Anal. Chem. 1999, 71, 1802-1808
Mass Spectrometry of Liquid Aniline Aerosol Particles by IR/UV Laser Irradiation Alla Zelenyuk, Jerry Cabalo, Tomas Baer,* and Roger E. Miller
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
The first results are reported from a new single-particle two-color laser time-of-flight mass spectrometer, incorporating a combination of infrared (CO2) and UV (excimer) laser irradiation. This combination of lasers has the capability to effectively separate the desorption or evaporation step from the ionization step, thereby greatly improving the analytical capabilities of such an instrument. The results on liquid aerosols, such as aniline, show that prior evaporation of the aerosol particle with the IR laser increases the ion signal produced by the excimer laser by more than 2 orders of magnitude. In the case of nitrobenzene aerosols, the excimer laser alone produces no ions, while a very large signal is observed when the aerosol is first irradiated with the CO2 laser. A simple model, based on the Coulomb explosion of the ionized aerosol, is used to estimate the number of ions generated by the excimer laser (∼105 ions). Experimental evidence based on the observed time delay of protonated aniline parent ions indicates that the laser irradiation of the liquid aerosol results in a stable neutral plasma which separates into positive and negative charges only after a 100-500-ns delay. Single aerosol particle mass spectrometry is a rapidly expanding field of research.1-5 In these experiments, particles with diameters from 0.1 to 10 µm, with corresponding masses from 5 × 10-16 to 5 × 10-10 g, are irradiated with a high-intensity pulsed laser and the desorbed/ionized ionic fragments are mass analyzed by time of flight (TOF). Sufficient numbers of ions are produced in a single laser shot to record a full TOF mass spectrum with excellent signal-to-noise ratio. Although this approach is already being used to monitor aerosol pollutants,1,3,6 there remain a number of important questions concerning the mechanism for ionization. Among these are the following: the extent to which the mass spectrum is representative of the composition of the whole particle (rather than just its surface), the effect on the mass spectrum of ion-neutral interactions during the ionization/ extraction process, and the difference between liquid and solid aerosol mass spectra. Some studies have shown that the mass spectra of certain components are affected by other constituent (1) Noble, C. A.; Prather, K. A. Geophys. Res. Lett. 1997, 24, 2753. (2) Murphy, D. M.; Thomson, D. S. Aerosol Sci. Technol. 1995, 22, 237. (3) Noble, C. A.; Prather, K. A. Environ. Sci. Technol. 1996, 30, 2667. (4) Gard, E.; Mayer, J. E.; Morrical, B. D.; Dienes, T.; Fergenson, D. P.; Prather, K. A. Anal. Chem. 1997, 69, 4083. (5) Johnston, M. V.; Wexler, A. S. Anal. Chem. 1995, 67, 721A. (6) Noble, C. A.; Prather, K. A. Phys. World 1998, 11, 39.
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species in the aerosol.7-9 As a result, it is still not at all clear how to relate the observed mass spectra to the composition of the particles. Nevertheless, research by Noble and Prather3,6 has clearly shown that it is possible to broadly classify particles according to whether they are primarily organic, marine in origin, or primarily inorganic, the latter containing ammonium, sulfate, or nitrate groups. To address some of these questions, and in an effort to make such measurements more quantitative, we have developed a two-color instrument in which a CO2 laser is used to vaporize the sample prior to ionization by an excimer laser. One of the problems with the use of a single laser that both desorbs and ionizes the molecular fragments is that the high laser power generates a large number of ions, resulting in TOF mass peaks that are broadened by space charge effects (Coulomb explosion). Reduction of the laser power reduces this problem but also decreases the signal, eventually limiting the analysis of the particle. For this reason, a two-step process that separates the desorption and ionization processes seems to hold great analytical potential. In this paper, we address the specific issues encountered in liquid aerosol mass spectrometry, such as delayed flight times of ions originating from the condensed phase, significant peak broadening, and the higher ionization threshold for the liquidphase particles. EXPERIMENTAL APPROACH The overall schematic of experimental apparatus is shown on Figure 1. It consists of three main regions: (1) an aerosol generation and vacuum introduction region, which includes three stages of differential pumping; (2) particle detection and velocity determination by light scattering; and (3) particle composition analysis by laser desorption/ionization and TOF mass spectrometry. Liquid aerosol particles are generated by bubbling air through a fritted disk which is immersed in a liquid of interest. The particles, in a range of sizes, are swept through several bends in a glass line (12-mm diameter) which eliminates the large particles. Particles of a specific size are selected and focused along the horizontal axis of the chamber by varying the air flow rate through a series of apertures forming what is known as an aerodynamic (7) Neubauer, K. R.; Sum, S. T.; Johnston, M. V.; Wexler, A. S. J. Geophys. Res. 1996, 101, 18701. (8) Neubauer, K. R.; Johnston, M. V.; Wexler, A. S. Int. J. Mass Spectrom. Ion. Processes 1995, 151, 77. (9) Ge, Z.; Wexler, A. S.; Johnston, M. V. J. Colloid Interface Sci. 1996, 183, 68. 10.1021/ac980971l CCC: $18.00
© 1999 American Chemical Society Published on Web 03/20/1999
Figure 1. Schematic diagram of experimental setup.
