DETECTION OF MOLECULAR MARKERS IN AQUATIC SEDIMENTS BY ION PROFILES OBTAINED BY GC/MS SYSTEM
An analytical method has been developed that permits the determination of molecular markers in aquatic sediments. The procedure consists of ultrasonic solvent extraction followed by a clean-up operation and then analysis by a GC-MS method. The molecular markers are visualised by ion profi le chromatography. The compounds were identifi ed based on mass spectra obtained by Electron Impact (EI) in full scan mode.
1. Introduction
The sedimentary organic matter results from a complex combination of both natural and anthropogenic sources [1-7]. The activities of present day civilisation have released into the aquatic environment a wide variety of natural and synthetic compounds not found previously. Many of these compounds when found provide a means of identifi cation of sources of input and pathways of movement of chemicals through ecosystem.
The concentrations of organic compounds in an environmental sample refl ect both the original source of organic matter and alteration processes, which have occurred in the environment. The anthropogenic and naturally occurring compounds are found mixed together in recent environmental samples and several of these compounds may be used as tracers to study natural processes affecting the fate and effects of chemical contaminants in aquatic sediments.
The sediments constitute archives of environmental processes. In order to differentiate between inputs of organic matter, molecular marker approaches have been developed on the potential of molecules to be source specifi c [2-4,8,9].
The purpose of this present paper is the characterisation by GC/ MS analysis of organic molecular markers detected in the river sediment to evaluate the degree of contamination and identify the sediment sources in a surface water catchment in Romania. The GC/MS method has the advantage of generating a number of parameters from a single analysis. The study presents data for several families of organic compounds used as molecular markers [8-10] in sediment organic mater investigation: n-Alkanes, Acyclic isoprenoid, Pentacyclic triterpanes (Hopanes), Polyaromatic Hydrocarbons (PAHs), Benzothiophenes, Linear Alkylbenzens (LABs), Trialkylamines (TAMs) and 2M-Ketones.
2. Experimental section
Sample sites The samples were collected from places along a 60 km section (between Cluj-Napoca and Dej, Romania) of the Somes River catchment and were analysed for a large number of organic compounds considering as molecular marker for sediment mater composition.
Ultrasonic solvent extraction Cl2
Figure 1. GC/MS chromatogram on m/z 85 for a sediment sample collected from river for visualisation of n-alkanes (n range 12-26)
every extraction. The sediment was centrifuged at 3,000 cycles/ minute for 10 minute. The extract was fractioned on Al2
O3
following organic phases: n-hexane, n-hexane+CH2 CH2
Cl2 Cl2 (1:2),
+methanol (1:1) and methanol respectively. The fi nal extracted was dissolved in 1ml in n-hexane [11,12].
Instrumentation
The GC/MS analyses were performed using a Thermo Electron Ultra Trace GC and a Polaris Q mass spectrometer operated in EI mode to 70 eV. The source temperature was 250ºC and emission current 300 µA. The gas chromatograph was equipped with a capillary column HP-5MS (30x0.25mm) with 0.25 micron fi lm thickness. The temperature was programmed from 90ºC (1minute) to 120ºC at 10ºC/minute and than to 200ºC at 3.5ºC/min and then to 315ºC at 5ºC/min (keeping this temperature for 11 min). Helium was used at a fl ow of 2 ml/minute as carrier gas.
with 3. Results and discussions
3.1. The n-Alkanes. The alkanes are detected in high quantity and show compounds with the number of carbon atoms ranging from 12 to 26. They are presented by the characteristic ion m/z 85 (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 ion chromatogram is unimodal with a maximum to 17 carbon atoms having a CPI (C14-C21) value of 1.3 leading to the conclusion that n-alkanes originate from aquatic algae [2,6]. The relative amount of odd/even carbon number n-alkanes (or CPI) is used to obtain information on relative maturities of organic mater from sediment.
An aliquot (2g) of freeze-dried sediment was extracted successively with 10 ml CH2
+methanol (2:1) 3 times for 15 minutes IET JANUARY / FEBRUARY 2021
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Figure 2. The detection of acyclic isoprenoids (Pristane and Phytane are main compounds) in a sediment river sample.
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