Gas Chromatography of Polycyclic Aromatic Hydrocarbons with Flame Ionization Detection

Gas Chromatography of Polycyclic Aromatic Hydrocarbons with Flame Ionization Detection

Abstract:

The purpose of the experiment was to use the gas chromatography method to analyze polycyclic aromatic hydrocarbons with flame ionization detection. The polycyclic aromatic hydrocarbons (PAHs) were analyzed by temperature gradient and isothermal gradient. A user temperature gradient program was created for the better quality separation and resolution of the peaks and to minimize the analysis time. In this experiment, the customized temperature gradient analysis came out to be more effective analysis as it resulted in 14 sharp and high- resolution peaks. Isothermal and temperature gradient programming for 16PAHS resulted in 2 and 13 peaks respectively. Neither of the gradient programs could separate all sixteen polycyclic aromatic hydrocarbons successfully.

Introduction:

Gas Chromatography method is used in this experiment to separate mixture of 16 PAHS in a methanol into individual components based on the physical properties, such as polarity. This modern method is highly useful and crucial because it qualitatively and quantitatively analyzes molecular species. The Varian CP-3380 capillary gas chromatograph used in the lab was a flame ionization detector. This analysis was done because FID responds to a wide assortment of organic compound and is sufficiently sensitive for most columns. FID has a large dynamic range of

which makes them relatively simple to operate and are considered highly reliable. This type of universal detector does not respond to few inorganic which include O₂, N₂, SO2, NO2, CO2, and H2O. By being insensitive to these compounds, FID becomes ideally suited for the analysis application for hydrocarbons in atmospheric and aqueous samples. Other techniques for the determination 16 PAHS is using the prominence-I integrated High-Performance Liquid chromatograph. In which the chromatographs are obtained by analysis of 16 PAHS using the wavelength switching mode. The liquid crystalline phase is another stationary phase type used for PAH separation due to its unique selectivity. Advantages of the gas chromatography are: they have high sensitivity and high resolution power compared to other methods. This technique relatively results in good accuracy and precision. Separation of the volatile substances is done in minimum amount of time. Freedom to even change the temperature program and control other parameters is also one of the pros of this technique. The major drawback of this method is that the compound to be chromatographed must be volatile; it must vaporize inside the temperature of the inlet. Proper attention is needed during the injection of the gaseous sample.

Theory:

In the gas chromatography process the vaporous analyte is transported through the column with the aid of a gaseous mobile phase, referred to as the carrier gas. By the means of He, N2, or H2 carrier gas, vapors are swept via the column. The choice of carrier gas is often dependent upon the type of detector used. Using a syringe, gaseous sample or the volatile fluid rapidly evaporates as it is injected through the septum (an elastic disk) into a heated harbor. Splitless injection was used for the trace analysis of the sample. Because if a split injection was used then there won’t be sufficient analyte injected on column to detect. The gaseous analytes then move along a long narrow, open tubular column. Inside the walls of the column; a non-volatile liquid stationary phase is bonded to a solid surface consisting of fused silica and polyamide coating. Partitioning of the solutes from the stationary liquid to the mobile phase leads to separation. The column must be sufficiently hot to supply adequate vapor pressure for analytes to be eluted in a reasonable time. But, the temperature shouldn’t exceed the boiling point or the analystes and the stationary phase will decompose. Next, the separated components stream through a detector. In Flame ionization detector, elute is burned in a mixture of hydrogen –air flame to produce ${\mathit{CHO}}^{+}$

ions. This hydrogen air flame helps to ignite and ionize the solutes as the carrier gas passes them into the detector. The detector is maintained at a higher temperature than the column so analytes could be gaseous.The product ions are gathered at the cathode, under the polarization of an electric field to deliver a current signal in the form of a peak. The current carried by these ions is proportional to the concentration of the analyte present in the detector. The data analyzer connected to the machine creates chromatogram with peaks equivalent to the relative amounts of the various chemicals within the sample. Finally, the respond is displayed on a computer screen. The partition coefficient is defined as= $\frac{\mathit{Molar\; concentration\; of\; analyte\; in\; the\; stationary\; phas}\mathrm{e}}{\mathit{Molar\; Concentration\; of\; the\; analyte\; in\; the\; mobile\; Phase}}$

