Robert F. Cassidy, Jr. and Conrad Schuerch SUNY College of Environmental Science and Forestry Syracuse. New York 13210
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Gas Chromatographic Analysis Gasoline A laboratory experiment
This experiment can be used to instruct lower division students in gas chromatography as a method of separation of complex mixtures. The analysis of data from the experiment can give an insight into the behavior of various classes of com~oundsin eas chromatoera~hv. - . " Bv. usine commercial gasolines, environmental relevance and general interest are introduced into the laboratorv. We have used this experiment with a mixed class of sophomore chemistry maiors and nonmaiors in a lahoratorv course accompanvine organic chemistry" lectures. The laboratory includes-eley ments of laboratory techniques, synthesis, separations, classical qualitative organic analysis, instrumental and volumetric analysis. The experiment (procedure 1) has been used in a three-hour lahu;atory together with a rather simple experiment on polymerization. The time required for the experimental work is brief hut the analysis of data more lengthy. Alternatively with minor modifications such as the use of known compounds to identify peaks, the laboratory work can he made suitable for an entire period assignment and more detailed introduction to gas chromatography. Much of the data on which this experiment is based .~ discuscomes from Blundell, et al.,' and M e ~ e r General sions of the principles of gas chromatography are found in McNair3 and standard organic laboratory texts, such as Roberts, et aL4 A simpler experiment on the glc separation of gasoline and a number of additional pertinent analytical methods can be found in "Elementary Theory of Gas Chromatography." The Experiment In this exneriment we take advantaee " of the hieh " resolving power and rapidity of gas chromatography to separate some commercial easolines into individual com~oundsand simple mixtures of compounds of similar volatility. For a complete separation of components a more powerful instrument would he needed. The table lists the major components of a typical commercial gasoline in order of their appearance in a gas chromatograph. Their order of appearance is determined both by their volatility and by their chemical nature, which determines their interaction with the column's stationary phase. Figure 1 shows a typical separation of a commercial gasoline on a student glc apparatus. The peaks are keyed by letter to the compounds listed in the table. The presence of these compounds in individual peaks in this case has been established by separation of the gasoline a t high resolution into individual compounds (Fig. 2). Then repeated
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'Blundell, R. V., Griffith, S. T., and Wilson, R. R., in "Gas Chromatography," (Editor:Scott, R.), Butterworths, London, 1960, p. 360-371.
2Meyer, R. A,, "Gas Chromatography," Academic Press, New York, 1958.p. 93.
3McNair, H. M., and Bonelli, E. J., "Basic Gas Chromatograohv." . .. Varian Aeroeraoh. - . . Walnut Creek. California. 1969. Toberts, R. M., Gilbert, J. C., ~ o d e h dL. , B., and Wingrove, A. S., "An Introduction to Modern Experimental Organic Chemistry," Holt, Rinehart and Winston, Inc., New York, 1969. "Elementary Theory of Gas Chromatography," Gow Mac Instrument Co.. 1973. p. 29.
COLUMN TEMP LIQUID PHASE DC 200 on CH. G CARRIER GAS N2 @ 4 5 DETECTOR CURRENT 200mA
Figure
1. Typical student GOW-Mac,model 69-100.
Chromatogram of gasoline
(Arm
Supreme) on
COL. TEMP 40"-150" LIQ. PHASE SE-30 CARRIER N2 @ 3oMAN
I A I B ? C id E F 6 G
4 I
I8 I H
10
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!14~
Figure 2. Temperature programmed chromatagram preme)on Varian mcdel2048.
'? of
20
"M
gasoline ( A r m
Su-
separations were made at decreasing degrees of resolution until the peaks coalesced to form a pattern comparable to that obtained on the student chromatogram (Fig. 1). Identity of the peaks in Figure 2 was established by comparison with the chromatogram in reference ( I ) , page 368. The location of individual compounds can also be established hv addition of known com~oundsto the easoline and comparisun of peak heights as in procedure 2 below, either before the laboratorv or bv the student (Fia. - 3). The composition of gasoline influences the combustion of easoline. A ooorlv . . formulated easoline causes engine knocking and poor combustion. Knocking characteristics of a fuel are measured in a test motor and designated by an octane number. Low octane numhers correspond to poor performance and high octane numbers to high antiknock characteristics. Normal paraffins have low octane rating (n-heptane = 0), branched paraffins and naphthenes intermediate ratings ("isooctane," 2,2,4-trimethylpentane = loo), and aromatics have high octane numhers (toluene = 120). Small amounts of tetraethyl lead (3-4 cclgal) enhance the octane ratings of gasoline hut produce some environmental problems. Unfortunately caution must be used in replacing tetraethyl lead with aromatics because of the toxicity of some aromatic hydrocarbons.
