Report
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INDIVIDUAL AEROSOL PARTICLES
A
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 (particle contamination). 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 10"18 to 10"9 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 ap-
Murray V. Johnston and Anthony S. Wexler University of Delaware 0003-2700/95/0367-721 A/$09.00/0 © 1995 American Chemical Society
A better understanding of atmospheric particles will lead to a better understanding of their effects 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 (1). 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 contribute 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 sulfur-containing particles. Anthropogenic sources include biomass burning, power plant emissions, industrial processes, and transportation systems. In addition to these so-called 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 crustal material can contain silicon dioxide and cal-
Analytical Chemistry, December 1, 1995 7 2 1 A
Report 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 C7. 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 10-nm to l-pm 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 of 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, environmental 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 as 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, are subject to poor temporal resolution and sampling artifacts. The ideal method would analyze individual particles in real time using instrumentation that is sufficiently compact and robust for field operation. A new generation of mass spectrometers that comes close to achieving these objectives has been developed.
Aerosol MS In situ analysis of single aerosol particles dates back to the 1970s (7-12). In these early experiments, chemical information was obtained by 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 1990s, Johnston and coworkers demonstrated that laser desorp-
Aerosol particles are emitted from both anthropogenic and natural sources. tion/ionization (LDI) coupled with time-offlight MS (TOFMS) could overcome these problems (13). LDI permitted a wider variety of components to be efficiently analyzed, and the TOF mass spectrometer permitted an entire mass spectrum to be obtained from each particle. A similar approach was suggested by Marijnissen and colleagues in the late 1980s (14). Sinha investigated the possibility of using LDI for single-particle analysis in the mid-1980s, but the experiment was constrained by low laser irradiance and the use of a scanning mass analyzer (15). McKeown's encouraging results stimulated rapid development of a new generation of instruments by several groups (1619). Although the layout of each instrument differs somewhat, reflecting the specific demands of the intended application, the fundamental design principles are very similar and encompass three basic components: an aerosol inlet, a particle de-
722 A Analytical Chemistry, December 1, 1995
tection and sizing system, and a mass spectrometer. Figure 1 shows the instrument used in our laboratory (19). Aerosol inlet. The aerosol inlet transmits 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 region, the flow expands rapidly and, because small particles have low inertia, the expansion drags them away from the centerline to where they cannot be detected or analyzed. Positioning the laser beams close to the inlet exit can minimize the effect of this divergence, but the improvement is limited by design constraints of the ion source assembly. Long thin capillary inlets have low flow rates because of fluid drag, but the long transit time in the capillary enables most particles to attain the velocity of the flow before exiting. Although a low flow rate reduces the number of particles that can be sampled, it also results in a lower acceleration, which reduces the deposition of large particles. Nozzles containing an orifice or a short capillary entrance have higher flow rates and, because the residence time is lower, the particles have velocities that depend on their aerodynamic size, which facilitates sizing. Both
Aerosol generator
Aerodynamic particle sizer
Vent
Reaction chamber
Gases Aerosol - inlet
A Mass spectrometer Β
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BS Personal computer
500 MHz transient digitizer
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Figure 1 . Experimental setup for studying aerosol reactions by MS. BS, beam splitter; BC, beam combiner; M, mirror. (Adapted with permission from Reference 19.)
designs suffer from clogging and diver gence. Overall efficiencies are 1/500 or less, but improvements are under develop ment. Particle detection and sizing. Particles emerging from the inlet are ana lyzed 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 synchro nized with the arrival of a particle. In the instrument depicted in Figure 1, this is ac complished by placing a continuous la ser beam upstream from the desorption/ ionization laser beam. When a particle passes through the laser beam, the burst of scattered radiation triggers the desorp tion/ionization laser. This approach works well for gas dis charge (excimer, nitrogen) lasers that can be fired within 1 ps of receiving an exter nal trigger. In this case, the continuous and pulsed laser beams are spatially sep arated by ~ 200-300 pm. When LDI is per formed with a Nd:YAG laser, the scatter pulse must precede the desorption/ ionization laser pulse by ~ 150 ps. (Unlike gas discharge lasers, the Nd:YAG 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
ser beams located 10-15 cm upstream from the Nd:YAG laser beam. The detection step also provides the op portunity to size each particle before analy sis. 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 inten sity of scattered radiation also depends on the particle composition and morphol ogy. In addition, many particles will be un dersized because they pass through the edge of the laser beam where the excita tion intensity is smaller. Nonetheless, the determination is sufficient to classify par ticles into broad size ranges, such as dis tinguishing 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 veloc ity can be related to size. An elegant approach developed by Prather and co-workers uses two continu ous laser beams to track particles as they travel through the vacuum 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 sec ond laser beam, the scattered radiation provides a stop pulse and the time be
tween the two pulses is recorded. The in verse of this time difference is propor tional to the particle velocity and, with nozzle-type inlets, the velocity is propor tional to the square of the aerodynamic diameter da. For spherical particles, da = p 1/2 d p where ρ is the specific particle den sity and dp is the actual diameter. This method gives a precise measure of ά.Λ, typi cally 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 laser-Doppler velocimetry (22). A single continuous laser beam is split into two beams and then recombined to produce an interference fringe pattern. When a sin gle particle passes through these fringes, the scattered radiation exhibits a periodic oscillation. The frequency of this oscilla tion is proportional to the particle velocity and can be used to determine ά.Λ. This method is often used in commercial parti cle sizing instruments. Although not yet incorporated into aerosol mass spectrome ters, it offers the possibility of both high precision for particle sizing and modest complexity. The choice of which method to use de pends on the precision required for parti cle sizing, the size and complexity of the optical setup that can be tolerated (sim ple 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 de 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 in struments, this step is performed directly in the source region of a TOF mass spec trometer in which the particle beam
Analytical Chemistry, December 1, 1995 723 A
Report 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 acceler ated from the source region into the flight tube. Because all ions are acceler ated to the same nominal kinetic energy, different ml ζ ions travel down the flight tube at different velocities and reach the detector at different times. Thus, a com plete mass spectrum can be recorded for each particle by simply monitoring the detector response as a function of time after the desorption/ionization laser pulse. The instrument shown in Figure 1 in cludes a reflecting field mass analyzer, which offers improved mass resolution over a simple linear configuration by com pensating 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 ki netic-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 not compensate for the kinetic energy distri bution of ions ejected in the opposite direc tion, because these ions must turn around and accelerate toward the flight tube. The net effect is that the resolution improve ment is not as dramatic as is observed in other surface desorption experiments in which ions are ejected in the forward di rection 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 com pact 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 acceler ating 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
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