46 February / March 2019
Figure 2. The detection of acyclic isoprenoids (Pristane and Phytane are main compounds) in a sediment river sample.
minutes every extraction. The sediment was centrifuged at 3000 cycles/minute for 10 minutes. To reduce molecular interference from different families of compounds, the extract was fractioned on Al2
O3 Cl2 (1:2), c) CH2 Cl2 with the
following organic phases: a) n-hexane, b) n-hexane:CH2
:methanol
(1:1) and d) methanol, respectively. The final extract, after evaporation, was dissolved in 1ml n-hexane [11,12]. The n-Alkanes, Isoprenoids and Linear Alkyl Benzenes were analysed from fraction a, Polyaromatic Hydrocarbons and Sulphonated Polyaromatic Hydrocarbons from fraction b and Trialkylamines and 2M-Ketones from fraction c.
Instrumentation
The GC/MS analyses were performed using a Thermo Electron Ultra Trace GC coupled with a Polaris Q mass spectrometer operated in EI mode to 70 eV. The ion source temperature was 250ºC and the emission current was 300 µA. The gas chromatograph was equipped with a HP-5MS (30mx0.25mm) capillary column
possessing a film thickness of 0.25 micron. The temperature was programmed from 90ºC (1minute) to 120ºC at 10ºC/minute, then to 200ºC at 3.5ºC/min, and finally to 315ºC at 5ºC/min (keeping this temperature for 11 min). Helium was used as carrier gas at a flow of 2 ml/min.
3. Results and discussions
3.1. The n-Alkanes. Alkanes are detected in high quantity and show compounds with the number of carbon atoms ranging from 12 to 26. They are represented by the characteristic ion m/z 85 on fraction a of extract (Figure 1). To investigate the origin of n-alkanes the results and information from ion chromatogram data analyses as carbon number range and Carbon Preference Index (CPI) [2,5,8] are used. This number can be used also as an indicator of the product origin.
The selected ion chromatogram on selected ion m/z 85 is unimodal with a maximum to 17 carbon atoms having a CPI (C14-C21) value of 1.3. This finding reveals that the observed n-alkanes originate from aquatic
algae [2,6]. The relative amount of odd/ even carbon number n-alkanes (or CPI) is then used to obtain information on relative maturities of organic matter from sediment.
3.2. Acyclic isoprenoids
The acyclic isoprenoid compounds have a large number of methyl branching points in their structure leading to the production of information rich spectral enabling, in most cases, the branching points in the molecules to be determined. They are represented with the fragment m/z 183 containing 13 carbon atoms. The profiles of acyclic isoprenoids based on drawing the signal to m/z 183 (on fraction a) is shown in the Figure 2.
This group of compounds includes pristane (C19) and phytane (C20) is often used as an indicator of depositional environments [5,8]. The idea is based on the premise that pristane is formed from oxidation and decarboxylation reactions of phytol while phytane is formed by hydrogenation and dehydration of phytol. The type of reaction is mainly dependent on the oxidisation and reducing-type environments. The samples
Figure 3. Detection of Hopanes in a river sediment (m/z 191) in the time range 40-51.2 min.
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