Gas Chromatography (GC) is an instrumental analytical technique used to separate, identify, and quantitatively measure volatile compounds in gas, liquid, or solid samples. Its operating principle is based on the difference in the tendency of various compounds to interact with a stationary phase, or column, compared with an inert carrier gas. GC is one of the most widely used analytical methods in refinery, petrochemical, environmental, and pharmaceutical laboratories.
What Is GC and What Does It Measure?
Gas Chromatography is a technique used to separate the components of a mixture based on differences in vapor pressure and interaction with the stationary phase of the column. Each compound passes through the column at a different speed and reaches the detector at a specific time, known as the retention time.
GC measures compounds that:
Are volatile or can be vaporized without decomposition
Have boiling points below 350°C, or up to 450°C with specialized columns
Can be detected even at very low concentrations, from ppm to ppb levels
History: The first gas chromatograph was developed in 1952 by James and Martin, and this work led to the Nobel Prize in Chemistry in the same year.
What Are the Main Components of a GC Instrument?
A GC instrument consists of six main sections, each with a specific function:
1. Carrier Gas Source
The carrier gas is an inert gas that transports the sample from the injection port to the detector. The choice of carrier gas directly affects separation efficiency and detector sensitivity.
| Carrier Gas | Advantages | Disadvantages | Best Application |
|---|---|---|---|
| Helium (He) | High efficiency, inert | Expensive, dependent on imports | Most applications, GC-MS |
| Nitrogen (N₂) | Low cost, readily available | Lower efficiency at high flow rates | Simple analyses, FID |
| Hydrogen (H₂) | Fastest carrier gas | Fire hazard | Alternative to helium |
| Argon (Ar) | Completely inert | Expensive | ECD detector |
2. Sample Injection System
This is the point where the sample enters the instrument. Its temperature is usually set 20 to 50°C higher than the highest oven program temperature. Common injection modes include:
Split: The sample is split, and only a small portion, typically 1 to 5%, enters the column. Suitable for high-concentration samples.
Splitless: The entire sample enters the column. Suitable for dilute samples and trace analysis.
On-Column: The sample is introduced directly into a cold column. Suitable for temperature-sensitive compounds.
Headspace: The vapor phase above a liquid sample is injected. Suitable for volatile compounds in complex matrices.
3. Chromatographic Column
The column is the heart of a GC instrument. There are two main types:
Capillary Column
Internal diameter: 0.1 to 0.53 mm
Length: 15 to 100 m
Made of fused silica
Very high separation efficiency
Standard choice in modern laboratories
Packed Column
Internal diameter: 2 to 4 mm
Length: 1 to 5 m
Filled with coated support particles
Still used for some permanent gas applications
4. Oven
The oven is the chamber in which the column is placed. Precise temperature control, typically with an accuracy of ±0.1°C, is one of its most important features. It can operate in two main modes:
Isothermal: A constant temperature is maintained throughout the analysis. This mode is suitable for simple samples with a narrow boiling-point range.
Temperature Programming: The temperature is gradually increased during the analysis. This mode is suitable for complex samples containing both light and heavy compounds.
5. Detector
The detector identifies and measures the compounds that leave the column. Selecting the right detector is one of the most important decisions when developing a GC method.
| Detector | Abbreviation | Operating Principle | Main Refinery Application |
|---|---|---|---|
| Flame Ionization Detector | FID | Ionization in a hydrogen flame | Hydrocarbons, gasoline, petroleum products |
| Thermal Conductivity Detector | TCD | Change in thermal conductivity | Permanent gases, CO, CO₂, H₂ |
| Electron Capture Detector | ECD | Capture of free electrons | Halogenated compounds, pesticides |
| Flame Photometric Detector | FPD | Optical emission in a flame | Sulfur and phosphorus in fuels |
| Mass Spectrometer | MS | Fragmentation of molecules into ions | Identification of unknown compounds |
| Nitrogen–Phosphorus Detector | NPD | Selective response to N and P | Amines, pharmaceuticals |
6. Data System
The data system is the software that records the chromatogram, integrates peaks, and calculates results. Modern systems provide capabilities such as:
Automatic peak integration
Calibration using multiple standards
Automatic reporting
Method validation
How Does GC Work?
The GC analysis process is carried out in five steps:
Step 1 — Sample Preparation and Injection
A small amount of sample, typically 0.1 to 2 microliters for liquids or a few milliliters for gases, is introduced into the heated injection port. The heat of the injection port immediately vaporizes the sample.
Step 2 — Transport by Carrier Gas
The sample vapor is carried into the column by a continuous flow of carrier gas. The pressure and flow rate of the carrier gas must be precisely controlled.
Step 3 — Separation in the Column
Each compound moves through the column at a different speed depending on two main factors:
Vapor pressure: Lighter and more volatile compounds elute earlier.
Interaction with the stationary phase: Compounds that interact more strongly with the stationary phase elute later.
Step 4 — Qualitative Identification
Each compound reaches the detector at a specific retention time. Under the same method conditions, this retention time is characteristic of each compound.
Step 5 — Quantitative Measurement
The peak area is proportional to the amount, or mass, of the compound. Using calibration standards, the exact concentration is calculated.
Applications of GC in Refineries and Petrochemical Plants
Process Gas Analysis
Natural gas composition: methane, ethane, propane, iso- and normal butane, and pentanes according to ASTM D1945
Overhead gas analysis in distillation columns
Measurement of CO and CO₂ in process gases according to UOP603
Hydrogen gas analysis in hydrocracker units
Gasoline and Light Product Analysis
Detailed gasoline composition: PONA, including paraffins, olefins, naphthenes, and aromatics, according to ASTM D5134
Measurement of benzene, toluene, and xylene, or BTX, according to ASTM D3606
Analysis of oxygenates such as ethanol and MTBE in gasoline
Simulated Distillation
Simulated Distillation, or SimDis, is one of the most important applications of GC in refineries. Using ASTM D2887, the boiling-point distribution of petroleum products such as diesel fuel, kerosene, and fuel oil can be determined within 30 to 90 minutes, while conventional distillation may take several hours.