.The greater the coefficient the ratio of partition coefficients between the mobile and stationary phase, greater is the separation between the components of the sample. In chromatographic column, there large number of separate layers called theoretical plates. In these “plates”, separate equilibrations of the sample between stationary and mobile phases occur. The analyst moves down the column from one plate to the next by transferring the balanced mobile phase. Compared to an inefficient column, an efficient column has more theoretical plates. The height of the plate is proportional to the chromatographic band variance, the smaller the plate height, the narrower the band. The van Deemter equation describes band broadening on the chromatographic column: H ≈ A + B/ux + Cux, where H is plate height, ux is linear flow rate, and A, B, and C are constants. The A refers to irregular flow paths, B/ux to longitudinal diffusion and the third to the finite rate of solvent transfer between mobile and stationary stations.

Procedure/Experimental:

Gas Chromatograph parameters are uploaded from the CompassCDS software package to the instrument. This package controls the Flame Ionization Detector which was installed above the oven.  A commercially available mixture (dichloromethane mixture 1:1) of 16PAHS in a methanol was diluted. This sample had been used for all the injections. First, the isothermal temperature program was performed. The initials settings like the temperature, rate, and hold time were already uploaded. The method data window was achieved by going to file to open to open method. After that Isotemp.METH was selected and opened. We choose the control in the Data Info window and clicked the overview button in the Information Window. Then we clicked the upload button to upload the Gas Chromatography method settings. The system tab was clicked to display the system window. The box next to 3800GC was checked to get connected to GC. Once the system and chromatogram windows were connected, it appeared on the screen. Then we clicked on Acquisition button and choose Quick Start. In the second window, information was added about the group and what was being run. As the bottom screen showed Waiting for Injection, 2uL of the provided PAH mixture was injected into the machine. All the above steps were repeated for running the temperature gradient program and customize temperature program. PAH Gradient.METH file was used for temperature gradient programming.

Data:

Table 1: Shows retention times of theisothermal analysis of polycyclic aromatic hydrocarbons.

Table 2: Shows the retention timesofgradient analysis of polycyclic aromatic hydrocarbons.

Table 3: Peaks and retention time ofcustom created gradient analysis of polycyclic aromatic hydrocarbons.

Table 4: Modification of the customize created gradient analysis of the 16 polycyclic hydrocarbons.

The Flow Rate = 0.30mL/min

Discussion:

The gas chromatography utilized in this test had a column that had a 30M×0.25mm inward distance across, the gas utilized was helium (He) and nitrogen (N₂), and the stream rate for the isothermal and temperature slope chromatography was recorded to be 0.1ml/min and the stream rate for custom made angle chromatography was 0.2ml/min. The crests in each chromatogram (Figure:1.3,1.4,1.5) speak to  they elution of the polycyclic fragrant hydrocarbons, distinguishing the partition from one another or in case they are eluting with other polycyclic fragrant hydrocarbons. The measure of atoms shows up at different areas on a range based on their extremity properties, temperature and on the machine’s particular working conditions. The mass of the polycyclic fragrant hydrocarbons is an imperative figure in deciding the speed with which the atoms would escape from the column. Atoms with littler masses will evade from the column speedier than those with bigger masses. 

Table 5: Molecular weights of all 16 polycyclic hydrocarbons in g/mol.

In this test the isothermal chromatograph come about in as it were 2 wrong and destitute crests. The examination was carried out at 200℃ at a rate of 0℃/min and kept going for 20 minutes, at that point expanded to 300℃ at a rate of 0℃ and held for minutes. The stream rate was 0.1ml/min. As it were one temperature was utilized in most of the investigation with no expanding temperature to partition the analytes. There was no held time when the temperature was expanded. This gave no comes about on the genuine investigation performed at the moment temperature. Analytes require an ideal increment in tsemperature for the tried test to be isolated. A run of temperatures can continuously and precisely isolate the analytes as this would offer assistance to gather in more and predominant peaks. In Fig 1.3 we will actually see destitute shape crest due to the extremity bungle between the stationary stage and test solvent. The temperature angle chromatography given 13 crests, giving exceptionally precise and dependable comes about, with a few crests that displayed.

