Identifying Hydrocarbons, pg. 4

m/z 83 and 85 Mass Chromatograms for the Common Refined Petroleum Products.

Based on systematic GC/MS studies of different commercial and military fuels conducted in our laboratory, we have determined that the cyclohexane homologous compound series, which exhibits a characteristic distribution pattern in m/z 83 mass chromatograms for each fuel type, provides another useful fingerprint for characterizing petroleum derivatives (Kaplan and Galperin, 1996).
Examples of alkane (m/z 85) mass chromatograms and alkylcyclohexane (m/z 83) mass chromatograms for the common refined petroleum products are shown in Figures 6A through 8C.

Click here to view the Mass Chromatograms
Figure 6A  -  Mineral spirits
Figure 6B  -  Stoddard solvent
Figure 6C  -  Naphta
Figure 7A  -  Kerosene
Figure 7B  -  Diesel#1
Figure 7C  -  Diesel#2
Figure 8A  -  JP-5
Figure 8B  -  Jet-A
Figure 8C  -  JP-8
Key for peak identification is given in the Appendix .

The range of hydrocarbons in each mass chromatogram is determined by the boiling range of a product, whereas the distribution pattern reflects its application-specific formulation.

A short description of patterns for selected reference products is provided below. These distribution patterns for alkylcyclohexanes in refined petroleum products were published previously by our laboratory (I. R. Kaplan et al., 1997). Since composition of modern fuels is controlled by stringent manufacturing specifications, the range and distribution pattern of a product varies only slightly for each fuel, depending on the composition of crude oil feedstock and refining practices used in manufacture.

  1. Gasoline m/z 83 mass chromatogram exhibits an asymmetric distribution pattern in the CH-1 (methylcyclohexane) to CH-7 (heptylcyclohexane) range. Peak CH-1 is the most abundant, with peaks CH-2 to CH-7 rapidly decreasing in intensity.
  2. Mineral spirits m/z 83 mass chromatogram, presented in (Figure 6A), shows a symmetric distribution pattern with peaks CH-3 and CH-4 dominant, whereas the CH-2 and CH-5 peak heights are much smaller.
  3. Stoddard solvent exhibits an alkylcyclohexane pattern ( Figure 6B) in the CH-2 to CH-9 range and demonstrates a distribution maximizing at CH-5.
  4. Naphtha produces an alkylcyclohexane distribution (Figure 6C) in the range CH-1 to CH-6 which maximizes at CH-3.
  5. Kerosene shows a characteristic alkylcyclohexane distribution ( Figure 7A) in the range CH-1 to CH-9, maximizing at CH-6.
  6. Diesel fuel No.1 exhibits an alkylcyclohexane pattern ( Figure 7B) in the range CH-1 to CH-14 with maximum at CH-5.
  7. Diesel fuel No.2 m/z 83 mass chromatogram ( Figure 7C) demonstrates a wide range of alkylcyclohexanes from CH-1 to CH-14, maximizing around the CH-9 and CH-10 peaks.
  8. Military jet fuel JP-5 and commercial fuel Jet-A ( Figure 8A and 8B) demonstrate distribution patterns in the kerosene range (CH-1 to CH-9) with a noticeable difference in the maximum peak of distribution: CH-5 for JP-5 and CH-4 for Jet-A.
  9. Military Jet fuel JP-8 shows an asymmetric distribution pattern ( Figure 8C) in the CH-1 to CH-14 range, with a maximum distribution at CH-3.
  10. Military jet fuel JP-4 is a kerosene-gasoline range fuel mixture. Its m/z 83 mass chromatogram displays an alkylcyclohexane distribution in the CH-1 to CH-9 range, with peak CH-1 the largest peak, due to a high proportion of gasoline-range hydrocarbons in this fuel.

It is apparent from these mass chromatograms that alkylcyclohexane distribution patterns are as product-specific as are those for alkanes and therefore, can also be used for fuel type identification in environmental samples in combination with alkane distribution patterns. The main advantage in utilizing alkylcyclohexane patterns for hydrocarbon fuel recognition, is that alicyclic compounds are resistant to environmental alteration and can be identified in samples even when all of the n-alkanes and most or all of the isoalkanes and alkylbenzenes have been degraded.

Case Histories Using Alkylcyclohexane Patterns. The two case histories described here illustrate the application of alkylcyclohexane pattern recognition for fuel-type identification in weathered environmental samples. Key for peak identification is given in Appendix .

