Real-Time Analysis of Individual Atmospheric Aerosol Particles

and spatial information on the origin, reactivity, and fate of atmospheric aerosols. ... variations of the aerosol phase, the method used for real-tim...
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AC Research Anal. Chem. 1997, 69, 4083-4091

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Real-Time Analysis of Individual Atmospheric Aerosol Particles: Design and Performance of a Portable ATOFMS Eric Gard, Joseph E. Mayer, Brad D. Morrical, Tas Dienes, David P. Fergenson, and Kimberly A. Prather*

Department of Chemistry, University of California at Riverside, Riverside, California 92521

Two portable aerosol time-of-flight mass spectrometers (ATOFMS) of identical design are described. These instruments are powerful new tools for providing temporal and spatial information on the origin, reactivity, and fate of atmospheric aerosols. Each is capable of analyzing the size and composition of individual particles from a polydisperse aerosol in real-time. Particles are introduced into the instrument through a particle beam interface, sized by measuring the delay time between two scattering lasers, and compositionally analyzed using a dual-polarity laser desorption/ionization time-of-flight mass spectrometer. These are the first dual-ion TOFMS instruments to utilize a dual reflectron design. The instruments measure 72 in. long × 28 in. wide × 60 in. high and weigh ∼500 lb. Pneumatic tires allow them to be transported through standard doorways, elevators, and handicap ramps, granting access to virtually any location. Furthermore, because of rugged construction they will be able to operate during transport by automobile, boat, or aircraft.

Since discovering the stratospheric ozone hole during the Antarctic spring,1 scientists have determined that suspended particulate matter in the stratosphere, in the form of polar stratospheric clouds, plays a vital role in stratospheric ozone depletion.2-5 This discovery has, in turn, led to a wider appreciation of the importance and complexity of heterogeneous chemical reactions that occur in atmospheric aerosols; of particular interest is the prominent role particles play in the chemistry of the (1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810-812. (2) Molina, M. J.; Tso, T.-L.; Molina, L. T.; Wang, F. C.-Y. Science 1987, 238, 1253-1257. (3) Solomon, S.; Garcia, R. R.; Rowland, F. S.; Wuebbles, D. J. Nature 1986, 321, 755-758. (4) Tolbert, M. A.; Rossi, M. J.; Golden, D. Science 1988, 240, 1018-1021. (5) Hamill, P.; Toon, O. B. Phys. Today 1991, 44, 34-42. S0003-2700(97)00540-4 CCC: $14.00

© 1997 American Chemical Society

troposphere. Photochemical smog, fog, dust, haze, clouds, and smoke are all commonly observed tropospheric aerosols.6 The chemistry of aerosols is rich but remains largely unknown due to its complexity and the limitations of the instruments used to investigate it.7 Of immediate concern to the scientific community is the positive correlation between high tropospheric particulate levels and increased respiratory illness, morbidity, and premature mortality in humans.8-10 This correlation provides a strong impetus to characterize biogenic, geogenic, and anthropogenic aerosols in an effort to determine their role in human health and in the larger global atmospheric cycle.6,11,12 Understanding aerosols in detail requires examining the size and chemical composition of the individual particles making up the aerosol. Information at this level of detail would allow each particle to be traced to its original source, a major goal of many aerosol characterization studies. By monitoring the chemical changes occurring in individual particles with high temporal resolution, the chemical dynamics within the aerosol can be unraveled. Furthermore, continuous real-time analysis at the single-particle level allows immediate detection of atmospheric phenomena, determination of exposure levels to harmful industrial aerosols,13 and detection of chemical warfare agents, a capability (6) Hinds, W. C. Aerosol Technology; John Wiley & Sons: New York, 1982; pp 1-11. (7) Faust, B. C. Environ. Sci. Technol. 1994, 28, 217A-222A. (8) Schwartz, J. Environ. Res. 1994, 64, 36-52. (9) Reichhardt, T. Environ. Sci. Technol. 1995, 29, 360A-364A. (10) Wolff, R. Developments in Toxicology and Environmental Science, Tsukuba, Japan; Ishinishi, N., Ed.; Elsevier Science Publishers: New York, 1986: pp 199-211. (11) Spurny, K. R. In Physical and Chemical Characterization of Individual Airborne Particles; Spurny, K. R., Ed.; John Wiley and Sons: New York, 1986; pp 6-15. (12) Friedlander, S. K.; Morton, L. Environ. Sci. Technol. 1994, 28, 148A150A. (13) Hatch, T. F.; Gross, P. Pulmonary Deposition and Retention of Inhaled Aerosols; Academic Press: New York, 1964.

