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erosols play an important role in a wide range of chemical and physical processes, including those involved in air pollution (urban and regional smog, acid deposition, stratospheric ozone, global warming); medicine and health (airborne microbes, respirable aerosols); combustion (soot and soot precursors): materials synthesis and processing (nanoparticles, coatings, mining operations); and clean-room technology @articlecontamination). Research and development in these areas require analytical methods that can monitor changes in aerosol particles as they grow or are transformed by condensation, evaporation, or chemical reaction. This can be challenging because the particle mass of greatest interest ranges from 1 0 F to lo-' g, and the concentrations of minor components in particles are often of considerable interest In this article, we will discuss recent advances in the development and application of MS for on-line, real-time analysis of single aerosol particles. We will cover the role of aerosol particles in air pollution, the characteristics of an ideal aerosol analyzer, the development of MS for this type of analysis, instrumentation, and a p
Murray V. Johnston and Anthony S. Wexler University of Delaware 0003-270019510367-721 A1$09.00/0 E3 1995 American Chemical Socieh
A better understanding of atmospheric particles will lead to a better understanding of their efects on our health and climate plications for laboratory and field investigations. Although we emphasize atmospheric aerosols, applications in other areas of aerosol science and technology are also relevant. Atmospheric aerosols Particles in the atmosphere contribute to almost every air pollution problem. In the urban troposphere, aerosols have been implicated in increased morbidity and mortality. The effect is acute-increased particulate concentrations correlate with increased incidence of health effects ( I ) . Aerosol particles also influence global climate directly by scattering solar radiation and indirectly by changing the albedo and occurrence of clouds (2).In the form of polar stratospheric clouds, particles are
the storehouse of chlorine and nitrogen compounds that cause the austral spring ozone hole (3).Finally, aerosols contrib Ute to acid deposition and visibility reduction over large portions of the globe. Aerosol particles are emitted from both anthropogenic and natural sources. For example, sub-Saharan Africa and the steppes of China are substantial sources of wind-blown dust; plants slough wax particles from their leaves; waves breaking in the oceans emit a tremendous number of sea salt particles: and volcanoes periodically blanket the globe with crustal and sulfurcontaining particles. Anthropogenic sources include biomass burning, power plant emissions, industrial processes, and transportation systems. In addition to these socalled primary sources, secondary sources emit vapors into the atmosphere and result in particle formation via condensation, chemical reaction, or nucleation. Once in the atmosphere, the primary particles and secondary compounds undergo many transport and transformation processes (4). Atmospheric particles are composed of a wide variety of compounds depending on their source and the atmospheric processing they have undergone between emission and analysis. Typical compound classes include crustal material, heavy metals, carbonaceous compounds, water, and inorganic electrolytes.The c ~ s t amal terial can contain silicon dioxide and cal-
Analytical Chemistry, December 1, 1995 721 A
cium salts. Heavy metals include lead, chromium, arsenic, vanadium, and selenium. The carbonaceous material is composed of elemental carbon (soot) and many organic compounds that are larger than C,. The inorganic electrolytes are usually acidic and very hygroscopic, resulting in atmospheric particles that may contain water, even at low relative humidities. Particle sizes in the atmosphere range from 10 nm to 100 pm. Particles in the l h m to 1-qm range are called "fine" and are the result of homogeneous nucleation processes. These particles grow until they enter the accumulation mode at 0.1-1 pm. Particles in the 2- to 20-pm range are termed "coarse" and usually result from abrasive processes. Particles larger than 20 pm are usually droplets in fogs or clouds composed predominantly 01 water. Particle numbers range from 10 to several thousand per cm3 of air (4). It is necessary to measure both the size and composition of atmospheric particles to understand their sources, environmen. tal effects, and processing in the atmosphere. Ambient particles are normally collected on a filter or impactor in devices that may also segregate particles by size; the bulk samples are then characterized by conventional analytical techniques ( 5 ) .Particle collection must take place over an extended period, usually hours, before a sufficient sample is acquired. This limits temporal resolution but may also enable particle reactions to proceed or condensation or evaporation of volatile compounds to occur on the substrate. Collection of a bulk sample usually implies that particle-to-particle variations in composition cannot be assessed. Analysis of individual atmospheric particles has been performed with off-line microanalytical techniques such a s electron probe microanalysis, particle-induced X-ray emission, secondary ion MS, or laser microprobe MS (6).These methods give particle-to-particle variations in composition but, like the bulk methods,sre subject to poor temporal resolution and sampling artifacts. T h e ideal method would analyze individual particles in real time using instrumentation that is SUBciently compact and robust for field operation. A new generation of mass spectrometers that comes close to achieving these objectives has been developed.