lens system.10,11 The total pressure in the aerodynamic lens can be varied from a few Torr to 1 atm, the typical value in this study being 20 Torr. The particles are then accelerated through a 1-mm nozzle into a chamber maintained at a pressure of ∼1 × 10-3 Torr by a roots blower. A 2-mm-diameter skimmer samples the particles into the next chamber, which is pumped to a pressure of ∼1 × 10-4 Torr by a 400 L/s Edwards diffusion pump. A second 2-mmdiameter skimmer separates this section from the main chamber, pumped by a 2000 L/s Edwards diffusion pump. This chamber has a background pressure (without an aerosol stream) of 1 × 10-8 Torr and an operating pressure in the range 1 × 10-6-1 × 10-5 Torr. The timing of the experiment is based on light-scattering detection of the individual particle using two parallel He/Ne laser beams that are focused to 0.15-mm spots and separated by 4 cm. After passing through the two lasers, the particles travel an additional 4 cm to the ionization region of the TOF mass spectrometer. The scattered light from the particles is collected by two lenses and detected by a single photomultiplier. The signal, which consists of pairs of light-scattering pulses, can be displayed on a multichannel scaler and is shown in Figure 2. The separation between the pulses in this experiment is 900 µs, which corresponds to a particle speed of ∼44 m/s. Depending on the particle size, backing pressure, and nozzle diameter, the velocity of the particles can vary between 40 and 400 m/s. Most of the particles investigated in this study had a diameter of ∼5 µm, determined by calibration of the apparatus with cellulose particles with a known diameter of 4.8 µm. These particles had velocities similar to the liquid aniline particles for similar backing pressures of ∼20 Torr. Determination of particle velocity allows synchronization of the firing of the desorption/ionization lasers with the arrival of the particle in the source region of the TOF mass spectrometer. (10) Liu, P.; Ziemann, P. J.; Kittelson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 293. (11) Liu, P.; Ziemann, P. J.; Kittelson, D. B.; McMurry, P. H. Aerosol Sci. Technol. 1995, 22, 314.
Figure 2. Multichannel scaler trace of light scattering by aerosol particles arriving at random times. Each aerosol particle gives two light-scattering pulses separated by a time that is proportional to its velocity.
Several methods have been used to trigger the desorbing and ionizing lasers. In the present approach, the amplified and discriminated photomultiplier signals are fed into a time to pulse height converter (TPHC) so that the direct signal provides the stop, while a delayed signal provides the start. The output from the time to pulse height converter is fed into a single-channel pulse height analyzer (SCA). By fixing the voltage level and the voltage window of the SCA, it is possible to select aerosols of a particular velocity. The output from the single-channel analyzer is then fed to a delay unit which triggers the lasers, digital oscilloscope, and pulse generator. With this setup, the hit rate of particles is greater than 90%. When a particle of a particular velocity arrives in the ionization region of the TOF mass spectrometer, the 193-nm ArF excimer laser can be fired alone or the CO2 and excimer lasers can be triggered sequentially. The probability of hitting a selected aerosol with both lasers is ∼90%. When both lasers are used, liquid aerosol particles are first vaporized by the CO2 laser and then, after a Analytical Chemistry, Vol. 71, No. 9, May 1, 1999
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Figure 3. TOF mass spectra of aniline vapor and aniline aerosol particles: (a) mass spectrum of background gas in the chamber; (b) mass spectrum of an aerosol particle obtained with the excimer laser only; (c) mass spectrum of an aerosol obtained when the CO2 laser fired 2 µs before firing the excimer laser. Note that the mass peaks associated with the aerosol particle are much broader due to space charge effects.
