Identifying Hydrocarbons, pg. 3

The evaporation ratios compare the distribution of different hydrocarbons that belong to the same class, such as paraffins or isoparaffins, but have different volatilities. Water washing ratios compare benzene and toluene with nonaromatic hydrocarbons of about the same molecular size and volatility. Total aromatic hydrocarbons are compared also with total nonaromatic hydrocarbons. The biodegradation ratios compare olefins with paraffins and isoparaffins, and naphthenes with paraffins. Examples of changes which occur in the bulk PIANO composition of gasoline when it enters soil and groundwater and when it accumulates as free product are shown in Table 5.

Table 5. Average and range of bulk PIANO composition for altered gasoline in free product, soil and groundwater

Sample Type* Paraffins Isoparaffins Aromatics Naphthenes Olefins
Relative %
Free product
(25)
Average 10.2 31.8 49.0 4.5 4.0
Range 4.3 - 18.3 20.9 - 40.2 27.1 - 70.1 1.2 - 9.0
1.9 - 5.7
Soil
(10)
Average 2.8 15.1 79.4 1.2 1.5
Range 0 - 5.3 0 - 31.6 60.4 - 98.7 0 - 2.6 0.8 - 2.1
Water
(15)
Average 4.7 22.2 63.1 7.1 5.3
Range 0.8 - 10.9 2.2 - 56.9
26.8 - 94.3
1.1 - 16.6 0.2 - 19.6

*Number in parenthesis denotes number of sample analysis averaged.

Because of their potential health hazard to humans, the most volatile aromatic hydrocarbons (benzene, toluene, ethylbenzene, and the xylenes, or BTEX) are analyzed as one group whenever there is a reason to suspect the release of a hydrocarbon fuel. Consequently, BTEX concentration data as well as total petroleum hydrocarbons (TPH) concentration are often the only available analytical results from most sites examined. The concentration of these hydrocarbons can be measured by EPA methods 8015M, 602/8020, 624/8240 (USEPA, 1990) and 8260B (USEPA, 1996). Average values and range of BTEX concentration in different grades of dispensed gasolines are shown in Table 6. Calculated hydrocarbon ratios based on available experimental data are presented in Table 7. Although BTEX measurements are mandated by the EPA for potential toxicological purposes, the relative concentration of individual volatile aromatic hydrocarbons in environmental samples reflects the effect of the gasoline exposure to an aqueous environment. The compilation of analytical results obtained in our laboratory, showing typical changes in the BTEX composition after gasoline enters the environment are presented in Table 8 for gasoline residues in free floating product, groundwater and soil.

Table 6. Average and range of BTEX hydrocarbon content (mg/ml) in different grades of gasoline

Gasoline Grade* Benzene Toluene Ethylbenzene Xylenes
Range Average Range Average Range Average Range Average
Regular Leaded (5) 6.6-14.8 9.2 18.6-64.4 32.6 6.2-14.0 9.1 32.1-77.4 47.2
Regular Unleaded (12) 5.0-19.4 10.2 17.9-56.6 28.7 5.8-15.4 8.0 27.1-76.6 39.6
Unleaded Plus (8) 7.1-18.2 11.9 21.6-62.8 36.7 6.0-15.1 9.3 28.6-81.5 45.7
Super Unleaded (12) 6.6-18.9 11.5 22.4-72.2  - 6.6-19.1 10.1 33.4-90.8 50.6

*Number in parenthesis denotes number of sample analysis averaged.

Table 7. Ratios calculated from the BTEX data of different grades of dispensed gasolines

Gasoline Grade* Benzene/
Toluene
Benzene
Ethyl-
benzene
Benzene/
Xylenes
Toluene/
Ethyl-
benzene
Toluene/
Xylenes
Ethyl-
benzene/
Xylenes
Benzene
+Toluene/
Ethyl-
benzene
+ Xylenes
Regular Leaded (5) Average 0.28 1.0 0.20 3.6 0.70 0.19 0.74
Range 0.23-0.36 0.92-1.10 0.19-0.20 3.0-4.6 0.59-0.83 0.18-0.19 0.67-0.98
Regular Unleaded (12)     Average 0.36 1.28 0.26 3.6 0.72 0.20 0.82
Range 0.28-0.41 0.86-1.7 0.18-0.32 2.9-4.2 0.57-0.83 0.19-0.22 0.64-0.95
Unleaded Plus
(8)
Average 0.32 1.28 0.26 3.9 0.80 0.20 0.88
Range 0.28-0.37 0.85-1.6 0.19-0.32 3.0-5.0 0.63-0.94 0.19-0.23 0.70-1.05
Super Unleaded (12) Average 0.28 1.14 0.23 4.0 0.81 0.20 0.86
Range 0.24-0.33 0.77-1.6 0.16-0.30 3.1-5.6 0.61-1.02 0.18-0.21 0.67-1.13

*Number in parenthesis denotes number of sample analysis averaged.

