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Aperture

Neutral' density filter

Detector

Aperture

Neutral' density filter

FIGURE 11.68 Schematic of integrating plate method (IPM) for measuring graphite carbon (adapted from Weiss and Waggoner, 1982).

a number of potential problems. For example, there may be other light-absorbing organics or other species present in the sample (e.g., Huffman, 1996); in addition, it is not clear what value should be used for the absorptivity of combustion-derived carbon particles. Thus, Horvath (1997) showed that the light absorption coefficient of carbon measured using the integrating plate method was systematically high and that this is expected theoretically for measurements made using a combination of transmission and integration of the scattered light.

Reflectance techniques, like the tPM, are based on the absorption of visible light by graphitic carbon. However, rather than measuring the decrease in light transmitted through a filter due to absorption, the decrease in light reflected from the carbon-containing surface is measured; the higher the elemental carbon, the more light will be absorbed and the less reflected. Thus log(R{]/R), where R I, is the reflectance in the absence of carbon and R the reflectance in its presence, has been shown to be linearly related to the elemental carbon concentrations (Delumyea et al., 1980). Because this is a light absorption/reflectance measurement, it suffers from the same types of problems as the IPM. However, it has an advantage in terms of its simplicity. In addition, in some urban areas there are historical records of filter sample reflectances that can be calibrated against more recent methods to examine historical trends in graphitic carbon (e.g., see Cass et al., 1983).

Although carbonate has been observed in some ambient samples (e.g., see Cunningham et al., 1984), it is generally believed to be present at insignificant concentrations compared to organic and graphitic carbon.

Although it has been generally assumed that elemental carbon is the only component of particles that absorbs visible light, as discussed in Chapter 9, this may not be the case. Instrumentation for measuring total light absorption by all particle components based on the heating of the surrounding gas caused by the absorbed energy is discussed by Moosmiiller et al. (1997).

d. Speciation of Organics

As seen in Chapter 9.C.2, a very wide variety of organics are found in particles in ambient air and in laboratory model systems. The most common means of identification and measurement of these species is mass spectrometry (MS), combined with either thermal separation or solvent extraction and gas chromatographic separation combined with mass spectrometry and/or flame ionization detection. For larger, low-volatility organics, high-performance liquid chromatography (HPLC) is used, combined with various detectors such as absorption, fluorescence, and mass spectrometry. For applications of HPLC to the separation, detection, and measurement of polycyclic aromatic hydrocarbons, see Wingen et al. (1998) and references therein.

Thermal desorption was described earlier with respect to differentiating organic and elemental carbon. Once organics have been desorbed by heating, they can be identified and measured individually using chromatographic techniques. While this technique works well for a number of organics, as discussed shortly, some compounds are thermally unstable and decompose during the desorption process. In addition, it may not completely vaporize high molecular weight compounds of low volatility.

Solvent extraction of the sample is also frequently used in the analysis of particulate matter. Through the appropriate choice of solvents, the organics can be separated into acid, base, and neutral fractions, polar and nonpolar fractions, and so on. This grouping of compounds according to their chemical properties using extraction techniques simplifies the subsequent analysis. Each fraction can then be analyzed by GC-MS, with the GC retention time and the mass spectrum used for identification and measurement.

A more recent extraction technique involves the use of supercritical fluids such as C02. This has a number of advantages (e.g., see Skoog et al., 1998) in that it avoids the use of large quantities of solvent and the need to concentrate the extract during which losses of the analyte may occur. Because the extracting fluid under atmospheric conditions is a gas, separating the fluid from the analyte only requires lowering the total pressure to release the "solvent" as a gas. In addition, it is quite fast since the rate of extraction depends on the rate of diffusion in the supercritical fluid and its viscosity, both of which are faster than for liquid extractions (e.g., minutes to hours versus hours to days). Low temperatures can be used for many supercritical fluids such as C02, minimizing thermal decomposition and/or reactions that may occur using thermal desorption techniques.

Supercritical fluid extraction (SFE) has been applied to ambient air particles with some success. For example, Hansen et al. (1995) developed a technique in which particles collected on a filter were extracted online using C02 and simultaneously transferred to the cooled head of a GC column, a process that took only 20 min. Recovery of long-chain alcohols and carboxylic acids spiked onto filters was much better for SFE than for thermal desorption but about the same or worse for some compounds such as nicotine. For a sample of urban aerosol particles, SFE detected some compounds not seen by thermal desorption such as larger alcohols. This was attributed to a lack of volatilization of these compounds with the temperatures used in the thermal desorption method and/or to decomposition during heating. On the other hand, some compounds such as benzaldehyde and the alkene 1-nonacosene were observed using thermal desorption but not using SFE. The alkene was thought not to be present in the aerosol, but rather was produced by thermal decomposition of some other compound and hence would not be generated during SFE. The amount of benzaldehyde may have been below the detection limit of the SFE-GC system.

