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Anal. Chem. 1986, 58,355-358
Reionization Agents for Neutralization-Reionization Mass Spectrometry Paul 0. Danis, Rong Feng, and Fred W. McLafferty* Chemistry Department, Cornel1 University, Ithaca, New York 14853
Oxygen Is the most efficient colllslon gas of nlne tested for reionlratlon of fast neutral specles In the mass spectrometer, in part because this can Involve the concomitant formation of stable 02-. Neutralization and relonlzatlon of multlkliovolt gaseous Ions can provide unique structural and analytical Information, as well as a method for synthesizing unstable and reactlve neutrals. All reloniratlon targets studied showed maxlmum efficiency at pressures corresponding to -30 % transmittance of the precursor beam. Oxygen reionization also mlnlmlzes fragmentatlon; when increased fragmentation Is desirable for structural information, Increased target pressure and/or helium target gas should be used. Kinetic energy changes from 3 to 10 keV affect the reioniratlon efficiency by less than a factor of 2. Efflclencles of relonlratlon vary substantlaliy between species and with Instrumental parameters.
Fast neutral species formed in the mass spectrometer can provide novel information promising for important structural and mechanistic problems (1-14). In neutralization-reionization mass spectrometry (NRMS) (6) the neutral species are generated from multikilovolt ions by charge-exchange neutralization (3-6, 9, 12-14); fast neutrals can also be formed by metastable ion dissociation (3,4,6-8,10, II), collisionally activated dissociation (CAD) (3,4,6,8,10,13),or dissociation of the neutrals resulting from these processes (13,14). The neutrals are then characterized by reionization and mass (or energy) analysis of the ionic products. The generation of fast neutrals from fast (multikilovolt) ions via CAD is most effective with He targets (13-16), while that via charge-exchange neutralization is most effective with metal vapor targets (6, 9,13). Optimization of the reionization process is the subject of this study. The removal of an electron from these fast neutral species, M, upon collision with target A
-.
+A M+ + A M
M+ + e - + A
(1)
M2++ e - + A
(2) (eq 1, upper bar denotes a fast species) is analogous to the charge stripping of a fast singly charged ion M+by target A to produce the doubly charged ion M2+ (eq 2). The efficiency of various targets for charge stripping has been shown to decrease O2 > N2 > He > CHI (17-19). Similarly, fast alkali-metal atoms are ionized with target efficiencies O2> N2 > H2 (20,21).For fast Ar reionization O2 has superior efficiency (22, 23), while for fast He2, H3, ND4, and N2D7Porter and Gellene (9) have recently shown the relative efficiencies to be NO2 > 0 2 > N2 > He. Other NRMS experiments have utilized helium (3,4,6-8,10-12) and H2,N2, Ar, Kr, and Xe (24)for reionization, with our most recent work (13,14)utilizing oxygen, as recommended here. It is the purpose of this study to elucidate the optimum NRMS conditions in terms of target, pressure, and kinetic energy under which larger fast neutrals can be reionized, fragmented, and collected. 0003-2700/86/0358-0355$0 1.50/0
EXPERIMENTAL SECTION The measurements were performed on a tandem mass spectrometer ( 3 , 4 )with a special collision interface region described in the companion paper (13, 25). Ions were formed by 70-V electron ionization, accelerated at 9.8 kV (unless othenvise noted), and mass analyzed in a double-focusingmass spectrometer (MS-I). These ions underwent charge exchange with Hg vapor at room temperature (-1.6 torr, 90-95% transmittance) in the metal vapor collision chamber (Cls-I) to yield the beam of fast neutrals. Polarization of lens A (Dfl-I) removed residual ions from the neutral beam, which could then be measured with the retractable multiplier detector (Mlt-I) in line. To determine the efficiency of various targets for reionization, the neutral flux was first measured at Mlt-I. The multiplier was then retracted and the reionizing target introduced as an orthogonal molecular beam (Cls-111)between Dlf-I and Mlt-I. The total ions formed could be measured by deflection (Dfl-11)of the resulting ion beam into the retracted Mlt-I, or the individual product ions could be measured by separation with the electrostatic analyzer (ESA-11) and collection at Mlt-11. The maximum efficiency of reionization was read from a pressure/signal plot obtained by attaching the output of pressure gauge located near lens A to the X axis and the output of the ion current amplifier to the Y axis of an X-Y recorder (26)and increasing the pressure at Cls-111. NR mass spectra were recorded with Hg vapor at Cls-I, ion removal at Dfl-I, reionizing target at Cls-I11giving (unless noted otherwise) 30% transmittance of the primary ion beam to Mlt-IT, linked scan of lens B (between Mlt-I1 and ESA-11) and MS-11, and detection at Mlt-11. Low (m/Am 2) resolution NR mass spectra were measured under similar conditions at Mlt-I (retracted) using Dfl-I1 as a crude energy analyzer, scanning its potential to divert ions into Mlt-I (13, 25).