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tection and sizing system, and a mass spectrometer. Figure 1 shows the instrument used in our laboratory (19). Aerosol inlet. The aerosol inlet t r a m mits particles from atmospheric pressure to the near vacuum of the source region. Current inlet designs use either a capillary (typically 0.5 mm in diameter) or a nozzle as shown in Figure 1. Often these entrances are followed by one or more stages of differential pumping to lower the gas load to the source region. Particles can be sampled only if they do not deposit on the inlet and are transmitted to a relatively small region, usually 200 pm in diameter, in the source. Ideally, the inlet should have both a high transmission efficiency and a high particle-flow rate so that a sufficient number of particles over a range of sizes are sampled and size bias is avoided. High transmission efficiency also minimizes clogging and associated downtime. Unfortunately, current inlet designs fall short of this ideal. Transmission efficiency decreases in two ways. First, air sampled through the inlet must undergo a rapid acceleration as it enters. Because large particles have substantial inertia, their paths may vary from fluid streamlines and hit the inlet walls, perturbing the flow stream and reducing transmission efficiencies for subsequent particles. Second. as particles enter the mass spectrometer source tionhonizatiou 0 1 ) coupled with time-of- region, the flow expands rapidly and, because small particles have low inertia, the flight MS (TOFMS) could overcome expansion drags them away from the centhese problems (13).LDI permitted a terline to where they cannot be detected wider variety of components to be effior analyzed. Positioning the laser beams ciently analyzed, and the TOF mass spectrometer permitted an entire mass spec- close to the inlet exit can minimize the effect of this divergence, but the improvetrum to be obtained from each particle. A similar approach was suggested by Marij- ment is limited by design constraints of the ion source assembly. nissen and colleagues in the late 1980s (14).Sinha investigated the possibility of Long thin capillary inlets have low flow rates because of fluid drag, but the long using LDI for single-particle analysis in the transit time in the capillary enables most mid-1980s. but the experiment was constrained by low laser irradiance and the particles to attain the velocity of the flow before exiting. Although a low flow rate use of a scanning mass analyzer (15). reduces the number of particles that can McKeown's encouraging results stimube sampled, it also results in a lower aclated rapid development of a new generaceleration, which reduces the deposition tion of instruments by several groups ( 1 6 of large particles. Nozzles containing an 19).Although the layout of each instruorifice or a short capillary entrance have ment differs somewhat, reflecting the higher flow rates and, because the resispecific demands of the intended application, the fundamental design principles are dence time is lower, the particles have velocities that depend on their aerodyvery similar and encompass three basic components: an aerosol inlet, a particle de- namic size, which facilitates sizing. Both
Aerosol MS In situ analysis of single aerosol particles dates back to the 1970s (7-12). In these early experiments, chemical information was obtained hy impinging particles on a heated surface in the source region of a mass spectrometer and thereby creating a burst of ions, either directly by surface ionization or indirectly by electron ionization of thermally generated neutrals. Although these instruments provided the first opportunity to analyze individual particles in real time, they were limited in that only a few components (primarily alkali metals) could be efficiently vaporized and ionized. Furthermore, they used quadrupole or magnetic sector mass analyzers that were incapable of scanning the entire mass spectrum for the burst of ions from each particle. In the early 199Os,Johnston and coworkers demonstrated that laser desorp
Aerosol particles are emittedfiom both anthropogenic and natural sources.
