🏭 Process Input — Sample Composition
SourcePipeline / Process Stream
Carrier GasHelium (He)
Inlet Pressure2.8 bar
Component Composition (%)
CH₄75%
C₂H₆12%
C₃H₈6%
CO₂4%
N₂3%
Total100%
🚨 Performance & Alarms
✅Separation Quality: Good
✅Column Temperature: Normal
✅Carrier Gas Flow: Normal
✅Sample Load: Normal
| Oven Status | HEATING |
| Column Pressure | 2.8 bar |
| Detector | TCD — READY |
| Run Status | STANDBY |
| Inject Count | 0 |
| Run Time | — |
⚙️ GC System — Process Flow Diagram
📈 Real-Time Chromatogram OutputRun Time: 0.0 min
🧠 Learning & Reference Panel
GC WORKING PRINCIPLE — STEP BY STEP
1
Sample Introduction: A precise volume of gas sample (typically 0.25–1 mL) is introduced via a 6-port switching valve. The valve switches from LOAD to INJECT position, pushing the sample slug into the carrier gas stream.
2
Carrier Gas Transport: An inert gas (Helium or Nitrogen) acts as the mobile phase. It carries sample molecules from the injection point through the entire column at controlled pressure and flow rate.
3
Column Separation: Inside the heated oven, molecules interact with the stationary phase coating. Lighter, less polar molecules (CH₄, N₂) interact weakly and exit first. Heavier molecules (C₃H₈) interact more strongly and exit later.
4
Thermal Effect: Column temperature controls interaction strength. Higher temp = shorter retention times but reduced resolution. Temperature programming can optimize both speed and separation quality.
5
Detection: The TCD (Thermal Conductivity Detector) measures changes in electrical resistance of a heated filament as each component passes through. FID is used for hydrocarbons. Each component produces a unique electrical signal peak.
6
Chromatogram Output: The data system plots signal intensity vs. time. Each peak represents one component. Peak retention time = component identity. Peak area = concentration (after calibration).
7
Quantification: Peak areas are compared to known calibration standards. Results are expressed as mol%, vol%, or mass%. Calibration is performed with certified reference gas mixtures (CRGM).
Carrier GasStationary PhaseMobile PhaseRetention TimeResolutionPeak AreaSelectivityEfficiencyvan Deemter
Retention Time (tR): Time from injection to peak maximum. Unique fingerprint for each compound under fixed conditions. Used for qualitative identification.
Resolution (R): Degree of peak separation. R = (tR2−tR1)/(0.5×(w1+w2)). R > 1.5 = baseline separation. R < 1.0 = peaks overlap.
Stationary Phase: Polydimethylsiloxane (DB-1, HP-1) coating on column wall. Interacts with sample via van der Waals and dipole forces. Phase polarity determines selectivity.
Peak Area: Proportional to component concentration. Must be calibrated with CRGM. Area% method common for natural gas analysis.
Column Efficiency (N): Number of theoretical plates. N = 5.54×(tR/w½)². Higher N = narrower peaks = better separation. Typical capillary: 20,000–100,000 plates.
van Deemter Equation: H = A + B/u + Cu. Describes height equivalent of theoretical plate vs. carrier gas velocity. Optimal flow rate minimizes H and maximizes N.
Partition Coefficient (K): K = Cs/Cm (concentration in stationary/mobile phase). High K = long retention. Determines relative order of elution for each component.
⚙️ INSTALLATION & COMMISSIONING
- Verify all connections for leak tightness using appropriate leak detection fluid or He detector
- Condition new column at 10–15°C below max temp for ≥30 min before first use
- Flush system with carrier gas for minimum 30 minutes before introducing sample
- Calibrate with certified reference gas mixture (CRGM) traceable to NPL/NIST
- Verify detector response linearity across full composition range expected
📐 OPERATING BEST PRACTICES
- Maintain carrier gas purity ≥99.999% (5.0 grade) for TCD applications
- Keep column head pressure stable — fluctuations cause retention time drift
- Perform daily calibration check (span and zero) per ISA-37.1 requirements
- Use isothermal temperature for consistent retention time reproducibility
- Avoid sample overloading — inject minimum volume required for detection
- Monitor baseline noise and drift — abnormal drift indicates column contamination
🔧 MAINTENANCE
- Replace sample valve seals/rotor every 6–12 months depending on cycle count
- Clean or replace sample loop if contamination or carryover is detected
- Check carrier gas cylinder pressure daily — maintain adequate supply
- Inspect and clean detector filaments per manufacturer schedule
- Record all maintenance activities in GC logbook for audit traceability
- Perform column aging (baking) periodically to remove high-boiling contaminants
⚠️ TROUBLESHOOTING
- No peaks: Check carrier gas supply, valve position, detector power
- Peak overlap: Reduce column temp, reduce flow rate, or use longer column
- Retention time drift: Check column pressure, temperature stability, carrier gas purity
- Tailing peaks: Active sites on column — bake out column or replace
- Negative peaks (TCD): Component thermal conductivity similar to carrier gas
- Split peaks: Two-phase injection, condensation in sample system
📋 STANDARDS & CODES
- ASTM D1945 — Natural gas analysis by GC
- ASTM D1946 — Refinery gas analysis
- ISO 6974 — Natural gas composition determination
- GPA 2261 — LNG/NGL composition analysis
- ISA-37.1 — Instrument performance standard
🎯 QUALITY ASSURANCE
- Replicate injections (n≥3) required for statistical confidence in results
- Acceptable RSD (relative standard deviation) ≤0.5% for custody transfer GC
- Use two-point calibration (zero + span) for linear detector response verification
- Archive all calibration records and chromatograms per site data retention policy
- Participate in inter-laboratory comparison (round-robin) programs annually
- Validate any method changes against certified reference material
⛽ OIL & GAS
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Natural Gas Metering: Calorific value, Wobbe Index, and compressibility factor calculation for custody transfer per ISO 6976. GC feeds real-time composition data to flow computers.
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Offshore Platform: Wellhead gas composition for reservoir management, flare gas emissions reporting, produced water hydrocarbon content monitoring.
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Pipeline: Gas quality monitoring for tariff compliance, odorization verification (THT/mercaptans), interoperability checks at interconnection points.
🧊 LNG & REFINERY
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LNG Terminal: Methane number calculation for marine fuel, boil-off gas composition monitoring, regasification product quality verification per GIIGNL standards.
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Refinery: Reformate composition (BTX), FCC off-gas analysis, C3/C4 splitter control, product specification verification, benzene/toluene/xylene purity.
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Crude Distillation: Naphtha true boiling point (TBP) analysis, light-ends recovery optimization, crude oil characterization by simulated distillation (SimDist).
🏭 PETROCHEMICAL
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Polymer Feed: Ethylene purity ≥99.9% for polymerization feed, propylene purity verification, vinyl chloride monomer (VCM) analysis, trace impurities.
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BTX Plants: Benzene, toluene, xylene purity monitoring for aromatic extraction units. Sulfolane solvent quality tracking.
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Emissions Monitoring: VOC analysis, greenhouse gas composition (CO₂, CH₄, N₂O), continuous emissions monitoring system (CEMS) per EPA Method 18.