Anal. Chem. 1881, 63,2105-2109 (14) Iwasaki, K.; Fuwa, K.; Haraguchi, H. Anal. Chlm. Acta 1988. 183. 239-249. (15) Ebihara, M.; Mlnai, Y.; Kubo. M. K.; Tomlnaga, T.; Aota, N.; Nikko, T.; Sakamoto, K.; Ando, A. Anal. Scl. 1985, 1 , 209-213. (18) Paimlerl, M. D.; Fritz, J. S.; Thompson, J. W.; Houk, R. S . Anal. Chim. Act8 1988, 184, 187-196. (17) Shabanl, M. B.; Akagl, T.; Shlmizu, H.; Masuda, A. An8/. Chem. 1990, 62, 2709-2714. (18) Karlberg, B.; Thelander, S. Anal. Chim. Acta 1978, 98, 1-7. (19) Furman, W. B. Contlnuous Flow Analysis, Theory and Ractice; M. Dekker; New York. 1978; pp 52 and 141. (20) Nord, L.;Karlberg, 8. Anal. Chim. Acta 1980, 118, 285-292. (21) Bergamln, F. H.; Medeiros, J. X.; Reis, B. F.; Zagatto, E. A. And. Chlm. Act8 1978, 101, 9-16. (22) Backstrom, K.; Danlelsson, L. G.; Nord, L. Analyst 1984, 109, 323-325. (23) Sweileh, J. A.; Cantwell, F. F. Anal. Chem. 1985, 57, 420-424. (24) Coello, J.; Danielsson, L. G.; Cassou, S. H. Anal. Chim. Acta 1987, 20 1 , 325-329. (25) Shelly, D. C.; Rossl, T. M.; Warner, I. M. Anal. Chem. 1982, 54, 87-91. (28) Rossi, T. M.; Shelly, D. C.; Warner, I. M. Anal. Chem. 1982, 5 4 , 2056-2061.
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(27) Backstrom, K.; Danlelsson, L. G. Anal. Chlm. Acta 1990, 232, 30 1-3 15. (28) Bengtsson, M.; Johansson, 0. Anal. C h h . Acta 1984, 158, 147-158. (29) Shabani. M. B.; Akagi, T.; Shlmizu, H.; Masuda, A. Paper presented at the 38th Annual Meeting of Japan Society for Analytical Chemistry, Oct 1989. (30) Nord, L.; Kariberg, B. Anal. Chim. Act8 1984, 164. 233-249. (31) Kraak. J. C. Trends Anal. Chem. 1983, 2 , 183-187. (32) Atallah, R. H.; Ruzlcka, J.; Christian, G. D. Anal. Chem. 1987, 59, 2909-29 14. (33) Atallah, R. H.; Christian, G. D.; Hartenstien, S. D. Analyst 1988, 113, 463-469. (34) Potts, J.; Rogers, N. W. Geostand. News/. 1988, 10 (2) 121-125. (35) Kawakami, 0. Ph.D. Thesis, Tokyo University, 1986. (36) Yoneda, S. Ph.D. Thesis. Tokyo University, 1988. (37) Govindaraju, K. Geosfand. News/. 1989, 13, Special Issue.
RECEIVED for review April 2,1991. Accepted June 28,1991. This work was supported in part by a Grant-in-Aid for Fundamental Scientific Research from the Ministry of Education, Science and Culture, Japan.
