Introduction

6.1 Gas chromatography

Chromatography is an analytical technique commonly used for separating a mixture of chemical substances into its individual components. Chromatography involves separating a mixture of analytes according to their partitioning between a stationary phase and a mobile phase. Liquid (or column) chromatography and thin-layer chromatography (TLC) has been introduced in the practical class of Organic Chemistry I. For TLC and column chromatography, the mobile phase is organic solvent and the stationary phase is the silica gel. In this practical session, gas chromatography (GC) will be introduced. In a broad sense, GC is a very powerful and one of the most common instrumental analysis techniques in use. When properly utilized, it provides both qualitative (i.e., what is it?) and quantitative (i.e., how much?) information about individual components in a sample.

The configuration of a GC system is straightforward (Figure 1). There are five main GC system components: the sample injection unit, which heats the liquid sample and vaporizes it; the column, which is used to separate each compound; the column oven which heats the column; the detector, which detects the compounds and outputs their concentrations as electrical signals; the data processing unit (computer and software), which converts the electrical signals into chromatogram (Figure 2a). For GC, the mobile phase is gas and the stationary phase is coated in the column (Figure 2b).

6.2 Gas Chromatography Separation within the column

Sample injection

In the GC analysis, the sample in the form of a mixture of organic compounds is injected into the injector (sample injection). The injector is heated and maintained at high temperature (e.g., 200 °C) to evaporate the sample instantaneously. The vaporized sample in gas form is then carried by the carrier gas (e.g., nitrogen or helium form cylinder) to the column (Animation 1). Separation by GC occurs within the column.

Separation and detection of compounds

The sample containing multiple compounds is introduced into the column together with the mobile phase. In GC, the mobile phase is a gas referred to as the carrier gas. Both the sample and the mobile phase travel through the column, but the rate of progression (or traveling) for compounds within the column is different (Figure 3). Accordingly, differences arise in the times at which the respective compounds arrive at the column outlet. As a result, a separation between each compound occurs. The rate of organic compounds that travel through the column is influenced by (1) the interaction between compounds and stationary phase of column, and (2) temperature of the column. Every compound may interact with the stationary phase at a different degree. For example, the time for a compound to elute out (moving out) from the column will be longer if it interacts strongly with the stationary phase. The compound which weakly interacts with stationary will be eluted out from the column with a shorter time. The primary interaction between compounds that introduced into the GC column with a stationary phase through Van der Waal forces.

In GC, the column is heated by the temperature controllable oven. The separation of compounds also depending on the temperature of the column as the movement of molecules in the column can be manipulated by changing the temperature. In GC, the column can be heated via 2 methods: isothermal, where the temperature is fixed at one selected temperature or temperature programming, where the temperature varies throughout the analysis (Figure 4). In gas chromatography, it is often difficult to properly separate components completely without using a technique known as temperature programming. With temperature programming, compounds can be separated according to their boiling point. In temperature programming, temperature always started from the temperature (e.g., 40 °C) that lower than the injector temperature (e.g., 200 °C).

Example of separation of compounds based on boiling point using temperature programming

Assume that you have a mixture of compounds A, B, C with the boiling point of 150, 170, and 190 °C, respectively. When this mixture is injected into the injector that heated at 200 °C, compound A, B, and C are vaporized into gas form. The carrier gas (mobile phase) then carry these compounds into the column. When these compounds reach the column with an initial temperature of 120 °C, all compounds are condensed into liquid form at the front part of the column. Then, the temperature of the column is increased (assume that the temperature is increased with the rate of 10 °C/min). At the time of 3 min, the temperature of the column is reaching 150 °C, which is the boiling point of Compound A. Under this condition, Compound A is vaporized into a gas and starts to travel along the column (with the interaction with stationary phase), whereas Compound B and C remain as a liquid. At the time of 7 min, the temperature of the column reaching 170 °C. At this condition, Compound B is vaporized into a gas, and Compound C remains as a liquid, whereas Compound A has moved further apart from B and C. At 9 min, Compound C is vaporized, and Compound B has moved further apart from it.

All compounds that eluted out from the column reached the GC detector. The detector is a device that converts compounds into the electrical signal. Flame Ionization Detector (FID) is the most commonly used GC detector. In the presence of air and hydrogen, FID burns the compounds that eluted from the column and converts it into ions. These ions are collected by an electrode and converted into the electrical signal.

When the electrical signals output from the GC detector are plotted on the vertical axis and the elapsed time after sample injection is plotted on the horizontal axis is called a chromatogram (Figure 2). The elapsed time is named as Retention Time. The vertical axis shows the signal intensity. The part at which nothing is detected is called the baseline, and the part where a component is detected is called a peak. The time from when the sample is injected into the system until the peaks appear is called the retention time. As the elution times for each component differ, each component can be separated and detected.

6.3 BTEX

BTEX refers to benzene, toluene, ethylbenzene, and xylenes. These compounds occurred naturally in crude oil and can be found in seawater, natural gas, and petroleum deposits. The primary human-made releases of BTEX compounds are through emissions from motor vehicles and aircraft, and cigarette smoke. BTEX compounds are created and used during the processing of petroleum products and the production of consumer goods such as paints and lacquers, thinners, rubber products, adhesives, inks, cosmetics, and pharmaceutical products. BTEX is an important class of volatile environmental contaminants, and are frequently analyzed in environmental and drinking waters. Regulations often required that all waste entering the municipal sewer system contain no more than 1 mg/L. In this experiment, the concentration of each of the component of BTEX will be quantified using GC-FID (gas chromatography equipped with a flame ionization detector) via external standardization technique.

Task

Prepare a technical report based on the given data in the above section. The result and discussion must include:

1. Assume that you are given a standard solution that containing toluene, ethylbenzene, o-xylene, and p-xylene with the concentration of 1000 ppm each. Describe how to prepare the standard solution that containing 10, 20, 30, 40, and 50 ppm of each component with hexane as solvent.

2. Obtain the boiling point of each selected compound. Identification of the compounds listed in Table 1.

3. Calibration plot for each compound.

4. Determine the concentration of selected BTEX in Unknown A, B and C.

5. Answer the following Question.

Question:

1. The separation obtained in this experiment was based on the temperature programming method. What is the expected result in terms of separation if the analysis of the above mixture is carried out using the isothermal method at 200 °C?

2. What is the expected result if we cool down the temperature of the injector to 0 °C?

3. What is the expected result if we turn off the detector?

4. What is the expected result if we remove the column and replacing it with a bare tubing?

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