2.3 Measuring the levels of weathered crude oil by GC-MS

The GC-MS has been widely used in recent environmental research due to its effectiveness with regards to many major classes of organic samples. Analyses of weathered crude oil in terms of TPH were performed on a Varian 43GC and a Varian 21-MS IT with an auto-sampler CP-8400 (Figure 3-1) in order to examine and assess traces of petroleum hydrocarbons in the soil sample. A DB-5 MS capillary column (15 m x 0.32 mm i.d. x 0.1 μm film) was used for the GC–MS. A GC-MS works by mixing the specimen solution with helium and air in the column. The GC- MS had the following conditions: injector temperature, 300 °C; oven temperature program, 50 °C for 2 min – 300°C for 20 min at 5 °C/min. 1 μl sample of the sample was introduced into the GC inlet by injection in which it vapourised and swept onto a chromatography column by helium which is used as the carrier with a constant flow rate of 1.0 ml/min at 300°C and then carried on to the MS console. Electron impact mode was used for ionisation of the sample. The MS was scanned from m /z35–550, with 3 min solvent delay and a 150-threshold count.
2.4 Gravimetric method for analysis of weathered crude oil
Gravimetric method is based on two phases, the first phase aimed to extract the weathered crude oil from sand while the second phase aimed to determine the percentage of total petroleum hydrocarbons recovered from contaminated sand. The oil was extracted from the sand based on the following procedure. One gram of contaminated sample was placed into a screw-capped bottle with 5 ml of acetone: hexane solvent (1:1 v/v) and washed for 5 minutes in ultrasonication bath. The solution of crude oil and mixture of acetone: hexane was pipetted into a clean and dry weighed round-bottomed glass flask. The ultrasonication cycle was then repeated for four times using fresh solvent in each cycle.
The isolated sand was left to air dry for 2 hours and then noted the dry weight. The collection apparatus, conical flask, distillation head with condenser side arm and water bath were prepared and fixed to determine the percentage of total recoverable petroleum hydrocarbons. The water bath temperature was maintained at 52oC, to accelerate the evaporation of the acetone: hexane mix. Finally, when the acetone: hexane mix was recovered, the round glass flask was dried, then the total weight of crude oil in the soil sample was re-weighed to calculate the concentration of isolated weathered crude oil.
\begin{equation} \frac{\text{mg\ Oil}}{\text{sample\ g}}=\frac{\left(Wb-Wa\right)\ \times\ 1000\ }{\text{g\ sample}}=.\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots..\ldots.\mathbf{\text{E.q}}\left(\mathbf{1}\right)\nonumber \\ \end{equation}
Where, the weight of the crude oil sediment plus the round flask is Wb, while the weight of the round flask is Wa. While, the percentage removal of weathered crude oil is calculated by
\begin{equation} \text{Weathered\ crude\ Oil\ }\%=\frac{Wd-Wf\ }{\text{Wd}}\ \times\ 100=.\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots\ldots..\mathbf{\text{E.q}}\left(\mathbf{2}\right)\nonumber \\ \end{equation}
Where, the initial concentration of the weathered oil in the soil (g) before washing is Wd, while the concentration of weathered crude oil in the soil (g) after washing is Wf.