Anal. Chem. 1994,66, 1403-1407
Real-Time Characterization of Individual Aerosol Particles Using Time-of-Flight Mass Spectrometry Kimberly A. Prather,' Trent Nordmeyer, and Kimberly Salt Department of Chemistry, University of California, Riverside. California 9252 1
Thispaper describes the recent development of a time-of-flight mass spectrometersystem designed specifically to allow in situ characterization of the size and chemical composition of individual aerosol particles in real time. The design, experimental optimization studies, and planned future applications for this particle analysis system are described. Research in the field of analytical chemistry during the 1970s and 1980s aimed primarily at the development of methods for detection of trace quantities of gaseous pollutants, with little attention paid to particulates. However, current reports suggest that particles play a much more vital role in the environment than originally believed, being directly responsible for approximately 60 000 deaths in the United States each year. Reports such as these make it increasingly difficult to ignore the tremendous impact atmospheric aerosols have on health, visiblity, and climate, making the lack of available aerosol analysis techniques an imminent problem. Microanalysis techniques, such as LAMMA, are currently the most utilized methods for complete characterization of individual aerosol particles.1.2 Microanalysis allows determination of the distribution of a particular molecule or class of molecules within various particle size fractions. Current microanalysis tools include mass ~pectrometry,~-~ optical spectro~copy,6~~ and electron microscopy.8.9 However, these techniques suffer from being off-line analysis methods, and the majority can only be used for quantitative determination of either particle size or composition (but not both). An ideal aerosol analysis instrument would allow simultaneous determination of all physical properties of an aerosol and, because many of these are transient in nature, obtain them by realtime sampling. As part of an effort toward accomplishing these goals, we have recently developed an aerosol time-offlight mass spectrometer (ATOFMS) designed to allow online determination of both the size and chemical composition of individual aerosol particles in real time. The size and composition of an individualparticle not only reflectsthe nature of the source of the particle but ultimately determines the particle's environmental and biological effects. This paper (1) Van Gricken, R.; Xhoffer, C. J. Anal. At. Spectrom. 1992, 7, 81-88. (2)Kaufmann. R. L. Laser Microprobe Mass Spectrometry (LAMMA) of Particulates. In Physical and Chemical Characterization of Individual Particles; Spumy, K. R., Ed.; Ellis Horwood Limited: Chichcster, UK, 1986. (3) Mauncy, T.; Adams, F.Sci. Total Environ. 1984.36, 215-224. (4) Sinha,M.P.;Griffen,C.E.;Norris,D.D.;Estes,T. J.;Vilker,V.L.;Friedlander, S.K. J. Colloid Interface Sci. 1982, 87, 140-153. ( 5 ) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63, 2069-2073. (6) Fung, K.; Tang, I. J . Aerosol Sci. 1992, 23, 301-307. (7) Grader, G.; Arnold, S.; Flagan, R.; Seinfeld, J. J . Chem. Phys. 1987, 86,
5897-5902. ( 8 ) Markowicz, A. A.; Van Grieken, R. E. Anal. Chem. 1984,56,241R-ZSOR. (9) Shaw, G. E. Atmos. Environ. 1983, 17, 329-339. 0003-2700/94/0368-1403$04.50/0 0 1994 American Chemical Society
gives a brief overview of the experimental details and presents initial results obtained using the ATOFMS system.
