20 TOTAL PETROLEUM HYDROCARONS
3. IDENTITY AND ANALYSIS OF TOTAL PETROLEUM HYDROCARBONS
During the refining process, crude oil is separated into fractions having similar boiling points. These
fractions are then modified by cracking, condensation, polymerization, and alkylation processes, and
are formulated into commercial products such as naphtha, gasoline, jet fuel, and fuel oils. The
composition of any one of these products can vary based on the refinery involved, time of year,
variation in additives or modifiers, and other factors. The chemical composition of the product can be
further affected by weathering and/or biological modification upon release to the environment. The
following subsections present overviews of petroleum products. Also, a master list of individual
aliphatic and aromatic compounds found in TPH is provided in Appendix D. Further information on
whole petroleum products, their identity, major components, and physical/chemical properties is
found in Appendix E.
Automotive Gasoline. Automotive gasoline is a mixture of low-boiling hydrocarbon compounds
suitable for use in spark-ignited internal combustion engines and having an octane rating of at least
60. Additives that have been used in gasoline include alkyl tertiary butyl ethers (e.g. MTBE), ethanol
(ethyl alcohol), methanol (methyl alcohol), tetramethyl-lead, tetraethyl-lead, ethylene dichloride, and
ethylene dibromide.
Other categories of compounds that may be added to gasoline include anti-knock agents, antioxidants,
metal deactivators, lead scavengers, anti-rust agents, anti-icing agents, upper-cylinder
lubricants, detergents, and dyes (ATSDR 1995a).
Automotive gasoline typically contains about 150 hydrocarbon compounds, though nearly 1,000 have
been identified (ATSDR 1995a). The relative concentrations of the compounds vary considerably
depending on the source of crude oil, refinery process, and product specifications. Typical hydrocarbon
chain lengths range from C
4
through C
l2
with a general hydrocarbon distribution consisting of
4-8% alkanes, 2-5% alkenes, 25-40% isoalkanes, 3-7% cycloalkanes, l-4% cycloalkenes, and
20-50% aromatics (IARC 1989a). However, these proportions vary greatly. Unleaded gasolines
may have higher proportions of aromatic hydrocarbons than leaded gasolines.
Table E-1.b (Appendix E) presents ranges and weight percentage means for a representative subset of
the hydrocarbon compounds identified in gasoline. In cases where data are not available, the range
and mean are left blank.