Abstract - Topic #8

Advances in Organic Characterization and Quantification Applicable to Organic Aerosols

Topic Leader: Reinhard Niessner
Contributors: Markus Kalberer, Kimberly Prather, Paul Ziemann (click on name to link to abstract text)

On-line (& in situ) Chemical Analysis of Organic Aerosol

Reinhard Niessner
Institute of Hydrochemistry, Technical University of Munich

For many years, aerosol scientists and/or analysts have asked for on-line & in situ measurement techniques. The reason for this demand is easily understood: artifact-free sampling and immediate analysis allows the direct observation of fast processes; e.g., aerosol formation or degradation of even labile aerosol species. In favourable cases, a further dimension can be opened: a tomographic information, which means x,y,z-oriented chemical analysis.

This review summarizes the principal techniques available with their special characteristics rather than what has been published so far. The intrinsic possibilities of a principle will be discussed, especially in view of new instrumentation (laser, synchrotron, surface-enhanced effects, intracavity arrangement, antibody, PCR, high-parallel detection schemes etc.).

Analytes: organic particulate matter in a size range of 3 nm < dp < 150 µm

- Molecules > 1 kDalton ; viable bioaerosols, proteins, peptides, natural polymers (low vapour pressure material)
- Molecules < 1 kDalton (high vapour pressure substances)
- Functional groups, inert or labile bonds, chirality, spin density ; chemically reactive or inert.

It is important to realize the presence of organo-metallic species in ambient aerosol. In case of a needed phase transfer, rapid sampling becomes important (gaseous state c liquid state).

The measurement techniques are based on:

interaction of electromagnetic radiation in a wavelength range of 1 nm < l < 1 mm with aerosol particles

- absorption of radiation as only detection principle (transmission, optothermal effects, ESR)
- emission of radiation as only detection principle (Raman, fluorescence, backscattering, laser plasma emission)
- formation of charged particles or fragments as detection principle (photoelectron emission, mass spectrometry, photofragmentation);

particle – particle interaction

- detection is based on ballistic measurement of particles (photons, ions, electrons, atoms) after aerosol collision (RBS. phoretic techniques)

- detection is based on measurement of quantifiable adduct formation (strong interaction : antibodies or peptides (ELISA, PCR); weak interaction (water condensation)

- detection is based on multiple effects in a multi-dimensional space (smell, viability or reproducibility).

Back To Top

Laser-mass spectrometry of organic aerosols ­ identification of specific compounds and bulk properties

Markus Kalberer, ETH Zurich

Depending on the scientific focus, single compound analysis or measurement of bulk properties is preferable in organic aerosol analysis. Studying toxic compounds (carcinogenic, mutagenic) compounds such as PAHs, compound specific analysis and quantification is required. Two-step laser time-of-fight mass spectrometry (L2MS) is a fast and one of the most sensitive methods measuring aerosol-bound PAHs and very selective due to the REMPI ionisation process. This allows measuring PAHs in several particle size classes with 10-15min time resolution, a time scale within which short term emission peaks close to sources such as a busy road can be followed.

Recent quantification methods can give estimates of PAH concentrations on aerosols (Kalberer et al, 2002, 2003a). These measurements showed that most PAHs in the mass range up to m/z280 have a half-life <3hrs.

Other laser ionisation MS techniques, e.g., (matrix assisted) laser desorption/ionisation MS (MALDI, LDI), are less specific ionisation methods.

This apparent disadvantage, however, allows capturing mass analysis of the particle bulk, because a wide range of compound classes are ionized. Recent measurements of smog chamber generated secondary organic aerosols (SOA) with this technique showed that high molecular weight compounds based on acetal polymers up to a mass of about m/z1000 comprise a major fraction of the SOA mass formed from aromatic precursors (Kalberer et al, 2003b).

Functional groups, important factors for the particle's hygroscopic or optical properties, however, are easier accessible with optical methods, such as IR or VU-VIS. Also for such questions the determination of an average chemical composition is probably more suited than single compound
analyses. Time resolved measurements of SOA show that the functional group distribution is varying significantly over time demonstrating the continuously changing chemical composition of particles with age.

M. Kalberer, B. D. Morrical, M. Sax, R. Zenobi. Picogram Quantification of Polycyclic Aromatic Hydrocarbons adsorbed on Aerosol Particles by Two-Step Laser Mass Spectrometry, Anal. Chem., 74 (14): 3492-3497, 2002.

M. Kalberer, S. Henne, A. Prevot, M. Steinbacher. Vertical transport and degradation of polycyclic aromatic hydrocarbons in an Alpine valley. Atmos. Environ., submitted, 2003a.

M. Kalberer, D.Paulsen, M.Sax, M.Steinbacher, J.Dommen, R. Fisseha, A. Prevot, V.Frankevich, R.Zenobi, U.Baltensperger. First Identification of Polymers as Major Components of Atmospheric Organic Aerosols. submitted, 2003b.

Back To Top

Organic Aerosol Analysis Using Thermal Desorption Aerosol Mass Spectrometry

Paul J. Ziemann, Air Pollution Research Center, University of California, Riverside

In the past few years, a large variety of mass spectrometry-based instruments have been developed and employed for studies of the chemical composition of ambient atmospheric aerosol particles.

