Single particle mass spectrometry provides a rapid and unique picture of the particle mixing state of atmospheric particles, showing associations between sulfates, nitrates, and other secondary species with dust, sea salt, elemental carbon, and organic carbon particles.  With many of these instruments, size-resolved composition can be obtained on individual particles (Sipin, Guazzotti et al. 2003).  Thousands of particles can be rapidly analyzed, providing temporal information on timescales as short as 10 minutes.  The single particle mass spectral patterns show clear distinctions between elemental and organic carbon.  The associations of chemical species within individual particles can be used in a number of ways including performing source apportionment, understanding health impacts of particles, and detailing the radiative properties of aerosols (Bhave, Fergenson et al. 2001; Guazzotti, Suess et al. 2003).  Using more efficient sample introduction methods, high efficiency single particle instruments have been developed which measure size-resolved composition of particles in the fine and ultrafine size modes (Liu, Ziemann et al. 1995; Rhoads, Phares et al. 2003; Su, Sipin et al. 2004). Fragmentation which commonly occurs at the typical laser fluences used for the laser desorption/ionization step can often be used to identify the organic species (Silva and Prather 2000; Angelino, Suess et al. 2001; Whiteaker and Prather 2003).  Separating the desorption and ionization processes into two steps (L²DI) offers great promise as a potentially quantitative tool for organic speciation as the molecules undergo little to no fragmentation (Morrical, Fergenson et al. 1998; Woods, Smith et al. 2001). The 2-step approach can even be used to gain insight into species in the core of the particle versus on the surface of the particle (Woods, Smith et al. 2002).  Recently, single photon ionization has been shown to be a sensitive tool and shows clear distinctions between particles from multiple sources based on organic tracers (Oktem, Tolocka et al. 2004).  Recent developments in unique ionization schemes such as PERCI by Pettruci et al. show on-line mass spectrometry of single particles can even be used to distinguish between isomeric organic compounds. 

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.

Bhave, P. V., D. P. Fergenson, et al. (2001). "Source apportionment of fine particulate matter by clustering single-particle data: Tests of receptor model accuracy." Environmental Science & Technology 35(10): 2060-2072.

Guazzotti, S. A., D. T. Suess, et al. (2003). "Characterization of carbonaceous aerosols outflow from India and Arabia: Biomass/biofuel burning and fossil fuel combustion." Journal of Geophysical Research-Atmospheres 108(D15).

Liu, P., P. J. Ziemann, et al. (1995). "Generating Particle Beams of Controlled Dimensions and Divergence .2. Experimental Evaluation of Particle Motion in Aerodynamic Lenses and Nozzle Expansions." Aerosol Science and Technology 22(3): 314-324.

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.

Oktem, B., M. P. Tolocka, et al. (2004). "On-line analysis of organic components in fine and ultrafine particles by photoionization aerosol mass spectrometry." Analytical Chemistry 76(2): 253-261.

Rhoads, K. P., D. J. Phares, et al. (2003). "Size-resolved ultrafine particle composition analysis, 1. Atlanta." Journal of Geophysical Research-Atmospheres 108(D7).

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.

Su, Y. X., M. F. Sipin, et al. (2004). "Development and characterization of an aerosol time-of-flight mass spectrometer with increased detection efficiency." Analytical Chemistry 76(3): 712-719.

Whiteaker, J. R. and K. A. Prather (2003). "Hydroxymethanesulfonate as a tracer for fog processing of individual aerosol particles." Atmospheric Environment 37(8): 1033-1043.

Woods, E., G. D. Smith, et al. (2001). "Quantitative detection of aromatic compounds in single aerosol particle mass spectrometry." Analytical Chemistry 73(10): 2317-2322.

Woods, E., G. D. Smith, et al. (2002). "Depth profiling of heterogeneously mixed aerosol particles using single-particle mass spectrometry." Analytical Chemistry 74(7): 1642-1649.