variable delay time, ionized by the excimer laser. If only the excimer laser is used, it results in both desorption and ionization of the liquid. Ions are formed in the middle of a 2-cm acceleration region, are accelerated further in an 0.5-cm region, and finally drift up through a 1-m field-free region. Voltages are adjusted to ensure space focusing of the ions by the usual Wiley-McLaren approach.12 Typical accelerating plate and drift tube voltages are +200, -200, and -2100 V, respectively. The resulting ion signal is detected by a microchannel plate detector and sent to a Tektronix 500-MHz digital oscilloscope that is interfaced to a computer. RESULTS AND DISCUSSION Comparison of Aniline Background, Excimer Only, and CO2 Laser Irradiation. Figure 3 shows three aniline TOF mass spectra. Because the aniline vapor pressure at room temperature is ∼0.6 Torr, there is always a background of aniline vapor in the experimental chamber. Figure 3a shows a mass spectrum of this background vapor, while Figure 3b shows a typical TOF-MS of a single aniline particle taken with a single excimer laser pulse at an energy of 2 mJ/pulse. The third spectrum, Figure 3c, was obtained by first vaporizing the aniline particle with a 400 mJ/ pulse CO2 laser, followed 2 µs later by the ionizing excimer laser pulse. These spectra differ in several respects. The background spectrum, which has relatively low intensity, is characterized by sharp peaks. The mass resolution was optimized on these peaks by adjusting the flight tube voltage that varies the space focusing condition. Most of the peaks in this spectrum correspond to fragment ions that result from multiphoton absorption by the ions during the 10-ns laser pulse. When the excimer laser is fired alone to intersect an aerosol particle, the mass spectrum changes only by the appearance of a new peak which is broad and located on the high-mass side of the m/z 93 peak. Although this peak is similar in height to that of the background peaks, its area is considerably greater (∼1 (12) Wiley: W. C.; McLaren, I. H. Rev. Sci. Instrum. 1955, 26, 1150.
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order of magnitude) due to its increased width. In considering the source of this broadening, it is instructive to consider the relative volume of the background ion source and that associated with an aerosol particle. For a background aniline pressure of 10-6 Torr and a laser interaction volume of 3 × 3 × 10 mm, the number of molecules in the laser volume is 3 × 109 molecules. On the other hand, a particle with a diameter of 5 µm contains ∼5 × 1011 molecules. Since the signal increase in going from vapor to an aerosol particle is much less than the increased number of molecules, we conclude that the efficiency of ionization of, or at least the efficiency of collecting ions from, the liquid aerosol is lower than that of the gas. Nevertheless, the fact that the ions are produced in a much smaller volume in the aerosol case makes space charge broadening more important, accounting for the broader peaks. We will have more to say on this subject below. When the particle is first evaporated by the CO2 laser and then ionized by the excimer laser, the signal intensities of both the fragment and parent ions increase significantly. We have evidence that the CO2 laser heating of the aerosol particle raises the temperature to above the critical point (700 K) and completely vaporizes the particle.13 This is supported by the fact that the observed ion signal is independent of the CO2 laser power, indicating that the particle is completely vaporized even at the lowest IR laser powers. The increased signal associated with prior evaporation supports the conclusion that the probability for ionizing and detecting gas-phase molecules is considerably higher than that for generating ions by UV laser irradiation of the liquid aerosol. The reduced efficiency for liquid aerosol ionization has been noted previously by other workers.14-16 Apparently, there are competitive energy dissipation channels in the liquid drop, which are absent in the free molecule. Among these are internal conversion and perhaps electron/ion recombination which in solution is the well-known process of solvent-induced recombination, or “caging” of photofragments.17-19 The excimer laser duration of 10 ns is insufficient for molecules to escape (evaporate) from the aerosol particle and be ionized and fragmented. This