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Volume 53. Number 1. January 1976 1 5 1
0.5 JJLARC0 SUPREME
METHYWENTANE INCREPSE IN PEAK D
PEAK NUMBER F!gure 4. Relation of shucture
to ale of hansport through gas chromato.
graph.
Hydrocarbons in Gasoline"
Peak No.
Figure 3. ldemification of &romtopphic peaks.
C
Peak No.
Name
C
Name
lropentane n-pentane 2-Methylbut-2sne 2-Methylpentam 2-Methylbut-2ene n-Hexme
Materials and Methods Samples of Commercial Gasolines: high test, low lead or lead-free, regular, or winter and summer samples of a single grade; micro-syringe. Samples of pure compounds: toluene, n-heptane, 2,2,44rimethylpentane, etc. Student Gas Chromatograph: such as Gow Mac 69-100 with 4-ft X %in comer column with nonoolar stationarv phase OV-10'1or ~ e ' 2 0 0 ,6% on Chromosbrb G, availabie from Suoelco. Inc.. S u ~ e l c oPark. Bellefonte. Pa. 16823. P.O. Box 440. State ~ o l : and Appiied science ~ a b s . Inc., , lege, Pa. 16801. Recorder: Strip chart recorder such as Varian A-10. Full scale voltage matches gas chromatograph output voltage (usually 1 mV or 10 mV). Variable speed chart drive or fixed speed of 1in./min. Instrument Seltings chart span
N nor He flow rate
samole size injector temperature oven temperature detector temperature chart speed attenuator current total time for running a single chromatogram -5 inin Procedure (1) Inject a l - d u l p l e of gasoline into injection port and record chromatograph. Label individual peaks and shoulders as is shown in Figure 1. (2) To a 2-ml sample of gasoline used in (1) add one drop of known component of the gasoline and repeat chromatograph. Identify peak with incieased size. Figure 3 shows a typical result when gasoline is "spiked"' with methylcydopentane in this proportion. Repeat with other components as necessary to confirm the retention time data given hy the high resolution chromatography. Calculations and E x d s e s Using the table and Figure 1 for reference, interpolate the probable position of other compounds in your chro52 1 Journal of Chemical Education
MethylcyClopentane Benzene
aAaa~t,ted from ref. ( 1 ) . p.
362.
matogram, and prepare your own table of position of compounds. Relative amounts of two components in a mixture are proportional t o the areas under the corresponding peaks only when the detector is equally sensitive t o the two compounds. This is not generally true. Propose how you could analyze a mixture of known compounds quantitatively by gas chromatography. Draw a base line in pencil to each peak in your chromatograph and extend the sides of each peak down t o the base line. Draw lines perpendicular t o the baseline between overlapping peaks. Estimate the area under desired peaks bv calculatine the areas of the corresnondine trianeles and r&angles. F& a comparison of winter andsumm;?r gasoline use the volatile cuts (peaks A and B). For a comparison of lead-free and regular gasoline, compare the size of toluene ~ e a k(Whv . not the tetraethvl lead oeak?) iota .graph of peak letter as-abscissa (A,B,C,D .. .) and number of carbon atoms in compounds as ordinate (4,5,6,7,8,9). Place straight chain aliphatic hydrocarbons, branched chain hydrocarbons, and aromatics on graph. Draw smooth curves through each family of homologs. (Figure 4 shows the resulting graph based on student data using conditions listed above.) Which class of compounds is retained longest on the column? Compare the boiling points of some of these compounds and establish whether retention time is dependent solely on volatility or whether i t also depends on interaction between volatile compounds and the stationary phase. Acknowledgment . We wish to acknowledge the able assistance of Mr. Fred-
erick S. Potter in developing this experiment.