Organic Aerosol Analysis Using Thermal Desorption Aerosol Mass Spectrometry

A number of methods have been developed that employ various forms of thermal desorption-mass spectrometry for on-line analysis of organic aerosols. The most widely used instrument of this type is the Aerodyne Aerosol Mass Spectrometer (AMS), which is commercially available and can analyze organic particle size and composition in real time (Jayne et al. 2000). In the AMS, particles are focused by an aerodynamic lens into a narrow beam as they enter high vacuum. Particle aerodynamic diameter is determined from the particle velocity measured using a mechanical 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 quantitative analysis of sulfates, nitrates and total organic mass. Although it is not possible to identify individual organic compounds with this method, information on specific source contributions can be obtained through the use of marker ions that are characteristic of particular sources, such as primary organics and secondary (or oxidized) organics. The identification of marker ions is based on time correlations with other measurements (e.g., CO and NO_x for primary emissions and O₃ and UV light for photochemical processes) and particle size-dependent mass spectra. One advantage of this method compared to the use of molecular tracers is that source apportionment is based on the analysis of the entire organic particle mass rather than a trace component whose relation to total organic mass may be difficult to determine. Future improvements to this method include the addition of an electron ionization time-of-flight mass spectrometer for quantitative analysis of single particles (this has already been demonstrated) and a thermal denuder to obtain information on the volatility of organics and some degree of component separation prior to analysis.

An instrument called a thermal desorption particle beam mass spectrometer (TDPBMS) is similar to the AMS, but differs in that it does not use a chopper and, most importantly, the vaporizer can be cooled so that particles can be collected and subsequently analyzed as they desorb under slow heating (Tobias and Ziemann, 1999). This method separates components according to volatility and thus provides composition information as a function of component vapor pressure. When coupled to a particle concentrator (Kim et al. 2001), this instrument can be used for speciated analysis of sulfates, nitrates, and various organic components (primary and oxidized aerosol as well as some single compounds) in fine and ultrafine particles.

A method for quantitative, semi-continuous, speciated analysis of organic aerosols has been developed by Hering (Aerosol Dynamics) and Goldstein (UC-Berkeley). This method employs an impactor to collect fine particles on a metal surface, which is subsequently heated to desorb the organic components. The vapor is collected on the front a gas chromatograph-mass spectrometer column for subsequent analysis. The time to collect and analyze one sample is about one hour. 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.

Jayne, J.T., D.C. Leard, X. Zhang, P. Davidovits, K.A. Smith, C.E. Kolb, and D.R. Worsnop, Aerosol Sci. Technol. (2000) 33, 49-70.

Tobias, H.J., and P.J. Ziemann, Anal. Chem. (1999) 71, 3428-3435.

Kim, S., P. Jaques, M. C. Chang, J. R. Froines, and C. Sioutas, J. Aerosol Sci. (2001) 11, 1281-1297.

Laser-Mass Spectrometry of Organic Aerosols – Determining Specific Compounds and Bulk Properties

Besides laser single particle techniques and thermal methods for organic aerosol analysis, new combinations of desorption and ionisation techniques are promising when the analytical focus is on the organic fraction of atmospheric aerosols. Especially when higher molecular weight compounds represent the analytes of interest, soft desorption and ionization techniques are required. Depending on the ionization method, these techniques can be either very compound selective or rather unselective measuring a broad variety of compounds. The following will give examples for these types of Laser-MS.

A major problem for many analytical techniques when analysing solid samples is the varying and often unpredictable influence of the sample matrix. In Two-Step Laser Mass Spectrometry (L2MS) such effects are minimized by decoupling desorption and ionization of the analytes with two different lasers. The first laser, typically an IR-laser, is heating up the sample desorbing the analytes from the matrix and/or substrate and the second laser ionizes the compounds of interest in the gas phase where matrix effects are usually much smaller than in the liquid or solid phase. Since the analytes are directly desorbed from the solid aerosol sample no work-up or preparation of the sample is needed minimizing possible artefacts introduced by these processes. Choosing an adequate ionization wavelength especially aromatic compounds can be ionized very softly (e.g., with resonance enhanced multi-photon ionization, REMPI) preventing fragmentation, which greatly simplifies mass spectra interpretation. In addition REMPI is a very efficient ionization process making L2MS a very sensitive method.

