Gas Chromatography - Outlineo Definitiono Stationary Phaseso GLCo GSCo Schematicso Instrumentationo Advantages and disadvantages
Gas Chromatography - Definition Gas Chromatography (GC) (IUPAC Compendium of Chemical Terminology):A separation technique in which the mobile phase is a gas. Gas chromatography is always carried out in a column.
Most Common Stationary Phases1. Separation of mixture of polar compounds Carbowax 20M (polyethylene glycol)2. Separation of mixtures of non-polar compounds OV101 or SE-30 (polymer of methylsilicone)3. Methylester of fatty acids DEGS (diethylene glycol succinate)
Gas Chromatography Gas-liquid chromatography, GLC.Comprises all gas-chromatographic methods in which the stationary phase is a liquid dispersed on a solid support.Separation is achieved by partition of the components of a sample between the phases. Mostly used nowadays.
Gas Chromatography Gas-solid chromatography, GSC.Comprises all gas chromatographic methods in which the stationary phase is an active solid (e.g. charcoal, molecular sieves).Separation is achieved by adsorption of the components of a sample.
Gas Chromatography Filters/Traps Data system H RESET Regulators Syringe/Sampler Inlets Detectors o gas Gas Carrier system HydrogenAir o inlet Column o column o detect or o data system
Gas Chromatography - InstrumentationMobile phase: carrier gas.He, N2, H2, CO2, Ar. The carrier gas must be chemically inert.The choice of carrier gas is often dependent upon the type of detector which is used.The carrier gas system also contains a molecular sieve to remove water and other impurities.
Gas Chromatography - InstrumentationStationary phase: nonvolatile liquid, sometimes solid.Two kinds of column are used: packed and open tubular (capillary).Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase.Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 – 4mm.
Gas Chromatography – Instrumentation (GC Columns)Capillary columns have an internal diameter of a few tenths of a millimeter. The inner walls are coated with thin layer of stationary phase.
Gas Chromatography – Instrumentation (GC Columns)Open-tubular (Capillary):o 10-100 meters long, 250-530 mm internal diametero 1000-2000 plates/m or 10,000-100,000 plateso Wall-coated (WCOT) or Support-coated (SCOT)o Highly inerto Wide range of stationary phases (selectivity)Packed:o Not widely used, mainly gas analysiso 1-6 m longo Low efficiency
Gas Chromatography –Instrumentation (GC Columns)
Gas Chromatography - InstrumentationAnalyte: gas or volatile liquid.Hydrocarbons, fatty acids, flavor compounds, essential oils, environmental pollutants (pesticides), especially modified substances.It is estimated that 10-20% of the known compounds can be analyzed by GC.To be suitable for GC analysis, a compound must have sufficient volatility and thermal stability.If all or some of a compound or molecules are in the gas or vapor phase at 400-450°C or below, and they do not decompose at these temperatures, the compound can probably be analyzed by GC.
Gas Chromatography –Instrumentation (Detectors) Properties of Ideal GC Detectors•Sensitivity•Stable and reproducible•Linear over several orders of magnitude•Temperature range from room temperature to about 400 °C•Short response time, independent of flow rate•Reliability, ease of use•Universal or class selective
Gas Chromatography –Instrumentation (Detectors) Flame Ionization Detector (FID)• Column effluent is passed through a H2-Air flame – Produces ions and electrons• Charged particles are accelerated by voltage applied between jet and collector – results in current (pA)
Gas Chromatography –Instrumentation (Detectors) Flame Ionization Detector (FID)• Number of ions depends on number of reduced (methylene) carbons in molecule – one molecule of ethane gives twice the signal of one molecule of methane – less sensitive for non-hydrocarbon groups – insensitive to H2O, CO2, SO2 and other noncombustibles• High sensitivity, good LDR (107) , low noise, destructive
Gas Chromatography –Instrumentation (Detectors) Flame Ionization Detector (FID)•Air-Hydrogen Flame•Organic solutespyrolyzed, produce ions andelectronsthat conduct electricitythrough flame.•Universal for organics•High sensitivity (10-13 g/s)•Large linear response range(107)•Rugged, ease to use
Gas Chromatography –Instrumentation (Detectors) Electron-Capture Detector (ECD)• Carrier gas (and analyte) passes over β- emitter, resulting in ionization and e- production• Produces current between electrodes• In the presence of other compounds (especially halogens, etc.) electrons are captured, causing decrease in current• Most commonly used for halogenated organics (insecticides, etc.), small LDR (102)
Gas Chromatography –Instrumentation (Detectors) Electron-Capture Detector (ECD)• Widely used for environmental compounds, like halogenated organics• Constant standing current from 63Ni or other source, highly electronegative atoms decrease current markedly.• Selective (halogens, peroxides, quinones, n itro-)• Highly sensitive to compounds for which it is selective•Small linear response range
Gas Chromatography – Instrumentation (Detectors) Thermal Conductivity Detector (TCD):• Element is electrically heated at constant power – Temperature depends on thermal conductivity of surrounding gas• Measure conductivity (resistance) with respect to a “reference”• Hydrogen and helium carrier gas provide best sensitivity – most thermally conductive – Organics are less so – when analyte comes off, filament temperature goes up, resistance goes down• Poorer sensitivity than FID, but more universal• Large LDR (105), non-destructive
Gas Chromatography –Instrumentation (Detectors) Other Detectors• Atomic Emission – Microwave induced plasma, grating monochromator, diode array detector• Mass Spectrometry Detection• Thermionic Detector – Sensitive to phosphorous and nitrogen
Gas ChromatographyAdvantages of GC● non-destructive method of analysis;● analysis is fast;● analysis is sensitive;● high resolution;● method is compatible with many types of detectors, including MS.
Gas ChromatographyDrawbacks of GC● suitable mostly for analytical purposes;● restricted choice of eluent “polarity”;
Gas ChromatographyVariable parameters in GC:● column;● carrier gas;● gas flow rate;● temperature.