Source Characterization of Polycyclic Aromatic Hydrocarbons by Using Their Molecular Indices: An Overview of Possibilities - Reviews of Environmental Contamination and Toxicology - page 75

5 Molecular Indices

Comparing PAH concentrations in a contaminated area with the PAH content of
potential/suspected sources can produce very useful results (e.g., Boehm et al. 1995 ;
Wang et al. 2001 ; Yunker et al. 2002 ). To evaluate compositional differences in PAH
profiles, the concentrations of PAHs are usually standardized into ratios that are
both source-specific and refractory (De Luca et al. 2004 ; Kim et al. 2008 ). These
indices are of interest, because during PAH formation the PAH distribution is
temperature-dependent (De Luca et al. 2004 , 2005 ; Yunker et al. 2002 ). The PAH
distribution patterns are governed by the thermodynamics of low-temperature pro-
cesses (e.g., the formation of petroleum), and the kinetic factors of high-temperature
processes (e.g., combustion).
Different ratios may be attributable to differences between the sources (Hwang
et al. 2003 ; Stout et al. 2004 ), such as the type of substrate and the available path-
ways and conditions of PAH formation (Yan et al. 2005 ). PAH ratios can also be
used as tracers of PAH transformation during transport from the PAH source to
deposition and burial (Mitra et al. 1999 ; Stout et al. 2002 ). Consequently, to opti-
mize data analysis, one should use diagnostic ratios of PAH isomers, PAHs within a
homologue category, or PAHs that have similar thermodynamic stabilities
(Christensen et al. 2004 ). The use of PAH ratios minimizes confounding factors
such as differences in volatility, water solubility and adsorption (Yunker et al. 2002 ).
Therefore, such ratios may be used in two-component mixing models 4 for PAH
sources in sediment (Page et al. 1996 ).
From a statistical point of view, by using diagnostic ratios the sample composi-
tional information can be condensed into fewer variables that are less affected by
analytical artifacts/errors (e.g., retention time shifts, changes in peak shapes, rela-
tive signal intensities, etc.; Christensen and Tomasi 2007 ). When degradation effects
are factored in, a “multiple mixing model” can be developed that assists in estab-
lishing the range of contributions of different PAH sources (Gogou et al. 2000 ; Stout
et al. 2003 ; Wang et al. 2001 ).

5.1 Practical Concepts

PAH ratios are expressed as the ratio of the thermodynamically most stable isomer
(S) to the most unstable isomer (U) (e.g., S/U), or vice versa (e.g., Mitra et al. 1999 ;
Stout 2007 ). Such ratios are sometimes called thermal parameters because they are

4 A two component mixing model to estimate the “a” % contribution of the source A to the sample, where

the ratio of the two isomers is “r s ”, given the ratios of the isomer components in the sources A and B

( ) ( )

r + 1

1

r − r

A B s

(rA and rB respectively), and only sources A and B contribute, would look like: a = .

( ) ( )

r + r − r

s B A