Case Number 1. Shown in Figure 9A , is a m/z 85 alkane mass chromatogram of a free floating product. This chromatogram is typical of a petroleum product which has lost all n-alkanes due to weathering in the subsurface environment.

The group of peaks identified on the chromatogram represents isoalkanes (isoprenoids) from i-C15 to Ph (phytane, i-C20). The distribution of isoprenoids suggests the presence of a diesel fuel, but because n-alkanes are missing, it is not clear if other petroleum products (e.g. kerosene, diesel fuel No.1 or Bunker C fuel) are also present.

However, when the m/z 83 mass chromatogram is extracted from the same analytical run ( Figure 9A , second chromatogram), a clear distribution pattern of alkylcyclohexanes emerges. Comparison of this pattern with data for reference fuels allows one to conclude that the floating product consists entirely of a weathered diesel fuel No. 2.

Case Number 2. The alkane mass chromatogram (m/z 85) of a free floating product ( Figure 9B ) shows no detectable n-alkanes in this environmentally altered sample. Presence and relatively high abundance of pristane (Pr, i-C19) and phytane (Ph) suggests the presence of a weathered diesel fuel, whereas high abundance of i-C15 and i-C16 peaks and the presence of a group of peaks representing i-C9 - i-C12 isoalkanes also suggests the presence of a kerosene-type fuel. The alkylcyclohexane distribution pattern ( Figure 9B , second chromatogram) demonstrates that the major component of the floating product is mineral spirits (characterized by pronounced CH-3 and CH-4 peaks) with reduced amounts of kerosene and diesel.

Environmental Alterations of PAH’s (weathering, biodegradation). Alkylbenzenes, naphthalene and methylnaphthalenes. Distribution patterns of PAH compounds provide another useful tool for monitoring environmental alteration of heavy-end petroleum fuels. Some of the alkylated monoaromatic and PAH compounds of diesel fuel and Bunker C (also referred to as heating oil No. 6) are among the least affected by weathering:

Fig. 10. Bar diagram for PAH in unaltered diesel fuel No. 2. Acronyms in Appendix .

In this group of compounds, the alkylbenzenes are first to be altered by biodegradation, followed by naphthalene and methylnaphthalenes. Certain PAH isomers are biodegraded more rapidly than others, an observation which may be useful in determining degree and rate of alteration changes of diesel fuel and Bunker C. A set of important PAH weathering parameters is presented in Table 10. These ratios have been found useful in monitoring changes of the heavy oil components in soil or groundwater.

Table 10. Biodegradation ratios of diesel No. 2 and Bunker C fuels using selected PAH ratios

  Diesel Fuel
  Bunker C
C 4-alkyl- benzenes
C 2-phenan- threnes
C 2-naph- thalenes
C 2-phenan-threnes
 C 2-dibenzo-thiophenes
C 2-phenan- threnes
phenan- hrene,
phenan- threne
New Fuel   1.6  4.3  0.4  0.5
Degraded Fuel      1.2  3.4  1.3  1.1
Severely Degraded Fuel      1.0  2.2  1.9  2.5

Interestingly, the PAH parameters suggested in the extensive studies of Douglas et al. (1996) for marine weathering of crude oil do not appear to be replicated in the terrestrial environment. This may be due to the more extensive oxygenation occurring under wave mixing and solar radiation in surface ocean water or in the sublittoral zone.

Terpanes and Steranes. Bunker C fuel represents the distillation residue of a crude oil and contains biomarkers such as terpanes and steranes. The C30-pentacyclic terpane (hopane) and certain tricyclic terpanes are among the most stable biomarkers (Peters and Moldowan, 1993). Compared to the regular tetracyclic steranes, the diasteranes are more resistant to biodegradation.

Most crude oils and Bunker C fuels contain modified steranes in which either one ring, or all three six membered rings are aromatized (by heat exposure) and are converted to monoaromatic and triaromatic steranes. These hydrocarbons are highly resistant to degradation.

Severely altered Bunker C fuel undergoes a number of changes in soil. The terpane pattern (m/z 191 mass chromatogram) shows that peak #K (C29 17a -21b-30-norhopane) and peak #N (C30 17a, 21b-hopane) dominate the terpane distribution pattern. Sterane changes are not large, although an increase in a C28-sterane (24-methyl-13a, 17b-diacholestane (20S)) is evident.

