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.