is most likely the reason we see no increase in the fragment ion signal upon particle ionization since multiphoton absorption by the solvated ions does not give rise to fragmentation due to rapid recombination and dissipation of the energy to the surrounding molecules in the liquid. It is highly likely that much of the aniline aerosol is ultimately vaporized by the excimer laser absorption. However, the products of this laser/particle interaction are mostly neutral molecules and/or neutral fragments that are not detected by our mass spectrometer. We have investigated a few other liquid aerosol particles, but in less detail. All our efforts to ionize nitrobenzene particles with the excimer laser proved futile. Even at the highest excimer laser powers, no signal above the gaseous background was observed. (13) Cabalo J.; Zelenyuk A.; Baer T.; Miller R. E. Aerosol Sci. Technol., in press. (14) Neubauer, K. R.; Johnston, M. V.; Wexler, A. S. Int. J. Mass Spectrom. Ion. Processes 1997, 163, 29. (15) Thomson, D. S.; Middlebrook, A. M.; Murphy, D. M. Aerosol Sci. Technol. 1997, 26, 544. (16) Thomson, D. S.; Murphy, D. M. Appl. Opt. 1993, 32, 6818. (17) Farrar, J. M. In Cluster Ions; Ng, C. Y., Baer, T., Powis, I., Eds.; John Wiley & Sons: Chichester, 1993; p 243. (18) Otto, B.; Schroeder, J.; Troe, J. J. Chem. Phys. 1984, 81, 202. (19) Castleman, A. W.; Bowen, K. H. J. Phys. Chem. 1996, 100, 12911.
Figure 4. Mass spectra of nitrobenzene using pulse-delayed (2.3 µs) extraction with a 150 V/cm pulse in order to narrow the mass spectral peaks. Upper trace: mass spectrum of a single nitrobenzene particle obtained by first vaporizing the particle with the CO2 laser and ionizing 2 µs later with the excimer laser. Lower trace: mass spectrum of either the nitrobenzene background gas or an aerosol particle using only the excimer laser.
Figure 5. Total aniline mass spectral intensities versus excimer laser power. Without prior particle evaporation by the CO2 laser, aniline aerosol signal can be detected only when the excimer laser intensity exceeds 7 × 106 W/cm2. However, when the aerosol was first evaporated by the CO2 laser, the signal increased by a 2 orders of magnitude and the threshold for ion signal was reduced to zero excimer laser power.
However, when the particle was first irradiated with the CO2 laser, the signal increased by 2 orders of magnitude over the background signal, as shown in Figure 4. We conclude that the probability of generating ions during the 10-ns-long 193-nm excimer laser pulse is essentially zero for the case of nitrobenzene particles. We have observed the same behavior for nitrotoluene aerosols and decyl alcohol particles. There are clearly some interesting processes at work here dealing with liquid energy transfer that are deserving of future study. Threshold for Laser Ionization of Liquid Aerosols. It has been noted in previous work14-16 that a laser power threshold exists for observation of ions from liquid aerosols. While in the case of the nitrobenzene particles, the threshold is apparently above our available laser power, we do observe such a threshold for aniline particles. Figure 5 shows the difference between the integrated ion signal from the aniline particle and background signal versus excimer laser power with and without prior CO2 laser irradiation. Without CO2 laser evaporation of the particle,
ions are only generated from the liquid when the excimer laser power exceeds 1.4 mJ/pulse. It is interesting that upon IR laser irradiation/evaporation not only does the signal increase by a factor of ∼30-100, but we observe signals even down to the lowest excimer laser powers available on our instrument. As expected, there is no threshold for the gas-phase molecules. TOF Parent Ion Peak Widths. We now return to the issue of the excess broadening observed in the aerosol mass spectrum shown in Figure 3b. It is apparent that the aerosol peak in the vicinity of 23 µs in Figure 3b lies to the high-mass side of the background gas peak. Second, this peak arising from aerosol ionization is broad (∼500 ns). The kinetic energy of the ions can be determined from the TOF peak width, given that the TOF difference between forward and backward ejected ions is given by20
∆t ) ((2m)1/2/eE)2Ek1/2
(1)