Precursor Ions for Gas-Phase Cation-Attachment Reactions in Laser Desorption/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Jeremiah D. Hogan and David A. Laude, Jr.* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
The mechanism by which cation-attached organic compounds are fonned and and trapped for detectlon In the Infrared laser desorption ionization (LDI)/Fourler transform ion cyclotron resonance (FTICR) mass spectrometry experlment Is evaluated. A combination of timeof-fllght (TOF), variable trap potential, and double-resonance experiments offers evidence that these ions resuH from gas-phase reactlons in the trappedkn ceH. LDI spectra of K B r M organk samples show that as the desorption sHe Is dlspiaced from the trapped-ion K)' slgnal decreases and the K+ signal Incell, the (M creases. Optlmum LDIIFTICR trapping potentials for (M K)+ and K' are less than 3 V and greater than 17 V, respectively, which lndlcates that substantial dlfferences exist In klnetk energy dlstrlbutlons for these Ions. I n contrast, salt adduct Ions formed by LDI, K,Br+ for example, exhibit trapping proflles that are slmllar to (M K)'. Doubleresonance experiments to eject suspected precursor Ions Indicate that lt is these adduct Ions rather than the bare cation whkh react with the neutral to form the cation-attached organic species. For example, in LDI/FTICR experlments on a mlxture of KCI and dHaurytthbdlpropkmate (DLTDP), ejectlon of K' yields an abundant (M K)' Ion, whlie contlnuous ejection of K,CI+ precludes formation of any product Ion species.
+
+
+
+
Laser desorption/ionization (LDI) phenomena including laser plasma ignition (1,2),matrix-assisted ultraviolet laser desorption ( 3 4 ,and infrared thermal desorption (6-8) are of interest in mass spectrometry for generating ions from otherwise intractable materials. In particular, pulsed, lowpressure laser operation is easily integrated with Fourier transform ion cyclotron resonance (FTICR) detection (7-16) 0003-2700/91/0363-2105$02.50/0
and numerous biochemical (&IO), polymer (11-13), and materials (14)applications are reported. In the most common LDI/FTICR experiment, a rapid thermal desorption process initiated by pulsed infrared laser light generates abundant cation-attached species from intact organic molecules. Efforts to understand the fundamental aspects of this type of infrared LDI come primarily from time-of-flight (TOF) studies (17-21], and in particular, Kistemaker demonstrated that cation-attached products of infrared LDI are formed in the gas phase rather than at the desorption site (6, 17, 18). Gross and co-workers have explored the infrared LDI process with FTICR (22,23)and, by wing split-probe experiments,verified that cation-attached species detected by FTICR also are formed in the gas phase. In all FTICR work to date, it has been assumed that precursors for this ion/molecule reaction were bare alkali-metal cations and intact analyte molecules that fortuitously overlapped in the gas phase, despite possibly different kinetic energy distributions, spatial distributions, and desorption times. Evidently, this coincidental overlap is often facile given the success of infrared LDI/FTICR in generating high signal-to-noise spectra for many classes of organic and biomolecules. One aspect of the LDI/FTICR experiment that has not been addressed to date is the mechanism by which cationattached products are ultimately retained in the trapped-ion cell for detection. The answer is not immediately apparent. On the one hand, most LDI/FTICR experiments are performed with static potentials continuously applied to trap plates during the LDI event and subsequent delays. This fact is inconsistent with formation of low-energy cation-attachment products outside the cell, since no obvious vehicle exists by which such low-energy ions might penetrate the trapping barrier (24). Alternatively, cation attachment might occur 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
in the trapped-ion cell, although this is inconsistent with mechanisms proposed by Cotter for temporal evolution of the desorbed species (25). To be presented here are preliminary results from an investigation of the LDI gas-phase chemistry as it relates to FTICR detection. Double-resonance, TOF, and variable trap potential studies indicate that at laser power densities in the 107-10s W/cm2 range, product ions detected by FTICR are formed in the trapped-ion cell. Moreover, the surprising result, which is the subject of this correspondence, is that although the bare cation is formed and can be trapped in abundance in the cell; it is not the immediate precursor of cation-attached species detected by FTICR. Rather, double-resonance experiments provide compelling evidence that it is a lower kinetic energy adduct of the salt that reacts with the intact neutral organic molecule, M. For example, in KBr-doped samples, it is primarily K2Br+rather than K+ that reacts with M to form the (M K)+ ion that is detected.