Analytical Chemistry, December 1, 1995
chamber
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Singiechanne dyzer
Figure 1. Experimental setup for studying aerosol reactions by MS. BS, beam splitter; BC, beam combiner; M. mirror. (Adaptedwith permission from Reference 19.)
designs suffer from clogging and divergence. Overall efficiencies are 1/500 or less, but improvements are under develop ment. Particle detection and sizing. Particles emerging from the inlet are analyzed by firing the desorption/ionization laser while the particle is in the laser beam path. Because particles enter the inlet at random, the duty factor for analysis is greatest when the laser pulse is synchronized with the arrival of a particle. In the instrument depicted in Figure 1. this is accomplished by placing a continuous laser beam upstream from the desorption/ ionization laser beam. When a particle passes through the laser beam, the burst of scattered radiation triggers the desorp tiodionization laser. This approach works well for gas discharge (excimer, nitrogen) lasers that can be fired within 1 ps of receiving an external trigger. In this case, the continuous and pulsed laser beams are spatially s e p arated by 200-300 pm. When LDI is performed with a NdYAG laser, the scatter pulse must precede the desorption/ ionization laser pulse by 150 us, (Unlike gas discharge lasers, the NdYAG laser requires a substantial amount of time to build up the population inversion and, hence, a longer pretrigger warning.) This procedure requires a more sophisticated arrangement, typically two continuous la-
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ser beams located 10-15 cm upstream from the Nd:YAG laser beam. The detection step also provides the o p portunity to size each particle before analysis. The simplest approach is to measure the height of the scatter pulse from the continuous laser beam and correlate it with particle diameter. This determination is approximate at best, because the intensity of scattered radiation also depends on the particle composition and morphology. In addition, many particles will be undersized because they pass through the edge of the laser beam where the excitation intensity is smaller. Nonetheless, the determination is sufficient to classify particles into broad size ranges, such as distinguishing accumulation mode from coarse particles (18).In the future, this method may be improved by correlating the height and width of the scatter pulse; the width of the scatter pulse is related to the velocity of the particle, and the velocity can be related to size. An elegant approach developed by Prather and co-workers uses two continuous laser beams to track particles a s they travel through the vacnnm chamber (20).When a particle passes through the first laser beam, the scattered radiation provides a start pulse for a clock. When this same particle passes through the second laser beam, the scattered radiation provides a stop pulse and the time be-
tween the two pulses is recorded. The inverse of this time difference is proportional to the particle velocity and, with nozzle-type inlets, the velocity is proportional to the square of the aerodynamic diameter da. For spherical particles, d, = p’”d, where p is the specific particle density and d , is the actual diameter. This method gives a precise measure of d,, typically 1-10% RSD, and is especially useful for characterizing aerosols that have a complex size distribution with several closely spaced modes (21). A third possible approach is based on lase-Doppler velocimetry (22).A single continuous laser beam is split into two beams and then recombined to produce an interference fringe pattern. When a single particle passes through these fringes, the scattered radiation exhibits a periodic oscillation. The frequency of this oscillation is proportional to the particle velocity and can be used to determine d,. This method is often used in commercial particle sizing instruments Although not yet incorporated into aerosol mass spectrometers, it offers the possibility of both high precision for particle sizing and modest complexity. The choice of which method to use d e pends on the precision required for particle sizing, the size and complexity of the optical setup that can be tolerated (simple optical configurations are best suited for robust, compact instruments), and the synchronization requirements of the pulsed laser used for LDI. All of these methods are limited in that the intensity of the scattered radiation from a single particle becomes vanishingly small as the particle size decreases much below the wavelength of the laser radiation. The d e tection limit for small particles by light scattering depends on the exact optical configuration and is typically 0.1-0.2 pm, although detection of particles as small as 70 nm in diameter has been reported (21).Smaller particles can be analyzed by LDI, but the desorption/ionization laser cannot be synchronized to the arrival of a particle (23).Thus, the duty factor for analysis may be too low to be practical. LDI-MS. After a particle is detected and sized, it is analyzed by LDI. In most instruments, this step is performed directly in the source region of a TOF mass spectrometer in which the particle beam
Analytical Chemistry, December 1, 1995 723 A
