Gas Chromatography with FID

Transcription

1 Gas Chromatography with FID Introduction Gas chromatography is an instrumental method for the separation and identification of chemical compounds. Chromatography involves a sample (or sample extract) being dissolved in a mobile phase (which may be a gas, a liquid or a supercritical fluid). The mobile phase is then forced through an immobile, immiscible stationary phase. The phases are chosen such that components of the sample have differing solubilities in each phase. A component that is quite soluble in the stationary phase will take longer to travel through it than a component that is not very soluble in the stationary phase but very soluble in the mobile phase. As a result of these differences in mobilities, sample components will become separated from each other as they travel through the stationary phase. The diagram of the separation process is shown on the Figure 1. Figure 1.

2 Our laboratory setup is presented in Figure 2. Figure 2. After the separation of the compounds, Flame Ionization Detector (FID) is used to identify each of them and determine their mass. The simple diagram of the FID is shown on the Figure 3. Figure 3. The effluent from the column is mixed with hydrogen and air, and ignited. Organic compounds burning in the flame produce ions and electrons that can conduct electricity through the flame. A large electrical potential is applied at the burner tip, and a collector electrode is located above the flame. The current resulting from the pyrolysis of any organic compounds is measured. FIDs are mass sensitive rather than concentration sensitive; this gives the

3 advantage that changes in mobile phase flow rate do not affect the detector's response. The FID is a useful general detector for the analysis of organic compounds; it has high sensitivity, a large linear response range, and low noise. It is also robust and easy to use, but it destroys the injected sample. After detection, a signal is sent to the recording device. You will see the development of the curve on the computer screen (Figure 4.) Figure 4. The time between sample injection and an analyte peak reaching a detector at the end of the column is termed the retention time (t R ). Each analyte in a sample will have a different retention time. The time taken for the mobile phase to pass through the column is called t M. A GC can separate the compounds, but cannot identify them itself. By calibrating GC you can find out at what time various organic compounds are being detected. The area under the curve (automatically calculated by the computer) may be expressed in terms of concentration of the pollutant, by running some calibration standards at known concentration. You will need: Laboratory procedure - 4 calibration samples of water contaminated with Toluene and MTBE. Concentrations are 1 ppm, 10 ppm, 25 ppm and 50 ppm - Your test samples from other experiments - Safety equipment (gloves, glasses, etc.) ml Beaker with ml Acetone ml Beaker with 50 ml distilled water - GC Syringe ( 10 µl) - SRI GC with FID detector

4 - Notebook ml Wastewater beaker Make sure that you read and understand the lab procedure. Follow the safety procedure for the work in the lab. Turn GC on. Let it warm up for about 15 minutes Open up the valves on two rightmost reservoirs with compressed air and Hydrogen. Ignite the flame in FID by pushing and holding Flame Ignite bottom on GC. You can see if it works by bringing a stainless steel surface (or a mirror) into the contact with FID outlet. The surface will get foggy. Make sure that the temperature is programmed to increase from 40 to 90 degrees. Rinse the syringe in acetone and then in distilled water several times. Be very careful. The syringe is very fragile and very expensive. Start testing lower concentration of one-component liquids first. Then go to higher concentrations, change to other compound and then to mixtures. Insert the needle into the testing vial through the lid. Don t open the vial. Withdraw some liquid. Discard this liquid in the wastewater beaker. Insert needle into vial again. Slowly withdraw 4 µl of liquid. Make sure that you don t have any air bubbles inside the syringe. The volume must be precise. Make sure that GC thermometer indicates it is at about 40 o C. Start a new file. Click on 0 bottom on the screen of the computer for Autozeroing. Press space on the keyboard to start the run. Inject testing fluid into Injector 2. Wait 6-7 minutes. Review the output curve. Click results at the bottom of the screen. Write down type of the pollutant tested, concentration of the sample and area under the curve calculated by the computer. Save the file in the new folder. Repeat the procedure for other samples. When finished, close valves on air and hydrogen reservoirs. Turn GC off. Using Excel, draw the calibration curve. Calibration curve: Prepare 4 vials with 25 ml DI (de-ionized) water using a graduated cylinder. The stock solution is water saturated with toluene or MTBE. For toluene, the water is at a concentration of 546 mg/l; for MTBE, the water is at a concentration of 54,000 mg/l. Add toluene and MTBE solutions to obtain the following concentrations: 10, 25, 50 and 100 mg/l. (1 ppm = 1 mg/l)

5 The required volume can be calculated according to the equation: C 1 * V 1 = C 2 * V 2 (Where C = concentration, V =volume.) For example to get a 10 mg/l toluene solution; Solving for V1; 546 mg/l * V1 = 10 mg/l* 25 ml V1= ml = 458 µl of toluene saturated water is needed. The final volume should be 25ml, so you have to get this amount of water (i.e. 458 µl) out from the vial with pipette, and then add this amount of toluene water back into the vial. Cap & shake well to make sure it is a homogeneous solution. References