where m is the mass of the ion, e is the ion charge, E is the electric field, and Ek is the kinetic energy of the ion. The forward and backward ejected ions yield the minimum and maximum possible TOF, respectively. Given an electric field (E) of 200 V/cm, the 500-ns TOF peak width (measured as a full width at halfmaximum) corresponds to an ion kinetic energy of ∼13 eV. This is important to realize given that the apparatus discriminates against ions with initial perpendicular (to the extraction axis) velocities. Indeed, in the absence of electrostatic focusing, ions with an Ek >0.2 eV perpendicular to the TOF axis will not reach the detector. For monoenergetic ions with 13 eV of kinetic energy, we would thus expect to observe forward and backward ejected ion peaks separated by 500 ns. The possibility that one of the peaks is missing was considered since it could be argued that ions ejected away from the ion detector are lost and only the forward ejected ions would be observed. If this were the case, however, the forward scattered ion peak would appear at a shorter TOF than the background ion peak, not greater as is in fact observed. Thus, we conclude that the backward scattered ions are not lost. The fact that we observe a more-or-less Gaussian peak thus indicates that the ions are ejected with a distribution of energies and that our aerosol TOF peak is a composite of doublets. The broad parent ion TOF peak can be explained by space charge repulsion in which the ions originating in the 5-µm aerosol repel one another through their Coulomb forces. During the course of Coulomb explosion, the potential energy of the ions in the aerosol is converted into kinetic energy. Consider a simple model of an aerosol particle treated as a uniformly charged spherical cloud that consists of positive ions and neutral molecules and fragments, created after the irradiation with the excimer laser and interacting only through their Coulomb repulsion. The ions initially located at the surface of this uniformly charged sphere will be repelled by the full charge of the aerosol (with the charge located at the center of the particle), yielding the maximum possible kinetic energy, Ek, given by (20) Cotter, R. J. In Time-of-Flight Mass Spectrometry; Cotter, R. J., Ed.; American Chemical Society: Washington, DC, 1994; p 16.
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Ek ) (1/4π0)(Zeffe2/r0)
(2)
where Zeff is total effective charge of the aerosol particle and r0 is the radius of the aerosol. Ions initially located at smaller radii (r) from the particle center will experience less repulsion due to the lower charge inside the sphere of radius r. This simple model in which the ions only interact via Coulomb repulsion can thus explain the broad distribution of observed kinetic energies. Of course this model ignores the diffusion processes that may also be important in this high-density medium. Treating the system as a liquid drop, we must first modify eq 2 by including the dielectric constant () of the liquid in the denominator. For aniline at 20 °C, ) 6.89.21 In addition, although ions initially at the surface can still be ejected with the maximum kinetic energy, those in the interior will need to diffuse to the surface. As the charge of the droplet decreases due to ion evaporation the resulting kinetic energy will also be reduced. Once again, this model also predicts an overall smooth peak shape for the ion kinetic energies. Making use of eq 2, we can therefore calculate an effective charge on the aerosol particle, using the experimental kinetic energy of 13 eV. For a particle of a radius of 2.5 µm and a dielectric constant of 6.89, we obtain a total charge, Zeff, of 105. The charge density in the aerosol particle of volume (4/3)πr03 is thus calculated to be 1015 cm-3. Recently, other groups have reported even higher kinetic energies (up to several hundred electronvolts) for the case of Coulomb explosions resulting from intense femtosecond laser irradiation of small clusters. Multiple charged Ar, Xe, Kr, ammonia, and acetone ions were observed with charges up to +20 per ion.22 For even larger noble gas clusters (10 000 atoms) heated with high-intensity laser pulses (>1016 W/cm2), ions with charge states as high as +40 have been detected with kinetic energies up to 1 MeV.23 These very high kinetic energies result from a combination of the small cluster size and the very high charge states generated by the femtosecond lasers. In contrast to the aerosol TOF spectrum, the background spectrum is characterized by sharp peaks, a result of the combination of the lower