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Petroleum Product Identification in Environmental Samples:Distribution Patterns of Fuel-Specific Homologous Series

Yakov Galperin, Henry Camp

Identification of fuel-type in environmental samples (hydrocarbon fingerprinting) is one

of the major objectives of forensic investigations conducted at many of the petroleum-

contaminated sites throughout the country. Among analytical methods commonly used toidentify fugitive refined petroleum products, are those that focus on hydrocarbon group-

type analysis, such as alkanes, polynuclear aromatic hydrocarbons (PAH) and thepolycyclic alkanes, often referred to as biomarkers. These classic methods are included inthe analytical arsenal of most laboratories conducting fuel fingerprinting. Basic principles

of their application have been exhaustively examined in this column by Battelle

researchers.

Alkane distribution patters are routinely obtained using gas chromatography with flame

ionization detection (GC/FID). For petroleum products that have not undergonenoticeable biodegradation, this method provides an adequate procedure for fingerprinting

and fuel recognition based on the n-alkane homologous series. However, GC/FID issignificantly less useful for identification of other hydrocarbon groups, which is often

necessary for petroleum product fingerprinting in weathered environmental samples.Conventional analytical methods for identification of isoprenoids, PAH and biomarkers

which are commonly used in characterizing crude oil, are of limited utility for light andsome middle distillate fuels.

The serious limitations for the application of normal alkanes arises from the fact thatupon release into the environment, refined petroleum products are subject to various

weathering changes. In weathered products, most n-alkanes could be lost due to

biological degradation, whereas distribution of the more recalcitrant isoprenoids andPAH may not provide conclusive information on the source or type of fugitive fuel.

Recent investigations demonstrated that the cyclohexane homologous series of

hydrocarbons also exhibits fuel-specific distribution patterns that allow for fingerprintingof weathered fugitive fuels.

Fuel-specific distribution patterns

Crude oil contains a wide range of hydrocarbons from light gases to heavy residue. Atthe refinery, crude oil is separated by distillation into three main products: naphtha,

middle distillate and bottoms fraction. Naphtha is mainly used for motor gasoline and

processed further for octane improvement. The middle distillate can actually be

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separated into two categories consisting of kerosene range products (light-end) and dieselrange products (heavy-end). The light-end products are used for specialty solvents

(mineral spirits, stoddard solvent, etc), certain jet fuels and light diesel fuels (diesel #1).

The heavy-end middle distillates are used for diesel fuels (diesel #2), some jet fuels andheating oils.

Based on the systematic studies of different commercial and military fuels, it wasdetermined that in addition to a well-known alkane pattern, cyclohexane homologous

series also exhibit characteristic distribution patterns. Examples of alkane and

alkylcyclohexane patterns for three common fuels are shown in Figures 1-3. The

alkylcyclohexane distribution in gasoline (Figure 1) exhibits an asymmetric rapidlydecreasing pattern from methylcyclohexane to heptylcyclohexane. Jet propulsion fuel Jet

A (Figure 2) is characterized by a distribution pattern in the range from

methylcyclohexane to decylcyclohexane with the maximum at butylcyclohexane. Dieselfuel exhibits alkylcyclohexane pattern from methylcyclohexane to tridecylcyclohexane

with maximum at pentylcyclohexane (Figure 3).

The range of hydrocarbons in each product is determined by its boiling range, whereas

the distribution pattern reflects its application-specific formulation. Since the composition

of modern fuels is controlled by stringent manufacturing specifications, the range anddistribution pattern of each fuel varies only slightly, depending on the composition of

crude oil feedstock and refining practices used in manufacture.

Fuel-type characterization in weathered samples

Distribution patterns shown demonstrate that alkylcyclohexane distribution patterns are

as fuel-specific as are the alkane distributions.The main advantage in utilizingalkylcyclohexane patterns for hydrocarbon fuel recognition is that alicyclic compounds

are more resistant to environmental alteration and could be detected in a sample even

when most of the alkanes are degraded. Two case studies illustrating the application ofalkylcyclohexane pattern recognition for weathered environmental samples are

summarized below.

