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421-821-chapter-27

421-821-chapter-27 - Gas Chromatography A Introduction Gas...

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Gas Chromatography A.) Introduction : Gas Chromatography (GC) is a chromatographic technique in which the mobile phase is a gas. GC is currently one of the most popular methods for separating and analyzing compounds. This is due to its high resolution, low limits of detection, speed, accuracy and reproducibility. GC can be applied to the separation of any compound that is either naturally volatile (i.e., readily goes into the gas phase) or can be converted to a volatile derivative . This makes GC useful in the separation of a number of small organic and inorganic compounds. B.) Equipment: A simple GC system consists of: 1. Gas source (with pressure and flow regulators) 2. Injector or sample application system 3. Chromatographic column (with oven for temperature control) 4. Detector & computer or recorder
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A typical GC system used is shown below ( a gas chromatograph ) Carrier gas : He (common), N 2 , H 2 P inlet 10-50 psig Flow = 25-150 mL/min packed column Flow = 1-25 mL/min open tubular column Column: 2-50 m coiled stainless steel/glass/Teflon Oven: 0-400 °C ~ average boiling point of sample Accurate to <1 °C Detectors: FID, TCD, ECD, (MS)
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C.) Mobile Phase : GC separates solutes based on their different interactions with the mobile and stationary phases. - solute’s retention is determined mostly by its vapor pressure and volatility - solute’s retention is controlled by its interaction with the stationary phase - gas mobile phase has much lower density decreased chance for interacting with solute increased chance that solid or liquid stationary phase interacts with solute Carrier gas – main purpose of the gas in GC is to move the solutes along the column, mobile phase is often referred to as carrier gas. Common carrier gas : include He, Ar, H 2 , N 2 Carrier Gas or Mobile phase does not affect solute retention, but does affect : 1.) Desired efficiency for the GC System - low molecular weight gases (He, H ) larger diffusion coefficients
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D.) Stationary Phases : Stationary phase in GC is the main factor determining the selectivity and retention of solutes. There are three types of stationary phases used in GC : Solid adsorbents Liquids coated on solid supports Bonded-phase supports 1.) Gas-solid chromatography (GSC) - same material is used as both the stationary phase and support material - common adsorbents include: ° alumina ° molecular sieves ( crystalline aluminosilicates [zeolites] and clay) silica active carbon Magnified Pores in activated carbon
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Gas-solid chromatography (GSC): advantages : - long column lifetimes - ability to retain and separate some compounds not easily resolved by other GC methods geometrical isomers permanent gases disadvantage : - very strong retention of low volatility or polar solutes - catalytic changes that can occur on GSC supports - GSC supports have a range of chemical and physical environments different strength retention sites non-symmetrical peaks variable retention times
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2.)
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