Gas Chromatography Lab Report

Purpose The purposes of this experiment are; 1) to understand the term “retention time” and peak area in a gas chromatograph 2) to identify unknown hydrocarbon and hydrocarbons in unleaded gasoline with calculated log retention time of a series of hydrocarbons 3) to determine quantitatively the toluene contents in gasoline samples using standard addition method Method The gas chromatography is a method of separation and analysis of volatile mixture using the very small amount of sample. Not like another chromatography, gas chromatography uses gas as the mobile phase which is called carrier gas. When the small amount of a sample is injected into a moving stream of gas, the stationary phase that consists of solid particles retain each component of the sample to different extents and separate the components. As a consequence, the components of the sample will be carried through the column in the certain retention time depending on the affinity of the column particle, the temperature, and the flowing rate of the carrier gas. Procedure Two units, unit A and B, of Hewlett-Packard 5890 Gas Chromatograph were used for gas chromatography experiment. To obtain retention time and a peak area of chloroform sample (0.8µl), five trials were performed with each unit. The settings for chloroform retention time were oven temperature of 120℃ , injector temperature of 300℃ , and detector temperature of 300℃ . Identification of Hydrocarbons Two drops of each pure hexane, heptane, and octane were delivered into a clean vial and mixed thoroughly, and the mixture (0.8µl) was injected into GC unit A to obtain the chromatogram. Two drops of each pure nonane, decane, undecane, and dodecane were well mixed in another clean vial, and the mixture (0.8µl) was injected into GC unit A to obtain the chromatogram. Dodecane (0.8µl) was injected into the GC and chromatogram of dodecane was obtained. Methane gas (1ml) was injected into the GC to obtain the mobile phase retention time. Two drops of ethanol, propanol, and butanol were mixed thoroughly in a clean vial, and the mixture (0.8µl) was injected into GC unit B to obtain the chromatogram. Two drops of pentanol, hexanol, heptanol, and octanol were mixed in another clean vial, and the mixture (0.8µl) was injected to obtain the chromatogram. Unknown carbon compound (0.8µl) was injected into the GC unit A, and the chromatogram was obtained. The identification of the unknown was determined with the peak calculated from chromatograms of pure carbon compounds. Unleaded gasoline sample (0.8µl) was injected into the GC unit A and peaks of carbon compounds were identified from the chromatogram. Toluene Content of Commercial Gasoline Pure toluene sample (0.2µl) was injected into the unit A and chromatograms were obtained. Gasoline 87 (0.8µl) was injected into the GC unit A to obtain the chromatogram. Gasoline sample was mixed with toluene in a small vial with a ratio of 5:5 and mixed thoroughly. The gasoline 87 mixed with toluene (0.8µl) was injected into the GC unit A and spiked chromatogram was obtained. With 1.000ml micropipette, 200µl, 400µl, 600µl, and 800µl of pure toluene was transferred to 10 ml volumetric flasks #1,2,3 and 4. Each flask was diluted to the mark with gasoline 87. Each sample was injected into the GC unit A, and the chromatograms were obtained. The same procedure was performed with gasoline 91 sample using unit B. Data and Result The five trials of chloroform sample were injected to determine operation of GC and to obtain clear chromatogram by adequate the sample preparation and injection technique. The five runs with unit A shows the coefficient of variation of 0.38% and 3.84% in the retention time and peak area, respectively. The