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RESULTS AND DISCUSSION Effect of Target Gas on Reionization Efficiency. The variation of reionization efficiency with target gas is shown in Table I for the fast neutrals CH3COCH3, CH3C1, and CH2NH2. CH3COCH3and CH3Cl are neutralized molecular ions, while CH2NH2is neutralized CH2=N+H2 from CH3NH2. The reionization efficiency reflects the total abundance of products collected as fast ions at Mlt-I1 produced from a given flux of fast neutrals as measured a t Mlt-I, using the target pressure producing the maximum ion abundance. For O2 as the target the absolute efficiencies (Table 11,uncorrected for multiplier response) found for CH3COCH3, CH,Cl, and CH2NH2 were 0.24, 0.45, and 0.46%, respectively, for ion collection at Mlt-11. Efficiencies for collection a t Mlt-I were higher by 18,19, and 39 times, respectively, similar to factors reported for CAD of ions in the companion paper (13). Table I data are generally similar to the target efficiencies found for charge stripping (17-19) and for reionizing small fast neutrals (9, 20-23). Although oxygen is the optimum target for all three fast neutrals tested, these show significant differences in relative efficiencies, as do the other neutral species of Table 11. Reionization Mechanisms. The fast neutrals studied by Gellene and Porter (9) were characterized by very low IE (ionization energy) values (14.7eV), and for reionizing these, NO2 was found to have efficiencies 1.3-3 times higher than 02 They proposed this could be due to the near resonance 0 1986 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
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Table I. Relative Reionization Efficiencies of Targets
1.0 0.64 0.41 0.43 0.42 0.29 0.16 0.07 0.09
0 2
NO
ClZ NO2 N2
He CHI SF6
1.0
41 19 25 18
0.38 0.29 0.43 0.35 0.28 0.12 0.11 0.043
11 3.7 2.7 2.5 1.5
51 48 47
1.0 0.94 0.77 0.65 0.81 0.96 0.37 0.17 0.12
58 24 14 19
11
8.1 15 17
10 5.1
0.8 3.9
3.3
Xe 17 7.4 ‘Total abundance (peak areas) of reionized species collected at Mlt-I1 with this target gas vs. total with 02,using the same flux of neutral precursors. Abundance of the reionized precursor relative to the total abundance of reionized species. Table 11. Absolute Reionization Efficiencies (% )” fast neutral
IE,beV
CH&OCH, CHZNH2 CH3Cl CH2C1 CHC1 CCl
9.7 6.2 11.3 9.3 11.0 12.9 13.0 9.4 10.4
*c1 SZ *S
cIt/PIo‘ C2+IPlQd 4.4 18 8.4
0.24 0.46 0.45 0.50 0.90 0.61 0.29
11 27
O2target gas pressure yielding 30% transmittance. *Ionization energy, ref. 27 and 28. cTotal abundance of reionized species vs. precursor neutral abundance, both collected at Mlt-I, without correction for response of Mlt-I for ions vs. neutrals (29); 5-keV S+ ions produce -2X the response of the corresponding neutrals (13). Reionized species collected at Mlt-11, not corrected for response of Mlt-I VS. Mlt-11. between the electron affinity (30) of NOz (2.3 eV; Oz = 0.44 eV) and the IE values of the neutrals. For fast neutrals of higher IE values they predicted that electron affinity should play a much less significant role in reionization efficiency, as found for fast Ar atoms (22) and here. Note that the relative efficiencies using target NOz are substantially higher for fast .CHzNHz than for the species of higher IE values. Electron removal from a fast neutral, M, upon collision with a target, A, can occur by electron detachment (eq 1) or by inverse neutralization (eq 3). Bukhteev et al. (21) investigated the ionization of fast alkali-metal atoms by C12 and 0 2 and
M
+A
M+ + A-
(3) found both mechanisms to operate in varying ratios depending on the collision pair and the kinetic energy, while with Nz as the target only eq 1is effective. Durup et al. (22) found inverse neutralization (eq 3) to be dominant for ground-state fast Ar on O2 and Nz. This yields ground-state Ar+, while the concomitant transition to the stable ground state of 02is adiabatic (nonvertical); Nz- has no stable states (22). T o determine if such mechanisms are effective in the collisional ionization of the larger CH3Cl and CH3COCH3,the experiment was reversed, which should produce the same result (31). Fast N2+-and 02+. were neutralized with Hg a t Cls-I to produce the fast neutral “targets” that collided with CH3Cl and CH,COCH, introduced a t Cls-111. NR mass spectra were then recorded a t Mlt-I (Table 111). Although the experiment does not give evidence for the relative importance of the electron ejection pathway (eq l),the predominance of 0, and 0- in these results indicates that inverse neutralization (eq 3) is active for the ionization of fast CH3C1 and CH3COCH3in Oz collisions. Equation 3 apparently is of little importance for N2 collisions, assuming that the unstable +
Table 111. NR Mass Spectra of Nz and Oz relative abundancea target CH&l CH3COCH3
N2
