ANALYTICAL CHEMISTRY | INSTRUMENTAL ANALYSIS

MASTERING GAS CHROMATOGRAPHY (GC)

Complete Theory, Principle, Instrumentation, Working Procedure & Industrial Applications

Comprehensive Study Notes for Chemistry, Chemical Engineering, Petrochemical and Refinery Students

ANIKET PETROTECH

About This Chapter

Gas Chromatography (GC) is one of the most important analytical techniques used in modern chemical laboratories, petroleum refineries, petrochemical industries, pharmaceutical laboratories and environmental testing facilities. In this chapter, we will understand how a gas chromatograph separates different chemical compounds, how its major components work, how to operate the instrument, and how to interpret the chromatogram obtained during analysis. These notes are designed for students, laboratory trainees, QC chemists, analytical professionals and candidates preparing for technical interviews.

1. Introduction to Gas Chromatography

Gas Chromatography is an advanced instrumental analytical technique used to separate, identify and quantify volatile and semi-volatile compounds present in a mixture. Suppose we have a liquid sample containing several different chemical compounds. These compounds may have different boiling points and different interactions with a stationary phase. Gas Chromatography uses these differences to separate the components of the sample. The separated compounds are detected individually, and the instrument generates a graphical representation called a Chromatogram. GC is widely used in:

Petroleum refineries
Petrochemical industries
Pharmaceutical industries
Food and beverage laboratories
Environmental monitoring laboratories
Forensic and chemical testing laboratories

2. Definition of Gas Chromatography

Gas Chromatography is a separation technique in which the components of a volatile sample are separated based on their distribution between a gaseous mobile phase and a stationary phase inside a chromatographic column. The mobile phase is generally an inert carrier gas such as helium, hydrogen or nitrogen. The stationary phase is either a non-volatile liquid film coated inside a capillary column or a solid adsorbent in a packed column. Important: GC is particularly suitable for compounds that can be vaporized without undergoing significant thermal decomposition.

3. Principle of Gas Chromatography

The principle of Gas Chromatography is based on the differential distribution of sample components between the mobile phase and stationary phase. When a sample is injected into the heated injection port, it vaporizes and mixes with the carrier gas. The carrier gas transports the vaporized sample through the chromatographic column. Inside the column, different compounds interact differently with the stationary phase. Some compounds interact weakly with the stationary phase and travel faster through the column. Other compounds interact more strongly and take longer to pass through it. As a result, the components leave the column at different times. This process is called chromatographic separation.

Example: Understanding the Principle

Imagine a mixture containing three compounds: A, B and C.

Compound A has relatively weak interaction with the stationary phase and elutes first.
Compound B has moderate interaction and elutes second.
Compound C has stronger interaction and elutes later.

Therefore, each compound produces a separate peak on the chromatogram when properly resolved.

Important Concept: Retention time depends on several factors, including volatility, column temperature, stationary-phase chemistry and carrier-gas flow. A higher boiling point does not always mean a longer retention time because interactions with the stationary phase also play an important role.

Mobile Phase and Stationary Phase

Parameter	Mobile Phase	Stationary Phase
Meaning	Moving phase that transports sample components.	Phase that interacts with and helps separate sample components.
Physical nature	Gas	Liquid film or solid adsorbent
Examples	Helium, Hydrogen, Nitrogen	Polysiloxane-based and polyethylene glycol-based phases
Function	Moves sample through the column.	Controls differential retention and separation.

4. Types of Gas Chromatography

Gas Chromatography is commonly classified into two major types.

4.1 Gas-Liquid Chromatography (GLC)

In Gas-Liquid Chromatography, the stationary phase is a non-volatile liquid film coated on the inner surface of the column or on a solid support. The sample components distribute between the gaseous mobile phase and the liquid stationary phase. GLC is widely used for the analysis of volatile organic compounds, hydrocarbons, solvents and petroleum products.

4.2 Gas-Solid Chromatography (GSC)

In Gas-Solid Chromatography, the stationary phase is a solid adsorbent. Separation occurs mainly through differences in adsorption of the sample components on the solid surface. It is useful for analysing permanent gases and certain low-molecular-weight compounds.

GLC	GSC
Uses a liquid stationary phase.	Uses a solid stationary phase.
Separation is mainly based on partitioning.	Separation is mainly based on adsorption.
Widely used for organic compounds.	Useful for gases and certain small molecules.

5. Instrumentation of Gas Chromatography

A Gas Chromatograph consists of several important components. Each component performs a specific function to achieve accurate and reproducible separation.

Major Components of GC

Carrier Gas Cylinder

↓

Gas Flow Controller

↓

Sample Injector

↓

Chromatographic Column

↓

Detector

↓

Data System / Computer

5.1 Carrier Gas Supply System

The carrier gas supply system provides the mobile phase required to transport the sample through the column. Common carrier gases include:

Helium (He)
Hydrogen (H₂)
Nitrogen (N₂)
Argon (Ar), for selected applications

The carrier gas should be of suitable purity and compatible with the detector and analytical method. Gas regulators, purification traps and electronic flow controllers help maintain the required gas conditions.

5.2 Flow Controller and Pressure Regulator

The flow controller regulates the flow of carrier gas through the GC system. A stable carrier-gas flow is essential for reproducible retention times and good chromatographic performance. Modern GC instruments may use electronic pressure control (EPC) to regulate inlet pressure and flow.