The most obvious change in free floating product relative to the BTEX contents in dispensed gasolines is the reduction of about 20 to 50 percent in benzene concentration. Toluene, ethylbenzene, and xylenes do not demonstrate such a noticeable reduction. The reason for this, is that benzene preferentially dissolves out of the product and partitions in the groundwater phase. This is also evident from the average concentrations of BTEX hydrocarbons in water samples, in which benzene displays an enrichment relative to toluene, ethylbenzene, and xylenes. By contrast, the reverse occurs in the soil in which toluene and especially ethylbenzene and xylenes are preferentially retained. Furthermore, ethylbenzene and xylenes are more resistant to biodegradation than benzene or toluene (Weidermeir et al., 1996). Recent studies also show that under anaerobic conditions, toluene may be degraded more rapidly than benzene (Bosma et al., 1996; Eganhouse et al., 1996). Therefore, when the concentration ratios of benzene/toluene, benzene/ethylbenzene, benzene/xylenes, toluene/ethylbenzene and toluene/xylenes in product, water and soil samples (see Table 8) are compared with ratios for dispensed gasolines (see Table 7), the changes discussed above become apparent.

Table 8. Ratios calculated from BTEX data of altered gasolines in free product, soil and groundwater samples

Gasoline Grade* Benzene/
Toluene
Benzene
Ethyl-
benzene
Benzene/
Xylenes
Toluene/
Ethyl-
benzene
Toluene/
Xylenes
Ethyl-
benzene/
Xylenes
B + T/
E + X
Free Product (25) Average 0.26 0.97 0.16 3.60 0.60 0.16 0.65
Range 0.18-0.34 0.40-1.6 0.05-0.30 1.8-6.3 0.29-1.0 0.12-0.23 0.30-1.1
Water (366) Average 2.90 2.70 0.56 2.90 0.61 0.21 0.97
Range 0.09-21.6 0.13-10.9 0.01-3.5 0.1-9.6 0.04-4.2 0.08-0.55 0.11-3.4
Soil
(21)
Average 0.26 0.23 0.06 0.87 0.21 0.24 0.48
Range 0.03-100 0.02-2.9 <0.01-25 0.03-2.5 0.02-0.59 0.18-9.5 0.07-2.6

*Number in parenthesis denotes number of sample analysis averaged.

Environmental alteration of gasoline can also be determined by using a set of weathering ratios based on bulk PIANO compositions. The ratios presented in Table 9 show that the aromatic compounds are strongly concentrated in the groundwater and soil and to a lesser extent in the free product, relative to the naphthenes, paraffins, and isoparaffins. The paraffins and isoparaffins are preferentially concentrated in the free product and especially soil, relative to the dispensed gasoline. However, in groundwater the olefins appear to display an enrichment relative to the paraffins and isoparaffins. This suggests that removal of the olefins by water solution is more rapid than their rate of biodegradation. The last column in Table 9 shows that the isoparaffins and naphthenes are enriched in the soil and particularly groundwater relative to the paraffin fraction.

Table 9. Weathering ratios for gasoline in free product, water and soil (based on average values)

Sample Aromatics/
Naphthenes
Aromatics/
Total Paraffins
Paraffins + isoparaffins/
Olefins
Isoparaffins + Naphthenes/
Paraffins

 
Dispensed gasoline 7.8 0.9 8.4 3.8
Floating product 11.6 1.1 11.8 3.6
Water 22.9 5.3 6.0 7.7
Soil 76.4 11.2 35 4.5

Another potentially important ratio is listed in Table 4 under the heading of octane rating. To boost the octane number, a process of alkylation is performed at refineries where volatile olefins are heated with sulfuric or hydrofluoric acids, producing branched-chain alkanes (Hoffman and McKetta, 1992). Due to alkylation, the content of the major isoalkane generated, 2,2,4-trimethylpentane (TMP) increases in the finished gasoline relative to methylcyclohexane (MCH), a common constituent of crude oil and refined volatile fuels. Low octane fuels (87) have a TMP/MCH ratio <2.5. High octane gasolines (92 or 93) have TMP/MCH ratios >5, whereas for intermediate grade gasolines the ratio falls in the range of 2.5-5.0. When measured in gasoline contaminated groundwater samples using a purge-and-trap technique, this ratio decreases, probably due to the difference in purging efficiency between TMP and MCH. However, it still correlates with octane grade of dissolved gasoline.