four runs with unit B shows the coefficient of variation of 0.49% and 7.17% in the retention time and peak area, respectively. The peak area of one trail on unit B was discarded after Grubbs test because it has a significant difference from the average peak area (Gcalculated:1.703, Gtable: 1.672, at 95% confidence). Table 1. Retention Time and Peak Area of Chloroform Sample Unit A Trial 1 2 3 4 5 Avg. s CV Retention Time 1.476 1.480 1.470 1.466 1.476 1.474 0.00555 0.38 % Peak Area 83.0318 79.2680 77.9035 81.0356 85.8204 81.4119 3.1267 3.84 % Unit B Retention Time 1.450 1.466 1.453 1.453 Peak Area 24.2828 28.6487 28.0242 27.4812 1.456 0.00714 0.49 % 27.1092 1.9437 7.17 % Identification of Hydrocarbons The mobile phase retention time of methane gas with GC unit A was obtained as 1.263 and 1.206 with unit B. Table 2 shows each retention time of hydrocarbon and alcohol obtained from chromatogram and calculated log retention time. Table 2. Log Retention Time of Hydrocarbons and Alcohols Hydrocarbon (Tm=1.263) Alcohol (Tm=1.206) Carbon # B.P. TR ln(TR-Tm) Carbon # B.P. TR ln(TR-Tm) 6 7 8 9 10 11 12 68 98.42 125 151 174.1 196 216.2 1.450 1.646 1.996 2.676 3.833 5.793 9.563 -1.677 -0.960 -0.310 0.346 0.944 1.512 2.116 2 3 4 5 6 7 8 78.37 97 117.7 138 157 175.8 194.8 1.273 1.363 1.523 1.813 2.363 3.430 5.256 -2.703 -1.851 -1.149 -0.499 0.146 0.799 1.399 The graphs of log retention time versus carbon number and log retention time versus boiling point of the pure hydrocarbons and alcohols were obtained as in figure 1 and 2. The slope of log retention time versus carbon number graph of pure hydrocarbon is 0.6276 with an R2 value of 0.9986 and slope of 0.6751 with R2 0.9974 of that of alcohol. 3 2 f(x) = 0.63 x − 5.37 R² = 1 f(x) = 0.68 x − 3.93 R² = 1 ln(TR-Tm) 1 0 1 3 5 7 9 11 13 -1 -2 -3 Carbon Number Hydrocarbon 3 2 f(x) = 0 x² + 0.02 x − 3.13 R² = 1 f(x) = − 0 x² + 0.04 x − 5.91 R² = 1 ln(TR-Tm) 1 0 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 -1 -2 -3 Boiling Point, ℃ Hydrocarbon Figure 1 and 1. Graph of Carbon Number of Compounds Vs ln(TR-Tm) (up), Boiling Point of Compounds Vs ln(TR-Tm) (down) The unknown had 1.643 of retention time which give -0.96758 of log retention time. Calculating with the obtained pure hydrocarbon equation gives the unknown identification as a hydrocarbon with seven carbons. Table 3. Log Retention Time of Unknown Sample TR ln(TR-Tm) Determined Carbon# 1.643 -0.96758 7 Carbon¿=x= x ❑unknown= ln ( T R −T m ) +5.3674 0.6276 4.39982 ≈7.0 0.6276 The retention times from chromatogram of unleaded gasoline gives identification of hydrocarbon compounds from carbon number 5 to number 12 with following calculations. x ❑1= −2.303 + 5.3674 ≈ 4.9 0.6276 x ❑2 = −1.514+5.3674 ≈ 6.1 0.6276 Table 4. Hydrocarbons in Unleaded Gasoline Determined ln(TR-Tm) TR Carbon# Peak 1 1.363 -2.303 5 2 1.483 -1.514 6 3 1.623 -1.022 7 x 3= −1.022+ 5.3674 ≈ 6.9 0.6276 x 4= −0.338 + 5.3674 ≈ 8.0 0.6276 4 1.976 -0.338 8 0.287 + 5.3674 ≈ 9.0 0.6276 5 2.596 0.287 9 x ❑5 = 6 3.853 0.952 10 x ❑6 = 0.952+ 5.3674 ≈ 10.1 0.6276 7 5.780 1.508 11 x ❑7 = 1.508 + 5.3674 ≈ 11.0 0.6276 x ❑8 = 2.121 + 5.3674 ≈ 11.9 0.6276 8 9.606 2.121 12 *All the values are rounded to the closest natural number to determine the carbon # Toluene Content of Commercial Gasoline For this part, temperature program was set up the initial temperature of 35℃ and the final temperature of 250℃ over a period of 10 minutes. The retention time from the spiked chromatogram of a mixture of toluene and gasoline 87 with a ratio of 5:5 was 3.610, and that of mixture toluene and gasoline 91 mixture is 4.523. Each toluene peak area and reference peak area was obtained from the chromatograms, and the scale factor was calculated from toluene