The temperature angle chromatography given 13 crests, giving exceptionally precise and solid comes about, with a few crests that shown different analytes eluting at the same point. The examination performed at 80℃ with a rate of 0℃ held for 1 miniature, at that point expanded to 300℃ at a rate of 10℃ held for 2 minutes. The stream rate was recorded as 0.2ml/min. The increment in temperature permitted more division and diminished the maintenance time of the analytes compared to the isothermal investigation. There were approximately 2 to 3 crests that were so near together which might moreover be seen within the chromatogram of the isothermal examination, this result in wrong representation of the crests.

The third analysis in this experiment was an attempt to optimize the gradient analysis where the program was customized to shorten the time taken between peaks to separate and avoid the duplicate peaks that were found close together. The increasing temperature was programmed to heat the system for the first elution quickly and then to slow down the elution portion for the nearby peaks. This did result in good peak because a peak did form with a retention time of 8.51 minutes that was not visible in the temperature and isothermal gradient analysis. There were about 14 peaks that were observed compared to that of the temperature gradient analysis which had 13 peaks visible. The peaks produced had a higher accuracy and were more defined peaks that had a sharp and narrow bottom. The customized gradient chromatography did represent better separation of the molecules in the analyte since the peaks were not as broad as that compared to the isothermal analysis.

The gas chromatography isolated the polycyclic fragrant hydrocarbon compounds through vaporization without going through the decay. It is critical to utilize ideal column temperature for tall proficiency in connection to the gas being utilized for the partition of the compounds (Poster.D, 2006). This may be a key since a lower temperature will help the crests evade with a better determination and with more honed crests. Exceptionally tall column temperature would not create clear or sharp crests and will cover into one another and will hold a temperature for a longer period time (Poster.D, 2006). In case the held temperature is kept for a longer period of time the crest of the compounds will be broader and shorter. This leads to a maintenance time twice of the hypothetical maintenance time

Error that happened amid the test may have been a miss quantization of an analyte giving comes about that were essentially off from the expected. This was redressed by changing the range and avoidance of test unsettling influence. Another mistake would be not taking care of the stream rate. The tall stream rate will abbreviate the expository time but cause broadening due to the mass exchanging the Van Deemeter plot, as the solute does not connected totally with the stationary stage.

Conclusion:

This experiment was effectively conducted to analyze polycyclic fragrant hydrocarbons employing a gas chromatography handle with fire ionization locator. The polycyclic fragrant hydrocarbons were analyzed beneath isothermal, temperature slope and custom- made angle for the quality of the chromatographic crest partition. Out of the examination, client temperature angle examination brought about in 14 exact and sharp crests. Among the three, isothermal examination was the most exceedingly bad one because it comes about as it were two crests without a solid examination. The temperature angle investigation delivered 13 crests .This was way better than the isothermal analysis but did not accomplish the precise comes about just like the user temperature angle examination. But generally, the altered programming customized a few crests that were not seen within the chromatogram of the temperature angle.

References:

1. Poster, D. L., Schantz, M. M., Sander, L. C., & Wise, S. A. (2006). Analysis of polycyclic aromatic hydrocarbons (PAHs) in environmental samples: A critical review of gas chromatographic (GC) methods. Analytical and Bioanalytical Chemistry, 386(4), 859-881. Retrieved November 11, 2018.
2. Harris, D. C. (2010). Quantitative chemical analysis (8th Ed.). New York, NY: Freeman Custom Publishing.
3. Nalin, F., Sander, L., Wilson, W. and Wise, S. (2017). Gas chromatographic retention behavior of polycyclic aromatic hydrocarbons (PAHs) and alkyl-substituted PAHs on two stationary phases of different selectivity. Analytical and Bioanalytical Chemistry, 410(3), 1123-1137.
4. Janini, G., Johnston, K. and Zielinski, W. (1975). Use of a nematic liquid crystal for gas-liquid chromatographic separation of polyaromatic hydrocarbons. Analytical Chemistry, 47(4), pp.670-674.