number of ions and the much larger volume in which they are initially created. Nevertheless, these widths also increase if the aniline vapor pressure is increased in the mass spectrometer due simply to the increased ion number density. Figure 6 shows the peak widths of the parent ion as a function of the background gas pressure. In this case, the 200-ns TOF peak width corresponds to an ion kinetic energy of ∼1.9 eV. Once again, eq 2 can be used to calculate the effective charge of the ion cloud generated in the laser focal spot (r0 ≈ 1 mm), yielding Zeff ≈ 106. The difficulty with this value is that it is inconsistent with the relative signals for the background and particle. Indeed, the value obtained from this calculation is 1 order of magnitude greater than that calculated for the particles, while the experimentally observed signals from the particles are clearly much larger than those for the background gas. This paradox can be resolved by noting that the excimer laser tends to consist
of numerous “hot spots”24 in which the density of ions will be much greater than the average. Under these conditions, it is possible to observe the same peak broadening with a much lower total number of ions. It is interesting to note that evidence for these hot spots is also seen in the aerosol mass spectra, where the shot-to-shot fluctuations are nearly 1 order of magnitude larger than for the background gas, even when the average power per laser shot is fairly constant. The implication is that the laser intensity experienced by a 5-µm aerosol particle varies by 1 order of magnitude even when the total number of photons in the 3 mm × 3 mm laser spot size is constant. These fluctuations with the use of excimer lasers, which have been noted by other workers,25,26 can make single-shot quantitative analysis of the aerosol particles unreliable. An advantage in the prior evaporation by the CO2 laser is the significant reduction in these fluctuations since in this case one is again ionizing the gas which, given enough time between the laser pulses, has expanded to fill the entire excimer laser volume. The mass resolution can be significantly improved by the use of time lag focusing12 or postsource pulse focusing.2 Figure 7 shows the parent ion peak shape when ions from the particle are produced in a nearly field free region and pulsed out by a 150 V/cm pulse delayed by variable times between 0 and 1.6 µs. The TOF mass peak narrows because ions are first permitted to spread out, so that when the electrical pulse is finally applied, the ions are separated and can be space focused in the usual WileyMcLaren fashion.12 Of particular interest is the fact that the parent ion peak originating from the aerosol particle has a mass of 94, which identifies it as the protonated parent ion, resulting from ion/molecule reactions in the dense environment of the aerosol particle. Aerosol Parent Ion Peak Shift. One of the most surprising results of this study has to do with the fact that the mass 94 ions arrive at the detector with a significant time delay that depends on the excimer laser power and the extraction conditions. Figure 8 shows a series of mass spectra in the vicinity of the parent ion in which the dc extraction voltage was varied from 70 to 225 V/cm. The spectra are arranged so that the parent ions of the gas-phase
(21) Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC Press: Cleveland, 1975. (22) Poth, L.; Castleman, A. W. J. Phys. Chem. A 1998, 102, 4075. (23) Ditmire, T.; Tisch, J. W. G.; Springate, E.; Mason, M. B.; Hay, N.; Marangos, J. P.; Hutchinson, M. H. R. Phys. Rev. Lett. 1997, 78, 2732.
(24) Rose, H. A.; DuBois, D. F. Phys. Fluids B 1993, 5, 590. (25) Carson, P. G.; Johnston, M. V.; Wexler, A. S. Aerosol Sci. Technol. 1997, 26, 291. (26) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63, 2069.
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Figure 6. Aniline parent ion peak shapes at different background pressure of aniline in the chamber. Peak widths increase from 60 to 280 ns when the background pressure increases from 5 × 10-6 to 2 × 105 Torr.
Figure 7. Parent ion peak shapes for pulsed extraction of the ions at various delay times. Ions were produced (excimer laser only) in a field-free region and pulsed out by a 150 V/cm pulse. Note the resolved protonated parent ion peak (m/z 94) originating from the particle and parent ion peak (m/z 93) from the background vapor.