EXPERIMENTAL SECTION Figure 1 shows a schematic diagram of the experimental configuration. The overall system includes three distinct regions: (1) an aerosol introduction interface, which includes three adjustable stages of differential pumping; (2) a light scattering region for particle detection and velocity/size determination; and (3) a reflectron-time-of-flight mass spectrometer for single particle composition analysis. Initial experiments are currently being performed for calibration and optimization of the ATOFMS instrument. Calibration studies described in this paper are accomplished using organic-based particles,containing small amounts (0.1%) of various salts, including NaCl, KCl, and Na2S04, formed under controlled conditions. Particles of known size and concentration are formed using a commercially available vibrating orifice aerosol generator (TSI, Inc., Model 3450). This particular aerosol generation system was chosen because of its ability to provide monodisperse, micrometer-sized particles, all of known size (within 1%) and chemical composition. Using this aerosol generation system, the majority of particles are created with some charge, making neutralization necessary for efficient transfer into the mass spectrometer. This is accomplished by passing the particles through a 85Kr neutralizer which can be heated to remove the solvent matrix from the particles. The particles then enter the aerosol interface (Figure 2), consisting of a capillary (160 pm i.d. X 12 mm) followed by a series of three differentially pumped regions separated by skimmers with diameters of 0.5,0.5, and 1.0 mm, respectively. The aerosol undergoes expansion through the capillary to produce particles with velocitiesranging between 100 and 400 m/s which travel through successive skimmers to produce a collimated aerosol beam. The three interface regions operate at pressures of 10-1, 10-3, and Torr, achieved using pumps with speeds of 5.5,5.5, and 150 L/s, respectively. The ultimate pressure in the detector region of the mass spectrometer is 1 P Torr. The nozzle-skimmer and all skimmer-skimmer distances used for these experiments were 0.50.5 and 1.0 cm, respectively, but are adjustable from 0.1 to 3.0 cm. This design permits empirical optimization of particle transmission into the source region of the mass spectrometer while scattering signals are monitored directly. Upon exiting the interface region, the particles travel in the form of a narrow beam into the light scattering region (Figure 2). Each particle first encounters light from a 40mW Ar ion laser operating at 488 nm focused to a 0.1-mm Analytical Chemistry, Vol. 66, No. 9, May 1, 1994
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Flgure 1. Experimental configuration of the ATOFMS instrument. The main components consist of an aerosol introduction interface, a light scattering region, and a reflectron-time-of-flight mass spectrometer.
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Figure2. Detailof experimentalconfigurationshowing pathotparticles through interface and light scattering regions into the source region of the time-of-flight mass spectrometer.
spot size. The scattered light from the particle is collected by a lens and fiber-optic system and detected by a photomultiplier detector. The particle travels a set distance further where it encounters a second laser, a focused 10-mW HeNe laser operating at 633 nm focused down to 0.1 mm. Scattered light from this laser is collected by a second lens and fiberoptic system and transferred to another photomultiplier detector. In the ATOFMS system, the two scattering lasers are oriented at right angles to one another to ensurethe detected particle is traveling the centered path necessary to reach the desorption/ionizationregion of the time-of-flight mass spectrometer. Both light scattering signals are passed on to an external electronic timing circuit used to track the particles through the system (see Discussion). The velocity of the particle can be determined from the measured distancebetween the two scattering lasers and the time delay between the two scattering pulses. The velocity and size of the particle are directly related, and upon performing the appropriate calibration studies, this relationship will be used for determination of individual particle size. Thus, the light scattering region serves two purposes: determination of particle size/velocity and detection of the incoming particle and its associated velocity, allowing synchronization of the firing pulse of the desorption/ionization lasers with the arrival of the particle in the source region of the mass spectrometer. 1404
