Gas Chromatograph, GC analyzer, normal syringes and one micro syringe, Beakers, Sample bottles and Electronic weight.

Transcription

1 PI 306 GAS CHROMATOGRAPHY AIM To study the Gas Chromatography, study of influence of various operating parameters on the performance of GC. To analyze the sample of unknown concentration using Gas Chromatography. APPARATUS Gas Chromatograph, GC analyzer, normal syringes and one micro syringe, Beakers, Sample bottles and Electronic weight. CHEMICALS Methanol, Isopropyl Alcohol and water THEORY A Gas Chromatograph is used to detect the components based on the selective affinity of components towards the adsorbent materials. The sample is introduced in the liquid/gas form with the help of GC syringe into the injection port, it gets vaporized at injection port then passes through column with the help of continuously flowing carrier stream (mobile phase), mainly H 2 (for TCD), and gets separated/detected at the detection port with suitable temperature programming. We visualize this on computer in the form of peaks. Carrier medium can be liquid (e.g. HPLC) or gas (e.g. GC) for the ease of separation/detection, if it is gas then called gas chromatography otherwise called liquid chromatography. Different chemical constituents of the sample travel through the column at different rates depending upon, 1. Physical properties 2. Chemical properties, and

2 3. Interaction with a specific column filling (stationary phase). As the chemicals exit the end of the column, they are detected and identified electronically. The function of the stationary phase in the column is to separate different components, causing each one to exit the column at a different time (retention time). Other parameters that can be used to alter the order or time of retention are the carrier gas flow rate, and the temperature. Physical Components involve inlet port, Adsorption column, detector port, flow controller (to control the flow of carrier gas), etc. Experimental setup for Gas Chromatograph Two types of columns are used in GC: Packed columns are m in length and have an internal diameter of 2-4 mm. The tubing is usually made of stainless steel or glass and contains a packing of finely divided, inert, solid support material (eg. diatomaceous earth) that is coated with a liquid or solid stationary phase. The nature of the coating material determines what type of materials will be most strongly adsorbed. Capillary columns have a very small internal diameter, on the order of a few tenths of millimeters, and lengths between meters are common. The inner column walls are coated with the active materials (WCOT columns). Some columns are quasi solid filled with many parallel micro pores (PLOT columns). Most capillary columns are made of fused silica with a polyimide outer coating. These columns are flexible, so a very long column can be wound into a small coil.

3 Temperature dependence of molecular adsorption and of the rate of progression along the column necessitates a careful control of the column temperature to within a few tenths of a degree for precise work. Reducing the temperature produces the greatest level of separation, but can result in very long elution times. The choice of carrier gas (mobile phase) is important, with hydrogen being the most efficient and providing the best separation. However, helium has a larger range of flow rates that are comparable to hydrogen in efficiency, with the added advantage that helium is non-flammable, and works with a greater number of detectors. Therefore, helium is the most common carrier gas used. Detectors A number of detectors are used in gas chromatography. The most common are the Flame ionization detector (FID) and the thermal conductivity detector (TCD). While TCDs are essentially universal and can be used to detect any component other than the carrier gas (as long as their thermal conductivities are different than that of the carrier gas, at detector temperature), FIDs are sensitive primarily to hydrocarbons, and are more sensitive to them than TCD. Both detectors are also quite robust. Since TCD is nondestructive, it can be operated in-series before an FID (destructive), thus providing complementary detection of the same eluents. PROCEDURE Prepare the samples for calibration with various compositions. Keep the amount of Isopropyl alcohol fix equal to 2 grams in each sample. Start the apparatus by switching on the hydrogen supply and set the parameters: Oven temperature = 170 o C TCD temperature = 150 o C Injector temperature = 200 o C Carrier Gas Pressure = 4 kg/cm 2. START THE ISOTHERM. Before injection of sample wait till the base line of recorder becomes perfectly horizontal; which indicates that GC is stabilized or conditioned properly. Inject the sample with a micro syringe at the injector port and START THE RUN.

4 After all the peaks attained stop the run and get the integration results. Note the retention time and area of peak of each of the constituents for each sample. Plot the CALIBRATION CURVES and find out the response factor (slope of the calibration curve) for each of the constituents with respect to. The curves are the straight lines passing through origin. These will be used for analysis of unknown sample. OBSERVATIONS & CALCULATIONS 1. Standard Settings Gas Pressure: TCD temperature: Oven temperature: Injector temperature: Amount of Sample injected: 2. Calibration Table Samp. RT* RT RT AOP # AOP AOP NO * RT = Retention Time # AOP = Area of Peak

5 3. Calculation Table Samp. Weight Ratio Area Ratio Weight Ratio Area Ratio No. / (x1) / (y1) / (x2) / (y2) DISCUSSION AND RESULTS Plot the calibration curves and fit the straight line passing through the origin. Find Response Factor. Find out the composition of the unknown sample. REFERENCES D.J. David, Gas Chromatographic Detectors, John Wiley & Sons, 1974 J.P. Bantley, Principles of Measurement Systems, Longman, Singapore, 1998 P.G. Jeffery and P.J. Kipping, Gas Analysis by Gas Chromatography, Pergamon, Oxford,