47
Figure 4. Polyaromatic Hydrocarbons (PAHs) detected by ion chromatograms at m/z 178, 202, 228, 252, 276 and 300 (shown above and in Table 1) respectively in the range time of 18-70 min.
derived from terrestrial sources have higher concentration of the long chain isoprenoids than samples derived from marine sources [8].
3.3. Pentacyclic triterpanes (Hopanes)
Hopanes are a class of compounds that has a great impact on petroleum geochemistry. They are very resistant to biodegradation and can serve therefore as conserved internal standards for assessing the biodegradation of more degradable compounds [13]. They have carbon atom numbers ranging from 27 to 35. The hopanes are detected by drawing the profile using the ion m/z 191 (fraction a). In addition to providing maturity information, the detected molecular ions of the hopanes are also useful as source indicators. The hopanes detected in a sediment sample collected sample from Somes River are shown in the Figure 3.
The Hopanes are molecular markers used currently for estimation of the mineral maturity [2,5]. In low maturity materials the group of hopanes are dominated by stereochemistry 17β(H),21β(H) and configuration R at C22
and higher homologues [5,8].
The chromatogram obtained to m/z=191 can be used directly for estimation of the maturity minerals by comparison of the two compounds at C27
: 18α(H)22,29,30-
trisnorhopane and 17α(H)22,29,30- trisnorneohopane (the peaks 1 and 2 respectively).
3.4. Polyaromatic Hydrocarbons (PAHs)
An abundant class of compounds detected in the river sediment samples are Polyaromatic Hydrocarbons (PAHs). These compounds show the molecular ion as the base peak in mass spectrum and therefore the molecular ion is used for selected ion chromatogram (Figure 4). The molecular ion was also used as the diagnostic ion for all PAHs in early papers [2].
. As the maturity level
increases 17α(H), 21β(H) stereochemistry predominates and 22S configuration for the C31
The presence of polycyclic aromatic hydrocarbons (PAHs) in sediment samples reflects the anthropogenic impact on sediment organic matter [2,7]. The input of PAHs into the environment are due to anthropogenic activities such as combustion of fossil fuel as well as natural processes such as forest fires. The PAHs source can be of petrogenic, pyrogenic or biogenic origin [2,7]. Combustion of fossil fuels and plant materials leads to pyrogenic PAHs with four to eight parental rings. The
pyrogenic markers include fluoranthene, benzofluoranthenes, benzoanthracenes, benzo[e]pyrene, benzo[a]pyrene and benzo[ghi]perylene, with low abundances of alkylated
surrogates.The petrogenic sources (ancient plant material) are characterised by alkylated structures. The values of characteristic source parameters[2] defined as ratio Fluorantene/(Fluorantene + Pyrene) and ratio Benz[a]antracene/(Benz[a] antracene + Chrysene) for investigated samples have the average values 0.47 and 0.40 respectively. These values show a pyrogenic origin of the detected PAHs.
3.5. Sulphonated Polyaromatic Hydrocarbons (SPAHs). Benzothiophenes
Benzothiophenes is another important group of pollutants used very often as biomarkers by the source information. The organosulfur compounds can be formed by the reaction of aromatic hydrocarbons with elemental sulphur [14]. They are similar to PAHs in the fact that they are both very persistent chemicals [15]. Because of their environmental impact, PAHs are included in the list of priority pollutants (U.S. EPA). This group of compounds can by analysed by selected ion chromatogram having also the molecular ion as selected ion (Figure 5).
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