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needed in recent conflicts such as the Persian Gulf war.14 In order to obtain a broad perspective of local, global, and temporal variations of the aerosol phase, the method used for real-time single-particle analysis ideally would be portable. Efforts to achieve such a complete and timely profile of the aerosol phase have spanned many decades. Cascade impactors provided the first size-resolved composition information for aerosols, but they suffer from a number of intrinsic limitations.15 Foremost among these limitations are the lack of real-time information and an inability to analyze composition on the singleparticle level.16-18 To analyze individual particle size and composition, Van Grieken et al. pioneered an off-line technique, laser microprobe mass spectrometry (LMMS).19-22 However, LMMS is a relatively slow analysis method capable of analyzing less than 1 particle/min, precluding its use for real-time analysis. Real-time composition analysis of airborne particles was first accomplished by a number of early investigators by impinging particles on a heated surface/filament and analyzing the resulting burst of ions by mass spectrometry.23-28 Unfortunately, because of limitations inherent to the ionization and mass spectral techniques used, only a small portion of the available composition information could be obtained. The next advance came when Sinha developed a technique that could analyze not only the composition of the particles in real-time but the size of the particles as well.29 In this scheme, an interface introduces particles into the instrument where they encounter a single scattering laser. The scatter pulse resulting from the particle passing through the laser beam triggers a Nd: YAG laser, displaced by a known distance from the scattering laser, to fire a set time later. The aerodynamic size of a particle determines its velocity through the instrument; thus, particles of different size can be selected by varying the delay time between the scattering pulse and firing the Nd:YAG laser. If the particle is traveling the proper speed to pass through the second (Nd: YAG) laser at the exact time it is set to fire, material will be desorbed and ionized from the particle. All particles traveling with different speeds (i.e., different sizes) will be missed which, for a polydisperse aerosol, occurs the majority of the time. If the particle is hit by the laser, the ions are then analyzed using a (14) Haley, R. H.; Kurt, T. L.; Hom, J. J. Am. Med. Assoc. 1997, 277, 215-222. (15) Appel, B. R. In Aerosol Measurement: Principles Techniques and Applications; Willeke, K., Baron, P. A., Eds.; Van Nostrand Reinhold: New York, 1993; pp 233-254. (16) Marple, V. A.; Rubow, K. L.; Olson, B. A. In Aerosol Measurement: Principles Techniques and Applications; Willeke, K., Baron, P. A., Eds.; Van Nostrand Reinhold: New York, 1993; pp 206-232. (17) Christoforou, C. S.; Salmon, L. G.; Cass, G. R. Atmos. Environ. 1994, 28, 2081-2091. (18) Ligocki, M. P.; Liu, H. I. H.; R., C. G.; John, W. Aerosol Sci. Technol. 1990, 13, 85-101. (19) Jambers, W.; De Bock, L.; Van Grieken, R. Analyst 1995, 120, 681-692. (20) Van Vaeck, L.; Struyf, H.; Van Roy, W.; Adams, F. Mass Spectrom. Rev. 1994, 13, 209-232. (21) Wouters, L.; Hagedoren, S.; Dierck, I.; Artaxo, P.; Van Grieken, R. Atmos. Environ. 1993, 27A, 661-668. (22) Tourmann, J. L.; Kaufmann, R. Int. J. Env. Anal. Chem. 1993, 52, 215227. (23) Allen, J.; Gould, R. K. Rev. Sci. Instrum. 1981, 52, 804-809. (24) Stoffels, J. J. Int. J. Mass Spectrom. Ion Phys. 1981, 40, 217-222. (25) Sinha, M. P.; Griffin, C. E.; Norris, D. D.; Esters, T. J.; Vilker, V. L.; Friedlander, S. K. J. Colloid Interface Sci. 1982, 87, 140-153. (26) Myers, R. L.; Fite, W. L. Environ. Sci. Technol. 1975, 9, 334-336. (27) Davis, W. D. Environ. Sci. Technol. 1977, 11, 587-592. (28) Ciggy, C. L.; Friedlander, S. K.; Sinha, M. P. Atmos. Environ. 1989, 23, 2223-2229. (29) Sinha, M. P. Rev. Sci. Instrum. 1984, 55, 886-891.