+
EXPERIMENTAL SECTION The FTICR spectrometer employed in this work is described in detail elsewhere (26). Briefly, the spectrometer is assembled from components that constitute the Extrel FTMS-2000 mass spectrometer and includes a 3.0-T superconducting magnet and differentially pumped dual-section cubic trapped-ion cells of 4.76-cm3dimensions maintained at pressures below 1X lo4 Torr. For this work only the source side of the dual section cell was used. LDI was accomplished with a Spectra Physics DCR-11 Nd:YAG laser operating at 1064 nm in the Q-switched mode with an 8 4 s pulse width. The laser light path included a Pellin-Brocaprism, two 90" quartz prisms, and a 10-cm focal length lens that focused the light to enter a 10 m length by 600 pm diameter fused-silica fiber optic that terminated less than 1 mm from the sample at a 45O angle to the surface (27). The close proximity of the fiber optic to the sample was necessary to achieve the required power densities for LDI from the unfocused exiting light. Specifically, for a typical surface spot size of just under 1mm2and maximum laser energies of 20-25 mJ through the fiber as measured with a Genetec ED-200 Joulemeter, average maximum power densities up to about 3 X los W/cm2 could be achieved. A translatable 90 cm length by S / r in. diameter hollow stainless steel probe housed both the fiber optic, which was sealed to vacuum with a Vespel ferrule, and a in. diameter stainless steel sample probe. This smaller sample probe could be rotated and translated inside the larger probe to obtain about 200 laser shots on fresh sample if necessary, without changing vacuum or sampling conditions. An additional degree of freedom allowed with the assembly was translation of the 3/4-in.probe along the center line of the cell over a distance of 40 cm from the trapped-ion cell to the edge of the vacuum chamber in the fringingfield of the magnet. Again, this could be done from laser shot to laser shot without altering the conditions for ionization or detection. Sampleswere prepared by dissolving equal amounts of salt and analyte in suitable solvents and depositing the solution dropwise or by spraying onto a demountable stainless steel probe tip. For this work, 1:l mixtures of dilaurylthiodipropionate (DLTDP) or gramicidin4 with KBr or KCl were used. At vacuum chamber temperatures below 300 K, vapor pressures for these samples were negligible, and with system pressures at the time of experiments between 1 and 2 X lo4 Torr, it was assumed that the materials involved in the subsequent gas-phase chemistry all originated at the desorption site with the laser firing. The standard LDI/FTICR pulse sequence commenced with a system trigger to fire the laser, followed by a variable delay to allow pressures to return to the low 10-8-Torr range prior to detection. For low-resolution work used to obtain relative mass abundances profiles, broad-band swept excitation was followed by acquisition of 16K data points over a 2.667-MHz bandwidth. For higher resolution measurement,64K data points were acquired over a 1.333-MHz bandwidth. With the exception of the quench event, continuous and uniform trapping potentials were applied throughout the pulse sequence. Three variations on the basic LDI/FTICR experiment were used to investigate the gas-phase chemistry following the de-
sorption event. Effects of desorption distance from the trapped-ion cell on the FTICR signal were evaluated with the probe-mountedfiber optic assembly that could be translated more than 40 cm in the vacuum chamber. If evolving ion and neutral populations in this variable TOF experiment exhibited different velocity distributions, then gas-phase chemistry involving the reactants would occw in different regions of the vacuum chamber. This experiment would potentially distinguish whether cationattachment reactions occurred inside or outside the trapped-ion cell. In a second experiment, systematic changes in the applied trapping potential for each laser firing yielded LDI/FTICR abundance profiles from which some idea of ion kinetic energy distributions could be obtained in an experiment similar to conventionalretarding grid studies. Finally, LDI ejection studies were conducted on possible salt precursor ions. To ensure that the resonance ejection pulse was active during the entire trapped-ion cell reaction process, the sample probe was positioned 15 cm from the cell to allow a brief arrival time offset for radio-frequency excitation to reach peak power. This selective ejection was maintained until detection to prevent the precursor ion from forming by uncontrolled ion/molecule reactions.