The methodology so far described in this section is based on analytical data obtained by widely used gas chromatography methods. These techniques provide adequate sensitivity for measuring volatile hydrocarbons in environmental samples, but they only allow identification of those hydrocarbons for which standards are available and for which a recognizable chromatographic pattern has been established. Upon release to the environment, gasoline range hydrocarbons are highly susceptible to degradation by evaporation, water washing and biodegradation. When advanced alteration to the pattern occurs because of environmental exposure, the utility of gas chromatography alone in identifying the petroleum-related fuel types in the environment becomes increasingly limited.

Gas Chromatography-Mass Spectrometry Characterization.Coupling a gas chromatograph to a mass spectrometer gives the environmental chemist access to additional information that can be used to identify the presence of a variety of products, determine the degree of product weathering, and, in some instances, estimate the age of the product. Characteristic mass-to-charge ratios (m/z) for the principal aliphatic, aromatic and polyaromatic constituents of hydrocarbon fuels together with compound identification keys are presented in the Appendix .

As environmental alteration progresses, the more volatile hydrocarbons are preferentially removed by physical as well as biological processes. The alkanes and isoalkanes (m/z 85) may be eliminated and are therefore no longer useful as alteration tracers. The alkylcyclohexanes display a significant reduction in CH-1 (m/z 83), which is the dominant alkylcyclohexane in freshly dispensed gasoline, and a systematic increase in the CH-2/CH-1 ratio. For the C4-alkylbenzenes (m/z 134), 1,3-diethylbenzene (peak No. 20) is rapidly diminished and the group of compounds from 1-methyl-3-propylbenzene (peak No. 21) to 1,3-dimethyl-4-ethylbenzene (peak No. 27) decreases in relative content. The four hydrocarbons which persist are Nos. 1,2-dimethyl-4-ethylbenzene (peak No. 28), 1,2,4,5-tetramethylbenzene (peak No. 31a), 1,2,3,5-tetramethylbenzene (peak No. 31) and 1,2,3,4-tetramethylbenzene (peak No. 32). There is also a noticeable reduction in the peak height of 1,2,3,5-tetramethylbenzene (peak No. 31) relative to 1,2,4,5-tetramethylbenzene (peak No. 31a). For aromatic hydrocarbons depicted as bar diagrams (Figure 3), the observable changes are in the reduction of alkylbenzenes and the relative increase in naphthalene and the alkylnaphthalenes. Additionally, C3-alkylbenzenes diminish more rapidly than C4-, C5- and C6-alkylbenzenes. These results are in general agreement with the observations that alkylbenzenes and alkylnaphthalenes are more resistant to biodegradation than most of the other gasoline hydrocarbons (Eganhouse et al., 1993), and that the longer the alkyl chain attached to the aromatic ring, the more resistant the hydrocarbon is to biodegradation (Volkman, 1984).

Dye Additive Characterization. Finished automotive and aviation gasolines may contain dyes, which are blended by some fuel marketing companies to distinguish gasoline grades. There are four major commercial dyes used in automotive gasoline: red (alkyl derivatives of azobenzene-4-azo-2-naphthol), orange (benzene-azo-2-naphthol), yellow (para-diethylaminoazobenzene) and blue (1,4-diisopropylaminoanthraquinone) (Lane, 1980). In aviation gasoline, blue, yellow and red dyes have also been used to differentiate octane ratings (Ward, 1984). Analysis of dye additives by UV-visible absorption spectrometry or thin layer chromatography (see Figure 4) in dispensed gasolines and free products, may allow differentiation between gasoline grades and manufacturers and establish a source relationship. With time, the dyes in a released free product deteriorate (probably by hydrogenation which destroys the conjugated structure) and the red and orange dyes assume a dark brown color. Degradation of dyes generally limits their usefulness for forensic purposes. Yellow pigments are present in almost every gasoline sample that has been analyzed by the authors.

Fig 4. Thin layer chromatography of dye additives in different gasoline brands.

Other Fuel Additive Characterization. Refined fuels used for automotive or aircraft propulsion contain non-petroleum additives (Lane, 1980), some of which are useful for fuel-type differentiation and time of release estimates (see below). Lead alkyls and lead scavengers are historic gasoline additives designed to increase octane rating. Modern automotive gasolines are blended with oxygenates (alcohols and ethers) as mandated by the US Clean Air Act Amendments of 1990. Identification of these additives in environmental samples require specialized analytical methods.

