Process and Ambient Air Gas Chromatography

How does gas chromatography work?

Gas chromatography (GC) is a method that first physically separates the components of a mixture and then measures them individually. The sample is fed into a carrier gas (for example helium, nitrogen or hydrogen) through an injection valve and passes through columns. The column packing retains components at different rates according to their chemical properties, so each component leaves the column at a different time.

A detector at the column outlet senses each component, and the signal is recorded over time as a peak. The peak's exit time identifies the component, and the peak area indicates its concentration. In process GCs, sample injection is automatic, at set intervals and with multi-stream sample selection.

Detectors

The detector is selected according to the component to be measured:

• FID: sensitive for hydrocarbons
• PID: suitable for aromatics and toxic components (benzene, butadiene, vinyl chloride)
• TCD: general purpose for permanent gases and main components
• FPD: selective for sulfur compounds
• ECD: sensitive to halogenated compounds

What gas chromatography measures and where it is used

Gas chromatography is used where multi-component analysis is required:

• Composition monitoring and sulfur compound analysis in refinery and petrochemical process streams
• Natural gas composition and quality control
• BTEX, benzene, VOC and methane/NMHC monitoring in ambient air
• Tracking toxic vapors at industrial boundaries and in neighboring areas

Where gas chromatography works well and where it struggles

Because GC can separate closely related components and measure them individually, it is selective and gives multi-component information. It can be adapted to different applications by changing the detector and column.

Limitations: measurement is not continuous and gives results at intervals (hourly or by the minute) tied to the analysis cycle. Carrier and auxiliary gas supply are required. Periodic calibration and maintenance are essential for column and detector stability. Sample preparation quality directly affects the result. For BTEX monitoring in ambient air, the automated pumped sampling and GC method described in EN 14662-3 is used.

Choosing an instrument for gas chromatography

When selecting, the target components, measurement cycle time, detector type, column configuration and hazardous area requirements are considered together. The supply of carrier and calibration gases and the design of the sample preparation system directly affect operating cost and measurement quality.

Tetra and gas chromatographs

Tetra supplies, installs, commissions and services process and ambient air gas chromatographs in Türkiye. The product range includes the Yokogawa GC8000 process gas chromatograph and the Synspec GC955 ambient air BTEX/VOC/benzene gas chromatograph. The Thermo Scientific 55i analyzer also combines a back-flushed GC column with an FID. Tetra also carries out sample preparation and ATEX analyzer cabinet integration for GC systems.