RESULTS AND DISCUSSION Although Gross demonstrated convincingly that cationattached products of infrared LDI are formed in the gas phase (22), the actual location of the attachment with respect to the trapped-ion cell remains the subject of speculation. Several observations about the LDI/FTICR experiment are, however, consistent with an argument that the chemistry occurs in the cell. For example, large ion abundances are observed in the conventional LDI/FTICR experiment even though trapping potentials are continuously applied during the desorption and detection events. If cation-attached product ions formed outside the trapped-ion cell, then a mechanism by which low-energy ions would penetrate the potential barrier at the front trap plate or by which high-energy ions would lose or redirect energy in the potential well during flight through the trapped-ion cell, would be required. The alternative explanation that product ions form by gas-phase reaction in the trapped-ion cell precludes the need for such arguments. In support of the gas-phase chemistry of externally generated ions occurring in the cell, we recently demonstrated an external source analogue to the LDI experiment in which high-energy ions injected into the cell undergo charge exchange with resident neutrals to yield abundant product ions for FTICR detection (28). A second piece of evidence supporting a gas-phase mechanism is Kistemaker's demonstration that kinetic energies of cation-attached products exhibit a low kinetic energy distribution, in his case less than 1eV (6);such energies are compatible with efficient storage in FTICR trapped-ion cells typically maintained at trapping potentials of a few volts. As a side note, we point out that although TOF studies support gas-phase reactions in the trapped-ion cell as a plausible mechanism for infrared LDI/FTICR, they are incompatible with such a mechanism for matrix-assisted UV LDI/FTICR. Specifically, products of matrix-assisted LDI are thought to form at or near the desorption surface and with much higher kinetic energies than would be consistent with retention in shallow trapping potential wells (29). These manifestations of the technique are inconsistent with FTICR detection in high yields and perhaps explain some of the initial difficulties in extending matrix-assisted UV LDI of higher mass ions to FTICR (30). In an effort to demonstrate that LDI/FTICR is in fact a gas-phase process, FTICR spectra were acquired from translational studies with the probe-mounted fiber optic assembly. Shown in Figure 1are the resulting spectra from these experiments, acquired for a sample of KBr and DLTDP. The spectra demonstrate that trapping of (M + K)+ is favored when the probe is positioned adjacent to the cell, while K+
ANALYTICAL CHEMISTRY, VOL. 03, NO. 19, OCTOBER 1, 1991
2107
(M+K)*
200
100
300
400
500
600
mh Figwe 3. Comparison LDIlFTICR spectra of a KCI and DLTDP mixtue acquired folbwin (a) continuous ejectbn of K+ ions and (b) continuous ejection of K,Ci Ions. 200
B
600
400
m/z Figure 1. LDI/FTICR spectra of a KBr and DLTDP mixture acquired as the probe mounted flber optic was displaced by distances of (a) 5 cm, (b) 10 cm, (c) 15 cm, and (d) 20 cm from the trapped4on ceH abng the center line of the magnetic field. Trap potentials were maintained at 2.0 V durlng the laser firing. A delay of only 10 ms between desorptionand detection events compromised FTICR performance but was necessary to avoid subsequent K+ consumption in side reactions.
'f
450
i i
a
ka
I8!
300
150
0 0
10
20 30 Trapping Voltage
Figure 2. Relative abundances of K+ (01(M ,
40
50
+ K)+ (V), and K,Br+
( 0 )species extracted from LDIlFTICR spectra of a 1:l KBr and
DLTDP mixture. Above 10 V an external power supply was used to provide necessary potentials to the trap plates.