Gasoline, diesel and jet fuels also contain a variety of additives. These are lubricants, antioxidants, corrosion inhibitors and gum inhibitors (Lane, 1980). Many of these additives are oxygen-containing compounds such as ethers or glycerols and some are also polymers. It is often believed that each manufacturer has a unique set of additives which can be used in tracing their source by identifying these additives in free floating products, soil or groundwater. Although the rationale for the concept is commendable, in practice, identifying such additives is generally not feasible or not useful. Because many of the additives contain oxygen in their structure, they are partially soluble and biodegradable. Further, the polymers tend to depolymerize rapidly in the environment and convert into their monomers, which may also be rapidly metabolized. Therefore, it becomes difficult to identify the intact additive. In addition, although many refined fuel marketers claim that their product additives are unique, in fact this is only partially true. The additives are often synthesized by specialty chemical companies and sold to the refineries. Therefore, the same additives may be purchased by many refineries and sold under their own label with minor or no alteration in chemical configuration.

Stable Isotope Ratio Characterization. If it is assumed that each refinery processes crude oil from a fixed geographic location over an extended time period, the isotope ratios for the corresponding fuel would be similar to that of the crude oil. The only way to change the starting ratios would be to fractionate the isotopic composition during manufacture of the product. The degree to which this is possible is limited by the efficiency of the refining processes used in producing the stocks that will be blended into fuels or lubricating oils. The production of very light gases during refining does cause small changes in carbon isotope ratios of the refined fuels by concentrating the heavy isotopes in the residues. However, this is limited by the amount of light gas lost in the processes. Ordinarily, this does not have the effect of masking the differences in the original isotopic ratios of the feedstocks (Silverman, 1971). Greater changes may occur in the hydrogen isotope ratios, because hydrogen may be deliberately added or removed during certain refining processes. In addition, hydrogen in the hydrocarbon fuel may also exchange with hydrogen of water in the subsurface environment.

The great utility of isotope ratios is their application as a tracer for organic materials that have undergone partial degradation through bacterial or other processes in the subsurface environment, where the molecular characteristics of the original contaminant are not easily recognizable. In large part, this is because the isotopic composition (especially for carbon and sulfur) does not change to the same extent as the molecular composition during the physical and biological processes that alter the chemistry of fuels. Stable isotope data can be conveniently displayed as a diagram, which helps in discriminating different sources. An example of this application, which was used to differentiate groundwater contamination in the vicinity of two gasoline service stations, is presented in Figure 5. From the isotope data shown, it is obvious that gasoline contamination found in groundwater at the site is closely related to dispensed gasoline from service station A.

Fig. 5. Correlation of Isotopic ratios in groundwater extracts and gasoline fuels from two gasoline service stations. MW-n refers to groundwater samples.

5.5 Middle Distillate and Heavy Fuel Oil Characterization by Hydrocarbon Group Fingerprinting

Among analytical methods currently used to identify fugitive crude and refined petroleum (Bray and Evans, 1961), are those that focus on hydrocarbon group-type analysis, such as alkanes, polynuclear aromatic hydrocarbons and polycyclic alkanes (biomarkers).

Alkane and Alkylcyclohexane Fingerprinting. Alkane distribution patterns over a wide carbon number range are routinely obtained using long column gas chromatography with a flame ionization detector (GC/FID) (Whittemore, 1979) or long column gas chromatography with a mass selective detector (GC/MS) (long column, experimental conditions similar to EPA method 8270C, 1996). For petroleum-related products that have not undergone noticeable biodegradation, GC/FID provides an adequate procedure for fingerprinting and fuel recognition based on the n-alkane homologous series.

However, GC/FID is significantly less useful for identification of other hydrocarbon groups, which is often necessary for petroleum product fingerprinting. Conventional geochemical methods for identification of isoprenoids, PAH and biomarkers which are commonly used in characterizing crude oil (Douglas et al., 1992), are of limited utility for light and some middle distillate fuels (Kaplan et al., 1996).

The limitations for the use of normal alkanes in characterization of fuel types arise from the fact that upon release into the environment, they are preferentially degraded, and the distribution of the more recalcitrant isoprenoid hydrocarbons, when present, may not provide conclusive information on the source or type of fugitive fuel (Kaplan et al., 1996). Further, due to overlap of n-alkane, alkylbenzene and alkylnaphthalene chromatographic patterns in mid-distillate fuels with similar boiling ranges, recognition of fuel type(s) present in an environment, often becomes a challenging and formidable task.

Read More: Identifying Hydrocarbons, pg. 4

NEXT: Principles of estimating the date of a Hydrocarbon Fuel Release

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