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After exiting the light scattering region, the particle enters the source region of a reflectron-time-of-flight mass spectrometer (R. M. Jordan, Inc., Grass Valley, CA). The electronic tuming circuit, based on the measured time delay between the Ar ion and HeNe laser scatter pulses and the relative distances between the lasers, triggers a pulsed COz (10.6 pm) and/or a Nd:YAG laser operating at the third or fourth harmonic (355 or 266 nm, respectively). Upon absorption of the laser pulse(s), the particle is heated in a rapid fashion, desorbing individual molecules from the particle. The resulting ion signal is detected by a dual microchannel plate detector and sent to a Tektronix 500-MHz digital oscilloscope interfaced to a personal computer. Using laser powers of 107-108 W/cm2, the fraction of ions created by laser desorption of molecules from a surfacehas been estimated to be 1% or less. A future paper will describe experiments used for quantification of the fraction of molecules desorbed from suspended particles under varying laser desorption conditions. In these studies, either the C02 or Nd:YAG laser has been used for both the desorption and ionization processes. In future experiments, both lasers will be used in tandem, initially desorbing the particle using low C02 laser powers (lo6W/cm2) and then ionizing neutrals with the Nd:YAG laser at an optimized delay time. This laser configuration should lessen the amount of molecular ion fragmentation, simplifying the interpretation of mass spectra obtained from multicomponent particles, as well as increasing the overall sensitivity of the laser desorption/ionization process.1° RESULTS Figure 3 shows the results from preliminary studies examining particle size vs measured aerodynamic velocity, using 1.O-, 2.1-, 4.2-, and 8.4-pm-diameter NaCl particles. The results are qualitatively similar to those reported in previous studies.11J2 (10) Kovalenko, L. J.; Maechling, C. R.; Clemett, S. J.; Philippoz, J. M.; a r e , R. N.; Alexander, C. Anal. Chem. 1992,64,682-690. (1 1) Sinha, M. P. Rev. Sci. Instrum. 1984, 55, 886-891. (1 2) Dahneke, B. Aerosol Beams. In Recent Developments in Aerosol Science; Shaw, D., Ed.;Wiley: New York, 1976.
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d z Flgurr 4. Mass spectrum of a slngie 0.7-pm KCi/2,5dlhydroxybenzolc acM particle (1:lOO) produced by laser desorption ushg a W Y A Q laser operating at 266 nm. The Inset shows the presence of three Isotopic combinations of chlorine and potassium.
In other studies in our laboratory, mass spectra of individual organic and inorganic particles were obtained using the ATOFMS system under controlled conditions for calibration. The most recent spectra have been obtained by firing the desorption/ionization laser at the maximum repetition rate, randomly desorbing particles that enter the source of the mass spectrometer at the appropriate time. Future experiments will utilize a more efficient means of analysis, tracking each particle using the aforementioned external timing circuit, which is currently being modified from a more simplified design. Figure 4 shows a mass spectrum obtained for a single 0.7-pm KC1/2,5-dihydroxybenzoicacid (DHB) particle (1 : 100) obtained upon laser desorption/ionization with a Nd: YAG laser operating at 266 nm. When pure micrometersized KC1 particles were desorbed, the resulting K ion signal saturated the detection system. Thus, in order to avoid saturation, particles were created using a higher concentration ratio of 2,5-dihydroxybenzoicacid to salt. Figure 4 illustrates the potential "softness" of this laser desorption technique, as clusters up to &C13+ are observed. In addition, the inset shows the detected K,CI,1 ion clusters in the expected isotopic abundance ratios. An interesting feature to note in this spectrum is the presence of gas-phase background ion peaks. These peaks are detected even when particles are not entering
the system and, therefore, are attributed to laser ionization of background gas in the mass spectrometer. As shown in this figure, the mass resolution of these peaks is much lower than that for ion peaks created upon laser desorption of particles. At this point, we speculate that these resolution differences are a result of differences in the initial energy spread of the ions caused by the different ion formation processes for gas-phase vs particle-phase ions. This theory is consistent with the observation that, upon varying thevoltages placed on the reflectron, the observed resolution may be reversed, with the gas-phase signal having higher mass resolution than the ion signal produced by particle desorption. These observations will be explored further as part of future studies probing the dynamics of particle desorption processes. Based on these preliminary observations, when performing sampling and analysis by ATOFMS, one may be able to perform real-time "tuning" of the mass spectrometer, preferentially obtaining spectra of either gas-phase or particlephase species. This presents advantages over other analysis methods which require separation of particles and gas-phase species prior to ana1y~is.l~ Figure 5 shows a mass spectrum obtained upon 266-nm laser desorption of a single 0.8-pm particle composed of KC1, NaCl, and Na2S04. This spectrum was obtained as part of Analyticel Chemistry, Vol. 66, No. 9, May 1, 1994
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lasers. Investigations probing the feasibility of using the ATOFMS system as a method for matrix-assisted laser desorption (MALDI) of biological aerosols are also being performed. The results of these studies will be presented in future papers.