5.3 Sample Injection Port

The injection port is the part of the GC instrument where the sample is introduced. It is generally maintained at a sufficiently high temperature to vaporize the sample rapidly. A microsyringe or an automated autosampler may be used for sample injection. The vaporized sample is carried into the column by the mobile phase. Common injection modes include:

Split Injection: Only a controlled portion of the vaporized sample enters the column. The remaining portion is vented through the split outlet.
Splitless Injection: Most of the vaporized sample is directed into the column. This is useful for trace-level analysis.
On-Column Injection: The sample is introduced directly into the column under controlled conditions.

5.4 Chromatographic Column

The column is the most important separation component of a GC instrument. It contains the stationary phase, which interacts differently with the individual components of the sample. Two major types of GC columns are used. A. Packed Column A packed column is filled with a solid support material coated with a suitable stationary phase, or with an appropriate adsorbent. Features:

Generally shorter than capillary columns.
Relatively larger sample capacity.
Used in certain routine gas-analysis applications.

B. Capillary Column A capillary column is a narrow, long tube, usually made of fused silica, with a thin stationary-phase film coated on its inner wall. Features:

High separation efficiency.
High resolving power.
Low sample capacity compared with many packed columns.
Widely used in modern GC laboratories.

Parameter	Packed Column	Capillary Column
Construction	Filled with packing material	Open tubular column
Efficiency	Generally lower	Generally higher
Sample capacity	Generally higher	Generally lower
Applications	Selected gas and process analyses	Complex mixtures and trace analysis

5.5 Column Oven

The column oven maintains and controls the temperature of the chromatographic column. Temperature has a major effect on the volatility of compounds and their retention times. The oven can operate using:

Isothermal temperature programming: The column temperature remains constant throughout the run.
Temperature programming: The oven temperature increases according to a predefined programme during analysis.

Temperature programming is especially useful for samples containing compounds with a wide range of boiling points.

5.6 Detector

The detector identifies compounds as they leave the column and converts their presence into an electrical signal. This signal is processed by the data system and displayed as chromatographic peaks. Different detectors are selected according to the nature of the sample and the analytical objective.

5.7 Data System / Recorder

The data system receives the electrical signal generated by the detector. It processes and displays the signal as a chromatogram. The software helps determine:

Retention time
Peak area
Peak height
Peak width
Compound identification using standards
Quantitative concentration using a validated calibration method

The graphical output is called a Chromatogram.

6. Carrier Gas in Gas Chromatography

Carrier gas is the mobile phase of Gas Chromatography. It transports the vaporized sample through the column without ideally reacting with the sample or stationary phase.

Important Properties of a Carrier Gas

High purity.
Chemical inertness for the intended analysis.
Compatibility with the detector.
Stable and controllable flow.
Low moisture and oxygen contamination.

Carrier Gas	Important Characteristics
Helium	Inert, widely used and compatible with many GC applications.
Hydrogen	High diffusivity and useful chromatographic performance; requires strict safety precautions because it is flammable.
Nitrogen	Inert and economical, but may require different flow conditions for optimum efficiency.

Carrier gas selection depends on the detector, column, method requirements, instrument configuration, safety and cost. There is no single carrier gas suitable for every GC application.

7. Sample Injection System

The injection system introduces a measured amount of sample into the GC instrument.

How Does the Injector Work?

1. A sample is taken using a suitable microsyringe or autosampler. 2. The sample is introduced into the heated injection port. 3. The liquid sample vaporizes rapidly under the selected conditions. 4. Carrier gas transports the vaporized sample towards the column. 5. The sample components begin their separation inside the column. Important Injection Parameters:

Injection volume.
Inlet temperature.
Split ratio.
Inlet pressure.
Sample concentration.
Sample cleanliness and compatibility.

8. Column Oven and Temperature Programming

The column oven controls the temperature of the column during the analytical run. There are two important operating approaches.

8.1 Isothermal Operation

In isothermal operation, the oven temperature remains constant throughout the analysis. It is suitable for samples whose components have relatively similar volatility and retention characteristics.

8.2 Temperature-Programmed Operation

In temperature programming, the oven starts at an initial temperature and increases according to a programmed rate. A typical method may include:

Initial oven temperature.
Initial holding time.
Temperature ramp rate.
Final oven temperature.
Final holding time.

The actual values depend on the sample, column specifications and validated analytical method.

9. Types of GC Detectors

The detector is responsible for sensing the separated compounds as they exit the column. Different detectors have different selectivity, sensitivity and operating principles.

9.1 Flame Ionization Detector (FID)

FID is one of the most widely used detectors in Gas Chromatography. Principle: The column effluent is introduced into a hydrogen-air flame. Many organic compounds produce ions during combustion in the flame. The resulting ion current is measured by an electrode system. The electrical signal is related to the amount of ionizable organic material reaching the detector. Applications:

Hydrocarbon analysis.
Petroleum product testing.
Solvent analysis.
Organic compound analysis.

Important Point: FID responds strongly to many organic compounds but has little or no response to many permanent inorganic gases such as nitrogen, oxygen and carbon dioxi

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Gas Chromatography (GC): Principle, Instrumentation, Working Procedure & Applications By Aniket Petrotech