The technique has been applied for on-line single aerosol analysis (e.g., B. D. Morrical, D. P. Fergenson, and K. A. Prather, J. Am. Soc. Mass Spectrom., 1998, 9, 1068–1073) and for off-line analysis, i.e., aerosols collected on filters or impactor plates (O. P. Haefliger T. Bucheli, R. Zenobi, Environ. Sci. Technol., 2000, 34, 2184-2189; C. Emmenegger, M. Kalberer, V. Samburova and R. Zenobi, The Analyst, 2004, DOI:10.1039/B401201A).

One of the biggest disadvantages of L2MS is, that this method cannot distinguish compounds of the same mass. Tuneable ionization lasers offer in principle the opportunity for a highly compound selective analysis, because ionization spectra, similar to adsorption spectra, offer the possibility to distinguish compounds, even isomers. This requires to cool the desorption plume (generated by the desorption laser) in order to access the highly specific ionization spectra. See D. M. Lubman, R. Tembreull, and C. H. Sin (Anal. Chem. 1985, 57, 1084-1087) for an example where this was achieved by means of a molecular beam and O. P. Haefliger and R. Zenobi (Anal. Chem., 1998, 70, 2660-2665) showing that without cooling such compound specific ionization is almost impossible due to the broad bands of the ionization spectra in the hot desorption plume. Another disadvantage of laser-MS techniques is, that they are often not quantitative or only semi-quantitative. Recently M. Kalberer, B. D. Morrical, M. Sax and R. Zenobi (Anal. Chem., 2002, 74, 3492-3497) presented for the first time a quantitative method for L2MS by adding an internal standard with an electrospray method on the aerosol sample achieving limits of detection in the low picogram range, which is orders of magnitudes below conventional methods such as gas chromatography – mass spectrometry. This high sensitivity can be used to perform ambient measurements with a high time resolution (C. Emmenegger, M. Kalberer, V. Samburova and R. Zenobi, The Analyst, 2004, DOI:10.1039/B401201A). Despite the somewhat complex experimental set-up of an L2MS instrument, this highly sensitive method has a large potential in aerosol analysis due to the minimal sample preparation necessary and due to the minimized matrix effects. Especially when taking advantage of the highly compound specific ionization spectra, L2MS could become a very powerful and much more widely used technique than nowadays.

Other laser ionisation MS techniques, e.g., laser desorption/ionization (LDI) or matrix assisted laser desorption/ionization (MALDI), are less specific ionisation methods. This apparent disadvantage, however, allows analyzing the particle bulk, because a wide range of compound classes is ionized. LDI and MALDI-MS techniques require almost no sample preparation minimizing artefacts due to sample work-up, such as a transfer into a liquid solvent. Another advantage of MALDI-MS is the soft ionization process minimizing fragmentation of the analytes and thus the possibility to measure large molecules. This technique seems promising to investigate high molecular weight compounds as recently found in organic aerosols from laboratory and field experiments (Zappoli et al., Tolocka et al., Kalberer et al.). Future more detailed analysis of these recently detected polymeric aerosol components should include exact mass determination with methods such as Fourier Transform Ion Cyclotron Resonance (FTICR) - MS or elucidation of their chemical structure with MSn. Both methods can be coupled with MALDI ionization techniques.

Another possibility to get information especially on the organic aerosol fraction is the combination of laser desorption techniques with other, non-laser based ionization techniques. A recently reported method comprises of an infrared (IR) laser pulse to desorb analyte species, followed by atmospheric pressure chemical ionization (APCI) with a corona discharge (LD-APCI) to effect ionization of the desorbed neutral analyte molecules (Coon et al.). Due to their MS/MS capabilities, especially ion traps appear to be appropriate mass analyzers for this purpose. The suitability of APCI as an ionization technique, in particular for the characterisation of secondary organic aerosol components, has already been demonstrated (Kueckelmann et al., Warscheid et al.). However, up to now the combination of IR-laser desorption and chemical ionisation techniques was only realized for off-line analysis of organic compounds, i.e. the analytes were deposited on target surfaces. In the future, it has to be shown that the concept also works for real-time analysis of organic aerosols.