mass spectrometer; viz., only one daughter ion, (CHZNCHzNNO2)-,(mlz 102), is observed. On the other hand, the high collision energy data for the m / z 102 anion shows peaks a t m / z 86 (CHzNCHzNNO-, 31%), m / z 74 (CHzNNOz-,95%), m / z 60 (NNOZ-or CH2N02-, 34%), m/z 54 (CH2NCN-, 12%), and 46 (NO2-,loo%),whereas on the ion trap the only daughter ion other than NOz- is observed a t m / z 58 (possibly CH,NNO-). An ion at m / z 58 also appears in the ion trap MS/MS experiment for the anion at m/z 129 but does not appear in the sector instrument data. The base peak in the ion trap for the m/z 129 anion is observed at m/z 85, probably resulting from the loss of NzO, whereas this ion is not a major ion in the sector instrument results. The reason(s) for these differences cannot be determined from these results and merit further study. There are several major differences in important aspects of the two experiments including conditions of ion formation, collision energies, numbers of collisions, time scales of the experiments, and chemical environments after the first mass analysis step. Any of these could at least in part account for the differences observed between the ion trap and sector mass spectrometer data. It seems likely, however, that the relatively reactive chemical environment in the ion trap after mass selection will have a major influence on the appearance of some
1673
MS/MS spectra. The importance of this situation will be clearer as more experience is gained with the ion trap as an instrument for MS/MS. A significant observation in these studies is the relatively wide variation with parent ion in the efficiency of conversion to daughter ions in the MS/MS experiment. Efficiencies recorded here were seen to vary by as much as 2 orders of magnitude (the anion at m / z 176 from RDX was converted to the daughter ion at m/z 10.2 at an efficiency of 95% whereas an efficiency of
