EXPERIMENT 4: ANALYSIS OF HYDROCARBONS IN COMMON FUELS BY SOLID-PHASE MICROEXTRACTION (SPME) AND GAS CHROMATOGRAPHY- MASS SPECTROMETRY (GC-MS)


CHM510
ANALYTICAL SEPARATION METHOD



EXPERIMENT 4:
ANALYSIS OF HYDROCARBONS IN COMMON FUELS BY SOLID-PHASE MICROEXTRACTION (SPME) AND GAS CHROMATOGRAPHY- MASS SPECTROMETRY (GC-MS)







INTRODUCTION

Solid-phase microextraction is a sample preparation technique that use fibre coated with extracting phase that extract different kinds of analytes for different fibre coating. The analytes can be in the form of liquid or gas either volatile or not. In this experiment, head space SPME is used where the fibre is exposed in the vapour phase above a gaseous sample. Head space SPME is suitable for the analysis of volatile organic compounds, and it is important to keep the headspace volume constant and keep the fibre position at the same depth for every analysis. The amount of analyte extracted by the fibre depends on the concentration of the analyte in the sample as long as equilibrium reached. After the extraction process was done, the fibre is transferred to the injection port of GC where desorption of the analyte takes place and the separation occur. The factors that affect the SPME performance are the coating material, thickness of fibre, sample agitation, sample pH, salt addition and the extraction procedure. The objective of this experiment is to identify the major hydrocarbons components in common fuels (diesel, petrol, thinner and an unknown) by using SPME-GC-MS.




REAGENT AND SOLUTION
Accelerants: Unleaded petrol, diesel, paint thinner and unknown.




INSTRUMENT
Gas Chromatograph (Agilent Technologies 6890N) equipped with HP 5971A mass selective detector (MSD) and a 30 m x 250 µm x 0.25 µm HP5-MS capillary column.




PROCEDURE
a)      Instrument Set-Up
Injector Temperature: 250oC
Detector Temperature: 300oC
Carrier Gas Flow Rate: 30 mL s-1
Column Temperature: 60oC to 170oC at 10oC min-1


b)      SPME Procedure
The fibre (100 µm polydimethylsiloxane[PDMS] in a GC Injection port was conditioned about 10 minutes at 250oC to remove contaminants. 5mL of sample in a glass vial was added with septum and the vial was placed on a hot plate. The sample was heated to 50oC and agitated the sample using a magnetic stirrer. The SPME fibre was exposed to the headspace of the vial for 20 minutes. The fibre was withdraw into the needle, pulled out from the vial and injected into the GC-MS with desorption time 80 seconds. The major compounds in each sample were identified using mass spectra library.




RESULTS


A.     Major compounds presence in petrol sample;

Retention Time (min)
Area (%)
Quality
Compound
4.09
12.07
97



o-Xylene
5.63
2.09
97



Mesitylene
6.53
1.28
97




Benzene, 1,2,3-trimethyl-
7.54
0.39
97




Benzene, 2-ethyl-1,4-dimethyl-



B.      Major compounds presence in diesel sample;

Retention Time (min)
Area (%)
Quality
Compound
10.66
10.17
94



Tridecane
11.79
3.11
98





Decahydro-1,1,4a,5,6-pentamethylnaphthalene
12.02
15.13
97



Tetradecane
14.79
4.26
98



Hexadecane




C.      Major compounds presence in thinner sample;

Retention Time (min)
Area (%)
Quality
Compound
2.80
100.00
91



Toluene

D.     Compound of unknown that match with compound in petrol;
Retention Time (min)
Area (%)
Quality
Compound
6.05
8.25
95





Benzene,1, 2, 3-trimethyl-
6.53
3.68
93



Mesitylene





E.      Compound of unknown that match with compound in petrol;

Retention Time (min)
Area (%)
Quality
Compound
9.22
9.84
95



Dodecane
11.79
2.85
99





Decahydro-1, 1, 1, 4a, 5, 6-pentametylnaphthalene
12.01
8.92
97


Tetradecane
13.29
3.69
96


Pentadecane





DISCUSSION

From the analysis done, 4 major compounds presence in petrol sample are o-xylene, mesitylene, benzene, 1, 2, 3-trimethyl- and benzene, 2-ethyl-1, 4-dimethyl- that lie from retention time of 20 minutes to 33 minutes of analysis. Diesel sample show the trend of major compound peaks from 7 minutes to 18 minutes that respond to tridecane, decahydro-1, 1, 4a, 5, 6-pentamethyl-naphthalene, tetradecane and hexadecane based on the mass spectra library.

The unknown sample analysis shows that the unknown sample is the combination of diesel and petrol based on the similarities of some of the major compounds presence in diesel and petrol. The major compounds presence in the unknown are Benzene,1, 2, 3-trimethyl- and mesitylene that is presence in petrol while dodecane,decahydro-1, 1, 1, 4a, 5, 6-pentametylnaphthalene, tetradecane, pentadecane presence in diesel. There is a retention time that is presence in thinner and the unknown that at 2.54 minutes, but since the quality is lower that is 64, the assumption cannot be made because it has the possibilities to be other compounds. The advantage of using SPME is that the extraction is simple, fast and can be done without any solvents. If the samples are properly stored, it can be analyzed days later without much loss of volatiles.




CONCLUSION

The major compounds presence in petrol o-xylene, mesitylene, benzene, 1, 2, 3-trimethyl- and benzene, 2-ethyl-1, 4-dimethyl-. The major compounds presences in diesel are tridecane, decahydro-1, 1, 4a, 5, 6-pentamethyl-naphthalene, tetradecane and hexadecane. Thinner only show a single peak that corresponds to toluene. It can be concluded that the unknown sample is the mixture of diesel and petrol since there are some major compounds of diesel and petrol presence in the unknown.





REFERENCE
1.  Nor’ashikin S., Ruziyati T., Mardiana S. (2012), Analytical Separation Methods Laboratory Guide       (2nd edition).

Faizzarul Mohd Fadzli

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