crosses a high-power pulsed laser beam and the laser pulse is synchronized with the arrival of a particle. The burst of ions produced from a single particle is accelerated from the source region into the flight tube. Because all ions are accelerated to the same nominal kinetic energy, different m/z ions travel down the flight tube at different velocities and reach the detector at different times. Thus, a c o r plete mass spectrum can be recorded for each particle by simply monitoring the detector response as a function of time aftero the desorptionlionization laser pulse. The instrument shown in Figure 1includes a reflecting field mass analyzer, which offers improved mass resolution over a simple linear configuration by compensating for the initial kinetic energy distribution of the ions produced by LDI. High kinetic-energy ions travel down the flight tube faster than low kinetic-energy ions, but they also spend more time in the reflector by penetrating further into the field. Thus, the flight times of different kinetic-energy ions are similar provided that the ions are ejected from the particle in the direction of the flight tube. However, the reflecting field does nor compensate for the kinetic energy distribution of ions ejected in the opposite direction, because these ions must turn around and accelerate toward the flight tube. The net effect is that the resolution improvement is not as dramatic as is observed in other surface desorption experiments in which ions are ejected in the fonvard direction only. With a configuration similar to that of Figure 1, adjacent peaks with masses up to several hundred daltons can be baseline resolved. Although the reflecting field analyzer gives sufficient resolution over the mass range of interest for atmospheric aerosols, it also tends to be bulky. Because a compact footprint is usually required, this can be a significant problem in designing a field instrument. One solution is to use postsource pulse focusing in combination with a short, linear flight tube (18).This method involves application of an accelerating voltage pulse to ions after they have left the source region. The timing and magnitude of the pulse are adjusted to compensate for the initial kinetic energy distribution of ions produced by LDI so that adequate resolution is achieved with 724 A
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Figure 2. Successive negative ion mass spectra of ambient aerosol particles.
microchannel plate. Each spectrum corresponds to a single particle. Laser shots 131t and 1313 either missed the particles or were false triggers caused by noise on the scattered-light signal. (Adapted with permission from Reference 18.)
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a relatively compact design. The resolution, however, is not as good as that achieved with reflecting analyzers. Ion trap mass spectrometers have also been used for singleparticle analysis (24, 2 9 Ion traps have several advantages for this application, including very small size, the ability to perform tandem MS and trap and store particles for an extended period of time before analysis, and the ability to detect positive and negative ions simultaneously. In normal opera-
Ana/ytica/ Chemistry, December 1, 1995
tion, a charged particle is levitated in the trap and then analyzed by LDI (25).Thus, both the particle and laser beams must pass through the trap, and the experimental conditions (pressure, electrode voltages) must be manipulated first to levitate a particle and then to analyze the ions produced from it. As an alternative approach, the particle detectionlsizing and LDI steps could be performed in an external ion source and the ions injected into the trap for analysis.
Applications
The analytical capabilities of on-line LDI are similar to those of laser microprobe MS (IAMMS), which has been used for almost two decades to analyze particulate matter (6).LAMMS, however, is an offline method because particles must be collected and mounted on a substrate, and the extended sample handling time allows semivolatile components to evaporate. Nonetheless, LAMMS has several proven advantages for single-particle analysis, such as detection of trace metals at the part-per-million level, speciation of inorganic compounds (especially those containing nitrogen and sulfur), detection of trace organics (especially aromatics), and the ability to distinguish the surface versus total volume composition of a particle. LAMMS also has a significant limitation: large pulse-to-pulse and particleto-particle variations of the ion signal intensity inhibit quantitation. On-line LDI is subject to these same advantages and limitations except for one important caveat. Because several particles can be sampled and analyzed per second, large data sets can be acquired in a very short time. ?his offersthe possibility of improving the precision of analysis by averaging the spectra of particles having similar s u e and composition. The spectra of individual particles are used to classify similar particles into groups. The average composition of each group is then quantitatively determined by comparison with spectra of standard particles having known size and composition. A simple example of this approach is the quantitation of ionic components in liquid microdroplets (26).Liquid droplets are unique in that the solvent provides a reproducible matrix composition surrounding the analyte. and preparation of calibration aerosols having a known amount of analyte is straightforward. Averaging the spectra of a sufficient number of "identical" droplets (- 25) permits the concentrations ofionic components to he determined to better than *lo%. Detection limits are lo-' to M. which is adequate for the range of concentrations normally found in cloud and fog droplets. A more complex example is sulfur speciation. Sulfur is ubiquitous in the atmosphere and plays an important role in acid deposition, urban smog, stratospheric