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Flgwe 5. Mass spectrum produced by laser desorption of a single 0.8-gm particle composed of NaCi, KCI, and NanS04. A Nd:YAG laser operatlng at 266 nm is used for desorption/ionization.
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a mass resdutkn of 1400. A Con laser operating at 10.8 pm is used for desorption/ionization.
initial studies probing the feasibility of using ATOFMS for real-time analysis of the molecular forms of sulfates and nitrates in atmospheric particles. At this point, typical mass resolutionfor spectra obtained using theNd:YAG desorption/ ionization method is approximately 650. Efforts are being made to increase this value, using a combination of computer simulations and empirical optimization. ATOFMS may also be used for characterization of organic particles. Figure 6 shows the mass spectrum of a 0.8-pm nicotinic acid particle, containing a small amount (0.1%) of KC1. The peak at mass-to-charge ratio 39 shows ringing, as a result of being larger in magnitude than the scale setting on the digital oscilloscope. The mass resolution of this spectrum is approximately 1400. To date, this is the first reported mass spectrum of an individual particle obtained using a COz laser for the desorption/ionizationprocess. These preliminary results will be complemented by results from additional optimization experiments in the near future. In addition to calibration experiments, initial experiments are currently being performed in our laboratory with the goal of elucidating the fundamental processesinvolved in desorption of single particles suspended in the gas phase. Specifically, the effectsof laser wavelength, power, and two-step desorption/ ionization on particles is being examined. We are also working on determining ion yields as a function of the delay time between the desorption (CO2) and ionization (Nd:YAG) (13) Cahill, T. A.; Wakabayshi, P. Compositional Analysis of Size-Segregated Aerosol Samples. In Measurement Challenges in Atmospheric Chemistry; Newman, L., Ed.; Advances in Chemistry 232; American Chemical Society: Washington DC, 1993.
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DISCUSSI ON Development of methods for analysis of single particles will allow many important problems to be investigated, and many promising experimentalapproachesare being developed to address such p r ~ b l e m s . ~ JSome ~ J ~ considerationsthat went into the design of the ATOFMS system, and the resulting key features that were chosen for incorporation, are described below. One of the most critical parts of the experiment involves creating a collimated beam of aerosol particles, while transferring them from atmospheric pressure down to timeof-flight operating pressures with minimal particle loss. A great deal of research has involved investigating the most effective way to accomplish this task.16J7 The ATOFMS system was designed with a unique aerosol/mass spectrometer interface,allowing in-depth studiesof the parameters affecting optimum particle sampling and transmission. As described in the Experimental Section, our interface was constructed to allow adjustment of nozzleskimmer and skimmer-skimmer distances under vacuum, permitting empirical optimization of particle transmission directly by monitoring the scattering signals. A concern, when aerosols are introduced through regions of differentialpumping, is that there will be preferential particle loss for a particular size range, depending on the distances between successive interface regions. This aerosol beam interface will allow direct investigation of trends in particle transmission as a function of the chosen distances between regions for particles varying in size from 0.2 to 10 CLm. One feature which makes our experimental arrangement a unique single particle analysis technique is incorporation of the method of aerodynamic sizing12J8to track particles in the time-of-flight mass spectrometer. A subtle, yet key, component that will make this a powerful combination is an external timing circuit which serves as a link between the aerodynamic sizing region and time-of-flight mass spectrometer. This circuit provides the logic for ATOFMS measurements, allowing the accurate setting of timing for the mass spectrometer experiments,while correlatingeach particle laser desorption mass spectrum with the corresponding particle velocity/size. The circuit is designed so that only those particles traveling through both the HeNe and Ar ion lasers cause the circuit to send a signal to trigger the desorption and ionization lasers, as these are the only particles which will arrive at the center of the source of the mass spectrometer. The circuit operates by detecting the first scattering pulse from the Ar ion laser, which starts a counter that counts up until a reverse pulse from the HeNe laser is received. At this (14) Thomson, D. S.; Murphy, D. M. Appl. Opr., in press. (15) Kievit, 0.;Marijnissen, J.; Verheijcn, P. J.; Scarlett, B. J . AerosolSci. 1992, 23, S301S304. (16) Browner, R. F. Microchem. J. 1989, 40, 4-29. (17) Estes, T. J.; Vilker, V. L.; Friedlander, S. K. J . Colloid Interface Sci. 1983, 93, 84-94. (18) Dahneke, B. Nar. Phys. Sci. 1973, 244, 54-55.