Two types of ionization sources are in widespread use—electron impact and chemical ionization. The traditional means of ionization by electron impact often causes extensive fragmentation of molecules so that only peaks corresponding to the fragments are seen in the mass spectrum. Particularly in a complex environmental sample, this may preclude positive compound identification. Chemical ionization complements electron impact mass spectra and is particularly useful for establishing the molecular weight of the compound. In chemical ionization sources, an electron beam is used to ionize a reagent gas such as CH4. The sample is then ionized by collisions with the ionized fragments from CH4. This often results in relatively strong peaks at masses one greater or one less than the parent peak, MH, through reactions such as the following:

Other types of mass spectrometry have also been used to examine ambient particulate samples. One such technique is secondary ion mass spectrometry (SIMS) in which the surface of the sample is bombarded with a beam of ions or neutral atoms that cause ejection of fragments from the surface. The fragments may be neutral atoms or molecules, positively or negatively charged species, electrons, or photons. The charged species, that is, the secondary ions, can be analyzed using MS, generating a SIMS spectrum. Elements such as potassium and sodium as well as functional groups such as COOH, sulfates, and nitrates can be detected by SIMS.

e. Artifacts

It is evident from the earlier discussion of sampling and collection of bulk samples of atmospheric particles that there is ample opportunity for the formation of artifacts, which can be either positive or negative, depending on the particular species measured and the techniques used. These can arise from a variety of processes such as the following: reactions of collected particles with other particle components or with gases (e.g., of NaCl with gaseous HN03), volatilization of compounds from collected particles (e.g., NH4N03), adsorption and/or reactions of gases on filters or on the particles previously collected [e.g., S02 uptake and oxidation on particles (Eatough et al., 1995) and adsorption of gas-phase organics on quartz fiber filters (e.g., Appel et al., 1989; McDow and Huntzicker, 1990; Turpin et al., 1994)], and reactions of gases with the filter medium, as discussed earlier for S02 on nylon filters, for example (e.g., Chan et al., 1986; Cadle and Mulawa, 1987). In addition, the particle composition may change during collection due to shifts in gas-particle equilibria from changes in temperature, pressure drop across the collecting medium, or composition of the sampled air (e.g., Zhang and McMurry, 1987, 1991; Kaupp and Umlauf, 1992).

In addition to these chemical artifacts, physical artifacts can also occur. For example, the problems of particle bounce (e.g., see Wedding et al., 1986) and reentrainment in impactors were discussed earlier. In addition, air turbulence is known to have a significant effect on the overall sampling efficiency of particle inlets (e.g., Wiener et al., 1988; Francois et al., 1995).

In short, care must be taken in sampling and analysis of airborne particles, as well as in the data interpretation, to minimize or at least recognize potential artifact problems. Such problems, along with a need to understand not only the bulk composition of a collection of airborne particles but also that of individual particles, have contributed to the development of realtime and single-particle analysis techniques discussed in the following section.

4. Real-Time Monitoring Techniques for Particles

Efforts to develop and apply real-time monitoring techniques for particles have been underway for more than two decades. Early approaches typically involved the impaction of particles on a hot filament, surface, or oven that volatilized and ionized the species in the particles, with the ions detected by mass spectrometry (e.g., Myers and Fite, 1975; Davis, 1977a, f997b; Allen and Gould, 1981; Stoffels, 1981a,b; Stoffels and Lagergren, 1981; Sinha et al., 1982, 1985; Sinha and Friedlander, 1986; Stoffels and Allen, 1986). Laser ionization was also investigated (Sinha, 1984). The ions produced were measured by mass spectrometry. Since many species volatilized but did not ionize, neutrals were detected in some studies by electron impact ionization subsequent to volatilization (e.g., Allen and Gould, 1981; Sinha et al., 1982). More recent developments of this approach (e.g., Tobias et al., 1999; Jayne et al., 1999) are discussed later in this section.

While the early studies paved the way for further development of real-time and single-particle monitoring techniques, their application to ambient air was limited by a number of factors. These included the fact that the burst of ions produced was very short, <f0 ms, so that scanning mass spectrometric methods such as quadrupoles could not scan sufficiently rapidly to capture the large mass range of interest. As a result, only a single mass or very limited number of masses could be recorded for each scan. Other limitations included extensive fragmentation of organics and a dependence of the efficiency of ionization on the composition of the particles. The use of time-of-flight mass spectrometry proposed by Allen and Gould in 1981 has helped to overcome the first problem. As we shall see, the fragmentation and variable ionization efficiencies continue to present challenges.

a. Single-Particle Laser Ionization Techniques

Since the first use of laser ionization by Sinha in f984 to detect single particles, there has been a great deal of activity and development of this method for application to the atmosphere (see Johnston and Wexler (1995) for a review). In the early work, Sinha (1984) developed a method for simultaneous sizing of particles in which the particle first scattered light from one He-Ne laser, followed by a second He-Ne laser. The time interval between the two was used to obtain the speed of the particle and hence its size. The Nd:YAG ionizing laser was collinear with the second He-Ne laser and was fired at a set delay time after the particle was detected by the first He-Ne laser. Only particles of a given size whose speed is such that they reach the ionizing laser as it fired were detected. Particles of different sizes could be detected by varying the delay time between the scattering and ionizing lasers.