ozone, and global climate change. In the marine environment, dimethyl sulfide is oxidized to sulfur dioxide or methanesulfonic acid. Sulfur dioxide is subsequently oxidized to sulfuric acid, which may then be neutralized to form sulfate salts. The relative amounts of methanesulfonic acid and sulfate depend on a variety of atmospheric conditions. Because its sole source is dimethyl sulfide oxidation, the presence of methanesulfonic acid indicates natural, as opposed to anthropogenic, sulfur emissions. For particles containing both methane sulfonic acid and sulfate, the mass spectra are not simply a juxtaposition of the spectra of each compound because the pres-
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ence of one compound can affect the distribution of ions produced from the other. However, chemometric techniques can be used to classify mixed-composition particles on the basis of mass spectral changes that occur as the composition changes. For example, a classification and regression trees ( C W algorithm can be used to classify particles according to their methanesulfonic acid to sulfate mole ratio (27).More than 80%of the particles are assigned the correct mole ratio, and the particles that are misclassified fall primarily in adjacent concentration ranges. Thus, the combination of large data sets and advanced chemometric techniques can significantly improve quantita-
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Figure 3. Negative ion spectra of NaCl particles exposed to reactive gases.
(a) Unreacted, (b) after exposure to 2.7 ppm nitric acid vapor and 1 .O ppm ammonia vapor, and (c)after exposure to 2.7 ppm nitric acid vapor. Each spectrum is the average of - 20 single-particle spectra. (Adapted with permission from Reference 19.) Analytical Chemistry, December 1, 1995 725 A
tion beyond the level that one would normally expect of LDI.These methods are of greatest use when particle size and matrix composition are well defined. In field measurements, aerosol MS can be used to assess particleto-particle composition variations (18).Figure 2 shows successive spectra obtained during 1min of data collection from an instrument located in a trailer near the continental divide at Idaho Hill, CO. The data set consisted of 5500 particles that were sampled and analyzed over a period of several days. Four of the particles show sulfate as the dominant anion, whereas the fifth particle is carbonaceous with no detectable sulfate. Based on light-scatteringmeasurements, the sulfate particles are estimated to be in the 0.3- to O.&um diameter range, and the carbonaceous particle is in the 0.7- to l y m diameter range. Unlike off-line analytical methods in which particles are collected over time, online LDI allows sampling of each particle at a well-defined time with minimal chemical transformation before analysis. Had these particles been mixed on a filter or impactor and subjected to bulk analysis, far different conclusions might have been drawn about their source and homogeneity. When the entire data set is examined, correlations among different components in the same particle, for example, nitrate and sulfate, can he drawn and compared with theory Some conditions may promote internal mixing (Le., the chemical constituents are equally distributed among all particles), and others may promote external mixing (Le., the constituents are differentially distributed). Aerosol MS can also be used in the laboratoly to obtain thermodynamic and kinetic information on aerosol reactions. NaCl particles, ubiquitous in the lower marine troposphere, are often observed in coastal locations over land. In clean locations, the particles are composed primarily of NaCI, but in more polluted locations condensation and evaporation of reactive gases may alter the particle composition. Results of a procedure in which NaCl particles were exposed to reactive gases before sampling into the mass spectrometer are shown in Figure 3 (19). Figure 3a shows an averaged spectrum of unreacted 3Spm diameter NaCl particles, and Figure 3b shows how the spec-
trum changes when the particles are exposed to two pollutants, nitric acid vapor and ammonia vapor, which react on the parti. d e surface to form an ammonium nitrate coating. Ammonium nihate is indicated by the appearance of ions such as 0; NQ. and NO, in the spectra as the partial pressures of the reacting gases increase. F i r e 3c shows the spectrum of particles that have been exposed to nitric acid vapor alone; because of diffusion limitation, at most a few monolayers of NaNO, were formed at the particle surface. A low-intensity N Q peak in Figure 3c is not observed with pure NaCl particles; thus, extremely thin particle coatings can be detected. As it stands Several instruments are now available that can determine the chemical composition of individual aerosol particles in real time,
References (1) Dockely, D. W. et al. N. Engl. J. Med. 1993,329,1753. (2) Charlson, R J. et al. Science 1992,255. 422. (3) Hamill, P.;Toon, 0. B. Phy. Today 1991, 44,s. (4) Pandis, S. N.; Wexler, A. S.; Seinfeld. J. H. J. By.Chem. 1995,99,9646. (5) Flagan, R C . In Measurement Challenges
in Atmosbheric Chemism Newman. L..