dominates the spectrum at increasing distances from the cell. One explanation for the reduction in (M K)+ intensity at increasing distance from the trapped-ion cell is that the product ions form by gas-phase reaction of the evolving neutral and ion distributions of differing velocities which fortuitously overlap in the cell. For identical laser and sample conditions then, reactions that form (M + K)+ in the same region with respect to the sample probe will at some point occur outside the cell as the probe is displaced. If these product ions are of low kinetic energy, then they will be unable to overcome the trapping barrier at tthe front trapping electrode and will not be detected. To prove this for the FTICR experiment, the variable trap potential profile for (M K)+ in Figure 2 was obtained. The signal exhibits a maximum at about 3 V
+
+
but disappears at higher trap potentials. The observation of K+ at increasing distances from the cell would be explained if these ions exhibited a larger average kinetic energy from the desorption event. Near the cell, the high kinetic energy of the bare cation would be sufficient to enter and then exit the potential well without being retained. However, at increasing distances from the cell these energetic ions would interact with a more intense radial magnetic field and convert z axis motion into radial motion. If the reduction in axial kinetic energies approached the scale of cell trapping potentials, then the likelihood of being retained upon passing through the cell because of collisions, loss of Debye shielding, or further conversion of axial to radial energy would increase (28, 31-33). The variable trap potential profile for K+ in Figure 2 does suggest a much higher kinetic energy distribution than (M + K)+. At low trapping potentials, no K+ is detected, but at 7 V it becomes the dominant ion in the spectrum. Leveling of the K+ signal beyond 15 V likely results from saturation of the cell with ions, so it is not possible to estimate an average energy from this data. Turning now to the gas-phase reaction that yields the cation-attached product, the data in Figure 2 suggest that direct reaction of a bare cation with M is not necessarily the sole manner in which (M K)+ is formed. This is because the arrival of M in the cell is independent of the trapping potential, so (M + K)+ should be observed at all times K+ is present in the cell. This is not the case a t higher trap potentials and is difficult to explain if K+ is a precursor ion. Alternatively, spectra in Figure 1 indicate that KBr adduct ions, specifically K2Br+( m / z 157 and 159) and K3Br2+(m/z 275,277, and 279), are produced by the desorption event and readily detected by FTICR. These ions have been observed in LDI/TOF studies (21) but have not been proposed as possible precursors to the (M + K)+ products. The variable trap potential profile for K2Br+presented in Figure 2 indicates the adduct approximatesthe energy distribution for (M + K)+, which as will be discussed, suggests it as a more plausible precursor to (M K)+ than the higher energy bare cation. FTICR double-resonance experiments were performed in which suspected precursor ions were continuously ejected from the cell by applying radio-frequency excitation at the appropriate cyclotron resonance frequency. Absence of the
+
+
2108
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991 M+K]
l l 1..
1. I 5d0
&
I " " j
loo0
Flgure 4. Comparison LDI/FTICR spectra of a KBr and gramicidin8 mixture acquired with (a) continuous ejection of K+ ions and (b) continuous ejection of K,Br+ ions. S I N values for the (M i-K)+ are 27 1 and 26, respectively.
To this point only the relative kinetic energies of ions and neutrals involved in the LDI cation attachment process have been considered in assessing the likelihood of overlap between reactant populations in the trapped-ion cell. Of equal significance is the desorption time profile of the reactants. Cotter demonstrated that at lo6 W/cm2, bare cations are expelled rapidly during the desorption event while neutrals are emitted on a longer time scale after desorption (24). These results are seemingly in conflict with the mechanism presented by us for cation attachment in the trapped-ion cell because it is not obvious how late-forming, low-velocity neutrals would ever encounter the fast moving preformed ions. Two explanations for the discrepancy are proposed here, although time-resolved experiments at higher laser power densities will be necessary to determine if either is correct. First, it is possible that as is the case for desorbed neutrals, salt adducts might continue to be formed long after the laser event. Indirect evidence for this comes from Hillenkamp's observation that at higher laser power densities, metal ions are desorbed for an increasing time period (34) and from Cotter's observation that late-desorbing ions exhibit a large drop in kinetic energy (25). A second explanation for our results is that the preformed adduct ions are ejected during the laser event but, upon arriving at the cell before the neutrals, are trapped and subsequently react with the later arriving neutrals. This mechanism, which remains consistent with Cotter's time-resolved experiment results, is appealing because it does not require the fortuitous overlap of evolving distributions of gas-phase species.
