GC General Technique

Transcription

1 Gas Chromatography Gas-liquid chromatography (GLC) is an accurate and rapid process for separating and analyzing the components of a volatile mixture. This type of chromatography is an example of partition chromatography, where the separation depends on partitioning of the mixture s components between two solvents. One of these solvents (called the stationary phase) is adsorbed on a solid support. The other solvent (called the mobile phase) passes through this support.

2 GC General Technique Gas chromatography involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid. Schematic diagram of a gas chromatograph:

3 GC - Instrumental components Carrier gas The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.

4 GC - Instrumental components Sample injection port For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapor - slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro syringe is used to inject sample through a rubber septum into a flash vaporizer port at the head of the column. The temperature of the sample port is usually about 50 C higher than the boiling point of the least volatile component of the sample. For packed columns, sample size ranges from tenths of a micro liter up to 20 micro liters.

5 GC - Instrumental components Packed Column Usually, a 5 10 ft length metal tubing, 3 6 mm in diameter, packed with a porous solid that is thinly coated with a high-boiling liquid. The tube is located in an oven that can be heated at a controlled temperature. Detector A device used to detect each component of the sample as it appears at the exit of the packed column. The intensity or area of the signal indicates the quantity of the eluted component. Recorder A device that provides a record of the analysis

6 GC Instrument Operation The injection block or sample injection port, column oven, and detector are heated to the desire temperature. The inert carrier gas is passed through the apparatus. Actually the carrier-gas stream is split. Part of it passes through the entire apparatus, and part of it goes directly to the detector. One common type of detector, called a thermal conductivity detector, consists of two heated wire filaments, one exposed only to the reference carrier-gas stream, and the other exposed to the effluent gas from the column.

7 GC Instrument Operation First, a small sample of the material to be analyzed is injected The sample vaporizes in the injection block and is carried through the column, through the detector, and eventually out of the apparatus. As the sample is eluted from the column and passed through the detector, one of the detector wires is exposed to a mixture of carrier gas plus sample, and the other wire is exposed only to the reference stream of carrier gas. The difference between the two gas streams causes a difference in the electrical conductivity of the wires, and this difference is automatically recorded. In this way a gas chromatogram is obtained.

8 How are the components of the mixture separated? The column is packed with a highly porous, inert powder (usually silica containing small amounts of metallic oxides). This powder is called the solid support. The support is coated with a small amount ( usually 5 10% by weight) of a high boiling liquid before it is packed into the column.

9 How are the components of the mixture separated? Many types of these liquid stationary phases are known: examples include silicone oils, polyesters, and polyethers. The rate at which a particular component of a mixture passes through the column depends on its boiling point, its solubility in stationary liquid phase, its polarity, the temperature of the column, and the flow of the carrier gas. By adjusting these variables, the components of a multicomponent mixture could be separated and analyzed by GC.