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quadrupole mass spectrometer. Unfortunately, because quadrupoles are scanning instruments and the ion burst from the particle is so brief, only a single mass-to-charge ratio can be analyzed for each particle. Spengler et al. improved upon Sinha’s scanning method by utilizing a time-of flight (TOF) mass spectrometer instead of a quadrupole mass analyzer.30 This greatly enhanced the sensitivity and allowed the acquisition of an entire mass spectrum for a single particle. Recently, Spengler et al. added a second flight tube to their instrument, making possible the simultaneous analysis of both positive and negative ions from a single particle.31 Unfortunately, in order to analyze a polydisperse aerosol with these instruments, the time between the scatter pulse and ionization pulse must be scanned. This capability has not yet been reported and would be, in principal, relatively slow, thus limiting its ability for real-time atmospheric monitoring. An alternate approach, first proposed by Marijnissen et al.32,33 and brought to fruition by Johnston et al., makes it possible to monitor the composition of a polydisperse aerosol in real time.34-36 This technique uses the scattering from a HeCd laser to detect particles and immediately fire an excimer laser to desorb and ionize material from the particle for analysis by TOFMS. Particle size is inferred from the scattered light intensity. Unfortunately, particle size information obtained in this way can vary by as much as 80%.37 Recently, Prather and co-workers developed the first analytical technique capable of simultaneously determining both the precise size and composition of individual particles from a polydisperse aerosol in real time. This technique, aerosol time-of-flight mass spectrometry (ATOFMS),38,39 uses light scattering from two lasers to determine aerodynamic diameter to within 1% and laser desorption/ionization (LDI) TOFMS for composition analysis. The laboratory-based ATOFMS instrument has been used for the analysis of atmospheric aerosols in Riverside, CA,40 and for the characterization of aerosols from a variety of particle sources.41-42 In addition, because of its fast sampling rate (6-8 s-1), it is ideal for rapidly screening both the size and composition of the entire aerosol distribution. Subsequently, Ramsey et al.43 used the scattering and ionization scheme developed by Prather and co-workers, coupled with an ion trap mass spectrometer (IT-MS) to perform tandem mass spectrometry (MS/MS) on individual particles. The additional information obtained by MS/MS can help positively identify higher molecular weight ions based upon fragmentation patterns. How(30) Hinz, K.-P.; Kaufmann, R.; Spengler, B. Anal. Chem. 1994, 66, 2071-2076. (31) Hinz, K. P.; Kaufman, R.; Spengler, B. Aerosol Sci. Technol. 1996, 24, 233242. (32) Kievit, O.; Marijnissen, J. C. M.; Verheijen, P. J. T.; Scarlett, B. J. Aerosol Sci. 1992, 23, S301-S304. (33) Marijnissen, J.; Scarlett, B.; Verheijen, P. J. Aerosol Sci. 1988, 19, 13071310. (34) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63, 2069-2073. (35) Murphy, D. M.; Thomson, D. S. Aerosol Sci. Technol. 1995, 22, 237-249. (36) Carson, P. G.; Neubauer, K. R.; Johnston, M. V.; Wexler, A. S. J. Aerosol Sci. 1995, 26, 535-545. (37) Salt, K.; Noble, C. A.; Prather, K. A. Anal. Chem. 1996, 68, 230-234. (38) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1403-1407. (39) Nordmeyer, T.; Prather, K. A. Anal. Chem. 1994, 66, 3540-3542. (40) Noble, C. A.; Prather, K. A. Environ. Sci. Technol. 1996, 30, 2667-2680. (41) Silva, P.; Prather, K. A. Environ. Sci. Technol. In press. (42) Liu, D.; Rutherford, D.; Kinsey, M.; Prather, K. A. Anal. Chem. 1997, 69, 1808-1814. (43) Yang, M.; Reilly, P. T. A.; Boraas, K. B.; Whitten, W. B.; Ramsey, J. M. Rapid Commun. Mass Spectrom. 1996, 10, 347-351.

Figure 1. Schematic showing the particle beam interface and particle-sizing region joined to the mass spectrometer region of the fieldportable instruments.

ever, the IT-MS currently has a very limited sampling frequency when not in MS/MS mode which becomes even smaller in MS/ MS mode. Taking all of the real-time particle mass spectrometry techniques as a whole, with one recent exception,44,45 a major shortcoming is an inability to routinely characterize the aerosol phase at more than a single geographic location, severely hindering their utility for atmospheric sampling. Therefore, a new generation of portable instruments is necessary to provide insight into the geographic variations, transport characteristics, and chemical dynamics of atmospheric aerosols. This article describes two identical instruments recently designed and built in our laboratory at the University of California, Riverside for such a purpose. EXPERIMENTAL SECTION Particles are sampled from ambient atmospheric conditions into the aerosol beam interface depicted in Figure 1. The particles first enter an inlet nozzle at atmospheric pressure and exit at a pressure of 2 Torr. This pressure differential causes the gas to undergo a supersonic expansion, during which small particles are accelerated to a higher terminal velocity than large particles. After expansion through the nozzle, the aerosol beam passes through two stages of differential pumping before entering the particle sizing region. The size-dependent velocity distribution within the (44) Murphy, D. M.; Thomson, D. S. J. Geophys. Res. 1997, 102, 6341-6352. (45) Murphy, D. M.; Thomson, D. S. J. Geophys. Res. 1997, 102, 6353-6368.