Figure 8. Delay in the flight time of the m/z 94 ions from the aerosol particles relative to the m/z 93 peak of the background gas as a function of the dc extraction voltage. Vertical lines show the expected flight times of the m/z 94 ions at different draw-out fields. The inset shows that increasing the laser power shifts the peak to longer times. These mass spectra were obtained with the excimer laser only.
spectra appear at zero time. Lines show where the m/z 94 peak is expected for the different draw-out fields. It is apparent that the delay of the m/z 94 peak relative to the expected time varies with extraction voltage, namely, the higher the field, the smaller the delay. The inset in Figure 8 also shows that this delay depends on the excimer laser power, namely the higher the laser power the greater the delay. This delayed ionization cannot be explained in terms of thermionic emission which occurs in systems where the ionization energy is less than the dissociation limit, a condition met in the case of C6027,28 and the recently discovered METcars.29-31 This condition is clearly not met for the case of aniline, nor are the (27) Campbell, E. E. B.; Ulmer, G.; Hertel, I. V. Z. Phys. D: At., Mol. Clusters. 1992, 24, 81. (28) Ding, D.; Compton, R. N.; Haufler, R. E.; Klots, C. E. J. Phys. Chem. 1993, 97, 2500. (29) Cartier, S. F.; May, B. D.; Castleman, A. W. J. Chem. Phys. 1996, 104, 3423. (30) May, B. D.; Cartier, S. F.; Castleman, A. W. Chem. Phys. Lett. 1995, 242, 265. (31) Kooi, S. E.; Castleman, A. W. J. Chem. Phys. 1998, 108, 8864.
TOF peak shapes (which are symmetric in the present study) consistent with thermionic emission, which gives asymmetric peaks that decay exponentially in time. Shifts in peak positions have been observed in other condensedphase ionization studies, namely, in matrix-assisted laser desorption/ionization (MALDI). Zhou et al.32 found that heavy ions (m/z >17 000) had an energy defect of ∼25 eV, as determined by varying the reflecting voltage in a reflectron TOF mass spectrometer. They found that the energy deficit increased with increasing signal intensity and with increasing ion mass. Surface charging, which prevents the rapid acceleration of the ions in the 15 kV/ cm electric field, was suggested as the likely cause of this effect..32 Vertes et al.33 observed asymmetric peaks, one component of which had an energy deficit, while the other had an energy surplus. The low-energy peak was explained by postionization of neutral fragments some time after application of the laser pulse, while the high-energy peak was attributed to a stream velocity due to the explosive desorption of the sample from the matrix. Similarly, Kinsel et al.34 found narrow and broad components in their TOF spectra, the broad component having an energy deficit for low-mass ions but an energy surplus for high-mass ions. These observations were interpreted in terms of two off-setting effects, one of which was the stream or entrainment velocity, which has the effect of increasing the energy surplus of the higher mass ions. Another possible explanation for the delayed ion peak is that the ions formed from the aerosol particle must pass through a plume of evaporated aniline molecules. Thus, collisions might slow the ions down during the initial stages of the ion extraction. This effect is profitably used in ion drift experiments35,36 in which ions reach terminal velocities that are governed by the ratio of the electric field over the gas density (E/N).37 Nevertheless, we can show that this is not a likely cause of the delayed ion peak. If the average speed of the neutral gas expansion is 500 m/s, a 5-µm aerosol particle will expand to a volume with a diameter of 100 µm in 100 ns following the ionizing laser pulse. This corresponds to a pressure (at 298 K) of ∼25 Torr. At this pressure, an electric field of 200 V/cm, and a reduced mobility of 30 cm2/(V s),37 the terminal drift velocity is 2 × 105 cm/s, which is 10 times greater than the velocity reached by the ion during 100 ns in a collisionfree environment. Thus, the ion velocity is not limited by collisions during the first 100 ns, and since the gas density decreases as the cube of the time, the effect becomes even less important at longer times. We propose here a model for the delay which is akin to the charging explanation of Zhou et al.32 in which the full accelerating field is shielded by the ion cloud formed by the laser. It is based on the fact that the ionization of the liquid drop produces a large number of positive and negative ions as well as electrons. In the (32) Zhou, J.; Ens, W.; Standing, K. G.; Verentchikov, A. Rapid Commun. Mass. Spectrosc. 1992, 6, 671. (33) Vertes, A.; Juhasz, P.; Jani, P.; Czitrovszky, A. Int. J. Mass Spectrom. Ion. Processes 1988, 83, 45. (34) Kinsel, G. R.; Edmondson, R. D.; Russell, D. H. J. Mass Spectrom. 1997, 32, 714. (35) Viggiano, A. A.; Morris, R. A.; Dale, F.; Paulson, J. F.; Giles, K.; Smith, D.; Su, T. J. Chem. Phys. 1990, 93, 1149. (36) Schmitt, R. J.; Krempp, M.; Bierbaum, V. M. Int. J. Mass Spectrom. Ion. Processes 1992, 117, 621. (37) McFarland, M.; Albritton, D. L.; Fehsenfeld, F. C.; Ferguson, E. E.; Schmeltekopf, A. L. J. Chem. Phys. 1973, 59, 6610.
