PAHs are compounds with two or more fused benzene rings and often contain alkyl side groups. PAHs are divided into two groups, low molecular weight PAHs (LPAHs) and the high molecular weight PAHs (HPAHs). LPAHs contain two or three aromatic rings and HPAHs are those with four or more rings. The distinction of low and high molecular weight is subtle because the actual difference in molecular weight between 2- and 6-ring compounds is not dramatic («120-300 Da). Even though these designations seem unimportant, they are relatively important for toxicity considerations because the LPAHs and HPAHs generally exhibit very different physicochemical properties, bioavailability, bioaccumulation, and toxic potential. Another important consideration is the designation of parent PAH versus alkylated homologs. Parent PAHs are those compounds consisting of the basic ring structure and no side groups. Many of the parent compounds exist as alkyl homologs. For example, the parent compound phenanthrene (Phen) may be alkylated producing several unique combinations such as methylphen, dimethylphen, ethylmethylphen, and others. There can be dozens of alkylated homologs for many of the parent compounds. One confusing aspect of all this is that many alkylated LPAHs have molecular weights that are higher than many of the so-called HPAH parent compounds and exhibit physiochemical properties that are more similar to parent HPAHs. In many cases, chemists will group the alkylated PAHs based on the total number of carbons in the alkyl moiety, for example, C1, C2, C3, or C4 Phen. A C2 Phen can be ethylPhen or dimethylPhen.
No one knows exactly how many PAH compounds exist; however, hundreds have been identified. In general, ecotoxicologists focus on a small number of PAHs (up to ^40) because of their relatively high frequency of occurrence. More recently, with the advancements made for gas chromatogram/mass spectrometers and standard reference materials that contain known quantities, an expanded list of PAHs can be identified and quantified for ecotoxicology assessments.
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