particle beam is the basis for determining the aerodynamic diameter of the particles in the particle sizing region of the instrument.46,47 Once in the sizing region, the particles pass through a continuous-wave argon ion laser beam, generating a pulse of scattered light which is collected by a photomultiplier tube (PMT). After traveling 6.0 cm, further the particle encounters a second laser beam oriented orthogonally to the first, generating another scatter pulse which is collected using a second PMT. Thus, particle velocity is determined by using the known distance between the scattering lasers and the measured time between the two scatter pulses. Ultimately, upon instrument calibration, this characteristic velocity is used directly to determine each particle’s aerodynamic diameter. The outputs of both PMTs are sent to a timing circuit, which is used to track the particles and control the firing of the desorption/ionization laser. As the particle is being “tracked”, it continues traveling through the scattering region and ultimately enters the source region of a laser desorption/ionization time-offlight mass spectrometer (LDI-TOFMS). As the particle reaches the center of the source, the timing circuit fires the desorption/ ionization laser. The TOFMS simultaneously monitors both the (46) Baron, P. A.; Mazumder, M. K.; Cheng, Y. S. In Aerosol Measurement: Principles Techniques and Applications; Willeke, K., Baron, P. A., Eds.; Van Nostrand Reinhold: New York, 1993; pp 381-403. (47) Wilson, C. J.; Liu, B. Y. H. J. Aerosol Sci. 1980, 11, 139-150.

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Figure 2. Size calibration curves used to convert particle velocity to aerodynamic diameter for both portable instruments. Exponential fit used for instrument 1: aerodynamic diameter, 67.906e-0.043velocity with R 2 ) 0.9964. Exponential fit for instrument 2: aerodynamic diameter, 61.816e-0.0109velocity with R 2 ) 0.9988.

positive and negative ions generated by the desorption/ionization event. Calibration of the particle-sizing region was accomplished by nebulizing polystyrene latex spheres (PSL), ranging in size from 0.2 to 7 µm in diameter. The PSLs were suspended in a 50:50 methanol/water solution and nebulized using particle-free air. To ensure that the particles were dry, the output of the nebulizer was sent through a diffusion drier. The particle flow was split and directed into an Aerosizer (API, Model 8050, Hadley, MA) to verify the particle size measurement and into the ATOFMS instrument to determine a characteristic particle velocity. The complete calibration curve ranging in size between 0.2 and 10 µm is shown in Figure 2. The points represent the experimental data, and the line is the best-fit exponential curve to the data. Each point on the graph is the average for 1000 particles, and the error bars indicate one standard deviation. The wood smoke particles analyzed in this paper were generated by burning a piece of wood and trapping the particles in a 20 L glass bottle equipped with inlet and outlet tubes. The outlet tube was attached to the ATOFMS instruments for direct sampling and analysis of the smoke particles. A HEPA capsule filter was attached to the inlet to allow particle-free air to be pulled through the container to ensure a consistent flow rate of air into the instrument. Direct atmospheric sampling was accomplished by two methods. In the early stages of the development process, a hole in the laboratory wall was used to run a copper sampling tube outdoors. Subsequently, the instruments were simply transported to various outdoor locations where aerosols were sampled directly. RESULTS AND DISCUSSION Design. Overview. A photograph of one of the portable ATOFMS instruments, designed and built in the chemistry department at U. C. Riverside, is shown in Figure 3. Each of the two identical instruments measures 72 in. long × 28 in. wide × 60 in. high and weighs ∼500 lb. An aluminum framework supports the main body of the instrument and its ancillary components. Due to their weight, the three roughing pumps and additional components are mounted toward the bottom of the aluminum framework, increasing the stability of the instrument. The assembled interface, particle sizing region, and mass spectrometer unit are located in the upper portion of the framework. 4086 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997

