Isomeric PAHs identification

GC-MS Analysis. PAHs produce stable molecular ions upon electron impact very little subsequent fragmentation takes place. This is advantageous in determining molecular weight and hence the empirical formula and ring system of a PAH molecule. However, lack of fragmentation makes it usually impossible to differentiate between structural isomers. Chromatographic separation is therefore essential for identification isomeric PAHs by GC-MS. 

The major problem in PAH analysis is separation and conclusive identification of individual isomeric compounds, since the biological properties of many PAHs are isomer-specific. Another problem is the unavailability of many reference standards, making optimization of GC-operating parameters and column preparation methods for isomer separation difficult. The best possible separation efficiency is crucial for identification and quantitation of PAHs in any environmental sample. In addition, PAHs must be separated from other classes of compounds mostly encountered in environmental samples. 

Figure 4 Tiered oil spill source identification scheme using molecular chemistry. The increasing level of source specificity required (down the diagram) is provided by global distributions of n-alkanes (level 1), PAH and biomarker distribution patterns (level 2), and isomeric and other diagnostic marker ratios (level 3), respectively (reproduced by permission of Nordtest from Revision of the Nordtest Methodology for Oil Spill Identification, 2002, 110).

Azulene, a bicyclic C,oHg hydrocarbon isomeric with naphthalene, was first reported as a tobacco smoke component by Ikeda (1857) and subsequently by Gilbert and Lindsey (1287, 1288), Lindsey (2365), and Lyons (2426). Despite improved analytical technology and hundreds of studies on the identification of PAHs in tobacco smoke, very few reports of its identification have appeared since those in the 1950s. This suggests... 

---