Post-source decay of alkylated and functionalized polycyclic aromatic compounds.

Post-source decay (PSD) is a valuable tool for providing structural information from large molecules by time-of-flight mass spectrometry (TOFMS). We used PSD to obtain this type of data from small molecules in the laser desorption/ionization (LDI) study of diesel engine exhaust particles. As the original nitrogen laser (lambda = 337 nm, E = 3.5 eV/photon) of our TOF mass spectrometer does not yield sufficient energy to ionize polycyclic aromatic hydrocarbons (PAHs), a second laser with a shorter wavelength has been coupled to the instrument. The fourth harmonic of a Nd:YAG laser (lambda = 266 nm, 4.6 eV/photon) has been chosen to achieve two-photon single-step desorption/ionization of PAHs. The PSD fragmentation of functionalized, alkylated and sulfur PAHs is discussed. Diesel engine exhaust particles are also studied as an example of a real complex sample. This technique is presented herein as a way to identify small molecules in environmental samples. Information provided by LDI-PSD-TOFMS can be a way to distinguish pollutants with very close molecular weights even if the resolving power of a TOF mass spectrometer is not sufficient.

Laser desorption/ionization mass spectrometry of diesel particulate matter with charge-transfer complexes.

Polycyclic aromatic hydrocarbons (PAHs) are often associated with complex matrixes such as exhaust diesel particulate matter (DPM), which complicates their study. In that case, laser desorption/ionization mass spectrometry is one of the techniques which ensures their direct analysis in the solid state. We demonstrate in this paper that the use of charge-transfer pi-complexing agents allows us to selectively detect by Fourier transform ion cyclotron resonance mass spectrometry PAHs adsorbed on diesel particles with high sensitivity. 2,4,7-trinitro-9-fluorenone and 7,7',8,8'-tetracyanoquinodimethane pi-acceptor compounds form charge-transfer complexes with PAHs and prevent their evaporation in the mass spectrometer during analysis. Moreover, the production of PAH molecular ions is dramatically increased by laser irradiation of these complexes at short wavelength (221.7 nm) and low power density (5 x 10(6) W cm(-)(2)). This methodology is applied for the first time to the examination of DPM collected during the new European driving cycle for light-duty vehicles. Differentiation criteria may coherently be assigned to engine operating mode (engine temperature, driving conditions). DPM samples can also be easily distinguished in negative ions according to the high sensitivity of this detection mode to sulfate compounds.