Finally, the computer is mounted above the main framework with the screen and keyboard at a convenient height for the operator. For mobility, the instruments are equipped with pneumatic tires affording them access to most locations through the use of standard doorways, elevators, sidewalks, and handicap ramps. In addition, they can be hoisted by helicopter, crane, or forklift for transportation to less accessible locations. However, the standard method for long distance transport is to simply load them into a truck using a lift gate and secure them for transport. During the design phase, a great deal of thought was given to making the system rugged enough for constant movement. In fact, after assembly of the first instrument, it was optimized in our laboratory, unplugged, loaded into a truck, driven 90 miles from Riverside, CA, to Long Beach, CA, unloaded, powered-up, and hitting particles 10 min later with no alignment necessary, thereby proving our design efforts effective. The specific steps taken to ensure this robustness are explained in the following sections. Interface/Vacuum System. The interface region draws in air and entrained particles at a rate of 20 mL/s. This sampling rate is determined by an inlet nozzle having an initial diameter of 5.08 mm tapered at 30° to a final diameter of 0.342 mm which is maintained for 3.53 mm (Figure 1, inset 1). The area between the nozzle and first skimmer is maintained at a pressure of 2 Torr by an 18 cfm mechanical pump (Edwards High Vacuum, Model E2M18). The first skimmer (Beam Dynamics Inc., Minneapolis, MN) has a 0.5 mm conductance-limiting orifice which is centered 2 mm from the exit of the nozzle. A second conductance-limiting skimmer (also 0.5 mm diameter) is located 2 mm from the first. The area between the two skimmers is maintained at 5 × 10-2 Torr by a second 18 cfm mechanical pump. The particle sizing region is evacuated to a pressure of 5 × 10-5 Torr by a 70 L/s turbomolecular pump (Varian, Model V-70). In addition, the mass spectrometer region, which is separated from the sizing region by a 0.3 × 3.0 cm conductance-limiting tube, is evacuated to a pressure of 2.0 × 10-7 Torr by two 250 L/s turbomolecular pumps (Varian, Models V-250 and V-250 SF)sone standard and one split flow. The low vacuum input of the split flow pump is used to back the standard 250 L/s pump as well as the 70 L/s pump on the sizing region. The split-flow pump is, in turn, backed by a third 18 cfm mechanical pump. Furthermore, should any one turbo or mechanical pump fail, the others can be rerouted to evacuate the system with a pump-down time of 30 min instead of the usual 10 min. Each mechanical pump is fitted with an oil mist filter and an in-line oil trap to prevent generation of oil mist particles and oil backstreaming, respectively. The turbomolecular pumps are aircooled and capable of operating in any orientation. Pressure measurement is accomplished using a vacuum gauge controller (HPS, Boulder, CO) with pirani gauges on the first two stages and cold cathode gauges on both the particle sizing and mass spectrometer regions. Ensuring accurate sizing and straight particle trajectories through the instrument requires a consistent expansion process through the inlet nozzle. During sampling, however, the nozzle slowly becomes clogged as particles accumulate and must be cleaned periodically (once every 2-4 h during high particle flux). Therefore, the particle interface has an interlocking ball valve (Figure 1) to allow nozzle extraction and cleaning while the instrument remains under vacuum in standby mode. The whole

Figure 3. A portable instrument in Long Beach, CA.

process generally takes 3-5 min. In addition, a nozzle extraction mechanism was designed and implemented to minimize wear, prevent accidental venting, and avoid nozzle damage while keeping the nozzle aligned. Manipulating the distances between the nozzle and skimmers provides control over the expansion conditions that accelerate the particles entering the instrument. These distances can be adjusted to maximize particle focusing and transmission through the interface region. For this reason, the upper stages of the instrument are adjustable and allow a minimum separation of 2 mm and a maximum separation of 9 mm between the nozzle and first skimmer as well as between the first and second skimmers. This is accomplished by simply inserting flat shims between the stages and is made possible because of the gland seal o-rings used between each stage. To preserve alignment during movement and assembly, each interface stage is designed to key into the subsequent stage. Each skimmer keys into a recess which is concentric with the outer alignment diameter of the stage. The last stage of the interface keys into the body of the sizing region which, in turn, keys into the mass spectrometer region, thereby maintaining alignment

throughout the instrument to well within 0.008 cm. The three interface stages and most other parts of the instrument were machined from solid pieces of 6061 T6 aluminum to minimize weight, and machining time and to eliminate the need for welds. All the vacuum seals are made with Viton o-rings to simplify assembly/disassembly procedures. Particle Sizing Region. Once the particles pass through the interface, they enter a sizing region (shown in Figure 1) where aerodynamic sizing takes place. The particles pass through two continuous-wave laser beams with the detected scattering pulses sent to a timing circuit. This circuit records the time required for the particle to pass between the two scattering laser beams and prepares a Nd:YAG laser to perform desorption/ionization at the appropriate time based on the particle velocity. In the portable instruments, fiber optics direct the scattering laser beams into the system. This eliminates the need to align the laser itself, making it possible to rack mount the laser and simply align the fiber-optic output. Therefore, an argon ion laser (Omnichrome, Model 532) is coupled onto the input side of a 100 µm multimode fiber-optic power splitter (AMP Corp., Model 2-107842-1). Each output fiber is inserted into an assembly that Analytical Chemistry, Vol. 69, No. 20, October 15, 1997

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Figure 4. Schematic of the timing circuit used to size and track the particles.

