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

${10}^7$

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.