Gas Chromatograph (GC) – Fundamentals & Field Perspective

Category: Analyzer · Gas Chromatograph · Fundamentals · Online Process GC

What a Gas Chromatograph Actually Does

A Gas Chromatograph (GC) separates and measures individual components in a gas mixture. Unlike optical analyzers, GC physically separates components before detection.

In refineries, petrochemical plants, LNG units, and gas processing facilities, GCs are primarily used for:

• Online composition measurement
• Calorific value calculation
• Product quality control
• Custody transfer
• Safety and process optimization

If composition matters for control or billing, GC is usually involved.

Working Principle – Separation by Retention Time

GC works on the principle of separation by retention time. Each component travels through the column at a different speed based on volatility and interaction with the stationary phase.

1. Sample is injected through a valve and fixed-volume loop
2. Carrier gas pushes sample into the column
3. Components interact with column packing/coating
4. Lighter or less interactive gases exit earlier
5. Heavier or more interactive gases exit later
6. Detector converts each component into a peak

Retention time is the fingerprint of each component.

GC Flow Diagram

Understanding the Chromatogram

Each peak represents one component. Peak height or area corresponds to concentration.

Main GC Components (Field View)

1. Carrier Gas System

• Must be high purity (99.999% typical)
• Moisture contamination causes baseline noise
• Pressure regulator stability affects retention time
• Hydrogen requires strict safety control

2. Sample Injection System

• Fixed loop ensures repeatable volume
• Valve leakage shifts peak areas
• Pressure imbalance causes retention drift
• Condensation creates distorted peaks

3. Column & Oven

• Column type defines separation selectivity
• Oven temperature affects retention time
• Temperature overshoot shortens column life
• Aging columns reduce peak resolution

4. Detectors

• FID – Hydrocarbons, high sensitivity
• TCD – Permanent gases (H₂, O₂, N₂, CO)
• ECD – Trace electronegative compounds

Incorrect detector selection leads to poor measurement reliability.

Calibration & Validation

• Certified calibration gas required
• Response factors must be periodically verified
• Retention time window must be monitored
• Validation more frequent than recalibration

Calibration failures often indicate sampling or carrier gas issues.

Common GC Field Problems

• Baseline drift (carrier gas contamination)
• Retention time shift (pressure/temperature change)
• Poor separation (column aging)
• Negative peaks (condensation or valve leakage)
• Detector noise (grounding or unstable power)

Logical Troubleshooting Sequence

1. Verify carrier gas purity and pressure
2. Leak test injection valves and fittings
3. Check oven and detector temperatures
4. Review chromatogram history, not just alarm
5. Confirm calibration gas composition

In most plants, sampling and carrier gas issues are responsible for GC instability.

Next GC Technical Pages

• GC Sampling System – Deep Dive
• GC Detector Comparison – FID vs TCD vs ECD
• GC Calibration & Response Factor Control
• GC Retention Time Shift Troubleshooting