Gasoline Analysis by GC-FID and GC-MS

8.f Gasoline Analysis by GC-FID and GC-MS T Introduction: This research examines the concentrations of various potentially hazardous compounds present in gasoline. Gasoline is a generic name for volatile and flammable petroleum fuels.1 The name itself therefore does not tell much about what is actually present in this incredibly complicated solution. What is sold as gasoline, can vary significantly in composition and must be heavily regulated. There are over 500 hydrocarbons present in standard gasoline plus a variety of artificial additives.2 These hydrocarbons include varying groups such as alkanes, alkenes, and aromatics. Of these, some immediately fall under closer scrutiny as they have previously been identified as carcinogenic in humans.2 Several of the most concentrated of these potentially hazardous compounds are benzene, cyclohexane, nheptane, hexane, pentane, toluene, and xylenes.2 A brief exposure to low concentration of these compounds has not been found to be hazardous, but long term exposure to the carcinogens could be cause for concern. Take for example the removal of lead from gasoline. While lead levels in gasoline were never extremely high, continuous exposure, especially in young children, led to elevated levels of lead in blood.3 The removal of lead from gasoline resulted in a decrease of 95 percent in blood lead levels across the U.S. population.4 This demonstrates the danger of even low level toxic components of gasoline given its ubiquity and volatility. Many individuals spend several hours a day around automobiles or other gasoline powered machinery. Gas attendants in particular have a high potential exposure to these chemicals. Clinical trials involving exposure of animals to high concentrations of gasoline vapors have been shown to can cause dizziness, muscle weakness, kidney damage, and even liver cancer.5 Toluene is of particular interest in gasoline. Toluene has a relatively high mass fraction in gasoline of approximately 0.055.6 Toluene has been found to have detrimental effects at only 500 ppm and the eight-hour exposure limit as determined by OSHA is 100 ppm.7 Benzene is another hazardous chemicals to human health and it has an exposure limit of only 1 ppm for an 8 hour period.8 Even at concentrations of less than 1% by volume in gasoline, benzene levels need to be strictly monitored. Hexane and cyclohexane have been found to have narcotic effects at high, 1000 ppm, concentrations.9 By formulating a methodology for the analysis of components in gasoline, it is possible to more closely monitor some of these compounds of interest. An effective methodology will allow for time and cost effective analysis of various sources of gasoline to ensure acceptable levels of hazardous compounds are being maintained at all times. The purpose of this study was to develop a procedure for use in an Instrumental Methods course to monitor vapors from a normal automobile gasoline filling operation. Issues with the collection system led to the decision to directly analyze gasoline in its liquid form. Materials and Methods: Chemicals: The components of gasoline to be analyzed were selected based on their relatively high concentration in gasoline. The components selected were heptane, benzene, toluene, m-xylene, p-xylene, cyclohexane, and ethylbenzene. An internal standard of decane at 50 ppm was also selected. Calibration Curve: In order to create a calibration curve, a 1 L stock solution of 50 ppm decane in pentane was prepared for dilution of the standards. A 1 L 1000 ppm solution of all target compounds was prepared in order to dilute into calibration curve standards (Figure 2). From these two stock solutions, 25 mL standards of 0.1, 0.5, 1.0, 5.0, 10, 20, 40, 60, 80, 100, 120, and 160 ppm of target compounds were prepared, each of which contained the 50 ppm internal standard of decane. External standards and diluted samples were analyzed by GC-FID. Retention Times: After the calibration standards had been taken, a small, but detectable amount of each target component was prepared in pentane in order to measure the retention times. Gasoline Sample: Regular grade gasoline was collected for analysis from a local gas station. It was diluted to 18 µL in a 25 mL volumetric flask with 50 ppm solution of decane in pentane. Instrumentation: An Agilent Gas Chromatograph/6890N Flame Ionization Detector with a 7683 series autoinjector was used to analyze all samples. An Agilent Gas Chromatograph/7820A Mass Spectrometer/5977E was used to analyze the gas sample in order to confirm the presence of the target compounds in gasoline. GC-FID Conditions: The GC-FID was run splitless and utilized a front injection DB-5 column with a 1 µL injection volume. The inlet temperature was 230º C and the detector temperature was 250º C. The starting oven temperature was 40º C held for 4 minutes, then ramped up at 1º C a minute until 60º C. The temperature was held at 60º C for 10 minutes at which point it ramped at 15º C a minute up to 240º C. A blank of 50 ppm decane solution was run before and in between each standard, and a rinse of pentane was used twice before and twice after each injection. This method was used for all standard samples, retention times and for the gasoline sample. GC-MS Conditions: The GS-MS was run with a front injection Agilent HP-5ms Ultra Inert column with a 0.2 µL injection volume. The inlet temperature was 250º C. The oven was ramped at the same rate as the GC-FID, starting at 40º C and ending at 240º C. A blank was run in between and before each sample, and the syringe was rinsed with pentane 3 times before each injection. This method was used for analysis of the gasoline sample, and one standard of 20 ppm. Results: Analysis of the GC-FID results could only be performed on 4 of the 7 target compounds due to issues with the solvent peak obscuring some of the more volatile compounds, and some compounds being indistinguishable from each other. The GC-FID was found to have a detection limit of roughly 1 ppm so the standards below that were discarded. The compounds that were analyzed were toluene, m-xylene, pxylene, and heptane. The GC-FID, Figure 1, response showed the percent composition of gasoline as 1.52% n-heptane, 4.50% toluene, 1.53% pxylene and 0.34% m-xylene (Table 1). These results are similar to the literature value for the percent composition of these compounds in gasoline.10 Analysis of the gasoline sample by GC-MS confirmed the presence of these compounds and shed light on the variety of other compounds present in relatively high concentrations. Lesson Learned: Although the analysis of gasoline by GC-FID is possible, it is difficult due to the complex composition of gasoline. The retention times are too similar to effectively resolve some of the peaks. Furthermore, the inconsistency between the retention times in single compound runs versus the calibration curve made it difficult to assign peak identities, particularly for m and p xylene and made it impossible for benzene and cyclohexane. It was also surprising to see the lack of confidence by GC-MS in the fragmentation pattern of the target compounds. The library had a difficult time determining between similar compounds, such as o and p xylene. Even some of the compounds identified were only with a confidence of ~70%. Most notably if this experiment is repeated, we recommend using carbon disulfide as the solvent. While carbon disulfide is a carcinogen, it produces a very small, if any, solvent peak in the GC-FID and as a result will allow separation of most component found in gasoline, especially the early eluting analytes. Toluene   Decane   p-­‐Xylene   n-­‐Heptane m-­‐Xylene   Figure 8.f-1. GC-FID spectra of gasoline sample with labeled peaks. Heptane   A)   1.2   Response   1   0.8   0.6   0.4   0.2   y  =  0.0116x  +  0.0247   0   0   10   20   30   40   50   60   Concentra