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GAS CHROMATOGRAPHY

GAS CHROMATOGRAPHY. ORG I LAB Dr. W. J. KELLY. THE GAS CHROMATOGRAPH. THE GAS CHROMATOGRAPH.

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GAS CHROMATOGRAPHY

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  1. GAS CHROMATOGRAPHY ORG I LAB Dr. W. J. KELLY

  2. THE GAS CHROMATOGRAPH

  3. THE GAS CHROMATOGRAPH • A gas mixture,called the Mobile Phase, composed of an inert “carrier” gas (usually Helium) and various other chemical components in the vapor phase introduced at the Injecton Port pass through a non-moving material, called the... • Stationary Phase, usually tightly packed into a copper tube called the Column… • which separates the gas stream into its particular components based on how “quickly” they move through the stationary phase. The components reach a... • Detector at different times, the detector generates an electrical signal which is sent to a... • Recorder to record the signal. • Since many chemical substances are solids or liquids at room temperature, the stationary phase-mobile phase must be heated to higher temperature in order to “vaporize” the components of the mobile phase. Thus an external heating device or... • Ovenis needed. The temperature of the oven need not be above the boiling points of the components but rather above the “dew point” of the least volatile component

  4. THE GC PROCESS

  5. THE STATIONARY PHASE • The mobile phase can be separated out by the stationary phases through several different processes. The two most common processes are Liquid-Partition Chromatography and Adsorption Chromatography. We will initially consider the former case. • In Liquid-Partition Chromatography, the stationary phase consists of a very porous, inert, finely powered material called the Solid Support. The solid support is coated with a thin film of material called the Liquid Phase. • Most liquid phases are very high boiling liquids, some are waxes at room temperature.

  6. THE SEPARATION PROCESS • As the mobile phase passes through the stationary phase, the different components of the mobile phase will dissolve into the liquid film of the stationary phase. The solubility of the gasses in the liquid phases depends on a number of factors, but one of the most important is the volatility of the liquid from which the component vapor drives. Gasses from low boiling liquids will spend less time dissolved in the liquid phase, whereas gasses from less volatile liquids will spend more time in solution with the liquid phase. The less time a gas spends dissolved in the liquid phases, the more time it spends flying along with the carrier gas. Thus it moves through the stationary phase more quickly.

  7. THERMAL CONDUCTIVITY DETECTOR

  8. PEAK RESOLUTION • Temperature • Carrier Gas Pressure • Column Length • Column Diameter

  9. Column Efficiency and HETP • The resolution in a gas chromatogram can be expressed, as in the case of distillaton, in terms of the number of theoretical plates. A simple formula can be used to derive the number of plates from the position and width of the GC peak: • n = 16(d/w)2 • Where the number of plates (n) is given by the width (w) and the retention time (d) measure in the same units ( mm, cm ,inch or minuits, etc.). Like distillation, the overall efficiency of a stationary phase can be expressed in terms of HETP where: • HETP = n/l

  10. Peak Analysis

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