GAS CHROMATOGRAPHY (GC)

Gas chromatography (GC) also sometimes known as vapor-phase chromatography (VPC), or gas–liquid partition chromatography (GLPC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. It is a term used to describe the group of analytical separation techniques used to analyze volatile substances in the gas phase.

Typical uses of GC include:

• Testing the purity of a particular substance
• Separating the different components of a mixture.
• It can also be used to prepare pure compounds from a mixture.

In GC, components of a sample are dissolved in a solvent and vaporized in order to separate the analytes by distributing the samples between a stationary phase and a mobile phase.

• Mobile phase: This is where a chemically inert gas or an unreactive gas such as helium, argon, nitrogen or hydrogen serves to carry the molecules of the analyte through the heated column. GC is one of the sole forms of chromatography that does not utilize the mobile phase for interacting with the analyte.
• Stationary phase: This is a microscopic layer of viscous liquid on a surface of solid particles on an inert solid support inside a piece of glass or metal tubing called a column. The stationary phase is either a solid adsorbent, termed gas-solid chromatography (GSC), or a liquid on an inert support, termed gas-liquid chromatography (GLC)

Advantages of using GC

1. Short Analysis Time
2. Wide Choice of Stationary Phase
3. Wide Choice of Detectors
4. Ease of Operation
5. High sensitivity
6. Good separation efficiency
7. Suitable for trace analysis
8. Both qualitative and quantitative analysis possible

Limitations of GC

1. Mainly suitable for volatile and thermally stable compounds
2. Some samples require preparation or derivatization
3. Non-volatile compounds are difficult to analyze directly

Types of GC

• Gas-solid chromatography (GSC): It based upon a stationary phase on which retention of analysis consequence of physical adsorption
• Gas-liquid chromatography (GLC): Is useful for separating ions or molecules that are dissolved at absolvent.

Main Components of a GC

Source: MSc. Yassen .H.jassim & MSc. Elham Faisa - Analytical Chemistry Lecture 6

i) Carrier gas reservoir

Inert gases like argon, helium, nitrogen may be used as a carrier gas. Hydrogen gas is less preferred because of it poses explosion hazards. Selection of carrier gas depends on the nature of the mixture to be separated, purity required and detector used for the analysis.

The main purpose of the gas in GC is to move the solutes along the column thus mobile phase is often referred to as carrier gas.

Carrier gas should be:

• Inert, Free from fire and explosion hazard
• Suitable for detector
• Easily available
• Have good flow rate

ii) Injector

Liquid sample is injected by means of a calibrated micro syringe and is injected through a rubber septum at the head of the column.

If the sample is gaseous then 1 to 10ml is injected while for liquid sample 0.1 to 10 micro liter is injected.

At the temperature of the injection port liquid sample is readily converted to vapors without decomposition.

iii) Column

This is the backbone of chromatography. Column is made up of stainless steel or glass and is 2 to 3 meter long and has an internal diameter 2 to 4 mm.

Types of columns

Packed column: It is made up of Teflon having internal diameter 2 to 4 mm and length 5 meter. Column is packed with finely divided solid as absorbent in gas solid chromatography.

Capillary column: These columns are 15 meters to 100 meter long and have internal diameter less than 1 mm (i.e., 0.25 to 0.30 mm). This column does not contain packing but contain stationary phase coated on their inner wall.

iv) Detectors

The most commonly used detectors in a GC machine are:

Flame Ionization Detector (FID): This detector has high sensitivity and selectivity for carbon containing compounds. FID has the following characteristics:

• This detector is 1000 times more sensitive than Thermal conductivity detector (TCD)
• It can detect component at ppb (parts per billion) level
• Fast sensitive and give response to almost all organic compound
• Not sensitive to inorganic compound
• It is widely used detector of Gas chromatography

Thermal Conductivity Detector (TCD): It works under the principles of:

• As the composition of gas changes the thermal conductivity also changes.
• Resistance of wire is the measure of it’s temperature.

Characteristics of TCD include:

• Simple and accurate
• Response is reproducible
• Give response to both organic and inorganic species
• Nondestructive (i.e., effluent can be collected and reused)

Electron Capture Detector (ECD): Detector consists of metal box acting as cathode (negative electrode). Inside this box there is beta (β) emitting source (i.e., ³H or ⁶³Ni). Collector electrode act as anode (Positive electrode).

Characteristics of ECD include:

• Very good detector for electronegative elements.
• Nitrogen gas can be used as carrier gas.
• Give very little response to electropositive elements.
• Can be used up to 350°C
• Less electronegative compounds can be detected by preparing their derivatives.

Suitable and good detector must have following properties:

• Good sensitivity
• Stability
• Selectivity
• Linearity
• Easy to use

v) Software / Data system

The software being used analyzes and displays a chromatograph for analysis and interpretation.

How a Gas Chromatography Works

Step 1: Sample Injection

A small amount of sample is injected into the system. If sample is solid, it is converted to liquid form by dissolving in suitable solvent.

Step 2: Vaporization

The sample is heated and converted into vapor inside the injector.

Step 3:  Carrier Gas Transport

An inert carrier gas (commonly helium, nitrogen, or hydrogen) carries the vaporized sample through the column.

Step 4: Separation inside the Column

Inside the column, compounds separate because they interact differently with the stationary phase based on:

• Volatility (boiling point)
• Polarity
• Molecular interactions

Step 5: Detection

Separated compounds reach a detector (such as FID, TCD, ECD, or MS) at different times.

Step 6: Chromatogram Generation

The detector response is converted into peaks called a chromatogram. Each peak corresponds to a compound, and the retention time helps identify it.

Chromatogram

This is a plot of the detector response against time. The number of peaks represents the number of components presents in the sample (mixture).

Separation of component is based on their partition coefficient. The separated component exit along with the mobile phase at the end of the column. These components are then passed through the detector and detector give response to read out device. Magnitude of the response depends upon the concentration of the component.

Example: 

Mixture of four components is analyzed by gas chromatography. Peak area corresponding to component A, B, C, D is 30cm², 15cm², 20cm², 25cm² respectively. Calculate % of each component.

Solution

Total peak area = 30 +15 +20 +25 = 90cm²

% of component A = (Peak area / Total area) x 100 = (30/90) x 100 = 33.33%

% of component B = (15 / 90) x 100 = 16.66%

% of component C = (20 / 90) x 100 = 22.22%

% of component D = (25 / 90) x 100 = 27.77%

Retention time (t_R)

This is the time taken by the sample to come out from the column after it’s injection

t_R = t₂ - t₁

t₂ = time of elution

t₁ = time of injection

Common Applications

• Solvent analysis
• Petrochemicals
• Food flavor compounds
• Environmental pollutants
• Pharmaceutical impurities
• Textile auxiliaries and finishing chemicals
• Forensic investigations

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