681. (7) Meyers, R. L.; Fite, W. L. Enuiron. Sci. Technol. 1975,9,334. (8) Davis, W. D. Enuiron. Sci. Technol. 1977. 11.587. (9) Sidi&, J. J. Int. I. M ~ Spectrom. S Ion pro c e s s 1981,40,217. (10) Allen, 1.;Gould, R. K. Rev. Sci Inst”. 1981.52,804. (11) Sinha. M. P. eta1.I. Colloidlnterface Sci. 1 9 8 ~87.140. . (12) Sinha. M. P.: Friedlander. S. K. Anal. Chenz. 1985.57.1830 ~~, (13) McKeown. P. J.;lohnston,M. V.; Murphy, D. M. Anal. Chem. 1991.63,2069. (14) Marijnissen,J.; Scarlett,B.; Verheiuen, P. J.Aerosol Sci. 1988,19,1307. (15) Sinha, M. P. Rev. Sci. Inst”. 1984,55. 886. (16) Prather, K. A.: Nordmeyer, T.; Salt, K. Anal. Cken. 1994.66.1403. Him, K-P.; Kaufmann,’ R.; Swngler. B. Anal, Chm. 1994,66,2071. Murphy, D. M.;Thomson, D. S. Aerosol Sci. Technol. 1995,22,237. Carson, P. G.; Neubauer, K. R; Johnston, M. V.; Wexler. A. S. I. Aerosol Sci. 1995, 26,535. (20) Nordmeyer, T.; Prather, KAnal. Chem. 1994,66,3540. (21) Prather, K. A. et al. Proceedings ofthe 43rd ~~
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The combination
of large data sets and chemometric techniques can improve quantitation. and some provide concurrent size information. Because particles are sampled and analyzed within a millisecond, temporal resolution is excellent and there is minimal opportunity for analyte alterations during sampling. Although current instruments are designed primarily for laboratory use, a number of field-portable versions are under development. These second-generation instruments will enable a far better understanding of aerosol particles that affect our environment. Eventually, they may also enable rapid diagnosis of particleladen streams in numerous industrial applications spanning the chemical, pharmaceutical, semiconductor, and mining industries. lhis work was supported by the National Sci.
ence Foundation and the Environmental Protection Agency. We thank D. Murphy for providing information and figures. and IC Prather for a critical reading of the manuscript
726 A Analytical Chemistry, December 1, 1995
ASMS Conference on Mass Spectmmehy andAIlied Topics, 1995, p. 1. (22) Wilson, J. C.: Liu, B. Y. J. Aerosol Sci. 198O,II, 139. (23) Reents, W. D. et al. J. Aerosol Sci 1995, 23,263. (24) Dale, J. M.;Yang, M.; Whitten, W. B.; Ramsey, J. M.Anal. Chem. 1 9 9 4 . 6 6 , 3431. (25) Yang, M.; Dale, J. M.; Whitten, W. B.; Ramsey, J. M.Ana1. Chem. 1 9 9 5 , 6 7 , 1021. (26) Mansoori, B. A: Johnston, M. V.; Wexler, AS. Anal. Chem. 1994,66,3681. (27) Neubauer, K. R; Sum, S. T.; Johnston, M. V.; Wexler, A. S. Proceedings of the 43rdASMS Conferenceon Mass Spectrometry andAIlied Topics. 1995,p, 5.
Murray V.Johnston is associate professor in the c h e m i s t y and biochemistry d e p a e ment a t the University of Delaware. Antkony S. Wexler is associate profssor in the mechanical engineering depaltment at the Uniuenitj of Delaware. Address correspondence to either author in his departntent, Univmity ofDelaware, Newark, D E 19716.