One approach that is particularly well suited for the analysis of organic aerosols involves the use of thermal desorption to vaporize the organic components prior to analysis using mass spectrometry.

The most widely used instrument of this type is the Aerodyne Aerosol Mass Spectrometer (AMS), which is commercially available and can analyze organic aerosol size and composition in real time. The AMS consists of an aerosol inlet, a particle sizing chamber, and a particle detection section. Particles are sampled from ambient pressure and focused by an aerodynamic lens into a narrow beam as they enter high vacuum. Particle aerodynamic diameter is determined by measuring the time a particle takes to reach the vaporizer after passing through a mechanical beam chopper. Particle chemical composition is determined via flash vaporization followed by electron ionization and quadrupole mass spectrometry. The separation of the particle vaporization and the vapor ionization steps enables linear chemical composition detection even for complex mixtures.

A similar instrument, called a thermal desorption particle beam mass spectrometer (TDPBMS), has been developed by the Ziemann group at UC-Riverside and primarily used for laboratory studies of secondary organic aerosol formation and diesel emissions. As with the AMS, particles are focused in an aerodynamic lens, sampled into a high vacuum chamber where they impact on a heated surface, and the resulting vapor is analyzed by a quadrupole mass spectrometer using electron ionization. One important difference between the TDPBMS and the AMS is that, in addition to operating in a real-time analysis mode, the vaporizer of the TDPBMS can be cooled so that particles can be collected and subsequently analyzed as they desorb under slow heating (temperature-programmed thermal desorption). This method separates components according to volatility and thus provides composition information as a function of component vapor pressure. The TDPBMS is not as sensitive as the AMS, however, and so normally cannot be used for ambient analysis. Furthermore, the TDPBMS does not incorporate a means to size-select particles for analysis. Recently, these problems have been addressed by coupling the TDPBMS to fine and ultrafine Particle Concentrators developed by the Sioutas group at USC, which has made possible its use for ambient organic aerosol analysis.

A different method has been developed for semi-continuous analysis of organic aerosols by Hering at Aerosol Dynamics and Goldstein at UC-Berkeley. This method employs an impactor to collect fine particles on a metal surface, which is subsequently flash-heated to desorb the organic components. The vapor is collected on the front a gas chromatograph-mass spectrometer column for subsequent analysis. This technique provides much more speciated information than the AMS or TDPBMS, but is not a real-time analyzer, and requires derivatization methods for the analysis of polar compounds.

In this talk I will discuss the capabililties, advantages and disadvantages of these methods for organic aerosol analysis and ways in which their performance might be improved.

Back to Top

On-line analysis of organic species using single particle mass spectrometry

Kimberly A. Prather, University of California, San Diego

Single particle mass spectrometry is a growing area of research, due to the fact it can provide unique insights into the mixing state of ambient aerosol particles (Sipin, 2003). Depending on the application, a number of different instrumental designs have been developed, each with its own advantages and disadvantages. Much of the focus of on-line single particle ambient measurements has been on inorganic particles. Under standard conditions of high laser fluence, extensive fragmentation can occur during the laser desorption process; often times these fragmentation patterns can be used to identify the compounds of interest upon comparison with mass spectral libraries (Silva, 2000). A number of groups have shown these techniques are capable of providing information on molecular organic species in individual particles (Angelino, 2001; Smith, 2002). New methods have been developed to reduce fragmentation, including two-step LDI and VUV photoionization (Morrical, 1998; Woods, 2001). Because the particles are analyzed on-line in less than 1 ms, these techniques hold promise for studying the partitioning of semivolatile organic compounds (SVOC). The potential for using single particle mass spectrometry for source apportionment will be discussed. Finally, fundamental differences in organic versus elemental carbon single particle mass spectral signatures can be used to differentiate between EC, OC, and EC/OC particles; a discussion of how this information might be used to provide insight into discrepancies in other standard EC/OC measurements will be presented. This presentation will focus on the potential for obtaining information on organic aerosols, detailing the advantages and disadvantages of this approach compared to alternative analysis techniques.

Angelino, S., D. T. Suess, et al. (2001). "Formation of aerosol particles from reactions of secondary and tertiary alkylamines: Characterization by aerosol time-of-flight mass spectrometry." Environmental Science & Technology 35(15): 3130-3138.

Morrical, B. D., D. P. Fergenson, et al. (1998). "Coupling two-step laser desorption/ionization with aerosol time-of-flight mass spectrometry for the analysis of individual organic particles." Journal of the American Society for Mass Spectrometry 9(10): 1068-1073.

Silva, P. J. and K. A. Prather (2000). "Interpretation of mass spectra from organic compounds in aerosol time-of-flight mass spectrometry." Analytical Chemistry 72(15): 3553-3562.

Sipin, M. F., S. A. Guazzotti, et al. (2003). "Recent advances and some remaining challenges in analytical chemistry of the atmosphere." Analytical Chemistry 75(12): 2929-2940.

Back To Top