For the PAH fraction, the naphthalenes and phenanthrenes are significantly reduced and the benzo-, dibenzo- and napthobenzo- thiophenes are reduced to values barely above detection limits. The hydrocarbons which desplay a significant increase in relative concentration, are the mono- and triaromatic steranes. In this example, the triaromatic steranes became the dominant PAH in the residual Bunker C component of the soil.

Based on the comparison of newly dispensed fuels with those in the environment, a scheme has been developed (Figure 12) (see also Volkman, 1984) to illustrate the relative degradation level of different hydrocarbon types in fuels with a volatility range from gasoline to Bunker C fuel.

 Fuel Type
Level of Biodegradation Chemical Composition
 1 Abundant n-alkanes
 2 Light-end n-alkanes removed
 3 Middle range n-alkanes, olefins, benzene & toluene removed
 4 More than 90% of n-alkanes removed

Alkylcyclohexanes & alkylbenzenes removed

Isoprenoids & C0-naphthalene reduced


Isoprenoids, C1-naphthalenes, benzothiophene & alkylbenzo-
thiophenes removed.
C2-naphthalenes selectively reduced

 7 Phenanthrenes, dibenzothiophenes and other polynuclear aromatic hydrocarbons reduced
 8 Tricyclic terpanes enriched. Regular steranes selectively removed C31 to C35-homohopanes reduced
 9 Tricyclic terpanes, diasteranes & aromatic steranes abundant

Aromatic steranes & demethylated hopanes* predominant
* = Present under certain conditions

Fig. 12 Change in gasoline, diesel fuel and bunker C oil compositions during biodegradations

5.6 Fuel Mixing Ratios by Simulated Distillation

Simulated distillation (ASTM, 1994, method D2887) is a gas chromatography technique routinely employed in the petrochemical industry for determining the boiling point range of petroleum products. When applied to environmental NAPL samples, this fast and inexpensive method often provides useful information regarding the fuel-type present, because the boiling point range is a major controlling parameter in the manufacture of petroleum products. An example of boiling point distribution for selected reference fuels is shown below in Figure 13.

Fig. 13 Simulated distillation curves for selected reference fuels: Gasoline, JP-4, JP-5, Diesel #2 and Bunker C.

Each fuel is characterized by initial and final boiling points, as well as by the shape of the distillation curve. Studies conducted in our laboratory demonstrate that in cases of two component hydrocarbon NAPL plumes, simulated distillation curves can be used to determine the relative amount of each fuel in the mixture at various locations. An example from a case history for a mixed gasoline-diesel plume, which demonstrates the method application, is presented here. The same approach was also successfully applied to other binary mixtures. Shown in Figure 14 is an alkane mass chromatogram of the representative sample (MW-14) from a NAPL plume.

Fig. 14. Alkane mass chromatogram of the free floating product. Keys for peak identification is given in Appendix.

Comparison of the alkane distribution pattern obtained with those for reference fuels indicate that the plume consists of two fuels: gasoline and diesel fuel No. 1. This conclusion is consistent with the sample distillation curve (Figure 15), which exhibits an initial boiling point of about 50°C characteristic of gasoline and a final boiling point at approximately 340°C, typical of diesel fuel No. 1.

Fig. 15 Simulated distillation curves for the free floating product and a gasoline (35%) and diesel (65)% mixture.

To determine a mixing ratio, artificial gasoline-diesel fuel No. 1 mixtures were prepared and analyzed by the simulated distillation method. The boiling point distributions obtained for each mixture are shown in Figure 16.

Fig. 16. Simulated distillation curves for artificial mixtures of gasoline and diesel

Next, by plotting amount of gasoline (%) for each mixture versus corresponding recovery (%) at the optimal for this particular fuel combination temperature of 180°C (at this temperature, all the mixtures exhibit a measurable recovery) we obtained a linear calibration curve (Figure 17).

Fig. 17 Calibration curve for simulated distillation of gasoline and diesel mixture (T = 180° C)

Using this calibration curve and the percent recovery value at 180°C for product sample MW-14 (Figure 15), the content of gasoline in the plume was estimated to be 35%. This result was confirmed by comparing the simulated distillation curve of the sample with the curve obtained for a 35% gasoline - 65% diesel fuel No.1 artificial mixture. Figure 15 shows that both curves match closely in shape and in initial and final boiling points.

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

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