Case 1. Shown in Figure 4 is an alkane distribution pattern in the product sample. The

product lost most of n-alkanes due to weathering in the subsurface environment. The

group of peaks identified on the chromatogram represents isoalkanes, whose distributionsuggests the presence of diesel fuel. Because of the alkane reduction, it is not apparent if

other middle distillate products are also present. However, evaluation of the

alkylcyclohexane pattern confirms that the product consists entirely of diesel fuel.

Case 2. Alkane distribution patterns of two samples collected at the same site are shown

in Figure 5. Sample A have lost most of n-alkanes and appears to represent a severely

weathered product, whereas a high abundance of n-alkanes in sample B attests to itsrelatively unaltered nature. A significant difference in the degree of product weathering

does not allow evaluating their source relationship. However, a comparison of the

alkylcyclohexane distributions (Figure 6) clearly indicates that both samples represent thesame fuel-type.

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Figure 1. Gasoline alkane (a) and alkylcyclohexane (b) distribution patterns

5.00 10.0015.0020.0025.0030.0035.0040.0045.0050.0055.0060.0065.000

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Time

Response_

09190036.D\FID1A

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Methylcyclohexane

Ethylcyclohexane

Propylcyclohexane

Butylcyclohexane

Pentylcyclohexane

Hexylcyclohexane

Heptylcyclohexane

Octylcyclohexane

Nonylcyclohexane

Decylcyclohexane

Undecanylcyclohexane

Dodecanylcyclohexane

Tridecanylcyclohexane

RelativeAbundance,%

(b)

(a)

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Figure 2. Jet A fuel alkane (a) and alkylcyclohexane (b) distribution patterns

5.00 10.0015.0020.0025.0030.0035.0040.0045.0050.0055.0060.0065.000

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Time

Response_

09190041.D\FID2B

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Methylcyclohexane

Ethylcyclohexane

Propylcyclohexane

Butylcyclohexane

Pentylcyclohexane

Hexylcyclohexane

Heptylcyclohexane

Octylcyclohexane

Nonylcyclohexane

Decylcyclohexane




Relative

Abundance,%

n-C9

n-C16

n-C12

(b)

(a)

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Figure 3. Diesel fuel alkane (a) and alkylcyclohexane (b) distribution patterns

5.00 10.0015.0020.0025.0030.0035.0040.0045.0050.0055.0060.0065.000

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Response_

09190044.D\FID1A

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Methylcyclohexane

Ethylcyclohexane

Propylcyclohexane

Butylcyclohexane

Pentylcyclohexane

Hexylcyclohexane

Heptylcyclohexane

Octylcyclohexane

Nonylcyclohexane

Decylcyclohexane




RelativeAbundance,%

n-C9

n-C21

n-C14

(b)

(a)

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Figure 4. Product sample alkane (a) and alkylcyclohexane (b) distribution patterns

5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.000

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Time-->

Abundance

Ion 85.00 (84.70 to 85.70): Y4504.D

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Methylcyclohexane

Ethylcyclohexane

Propylcyclohexane

Butylcyclohexane

Pentylcyclohexane

Hexylcyclohexane

Heptylcyclohexane

Octylcyclohexane

Nonylcyclohexane

Decylcyclohexane




Relative

Abundance,%

i-C18

i-C15

i-C15

Ph

Pr

(b)

(a)

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Figure 5. Product samples alkane distribution patterns

20.0025.0030.0035.0040.0045.0050.0055.0060.0065.0070.0075.0080.0085.000

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Time-->

bundance

Ion 85.00 (84.70 to 85.70): V4978.D

20.0025.0030.0035.0040.0045.0050.0055.0060.0065.0070.0075.0080.0085.000

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Time-->

Abundance

Ion 85.00 (84.70 to 85.70): V4687.D

Sample A

Sample B

n-C11

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Figure 6. Product samples alkylcyclohexane distribution patterns

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Methylcyclohexane

Ethylcyclohexane

Propylcyclohexane

Butylcyclohexane

Pentylcyclohexane

Hexylcyclohexane

Heptylcyclohexane

Octylcyclohexane

Nonylcyclohexane

Decylcyclohexane




RelativeAbundance,%

0

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Methylcyclohexane

Ethylcyclohexane

Propylcyclohexane

Butylcyclohexane

Pentylcyclohexane

Hexylcyclohexane

Heptylcyclohexane

Octylcyclohexane

Nonylcyclohexane

Decylcyclohexane




RelativeAbundance,%

Sample B

Sample A

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