Characterization of 2‐methylglyceric acid oligomers in secondary organic aerosol formed from the photooxidation of isoprene using trimethylsilylation and gas chromatography/ion trap mass spectrometry

Abstract In the present work, we have characterized in detail the chemical structures of secondary organic aerosol (SOA) components that were generated in a smog chamber and result from the photooxidation of isoprene under high‐NO x conditions typical for a polluted atmosphere. Isoprene high‐NO x SOA contains 2‐methylglyceric acid (2‐MG) and oligoester derivatives thereof. Trimethylsilylation, in combination with capillary gas chromatography (GC)/ion trap mass spectrometry (MS) and detailed interpretation of the MS data, allowed structural characterization the polar oxygenated compounds present in isoprene SOA up to 2‐MG trimers. GC separation was achieved between 2‐MG linear and branched dimers or trimers, as well as between the 2‐MG linear dimer and isomeric mono‐acetate derivatives thereof. The electron ionization (EI) spectra of the trimethylsilyl derivatives contain a wealth of structural information, including information about the molecular weight (MW), oligoester linkages, terminal carboxylic and hydroxymethyl groups, and esterification sites. Only part of this information can be achieved with a soft ionization technique such as electrospray (ESI) in combination with collision‐induced dissociation (CID). The methane chemical ionization (CI) data were used to obtain supporting MW information. Interesting EI spectral differences were observed between the trimethylsilyl derivatives of 2‐MG linear and branched dimers or trimers and between 2‐MG linear dimer mono‐acetate isomers. Copyright © 2006 John Wiley & Sons, Ltd.