LITERATURE CITED product ion from the resulting FTICR spectrum would provide confirmation that the ejected ion participates in the reaction. Shown in Figures 3 and 4 are double-resonance LDI/FTICR spectra for KC1 mixed with DLTDP and KBr mixed with gramicidin-S, respectively. In the DLTDP example, ejection of K+ yields an abundant (M + K)+ ion in Figure 3a, while continuous ejection of K2Cl+ suppresses formation of any product ion species. This reduction resulting from K2Cl+ ejection verifies that the adduct and not the bare cation is the immediate precursor of (M + K)+. Likewise, continuous ejection of K+ yields the gramicidin8 spectrum in Figure 4a with a S I N of 271 while ejection of K2Br+ reduces S I N by a fador of 10 to about 25 in Figure 4b. It should be mentioned that in control experiments in which no double-resonance events were used, (M + K)+ signal intensities were a factor of 2-5 less than when K+ was ejected; this can be attributed to elimination of unnecessary matrix ions that compete for space in the trapped-ion cell. The demonstration that K2C1+and K2Br+are the primary reactive ions in these LDI/FTICR experiments does not necessarily contradict statements by Gross (22) and others that the bare alkalai-metal cation participates in the reaction. In separate experiments in which we isolated K+ in the trapped-ion cell, we find it to be highly reactive with gas-phase neutrals. We believe the distinguishing feature between our work and Gross's in evaluating the thermal desorption mechanism is a 100-fold difference in laser power density. Work by Cotter and co-workers (25) in the 106 W/cmZregime used by Gross indicates that the bare cation forms with kinetic energies of less than 1eledronvolt; it is conceivable that under these conditions an effective overlap in the cell between K+ and M populations occurs. However, as the variable trapping profiles and double-resonance data in Figures 2-4 clearly indicate, at the lo8 W/cm2 power densities we used, higher kinetic energy K+ does not react in the trapped-ion cell. Evidently, the kinetic energy of the various preformed ions is critical to formation of (M K)+, either in establishing the appropriate gas-phase overlap with M in the cell or by enhancing the cross section for reaction.
+
Vertes, A.: Juhasz, P.; Jani, P.; Czyhovszkyp, A. Int. J . Mass Spectrom. Ion Processes 1988, 83,45. Vertes, A.: DeWoif, M.; Juhasz, P.; Gijbels, R. Anal. Chem. 1989. 67. 1029. Karas, M.; Bachmann, D.; Hiiienkamp, F. Anal. Chem. 1985, 57, 2935. Karas, M.; Bachmann, D.; Bahr, U.; Hillenkamp, F. Int. J . Mass Spectrom. Ion Processes 1987, 78,53. Beavis, R. C.; ChaR, B. T. Rapid Commun. Mass Spectrom. 1989, 3 , 233. Van der Peyl, G.; Van der Zand, W.; Kistemaker, P. Int. J . Mass Spectrom . Ion Processes 1984, 62, 5 1. McCreafy, D. A.; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1982, 5 4 , 1435-1431. Wilkins, C. L.; Well, D. A.; Yang, C. L.: Ijames, C. F. Anal. Chem. 1985, 57,520-524. Wilkins, C. L.; Yang, C. L. C. Int. J . Mass Spectrom. Ion Processes 1986, 72,195. Coates, M. L.; Wilkins, C. L. Anal. Chem. 1987, 59, 197. Brown. C. E.; Kovacic, P.; Wilke, C. E.; Cody, R. B.; Kinsinger, J. A. J . Polym. Sci., Polym. Lett. Ed. 1983, 23,453-463. Brown, R. B.: Weil, D. A.; Wilkins, C. L. Macromolecules 1988, 19, 1255-1260. Asamota, B . ; Young, J. R.; Citerin, R. J. Anal. Chem. 1990, 62, 61-70. - .