Subsequently, Marijnissen et al. (1988) proposed a single-particle system in which the amount of scattered light could be used along with the index of refraction of the particle to calculate its size. Shortly thereafter, McKeown et al. (1991) demonstrated the analysis of single particles using this approach, combined with time-of-flight mass spectrometry.

Since then, there has been a substantial development of such instruments. For example, Hinz et al. (1994) reported the first real-time monitoring of ambient particles in laboratory air using this technique of laser ionization combined with time-of-flight mass spectrometry. Carbon peaks from soot, metals attributed to abrasion of laboratory devices, and nicotine after "enriching the ambient air with tobacco smoke" were observed. Prather et al. (1994) and Mansoori et al. (1994) reported the application to inorganic and organic particles prepared in the laboratory, and Dale et al. (1994) and Yang et al. (1995a) reported analysis of organics adsorbed to the surface of particles of silicon carbide generated in the laboratory, using laser desorption combined with ion trap mass spectrometry. Reilly et al. (1998) have used laser ablation with an ion trap MS to identify polycyclic aromatic hydrocarbons in the particles from diesel engines.

Subsequently, Carson et al. (1995) and Neubauer et al. (1996, 1997) reported the ability to provide specia-tion of some aerosol components such as ammonium sulfate, ammonium sulfite, and methanesulfonic acid through control of the ionizing laser pulse energy, and Reents et al. (1995) showed that parent peaks could be obtained even for components such as Si02 that are difficult to ionize. Hinz et al. (f996) reported the simultaneous detection of both positive and negative ions produced by laser ionization of a single particle using a dual TOF system.

Prather and co-workers (Prather et al., 1994; Noble et al., f 994; Nordmeyer and Prather, 1994) introduced a significant improvement in particle size measurement and determining which particles are ionized. Light from the first laser is scattered by the particle and detected by a photomultiplier. It then travels a known distance where it encounters and scatters light from a second laser, which is also detected. The delay time between the two scattered light pulses is determined by the speed, i.e., the size, of the particle. This delay time is used to trigger the ionizing laser located further downstream at exactly the time that the particle should be in its optical line of sight. This use of three lasers allows both the determination of particle size and synchronization between detection of the particle and its ionization.

As illustrated below, the mass spectra of particles in ambient air can be (not surprisingly) quite complex. The use of tandem mass spectrometry would therefore be quite valuable, and indeed, such an instrument has

11. ANALYTICAL METHODS FOR GASES ANL) PARTICLES IN THE ATMOSPHERE

Particle trajectory

11. ANALYTICAL METHODS FOR GASES ANL) PARTICLES IN THE ATMOSPHERE

Particle trajectory

Reflectron adjustment rod

Reflectron

Nd:YAG laser

FIGURE 11.69 Schematic diagram of single-particle laser ionization mass spectrometer (adapted from Gard et al., 1997).

Reflectron adjustment rod

Reflectron

Nd:YAG laser

FIGURE 11.69 Schematic diagram of single-particle laser ionization mass spectrometer (adapted from Gard et al., 1997).

been developed using an ion trap mass spectrometer (March, 1992). Its application to aerosols generated in the laboratory has been explored for relatively simple systems (Yang et al., 1995a, 1996) and looks promising, although application to ambient air awaits further investigation.

Figure 11.69 is a schematic diagram of a single-particle laser ionization mass spectrometer with the particle sizing and ionization synchronization scheme of Prather and co-workers (Gard et al., 1997). Ionization is produced using light at 266 nm using a Nd:YAG laser and both positive and negative ions from the single particle are detected using a dual-ion coaxial set of TOF mass spectrometers. Figure 11.70 shows both the positive and negative mode mass spectra acquired from a single particle that was generated in the laboratory from wood burning. Hydrocarbon fragments are seen in both positive and negative ion modes, with potassium also present in the positive ion spectrum and HS04 in the negative ion spectrum.

Figure 11.71 shows some single-particle mass spectra obtained in the positive ion mode in a rural area in Colorado using a laser ionization single-particle mass spectrometer (Murphy and Thomson, 1995, 1997a,b). Figure 11.71a is an example of a mass spectrum of a particle containing organics with fragments occurring up to higher amu; indeed, there are peaks appearing at most masses, suggesting a complex mixture. On the other hand, the spectrum in Fig. 11.71b shows mainly peaks due to C„, which has been assigned to elemental carbon (soot particles). Figure 11.71c shows evidence for ammonium ions, perhaps due in part to ammonium nitrate as seen from the peaks in this mass spectrum.

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