Petrochemical Quality Control
Determination of the purity of polymer-grade products such as ethylene and propylene
Identification and quantification of impurities
Process control in reforming and isomerization units
ASTM Standards and GC-Related Methods
| Standard | Title | Application |
|---|---|---|
| ASTM D1945 | Natural gas analysis by GC | Pipeline gas composition |
| ASTM D2163 | Light hydrocarbons by GC | LPG, propane, butane |
| ASTM D2887 | Boiling-point distribution by SimDis | Diesel fuel, kerosene, fuel oil |
| ASTM D3606 | Benzene and toluene in gasoline | Environmental compliance control |
| ASTM D4815 | Oxygenates in gasoline | MTBE, ethanol |
| ASTM D5134 | Detailed gasoline analysis | PONA by capillary GC |
| ASTM D6729 | Single-column gasoline composition | 100+ compounds in one analysis |
| ASTM D7169 | High-temperature SimDis | Crude oil, waxes, heavy fractions |
| UOP603 | CO and CO₂ in gas | Reforming units |
GC vs. HPLC — Which Method Should You Choose?
| Criterion | GC | HPLC |
|---|---|---|
| Sample type | Volatile or vaporizable compounds | Volatile and non-volatile compounds |
| Mobile phase | Inert gas | Liquid solvent |
| Temperature range | Up to 450°C | Ambient temperature |
| Sensitivity for hydrocarbons | Very high | Moderate |
| Suitable for | Fuels, gases, volatile compounds | Pharmaceuticals, proteins, sugars |
| Analysis time | 5 to 90 minutes | 10 to 120 minutes |
Simple rule: If the sample can be vaporized without decomposition → GC. If not → HPLC.
Common Errors in GC Analysis and Their Solutions
1. Injection System Contamination
Symptom: Interfering peaks at the beginning of the chromatogram and peak tailing
Cause: Deposition of non-volatile materials in the liner and septum
Solution: Replace the septum every 50 to 100 injections and replace the liner every 200 injections
2. Gas Leak
Symptom: Change in retention time, reduced sensitivity, and increased baseline noise
Cause: Loose column connections at the injector or detector
Solution: Regular leak checking using a soap solution or an electronic leak detector
3. Improper Temperature Program
Symptom: Overlapping peaks or excessively long analysis time
Cause: Too high a temperature causes peaks to merge; too low a temperature makes the analysis slow
Solution: Optimize the initial temperature, heating rate, and final temperature
4. Reduced Column Lifetime
Symptom: Gradual loss of efficiency, peak tailing, and increased column bleed
Cause: Repeated injection of non-volatile materials or operating the column above its maximum allowable temperature
Solution: Trim 30 to 50 cm from the front end of the column and perform regular conditioning
5. Poor Repeatability
Symptom: Different results for identical samples
Cause: Poor calibration, carrier gas pressure fluctuation, or unstable oven temperature
Solution: Perform daily checks using a check standard and verify carrier gas pressure and flow rate
Service and Maintenance of GC Instruments
Daily Maintenance:
Check carrier gas pressure and flow rate
Monitor oven, injector, and detector temperatures
Run a check standard and record the result
Monthly Maintenance:
Replace the injector septum
Check connections and leaks
Clean the FID detector, including inspection of the jet and collector
Six-Month Maintenance:
Replace the injector liner
Condition the column
Perform full calibration using reference standards
Annual Maintenance:
Evaluate column efficiency using the HETP test
Perform full detector service
Replace carrier gas filters
Frequently Asked Questions
Can GC analyze non-volatile compounds?
Not directly. However, through chemical derivatization, which converts compounds into volatile derivatives, some non-volatile compounds such as fatty acids, sugars, and amino acids can also be analyzed by GC.
What is the difference between GC and GC-MS?
GC separates compounds, while MS identifies each compound based on its molecular mass. GC-MS is an unmatched tool for identifying unknown compounds. GC alone can only identify a compound by comparing its retention time with that of a standard.
How long does a GC analysis take?
A GC analysis can take anywhere from 5 minutes for simple gases such as CO and CO₂ to 90 minutes for detailed gasoline analysis involving more than 100 compounds. Most refinery GC analyses take between 15 and 45 minutes.
Helium or nitrogen — which carrier gas is better?
Helium provides better separation efficiency and is required for GC-MS. Nitrogen is less expensive and more readily available, but it has lower efficiency at high flow rates. Hydrogen is the fastest carrier gas, but it requires additional safety considerations.
Is GC suitable for measuring water?
Yes. With a TCD detector and a suitable column, such as Porapak Q or molecular sieve, water can be measured by GC. However, for accurate moisture determination in petroleum products, the Karl Fischer method, such as ASTM E1064, is more accurate and more widely accepted as a reference method.
What is the difference between a packed column and a capillary column?
Packed columns are still used for simple permanent gases such as O₂, N₂, CO, CO₂, and CH₄. Capillary columns provide separation efficiency that is hundreds of times higher and are the standard choice for complex analyses.
When should a GC column be replaced?
A GC column should be replaced when its efficiency, measured by the number of theoretical plates, drops below 50% of its original value, or when peaks become tailing and broad and do not improve after trimming the front end of the column.