Gas Chromatography -Outline

o 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 Phases

1. Separation of mixture of polar compoundsCarbowax 20M (polyethylene glycol)

2. Separation of mixtures of non-polar compoundsOV101 or SE-30 (polymer of methylsilicone)

3. Methylester of fatty acidsDEGS (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.

Filters/Traps

Air

Hyd

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en

Gas C

arrier

Column

Gas Chromatography

o gas system

o inleto colum

no detect

oro data

system

Data system

Syringe/Sampler

Inlets

Detectors

Regulators

H

RESET

Schematic Diagram of Gas Chromatography

Gas Chromatography - Instrumentation

Mobile 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 - Instrumentation

Stationary 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 - Instrumentation

Analyte: 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

Gas Chromatography - Instrumentation

Gas Chromatography - Instrumentation

Gas Chromatography - Instrumentation

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 solutes pyrolyzed, produce ions and electronsthat conduct electricity through 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, nitro-)• 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 respectto 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 Chromatography

Advantages 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 Chromatography

Drawbacks of GC

● suitable mostly for analytical purposes;

● restricted choice of eluent “polarity”;

Gas Chromatography

Variable parameters in GC:

● column;● carrier gas;● gas flow rate;● temperature.

ANY QUESTIONS ?