GC/MS Analysis of Gasoline
Courtney Richardson
Dr. Anne Gaquere
Chemistry Department
Abstract
Three different octane levels of gasolines have been gathered from five different
gas stations. In a previous experiment, their heat of combustion results have been
determined. The goal of this project is to determine if there are any detectable
similarities or patterns between these different types and suppliers of gasoline. Gas
Chromatography accompanied by Mass Spectrometry is the method of choice because it
allows the separation of the different structures of the gasoline and then identifies each
one of them. The differences and patterns within the many gasolines from different
octane rates and different gas stations have now been determined.
Introduction
Gas prices are likely to continue rising through the summer and even on into the
fall because of increased demand and tight supplies according to the United States
Energy Information Agency. The government agency predicts that retail gasoline prices
will average $3 a gallon this month (July 2007) and climb to $3.07 in August of 2007.
Because of this inconsistent yet constantly increasing price of gasoline and the ever
increasing demand for gasoline as personal vehicles (as the main means of American
transportation) as well as heating of homes and businesses, it seems interesting to analyze
the three main grades of gasoline used during daily even hourly activities. This
qualitative study, not quantitative, will allow us to understand the differences between the
gasolines. For this particular experiment the samples of gasoline chosen to be analyzed
are the three major octane levels 87, 89, and 93 from five different local gas stations: BP,
Chevron, Cowboys, Racetrac, and Wal-Mart.
Method
A Gas Chromatography/ Mass Spectrometry Machine (GC/MS) was the main
machine used in this analysis. In order to use this machine a temperature method and
dilution ratio had to be determined first. These two prerequisite steps to the experiment
can be done at the same time to some extent. Since this particular data analysis had never
been recorded before, there was no advice as to determining the correct dilution
quantities. Several solutions were attempted before a final dilution which would work
was found. Different temperatures and rates of increased temperature over time then had
to be determined as to give the best possible results. Two different methods along with
some variations were used before the desired results were given. The samples tested
were run overnight due to excellent time management, but to do so these samples had to
be prepared just a few minutes prior to injection. Gasoline evaporates fairly quickly and
with the small amount used evaporation would not take much time. Also because of this
factor, only small numbers of tests could be run at one time. Before, after and between
every analysis a “Burnoff” was required to be run in order to keep well GC/MS column
clean. This was extremely important because if any of the samples were to get mixed, the
results would not be accurate.
Gas Chromatography is a helpful method used for the separation and
identification of compounds (SLC). A sample is inserted in the heat injector, transported
through a column by an inert gas, and distinguished as a series of peaks. These peaks are
recorded when the compounds leave the column. This separation includes the use of an
immobile and a mobile phase. Components of a mixture transported in the mobile phase
are attracted to the immobile phase and then move through the immobile phase at many
different speeds. In gas chromatography the mobile phase is an inert gas transporter and
the immobile phase is a solid or a liquid coated on a solid contained in a coiled column.
Columns can be short and large in diameter; a packed column or long and very small
diameter; a capillary columns (SLC). In this particular experiment a capillary column
was used. The mobile phase or transporter gas flows through the instrument within a
pressurized tank. The column is contained in a heated oven that is preceded by a heated
injector and followed by a detector unit which produces the output. Samples may be pure
compounds. However, they are often prepared as dilute solutions due to the sensitivity of
the detection methods. This particular experiment required a diluted solution in order to
give a clear spectrum. When the system is ready, indicated by the ready light, samples
are injected into the injector where they are vaporized and transported into the column by
the transporter gas. Separation of components of the mixture occurs in the column
(SLC).
Mass spectrometers use the difference in mass-to-charge ratio (m/e) of ionized
atoms or molecules to distinguish them from one another (“Introduction”). Mass
spectrometry is useful when determining chemical and structural information about
molecules. Molecules have distinctive patterns that supply structural information to help
identify structural components (“Introduction”). Compounds retained in the immobile
phase reach the detector at different times to produce a set of peaks along the time line (a
spectrum) (SLC). The detector response is sent to a computer system where the progress
of the sample is monitored on the computer monitor in graphical form that displays
detector response as a function of run time. Each component of the mixture reaches the
detector at a different time and produces a signal at a characteristic time called a retention
time. The detector information can also be sent to a printer that produces a hard copy of
the chromatographic run (SLC).
Before the analysis of the peaks shown on a GC spectrum can begin, the
temperature method needs to be proven to allow a good resolution of the peaks. To do so,
several attempts with different dilutions had to be made. The sample of pure gasoline
gave a very crowded spectrum with overlapping peaks, which indicated the need for
dilution. As a result, the final dilution which worked along with the temperature method
produced was 12.50 microliters (uL) of gasoline and 1487.50 uL of acetone. A total of
1.50 milliliters was used because the GC/MS sample vials are 1.50 milliliters.
Then, the right temperature program had to be found. Two total methods were
used to determine which would produce clearer results. The first was method, “Anne,”
began running at a temperature of 70°C for two minutes then started to increase at a rate
of 10°C per one minute for a total of thirty minutes equaling out at a final temperature of
300°C. This method was not the method of choice because it heated the components too
quickly. The results did not show clear peaks over an interval of time but produced many
overlapping peaks in a short time period.
The next method, “Gaslowt,” began running at 50°C for twenty minutes then started to
increase at a rate of 5°C per one minute for a total of sixty-four minutes of runtime and a
final temperature of 250°C. This method seemed to work better than the previous one.
The peaks were spread out over a longer period of time which made analysis and
comparison quicker and easier.
One particular element both of these methods had in common was the solvent delay
employed at the beginning of each test. This delay lasted for approximately three
minutes. When looking at the results in a graph, the peaks (which represent compounds)
began at the very second the solvent delay was over. Since the peaks began so soon it
was very possible that some components were being lost during the solvent delay period.
The final method in which all the analyses were run did not use a solvent delay.
According to these results, it appears that no solvent delay, plus a dilution of
0.0125mL in addition of a programmed temperature described as program “Gaslowt” is
the best way to separate all the chemicals present in gasoline. This experiment did work
correctly to the expectations of giving results.
Results
Comparisons of data collected is in attached excel spread sheet along with lists of all
compounds present in the different types and brands of gasolines.
Comparisons which needed to be better explained than a table could do:
•
•
All regular grades of gasoline (87oct) had lots of small peaks (which were
difficult for the GC/MS to determine) than Plus and Premium grades, which
indicates the presence of impurities in this kind of gasoline.
1,3,5-tris(methylene)-Cycloheptane or one of its derivatives was present in at least
one grade from every gas supplier but never in Regular grade.
Conclusion
Many results came out of the analysis’s but most were not in a particular pattern
which was easy or even possible to trace. This study showed that different octane levels
of gasoline as well as different suppliers contain components which are not common to
all types. However few patterns were found, ones that were not particularly anticipated.
More data needed to be collected and analyzed in order to get the whole finger print of
these gasolines and to be able to understand the differences between them from a
qualitative point of view.
References
“Introduction to Mass Spectrometry (MS).” The Chemistry Hypermedia Project. June
2003. 14 June 2007 <http://www.chem.vt.edu/chem/ms/ms-intro.html>
SLC. “Introduction to Gas Chromatography.” The University of Michigan-Dearborn:
Department of Nature Sciences: Science Learning Center. 25 April 2001. 14
June 2007 <http://www.umd.umich.edu/casl/natsci/slc/slconline/GC/>