GAS CHROMATOGRAPHY

           Gas Chromatography 

Gas Chromatography (GC) is a chromatographic technique in which the mobile phase is a gas.
According to stationary phase, there two major types
1. Gas-solid chromatography stationary phase = solid, retention mech.
2. Gas-liquid chromatography stationary phase = immobilized liquid

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.

           Components of Gas Chromatography 

❖Carrier Gas, N2 or He, 1-2 mL/min
❖Injector
❖Oven
❖Column
❖Detector

                      Principles of GLC 

❑ 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.
❑ The partition coefficients are inversely proportional to the volatility of the analytes so that the most volatile elute first.
❑ The temperature of the column is raised to 50 - 300 oC to facilitate analyte volatilisation.
❑ Gas chromatography is widely used for the qualitative and quantitative analysis of a large number of low-polarity compounds because it has high sensitivity, reproducibility and speed of resolution.

                             Carrier gas

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.
1. Inert,
2. suitable for the detector,
3. high purity,
4. easily available,
5. cheap

Common carrier gas: He, Ar, H2, N2 

The mobile phase consists of an inert gas such as nitrogen for packed columns or helium or argon for capillary columns. The gas from a cylinder is pre-purified by passing through a variety of molecular sieves to remove oxygen, hydrocarbons and water vapour. It is then passed through the chromatography column at a flow rate of 40-80 cm3 min–1. A gas-flow controller is used to ensure a constant flow irrespective of the back-pressure and temperature of the column.

                          Carrier Gas system 

▪Carrier gases are available in pressurized tanks. Pressure regulators, gauges, and flow meters are required to control the flow rate of the gas. In addition, the carrier gas system often contains a molecular sieve to remove impurities and water. 

• Inlet pressures usually range from 10 to 50 psi (Ib/in2) above room pressure, which lead to flow rates of 25 to 150 mL/min with packed columns and 1 to 25 mL/min for open tubular capillary column 

• Generally, it is assumed that flow rates will be constant if the inlet pressure remains constant. 

• Flow rates can be established by a Rotometer at the column head; this device, however, is not as accurate as the simple soap-bubble meter which is located at the end of the column. 

• A soap film is formed in the path of the gas when a rubber bulb containing an aqueous solution of soap or detergent is squeezed; the time required for this film to move between two graduations on the buret is measured and converted to volumetric flow rate. Many modern computer-controlled gas chromatographs are equipped with electronic flow meters that can be regulated. 

                      Sample Injection System 

➢ Column efficiency requires that the sample be of suitable size and be introduced as a “plug” of vapor; slow injection of oversized samples causes band spreading and poor resolution. 

➢Direct Injection: The most common method of sample injection involves the use of micro-syringe to inject a liquid or gaseous sample through a self sealing, silicone-rubber diaphragm or septum into a flash vaporizer port located at the head of the column (the sample port is ordinarily about 50oC above the boiling point of the least volatile component of the sample). 

➢ sample size 

(i) 1-20 μL packed column 

(ii) 10μL capillary column 

➢ a sample splitter is often needed to deliver a small known fraction (1 :50 to 1;500) of the injected sample, with the remainder going to waste. 

Split injection:  routine method 0.1-1 % sample to column remainder to waste 

Splitless injection: all sample to column best for quantitative analysis only for trace analysis, low [sample] risk of backflash

On-column injection: for samples that decompose above boiling point no heated injection port column at low temperature to condense sample in narrow band heating of column starts chromatography 

                 Column Configurations

Two general types of columns are encountered in gas chromatography, 

1) packed and 2) capillary (open tubular). 

PACKED COLUMN:

 ➢ 1 - 4mm ID; 

➢1 - 5 m length 

➢ Glass / stainless steel coil ( fused silica) / teflon ) 

➢ Packed solid particles either porous/non-porous coated with thin (1 μm) film of liquid 

➢ To fit into an oven, they are usually formed as coils having diameters of 10 to 30 cm 

CAPILLARY COLUMN:

 ➢ 0.1 - 0.5 mm I.D.

 ➢ 2 - 100 m length 

➢Thin fused-silica. 

➢Inner wall coated with thin (0.1-5 μm) film of liquid (stationary phase) 

                         Column Ovens

➢ The column is ordinarily housed in a thermostated oven. 

➢ Column temperature is an important variable that must be controlled to a few tenths of a degree for precise work. 

➢ The optimum column temperature depends upon the boiling point of the sample and the degree of separation required. 

➢ Roughly, a temperature equal to or slightly above the average boiling point of a sample results in a reasonable elution time (2 to 30 min). 

➢ For samples with a broad boiling range, it is often desirable to employ temperature programming, whereby the column temperature is increased either continuously or in steps as the separation proceeds. 

                       Detection Systems 

The choice of detector will depend on the analyte and how the GC method is being used (i.e., analytical or preparative scale) 

Characteristics of the Ideal Detector:

 1. Adequate sensitivity 

2. Good stability and reproducibility. 

3. A linear response to solutes that extends over several orders of magnitude. 

4. A temperature range from room temperature to at least 400oC. 

5. A short response time that is independent of flow rate. 

6. High reliability and ease of use. 

7. Similarity in response toward all solutes or a highly selective response toward one or more classes of solutes. 

8. Nondestructive of sample.



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