Analysis of Hydrocarbon in Common Fuels by Spme (solid Phase Microextraction) and Gc-Ms (gas Chromatography-Mass Spectrometer)

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Unknown Compound

Petrol

Compounds

Quality (%)

Retention Time (min)

Compounds

Quality

%

Retention Time (min)

Toluene

91

2.52

Toluene

94

2.59

P-xylene

95

3.76

P-xylene

97

3.87

Benzene, 1,2,4-trimethyl-

95

5.72

Benzene, 1,2,3-trimethyl-

95

5.82

Compound of unknown that match with compound in diesel.

Unknown Compound

Diesel

Compound

Quality (%)

Retention Time (min)

Compound

Quality (%)

Retention Time (min)

Naphthalene, 1,6-dimethyl-

96

12.16

Naphthalene, 1,6-dimethyl-

96

12.16

Discussion

Based on the objective, major compound must been identified in each sample that were petrol, diesel, thinner and unknown sample. In order to identify the compound in the sample, firstly the solid-phase microextraction (SPME) technique was applied followed by gas chromatography-mass spectrometry (GC-MS) for a separation. Microextraction means that the amount of extraction solvent was very small compared to the sample volume. The SPME fibre was exposed to the headspace of the vial for 20 minutes before it was injected into GC. Since the compounds were volatile, headspace technique was chosen for extraction because it much clean than the fibre place directly into the sample. In this experiment, fibre used was polydimethylsiloxane (PDMS) for the extraction of non polar compounds. The film in PDMS acts like a ‘sponge’, concentrating the analytes on its surface during adsorption from the sample matrix. After the extraction time was completed, the SPME fibre was introduced into the gas chromatography injector for desorption for about 80 seconds. Then, fibre was transferred immediately as it did not want any of sample components collected to be disappeared into surrounding. Purpose of desorption was to transfer analytes from the SPME fibre to the column chromatography. The first sample injected was petrol. There were a lot of components found in the sample. However, only components with high quality were chosen and the structures were taken from the library of the mass spectrometer. For good selection compounds, the quality must be 90 and above. The components taken in petrol were toluene, ethylbenzene, P-xylene, and benzene, 1,2,3-trimethyl-. The quality of toluene, ethylbenzene, p-xylene, and benzene, 1,2,3-trimethyl- were 94,91,97, and 95 respectively. Based on the chromatogram, 10 out of 15 compounds of benzene was discovered in the petrol. It can be deducted that major compound in the petrol was benzene. Compounds that contain in petrol more volatile than diesel and thinner sample since the retention time was shorter that was 1.97 min compared to diesel (2.52 min) and thinner (2.63 min). For the second sample, diesel there were 20 compounds found in diesel. The major compounds identified were tridecane, naphthalene, 1-methyl-, naphthalene, 1,6-dimethyl-, and Cycloheptasiloxane, tetradecamethyl-. The quality for tridecane and 1,6-dimethyl- were 92 and 96 respectively while, cycloheptasiloxane, tetradecamethyl- and naphthalene, 1-methyl- were shared the same quality which 93. Major content in diesel were naphthalene since it was find in majority in the compounds. The retention time of diesel was about 20 minutes. Next sample was thinner. There was only one component detected which was toluene with 91 quality and 2.63 min of tR. Lastly, the sample injected was an unknown sample. The unknown sample analysis shows that the unknown sample was the combination of diesel and petrol based on the similarities of some of the major compounds present in diesel and petrol. The major compounds presence in unknown were Toluene with 91 quality, p-xylene with 95 quality, and benzene, 1,2,4-trimethyl- with 95 quality that also presence in petrol while naphthalene, 1,6-dimethyl- with quality 96 has exactly the same analysis as in diesel. The advantages of using SPME was that the extraction was simple and fast, also can be done without any solvents. If the sample properly stored, it can be analysed days later without much loss of volatiles.

Conclusion

The major compound presence in petrol were toluene, ethylbenzene, p-xylene, and benzene, 1,2,3-trimethyl-. The major compounds in diesel were tridecane, naphthalene, 1-methyl-, naphthalene, 1,6-dimethyl- and cycloheptasiloxane, tetradecamethyl-. Thinner only show a single peak that corresponds to toluene. It can be concluded that the unknown sample was the mixture of diesel and petrol since there were some major compounds of petrol and diesel presence in the unknown.

References

- Nor’ashikin S., Ruziyati T., Mardiana S. (2014), Analytical Seperation Methods Laboratory Guide (2nd edition),

- Solid-Phase Microextraction, 12/7/2015,

http://en.wikipedia.org/wiki/Solid-phase_microextraction.

3. Mardiana Saaid, 12/7/2015, SPME sample preparation notes