Gas Chromatography – Fundamentals & Troubleshooting
This engineering deep dive covers chromatographic theory, system suitability, performance calculations, detector diagnostics and structured troubleshooting logic.
1. Retention & Capacity Factor
Capacity factor (k'):
k' = (tR − t0) / t0
Where:
tR = retention time
t0 = dead time
Ideal k' range: 1 – 10
2. Theoretical Plates (Column Efficiency)
N = 16 (tR / W)^2
Where:
W = peak width at baseline
Higher N = better column efficiency.
Worked Example – Theoretical Plates
tR = 5.0 min W = 0.2 min
N = 16 (5 / 0.2)^2 N = 16 (25)^2 N = 16 × 625 N = 10,000 plates
3. Resolution Between Two Peaks
Rs = 2 (tR2 − tR1) / (W1 + W2)
Acceptable separation: Rs ≥ 1.5
Worked Example – Resolution
tR1 = 4.0 min tR2 = 4.5 min W1 = 0.2 W2 = 0.2
Rs = 2(0.5) / (0.4) Rs = 2.5 → acceptable
Detector Diagnostics
FID
- No flame → Gas supply issue
- Noise → Contaminated jet
- Low sensitivity → Incorrect air/H₂ ratio
TCD
- Drift → Filament aging
- No response → Bridge imbalance
- Noise → Carrier gas purity
System Suitability Parameters
- Retention time repeatability
- %RSD of peak area
- Resolution (Rs)
- Tailing factor
- Theoretical plates (N)
%RSD = (SD / Mean) × 100
GC Troubleshooting Logic Flow
- Ghost peaks → Septum bleed
- Peak tailing → Column contamination
- Retention shift → Oven temperature drift
- High baseline → Carrier gas leak
- Low response → Detector sensitivity drop
ISO 17025 Technical Controls
- Method validation documentation
- Calibration traceability
- Uncertainty estimation
- Preventive maintenance logs
- Performance qualification records