contains irises and a collimating/focusing lens which brings the beam to a 1 mm spot size. This assembly is placed into an XZ translator (Thor Labs, Newton, NJ) which is then adjusted to optimize the light-scattering intensity. The entire beam probe assembly is modular and easily removed from the main body for maintenance or modification. This optical arrangement retains alignment under normal transport conditions even when substantial shocks are encountered. If alignment is lost, the system has proven to be easy to realign. To maximize light collection efficiency, each scattering laser intersects the particle beam at one focus of an elliptical mirror with the resultant scattered light concentrated to the other focus of the ellipse, where it is detected by a PMT mounted directly in the system (Hamamatsu, Japan, Model H5783) (Figure 1, inset 2). Using this detection scheme, nearly 50% of the scattered light is collected. Each elliptical mirror and PMT pair is held in a precision-made module to maintain proper spatial and angular orientation with respect to the particle beam. The mirror/PMT module is easily removed from the main body of the scattering region for cleaning and maintenance. Currently, the smallest detectable particle has an aerodynamic diameter of 0.2 µm. Timing Circuit. The timing circuit in the field-portable ATOFMS is crucial for obtaining particle velocity, which is related to aerodynamic diameter, and for ensuring that each sized particle is desorbed and ionized by the Nd:YAG laser. A block diagram of the internal function of the timing circuit is shown in Figure 4. The signal first enters the timing circuit as an analog pulse from the upper PMT in the particle-sizing region and is converted by a discriminator to a TTL pulse. This pulse starts a 16-bit digital counter running at 20 MHz. Concurrently, the count-up trigger locks out other TTL pulses until it is reset, either by reaching a maximum value corresponding to 800 µs (overflow reset) or by 4088

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completing a “track and fire” sequence (master reset from the computer). The signal from the second PMT starts a counter operating at 10 MHz and signals the flight time data latch to hold the value of the counter. The slower countdown rate (10 vs 20 MHz) accommodates the 1:2 distance ratio between the two lightscattering lasers (6.0 cm) and the distance between the second scattering laser and the center of the mass spectrometer source (12.0 cm). This additional distance is required because of the time needed to fire the Nd:YAG laser (200 ( 5 µs). A trigger pulse is sent to the Nd:YAG flash lamps and a fire pulse is sent to the Q-switch of the Nd:YAG when the counter reaches +200 and 0 µs, respectively. Once the laser has fired, the computer reads the flight time data latch, acquires the mass spectrum, and sends the master reset signal to prepare the timing circuit for the next track and fire event. Mass Spectrometer Region. The time-of-flight mass spectrometer is designed to simultaneously analyze both positive and negative ions generated by the desorption/ionization of a single particle. It is the first dual-ion reflectron design, thus providing enhanced spectral resolution by compensating for the ion kinetic energy distribution.48 Furthermore, a coaxial design was selected to minimize instrument size and complexity. During the design phase of the dual-polarity TOF-MS, the ion optics were simulated using MacSimion 2.0 (Don McGilvery and Richard Morrison, Department of Chemistry, Monash University) on a Macintosh computer. Electrode geometry was refined to maximize resolution using the Simretof program (Dr. Gary Kinsel, University of Texas, Arlington) operating on a PC. Each of the flight tubes in the instruments is 50 cm long, providing resolution of 600 at m/z 165 when analyzing particles. The dual-ion coaxial reflectron mass spectrometer requires that the two primary detectors be mounted back to back close to the source region and have center holes for the ions to pass through as they exit the source, as seen in Figure 1. The primary detectors are 70 mm diameter MicroSphere Plates (MSP) (El-mul Technologies, Yavne, Israel), with a 6 mm center hole and a flat anode. The detectors are floated at high voltage to maintain a field-free region and are capacitively decoupled from the data acquisition electronics. In addition to the primary detectors, each instrument contains two 18 mm diameter MSPs with flat anodes behind the reflectrons. These may be used to detect ions in linear mode as a backup to the primary detector, for diagnostic purposes, or to detect neutral losses. Typical operating voltages in dual ion mode are as follows: +1700 and -4700 V for the two inner source plates, +7800 and -7800 V for the second source plates, flight tubes, and detectors, +1200 and -4525 V for the front and back of the negative ion reflectron, and -4458 V for the front and +900 V for the back of the positive ion reflectron. For particle desorption/ionization, a Nd:YAG laser (Minilight I, Continuum, Santa Clara, CA) was chosen which, when operating at 266 nm, has a 5 ns pulse width and generates 1.0 mJ/pulse. When focused to a 400 µm spot size, as in this application, a power density of ∼1 × 108 W/cm2 is achieved. The beam exits on the far side of the chamber with its power read by a laser power meter (Molectron, OR, Model EPM1000). To enhance the durability of the mass spectrometer, no grids (48) Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Sov. Phys. JETP 1973, 37, 45-48.

