Research Program Area: Health & Exposure
Laser Desorption Ionization Time of Flight Mass Spectrometry LDI/TOFMS has been developed for the direct screening of polycyclic aromatic hydrocarbons (PAHs) and nitro-substituted polycyclic aromatic hydrocarbons (nitro-PAHs). An ultraviolet 266-nm laser desorbs and ionizes PAHs and nitro-PAHs from the filter support material. The ions are mass analyzed in a custom built time of flight mass spectrometer. The laser power dependence of ionization and fragmentation of positive ions for the PAH compounds listed in ARB method 429 has been examined. The negative ion yield of nitro-PAH that was targeted by the MLD source test method has also been systematically examined for fragmentation and ionization efficiency.
The fragmentation/ionization studies show that there is a range of power densities that can produce molecular ion without significant fragmentation of the parent molecule for all but three of the compounds listed in Method 429. Naphthaiene, acenaphthalene and acenaphthene have relatively high vapor pressures. The vapor pressure makes analysis of the three small PAH compounds very difficult by LDI/TOFMS, however these are the three compounds most easily detected by GC/MS. The optimal 266 nm laser power densities for detection and identification of different PAHs and nitro-PAHs are similar.
Using laser power densities ranging from 1.3 to 2.4 X 106 W/cm2 it is possible to screen ambient samples for detection of a wide range of PAHs and nitro-PAHs. The listed nitro-PAHs yield [M - NO] as the most identifiable marker; in the case of 5-nitroacenaphthene and 2-nitrofluorene, [M - H] product ion serves as identifier.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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