Brenna, J. T.; Creasy, W. R.; McBain, W.; Sorb, C. Rev. Sci. Instrum. 1988. 59. 873-879. Ijames, C: F i Wilkins, C. L. J . Am. Chem. SOC. 1988. 770, 2687-2688. Nuwaysir, L. M.; Wilkins. C. L. In Lasers and Spectrometry; Lubman. D. M., Ed.; Oxford Series on Optial Sciences; Oxford University Press: New York, 1989; Chapter 13. Posthumus, M. A.; Meuzelaar, H. L.; Kistemaker, P. G. Anal. Chem. 1978, 50, 985-991. Van der Peyl, G.;Isa, K.; Haverkamp, J.; Kistemaker. P. 0.Org. Mass Spectrom. 1981, 9 ,416-420. Cotter, R. J. Anal. Chem. 1981, 53. 720-721. Baiasanmugam, W.; Dang, T. A.; Day, R. J.; Hercules, D. M. Anal. Chem. 1981, 53,2296-2298. Hardin, E. D.; Vestal, M. L. Anal. Chem. 1981, 53, 1492-1497. Chiarelii, M. P.;Gross, M. L. Int. J . Mass Specrrom. Ion Processes 1987, 78,37-52. Chiarelii, M. P.; Gross, M. L. J . phvs. Chem. 1989, 93. 3595-3599. Hofstadler, S. A.; Laude, D. A., Jr. Int. J . Mass Spectrom. Ion Processes 1990, 101, 65-78. Van Breemen, R. B.; Snow, M.; Cotter, R. J. Int. J . Mass Spectrom. Ion Processes 1983, 49, 35-50. Beu, S. C.; Laude, D. A., Jr. Anal. Chem. 1989. 67, 2422-2427. Hogan. J. D.: Beu. S. C.; Laude. D. A,. Jr.; MajMi, V. Anal. Chem. 1961, 63, 1452-1457. Beu, S. C.; Laude, D. A., Jr. In?. J . Mass Spectrom. Ion Processes 1990. 97. 295-310. ~
Pan, Y.; Cotter, R. J. Proceedings of 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 19-24, 1991.
Anal. Chem. 1991, 63, 2109-2114 (30) Hettlch, R. L.; Buchanan, M. V. J . Am. Soc.h s s Spectrom. 1991, 2, 22-28. (31) Hanson, C. D.; Castro, M. E.; Russell, D. H.; Hunt, D. F.; Shabanowltz, J. ms Of L e f (m’r’5000) ~ mmokuks; ACS rles 359 (FTMS); American Chemical Society: Washington, DC, 1987; pp 100-115. (32) Ke&y, E. L.; hnson, C. D.; Castro, M. E.; Russell, D. H. Anel. C t ” . 1988, 8 1 , 2528-2534. (33) Bamberg, M.; Wanczek, K. P. 37th Annual Conference on bss Spectrometry and Allied Topics, Miami Beach, FL, 1986; p 456.
se-
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(34) Feigl, P.; Schueler, B.; Hlllenkamp, F. Inf. J . Mass Spectrom. Ion Processes 1983, 47, 15-18.
RECEIVED for review February 19,1991. Accepted June 27, 1991. This work is supported by the Welch Foundation (Grant F-1138), the Texas Advanced Technology and Research Program (Grant No. 45151, and the National Science Foundation (Grants CHE9013384) and CHE9057097).