are used in the ion optics as they are prone to being damaged even under ideal conditions. Modified automotive spark plugs are used as high-vacuum electrical feedthroughs because they are commonly available and inexpensive. All internal parts of the mass spectrometer are insulated and held by Kel-F, which is much easier to machine and more resistant to cracking than ceramic. To greatly speed and simplify assembly/disassembly procedures, all vacuum seals are made with Viton o-rings. To modify or inspect any part of the mass spectrometer, one only need remove the chamber lid, which grants access to all electrical and mechanical components of the mass spectrometer. The source, flight tubes, reflectrons, and detectors are secured in place with a total of six fasteners, and all electrical connections are made with quick disconnect in-line connectors, making all the components easily accessed and removed. Once the fasteners and electrical connections are removed, the flight tubes and reflectrons can be removed. Furthermore, the source and detectors are all mounted in a single cassette which is fastened to one of the flight tubes for ease of installation and removal. The rectangular chamber that contains the mass spectrometer components was machined out of a solid block of 6061 T6 aluminum and thus contains no welds. This robust construction allows one to mount the ionization laser and associated optics directly to the outside wall, thus preventing loss of laser alignment during transportation. The flat exterior also makes it easy to mount the electrical feedthroughs, pressure gauges, and vacuum pumps directly to the chamber. The 21 plates for each reflectron are made of 304 stainless steel and connected in series with 10 MΩ vacuum-compatible resistors (Caddock, CA, Model MG 608). The steel disks are held rigid with four Kel-F rods and slipped into an aluminum canister to make a complete reflectron assembly. The source plates and flight tubes are also made of 304 stainless steel. The design accommodates the thermal expansion coefficients of the different materials used to build the instruments, thus preventing damage by thermal expansion/contraction in the field. Data Acquisition and Control Software. The data acquisition and control software was written in-house (Microsoft Visual Basic 4.0 running under Windows 95 on a Pentium-90 computer) to control the components of the instrument, data acquisition, and data storage. The program interfaces with the instrument using TTL signals from an I/O board (Keithly-Metrabyte PIO-96) which are used to send/receive information from the timing circuit and are also used by the control unit to activate/deactivate specific instrument components. By this means, the program orchestrates the startup and shutdown sequences for all components of the instrument. Furthermore, the program can place the instrument in “idle” mode in which unnecessary components (such as the lasers, PMTs, and high voltage) are set to low-power modes when data are not being acquired. Each time the system goes through a track and fire sequence, the acquisition software reads the (1) count number from the timing circuit and calculates the velocity of the particle, (2) laser power from the laser power meter via an IEEE-488 bus, (3) positive and negative ion spectra from the data acquisition boards, and (4) acquired data to determine if the particle was hit or missed by the desorption/ionization laser. If a particle is hit, the spectra (both positive and negative) and associated data (velocity, laser power, date/time, operating conditions) are saved in a hit-particle file. If, on the other hand, the particle is missed, no mass spectra

are acquired but the other data (velocity, date/time, operating conditions) are stored in a missed-particle data file. On the basis of this information, the program generates a real-time display containing either particle size histograms for both hit and missed particles or the real-time mass spectra of the particles as they are being analyzed. The program also displays the current operating conditions of the instrument, including the vacuum gauge readings and the status of each component. With the current hardware and software combination, the instrument can execute an entire track and fire sequence in less than 100 ms if the particle is missed and ∼300 ms if the particle is hit. The 200 ms difference represents the time it takes to compress and store the spectra. Therefore, computer processing time limits the maximum track and fire rate to slightly less than the 10 Hz maximum repetition rate of the Nd:YAG laser. The firing rate is typically 6-8 Hz when particles are abundant. Electrical Systems. All of the pumps, lasers, power supplies, and other electrical devices on the instrument are controlled by software via an I/O interface connected to a custom-built control unit. The control unit serves as a power distribution center for the entire instrument and can separately switch on and off all instrumental components. The input power (30 A at both 110 and 220 V) can be supplied by an existing power grid or, in remote locations, by natural gas-powered generators. The control unit also contains the timing circuit, a power supply and control interface for the PMTs, and an interlock circuit which shuts off the high-voltage power supplies if the pressure in the mass spectrometer exceeds a preset threshold. High voltage is provided by six negative and six positive 15 kV adjustable power supplies (Spellman, Model MP-15) which are built into a compact box with a low-voltage power supply, adjustment controls, a voltmeter, and output connectors. Frame Construction. The frame, to which the main body of the instrument and all its components are mounted, is constructed using a lightweight extruded aluminum assembly system (80/20 Inc., Columbia City, IN). The supporting electronics and lasers are mounted on three standard 19 in. rack mounts near the bottom of the frame. The entire interface, light scattering, and mass spectrometer assembly are mechanically separated from the aluminum support structure by rubber shock mounts for vibration isolation. In addition, the assembly has two pivot points located on the ends of the rectangular chamber allowing the entire mass spectrometer to be flipped on its side, providing easy access to the mass spectrometer region. Performance. To initially characterize the instruments, we used wood smoke particles because they have been extensively characterized in our laboratory, are easily generated, and provide a broad distribution centered at relatively small sizes.49 Wood smoke is also an atmospherically significant contributor to particulate pollution in the Los Angeles basin during the summer months due to brush fires and during the winter months because of wood fires used for home heating.50 Simultaneously acquired dual-ion mass spectra of a wood smoke particle are shown in Figure 5. The particle’s velocity (361.5 m/s) corresponds to an aerodynamic diameter of 1.2 µm. The positive ions present are assigned as carbon/hydrogen envelopes ranging from C2+ to C5+, a very characteristic potassium signal, and a prominent CH3CO+ (49) Silva, P. J.; Liu, D.; Noble, C. A.; Prather, K. A. Manuscript in progress. (50) Hildemann, L. M.; Markowski, G. R.; Cass, G. R. Environ. Sci. and Technol. 1991, 25, 744-759.