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simple electrostatic model used to describe a Coulomb explosion process, we assumed that the negative ions and electrons were instantaneously ejected from the aerosol so that only the positive ions remain. For small clusters, this is known to be the case for electrons22 since they have a mean free path that is large with respect to the size of the cluster. However, for larger clusters with diameters greater than 50 Å, the electrons will undergo multiple collisions within the cluster, allowing them to become thermalized and solvated, thus preventing their escape. There is, however, another factor that prevents their escape. According to a recent study by Villeneuve et al.38 the formation of a highly charged, but overall neutral microplasma generates a trapping potential which prevents electrons and ions from escaping. Application of an external field establishes a potential barrier through which the electrons can tunnel with a rate that depends on the strength of the applied field. This model was developed to explain the loss of resolution in zero kinetic energy (ZEKE) photoelectron spectroscopy (or pulsed-field ionization) when the density of ions created by the laser pulse became too high. This high-resolution photoelectron technique depends on the removal of promptly formed electrons prior to field ionization of the long-lived Rydberg states. If the number of charged particles becomes too great, the electrons cannot be removed with the very small stray fields usually employed, and as the field is increased, the resolution suffers. If we apply this quantitative model by Villeneuve et al.38 to our experiment with an applied electric field of 200 V/cm, plasma effects are possible for a spherical ion cloud with a diameter of 5 µm if the number density of charges exceeds 1010 cm-3. As noted before, the charge density generated in the ionization of the aerosols in this study was shown above to be ∼1015 cm-3, which is orders of magnitude greater than the charge density required to stabilize the plasma. (38) Villeneuve, D. M.; Fischer, I.; Zavriyev, A.; Stolow, A. J. Chem. Phys. 1997, 107, 5310.
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The implication for our experiment is that the ions created by the excimer laser pulse are initially trapped by the plasma. As electrons tunnel out of the plasma, a net positive charge begins to build up and the plasma accelerates in the applied field eventually leading to full acceleration of the individual ions once the positive and negative charges are separated. This model appears to have the same phenomenological effect on the ion TOF or the ion energy as the shielding model of Zhou.32 With either model, the ions arrive at the detector with an energy deficit or a delayed TOF. In summary, we propose that the observed time delay in the appearance of ions produced by aerosol ionization is due to the formation of a stable neutral plasma, in which electrons and negative and positive ions are trapped by their Coulomb attraction. Although we do not yet understand all the quantitative details associated with the observed delay, given that the mechanism is likely to be quite complex and probably depends on the properties of the liquid, it is apparent that such measurement could provide important new insights into the nature of ion and electron solvation. Further work will be required to fully appreciate these features of the experiment. The present study clearly shows that pre-evaporation of aerosols using a CO2 laser can greatly increase both the sensitivity and species selectivity in single-particle laser ionization mass spectrometry. Future studies will include the application of this method to more complex, multicomponent aerosols and ultimately to actual atmospheric samples. ACKNOWLEDGMENT We thank the U.S. Air Force Office of Scientific Research for financial support of this research. Received for review August 31, 1998. Accepted February 8, 1999. AC980971L
