GC (1).ppt
- 2. • Basic Principle of GC: • Volatile or Semi-volatile sample vaporized by injection into a heated injector, eluted through a capillary column by inert gaseous mobile phase (carrier gas) & gradient temperature and finally detected by detector • Separation is based on • A. differences in boiling points of the solutes: as boiling point increases retention time increases • B. and the solutes interaction with the stationary phase
- 3. Schematic of a GC
- 4. Components of GC • Carrier Gas, N2 , H2, He, Ar • Injector • Oven • Column • Detector
- 6. Chromatogram
- 7. Carrier gases (mobile phase) – must be chemically inert He, Ar, N2, CO2 even H2 Often detector dictates choice of carrier gas In GC sample doesn’t really interact with carrier gas (unlike HPLC), temperature controls partitioning Often necessary to purify cylinder gas with a trap, or cartridge of molecular sieves (or use high purity gas 99.999) The move today is away from gas cylinders toward gas generators (extract pure carrier gas from air)
- 9. Injector: use micro syringe injecting 1 to 2 μL, rapidly shoot in plug of sample Old GCs had separate injection area Today use on-column & microflash vaporizers – all have septum of synthetic rubber which is punctured by syringe Injector usually 50 oC hotter than boiling point of sample – also hotter than column.
- 11. GC columns: Column housed in Column Oven to maintain temperature. Types: 1. Packed – tube packed with material. Length 2-10 meter 2. Capillary – coated on walls, long & narrow (newest, took over) Length range – 10 to 100 m (typically 30 meter) -Liquid stationary phase is coated (as a thin film) on a high surface area solid Support - Capillary column
- 16. Typical Temperature Program Time (min) 0 60 50C 220C 160C
- 18. Flame Ionization Detector-FID https://www.youtube.com/watch?v=BJzqdzLY2KU -Most widely used and generally applicable. -Effluent from column is mixed with H2 and air and ignited electrically. -Organic compounds, when pyrolyzed, produce ions and electrons. -Number of ions produced is a function of the number of carbon atoms in the molecule. -Good for the analysis of most organics. -Detector insensitive toward noncombustible gases, such as H2O, CO2, SO2 and NOx. *LOD = low ppb range (sensitive) – Destructive to the sample!
- 19. Thermal Conductivity Detector-TCD https://www.youtube.com/watch?v=FEcA93SL2Zo -A very general or universal detector. -Response results from changes in the thermal conductivity of a gas stream brought about by the presence of an analyte. -Thermal conductivity of He and H2 (carrier gases) is roughly 10x greater than that for most organic molecules. -Filament heated resistively to a constant temperature. The electrical power needed to maintain a constant temperature depends on the thermal conductivity of the surrounding gas. -Small amount of analyte produces a big temperature change (increase) in the filament. LOD = ppm range (S/N>3) not sensitive Nondestructive Thermal conductivity cell