and reference peak value as shown in table 5. Standard deviations of gasoline 87 acquired from excel spreadsheet are 0.1849 for Sy, 0.0292 for Sm, and 0.1432 for Sb. Standard deviation values for gasoline 91 are 0.2863 for Sy, 0.0484 for Sm, and 0.2218 for Sb. Table 5. Peak Area and Scale Factor of Gasoline samples Gasoline 87 Percent Scale Factor, Percent PaT PaR PaT/PaR (v/v) (v/v) 0 27.4204 21.3029 1.2872 0 2 57.8371 20.8399 2.7753 2 4 37.6452 8.3704 4.4974 4 6 38.9624 6.8777 5.6650 8 8 41.1466 5.4081 7.6083 Gasoline 91 PaT 872.5576 126.1343 25.0510 2237.2780 PaR 242.9420 21.5140 3.0490 160.0636 Scale Factor, PaT/PaR 3.5916 5.8629 8.2161 13.9774 The least square function graph shown in figure 3 has a slope of 0.7766 for the standard addition of toluene to gasoline 87 and 1.319 to gasoline 91. Standard addition function of gasoline 87 has an R2 value of 0.9958 and gasoline 91 has 0.9962. 16 14 f(x) = 1.3 x + 3.35 R² = 1 Scale Factor 12 10 8 f(x) = 0.78 x + 1.26 R² = 1 6 4 2 0 0 1 2 3 4 5 6 7 8 9 Percent (v/v) Toluene Added Gasoline 87 Linear (Gasoline 87) Gasoline 91 Linear (Gasoline 91) Figure 2.Toluene Contents in Gasoline Samples Toluene (v/v) % in the gasoline samples and errors in the toluene content are acquired with the following calculations. ≈ 1.623( ± 0.335 ) % | −1.2603 0.7766 | = ≈ 2.567( ±0.392 ) % | −3.3479 1.304 | % Toluene∈ gasoline87= 9 % Toluene∈ gasoline91 8 0.7766 ¿ ¿ ¿ 2(40) ¿ 2 1 1 (0−4.367 ) + + ¿ 1 5 ( S cgas87= √ 2 S y 1 1 ( ´y c +´y ) 0.1849 √¿ + + 2 = m L N m S xx 0.7766 ) 1.304 ¿ ¿ ¿ 2(35) ¿ 2 1 1 (0−7.912 ) + + ¿ 1 4 ( S cgas91= ) √ 2 S y 1 1 ( ´y c +ý ) 0.3376 √¿ + + 2 = 1.304 m L N m S xx Conclusion and Results The chloroform sample chromatograms show fairly precise results. The discarded result may have caused by injecting a comparably small amount of sample into GC because the peak area would be proportional to the amount of the compound that the injected sample contains. Both graphs of hydrocarbon and alcohol provide a conclusion that the log retention time is proportional to the carbon number of the compound. A smooth curve can be observed from the graphs log retention time versus boiling point but acquiring more data would make the function precise and accurate. The unknown hydrocarbon sample with a retention time of 1.643 is identified to have seven carbons, and the unleaded gasoline is determined to have eight hydrocarbon compounds. The toluene (v/v) % in gasoline samples were determined to be 1.623(±0.3359) % and 2.567(±0.3928) % for gasoline 87 and 91. These values have fairly low accuracy, and it can be improved by running more replicates of each sample. Overall, it can be concluded that the gas chromatography is a useful method for quantitative and qualitative analysis of a mixture. Question 1. It can be estimated that there are eight hydrocarbon compounds are in the gasoline sample. 2. It can be assumed that there are similar kinds of hydrocarbon compounds in both gasoline samples. The peak area of heptane and octane is noticeably different. The ratio of octane to heptane determines the name of gasoline which gives a larger amount of octane to the gasoline 91. 3. Hydrocarbon compounds in diesel would have a larger value of retention time because of the higher carbon number. The boiling points of compounds in the diesel would also be higher than the gasoline samples with the same reason. 4. 30µl of benzene is more likely to overload the column. 30µl of gasoline contains several compounds which would not give a significantly high peak to overload. The height and area of a peak would be affected by the amount of the compound in the injected sample. Since 30µl of benzene only contains benzene itself, it is more like to overload the capillary column.