point, the counter counts down at one-third the speed it went up because the distance between the distance between the scattering laser and the center of the source is 3 times the distance between the two scattering lasers. As described previously, the initial circuit operated in this simplified manner, incorporating a minimum amount of logic. The logic changes we have added to the new design include the circuit resetting if no particle passes through during a time interval which matches the minimum repetition rate of the desorption laser (100 ms). This is a requirement we havedetermined necessary for optimum operation of the Nd:YAG laser. For the condition where two particles are traveling too closely, the new circuit ignores the second particle because the start count from the Ar ion laser is not reset until after the stop pulse from the HeNe laser is received. This prevents the circuit from falsely triggering on closely spaced particles. The total timing scheme accounts for any spread in velocity/size of the particles, allowing for size analysis of polydisperse aerosols. These features of the optimized timing circuit will make the ATOFMS system, as a whole, an accurate and more efficient method for measuring individual particle size and composition in real time. The method of aerodynamic sizing has proven to be extremely sensitive to small changes in particle size and is highly reproducible on a day-to-day basis. Unlike single laser light scattering methods for particle size determination,'* the two-laser method is virtually independent of the way the particle passes through the laser beams and does not require prior knowledge of the refractive index of the particles. Thus, ATOFMS is an ideal choice for analysis of multicomponent particles of unknown composition. This will be an important feature for future studies which will involve using ATOFMS for direct atmospheric sampling measurements.
CONCLUSIONS This paper describes the recent development of the ATOFMS system, a system designed for on-line analysis of individual aerosol particles. The incorporation of dual-beam
light scattering for tracking particles in the mass spectrometer makes this system unique from other mass spectrometric methods, in having the ability tosimultaneously obtain particle size and composition in real time. Results are reported for laser desorption of submicrometer size organic and inorganic particles using a Nd:YAG and COZlaser. Ultimately, plans for the ATOFMS system include applications in a wide range of areas, obtaining unique information about the chemical compositionas a function of size of small particles of biological, industrial, and environmental concern. The system is being optimized to allow detection and characterization of particles down to 50-100 nm. A major need exists for the development of analytical methods which allow studies of the sources of potential carcinogenic components, their transport into the environment, and their reactivity. In addition to characterization studies, future applications of this system will include the adaptation of ATOFMS into an instrument capable of monitoring tropospheric and stratospheric gas/particle reactions, specifically probing the catalytic effects of particles on atmospheric chemistry processes. Monitoring controlled reactions in real time will assist in sorting out complex field data on atmospheric heterogeneous reactions, ultimately allowing the generation of new models for atmospheric chemistry, which will be directly applicable in efforts to control ozone depletion and improve air quality. The study of heterogeneous gas/particle reactions is a new and important fundamental area of research that remains virtually unexplored relative to atmospheric gas-phase reactions.
ACKNOWLEDGMENT The authors acknowledgeJoseph E. Mayer for his dedicated time and effort required in machining the ATOFMS system, as well as his invaluable input into its design. Recelved for review November 10, 1993. Accepted February 16, 1994.' *Abstract published in Advance ACS Absrracrs, April 1, 1994.
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