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Figure 5. Simultaneous dual-ion mass spectra from a single wood smoke particle having an aerodynamic diameter of 1.2 µm.

The dual-ion mass spectra of a single atmospheric aerosol particle are shown in Figure 7. The positive ion spectrum has a prominent peak at mass-to-charge 18 (NH4+) which, in combination with the peak at mass-to-charge 30 (NO+), strongly indicates the presence of ammonium nitrate in the particle. Other components are assigned in the positive ion mass spectrum including K+, V+, VO+, and several organic peaks (C2H3+ and CH3CO+) . Furthermore, the corresponding negative ion mass spectrum is replete with nitrogen-containing peaks (NO2-, NO3-, NO3‚HNO3-). A small HSO4- component is also present as it was in the wood smoke particle.

Figure 6. Comparison of the hit and missed particles taken over the course of 1 h on 10/2/96 in Fullerton, CA. The light bars represent the missed particles and the dark bars those that were hit. The solid line is the hits multiplied by 8.6 times for direct comparison to the missed particles.

peak. The negative ion signal is principally comprised of oxygencontaining hydrocarbon peaks as well as small C2H4- and HSO4signals. The assignments made to the ion peaks in the mass spectra are based on previous studies performed in our laboratory using particles of known composition. It is important to note that there are other possible assignments but these are the most probable. Having characterized the instruments using calibrant particles, we then proceeded to sample actual atmospheric particles. Figure 6 shows a comparison between the particles that were sized and chemically analyzed (i.e., hit) and those that were sized but not chemically analyzed (i.e., missed) on 10/2/96. The overall shape of the hit particle histogram compares well with that of the missed particles, which demonstrates no size biasing in the 8.6% that were hit. 4090

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CONCLUSIONS We have designed, built, and demonstrated a new generation of aerosol analysis instruments which are portable and provide the size and composition of individual particles from a polydisperse aerosol in real-time. The instruments are designed to simultaneously analyze both positive and negative ions from individual particles and can be moved to virtually any location. They are designed to be capable of operating while in motion during transport by aircraft, ocean-going vessel, or ground-based vehicle. Furthermore, the instruments are durable and easily maintained, critical features for any field instrument. These portable ATOFMS instruments are potent new tools for understanding the production, chemistry, and ultimate fate of atmospheric aerosols. In fact, the instruments have already been involved in a field study conducted during the fall of 1996. This study was performed in conjunction with Professor Glen Cass’ group at the California Institute of Technology to monitor marine aerosol chemistry during a typical inland migration event in southern California. By comparing the standard aerosol analysis techniques, used by the Caltech group, with the novel information obtained with our portable ATOFMS instruments, proper scaling factors will be determined to ensure that ATOFMS can reliably

Figure 7. Simultaneous dual-ion mass spectra from a single atmospheric aerosol particle in Riverside, CA, having an aerodynamic diameter of 1.87 µm.

be used to measure absolute particle number concentration, while providing quantitative chemical composition information. The results of this effort will be presented in a series of forthcoming articles. ACKNOWLEDGMENT The authors thank the other members of their research group who contributed their time, energy, and experience at various points during this development effort. These include Kim Salt, Chris Noble, Don-Yuan Liu, Phil Silva, Deborah Gross, Markus Gaelli, Rich Carlin, and Sylvia Wood. We also acknowledge Matt McCormick for his work manufacturing the intricate inlet nozzle

as well as for the plethora of additional advice and shop cleanup tutorials that he so freely bestowed. This work was supported through a grant from the California Air Resources Board (ARB Contract 95-305).

Received for review May 27, 1997. Accepted July 28, 1997.X AC970540N

X

Abstract published in Advance ACS Abstracts, August 15, 1997.

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