Mechanism of Electrospray Mass Spectrometry. Electrospray as an Electrolysis Cell Arthur T. Blades, Michael G. Ikonomou, and Paul Kebarle* Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
If lt Is assumed that the charglng of the unipolar droplet spray In electrospray (ES) is due to a separatlon of posltlve from negative electrolyte ions present in the solution (electrophoretlc charglng), then charge balance requires that a conversion of ions to electrons should occur at the metal-liquid Interface of the ES caplllary, when the caplliary Is the poSnlve electrode. A proof for the occurrence of such an electrochemical process Is provided. When a Zn capMary tip is used, Zn2+ Ions are detected in the sprayed solution. The Zn2+ concentration matches the Zn2+ production expected on the basis of the measured ES current. Slmliar results are obtalned for Fez+ Ions when a stainless steel caplliary is used.
INTRODUCTION Electrospray mass spectrometry is a new technique of extraordinary potential (1-3). The exciting applications of this technique have also created a great deal of interest in the mechanism by which the gas-phase ions required for the mass spectrometric detection are produced. There are two important stages: the production of the charged droplets and the production of gas-phase ions from the charged droplets. The present work addresses mainly the first stage, i.e., the source of charge on the droplets. The electrospray (ES)device as used in ES mass spectrometry (1-4) can deliver a continuous stream of fine droplets that carry charge of the same polarity. Thus, when the ES capillary is at a positive voltage relative to the large planar counter electrode, the droplets are positively charged and a continuous positive current arrives at the counter electrode. This ES current is generally within the range 0.1-1 MA. It depends on the presence of ions in the solution that are due to electrolytes dissolved in the solvent used. The current increases with the concentration of total ionized electrolyte, taken to the -0.35 power (4-7). The mechanism by which the droplets are charged is assumed by some authors (4-9) to be electrophoretic, although this point of view seems not to be accepted by many ES mass spectrometrists. Electrophoretic charging occurs when, due to the imposed electric field, a partial separation of the positive ions from the negative ions present in the solution occurs, which leads to an excess of positive charge on the surface of the liquid at the capillary tip. This excess charge destabilizes the surface and leads to emission of positively charged dro0003-2700/91/0363-2109$02.50/0
plets. The central point here is that the excess ions in the droplets are electrolyte ions that were present in the solution and ion ions created from neutral molecules by processes like field ionization. Considering the requirements for charge balance in such a continuous electric current device and the fact that only electrons can flow through the metal wire supplying the electric potential to the electrodes, one comes to the conclusion that the electrophoretic charge separation mechanism requires that the electrospray process should involve an electrochemical conversion of ions to electrons. In other words, the ES device can be viewed as an electrolytic cell of a somewhat special kind. The cell is special insofar as part of the ion transport does not occur through the solution but through the gas phase. The suggested scheme (8, 9) is illustrated in Figure 1. An essentially conventional electrochemical oxidation reaction should be occurring at the liquid-metal interface of the capillary tip. The actual oxidation reaction(s) will depend on the electrical potential present a t given locations of the metal-liquid interface and on the electrochemical oxidation potential for the given reaction(s). Kinetic factors governing the reaction could also be involved although the kinetic constraints, leading to “overpotentials”, should be small, considering the low currents involved. The net effect of the oxidation reaction at the capillary tip will be the creation of an excess of positive ions over negative ions in the solution and the production of electrons that enter the metal. The excess of positive ions could result from a removal of negative ions from the solution or the production of positive ions. Thus, when NaCl is the electrolyte and wet methanol the solvent, reactions 1 and 2 could be involved in removing negative ions and reaction 3 could be a process 2C1-(,)
= Cl,,,,
40H-(,,) = o~(,,
+ 2e
Eored= 1.36 V
+ 2H,O(,) f 4e
[OH-] =
M
+
(Emd
2H,00, = 02(,) 4H+(aq)+ 4e [H+] = lo-’ M
Eared
(1)
= 0.40 (2)
= 0.81)
Eord = 0.68 (3)
(Er& = 0.27)
producing positive ions. Also given with the reactions are the standard electrochemicalreduction potentials. The tendency for oxidation increases as the reduction potential decreases. The reduction potentials, when the activities of H+ and OH0 1991 American Chemical Society
