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The purpose of this article is to provide a stepwise, expert guide to diagnose and fix flame ignition failures on GC flame ionization detectors with safe, reproducible methods.
Understand How FID Ignition Works
The FID burns a premixed stream of hydrogen and oxidant at the jet to form ions that generate a small current between the collector and the jet held at high potential relative to ground. Ignition succeeds when the correct gas composition, flow, jet geometry, and ignitor energy align. Any leak, blockage, or wrong setpoint prevents stable combustion or the controller aborts ignition.
Safety First
Caution: Hydrogen accumulates invisibly. Vent the GC enclosure well, stop flows before disassembly, and leak-test with a non-reactive leak detector. No open flames near the instrument.
Caution: Disable automatic ignition attempts during troubleshooting to avoid unplanned sparks while lines are open.
Rapid Checklist Before Deep Work
- Confirm correct gases and purity. Hydrogen and zero air must meet instrument specifications. Makeup gas must match method requirements.
- Verify cylinder pressures, two-stage regulator delivery pressures, and that valves are fully open.
- Check for leaks at the FID base, jet, and gas fittings. Repair any leak before retrying ignition.
- Inspect the FID jet for polymerized deposits or salt crystals. Clean or replace if dirty.
- Verify correct flows and ratios for hydrogen, air, and makeup. Do not exceed the detector flow limits.
- Confirm ignitor function. Ensure the glow plug heats or the spark electrode arcs consistently.
- Confirm oven, detector, and pneumatic modules are at setpoint and not in fault state.
Typical FID Setpoints That Ignite Reliably
| Parameter | Capillary Column FID | Packed Column FID | Notes |
|---|---|---|---|
| Hydrogen (H₂) | 30–40 mL/min | 250–300 mL/min | Lower H₂ reduces flame speed. Too high causes blow-off. |
| Air (oxidant) | 350–450 mL/min | 300–400 mL/min | Use zero air. Avoid oil contamination. |
| Makeup gas | 20–40 mL/min | N/A | N₂ or He common. CO₂ is not suitable. |
| Detector temp | 250–320 °C | 250–320 °C | High enough to avoid condensation. |
| Jet type | Capillary jet, correct part no. | Packed jet | Wrong jet prevents ignition and stability. |
| Ignition attempts | 2–3 per trial | 2–3 per trial | Investigate if all attempts fail. |
Caution: Always use the jet type that matches column format. A packed-jet on a capillary method will flood the flame or stall ignition.
Stepwise Troubleshooting Workflow
1. Verify Gas Identity and Purity.
Use certified hydrogen, zero air, and high-purity makeup gas with hydrocarbon and moisture filters. Replace exhausted traps. If the zero air generator is in use, check catalyst temperature and dew point performance.
2. Set Flows to a Known-Good Baseline.
Set H₂ 35 mL/min, air 400 mL/min, makeup 30 mL/min for capillary methods. Allow flows to stabilize for two minutes before ignition. Record actual measured flows using a bubble meter or calibrated flowmeter at the detector exhaust to confirm controller accuracy.
3. Inspect and Clean the Jet.
Cool the detector. Remove the jet with the correct wrench. Soak in solvent that dissolves residues used in your method. Sonicate if allowed. Dry fully, then reinstall to the specified torque. Replace the jet if orifice erosion or blockage is visible.
4. Check Ignitor Health.
- Glow plug types must heat to visible dull red within a few seconds. Replace if cold or intermittent.
- Spark electrode types must show a strong blue arc at the tip. Set gap per the service manual. Clean carbon tracks on the insulator.
- Inspect ignitor wiring and connectors for heat damage.
5. Confirm Detector Body Temperature.
Ignition fails at low detector temperatures due to condensation and quenching. Verify the detector is at the target temperature and that the sensor reads correctly. Replace faulty RTDs or thermocouples as needed.
6. Eliminate Leaks and Backpressure Errors.
Perform a pressure decay or electronic leak check on H₂ and air lines. Tighten or re-ferrule fittings that leak. Check FID vent for obstructions that elevate backpressure and disrupt mixing at the jet.
7. Re-Ignite With a Controlled Sequence.
# Standard FID ignition sequence 1. Set flows to baseline: H2 35 mL/min, Air 400 mL/min, Makeup 30 mL/min. 2. Stabilize flows for 120 s. 3. Trigger ignition. Observe flame status. 4. If failure, stop H2 for 30 s to purge with air, then retry. 5. Limit to 3 consecutive attempts. Investigate before further tries.Symptom-to-Cause Matrix
| Symptom | Likely Cause | High-Leverage Fix |
|---|---|---|
| Ignites then dies within 1–3 s. | Wrong H₂:air ratio or draft at the jet. | Set H₂ 35 mL/min and air 400 mL/min. Shield from drafts. Verify makeup gas flow. |
| No ignition spark or heat. | Failed ignitor or wiring fault. | Replace glow plug or adjust spark gap. Inspect connectors. |
| Multiple failed tries with sooty exhaust. | Jet contaminated or mis-sized. | Clean or replace jet with the correct part number. |
| Ignites only at high H₂ then blows off. | Backpressure or vent restriction. | Clear detector vent. Verify exhaust line is short and free-draining. |
| Baseline current near zero after “lit”. | Flame not actually lit or collector fouled. | Confirm flame with water drop test at exhaust. Clean collector and insulator. |
| Intermittent flameouts during runs. | Carrier or makeup flow instability. | Stabilize EPC modules. Replace leaking septum or liners. Service carrier gas regulator. |
Proven Repairs and Preventive Actions
- Replace the FID jet and ignitor annually in high-throughput labs. Keep one full spare kit per instrument.
- Standardize a single ignition baseline in methods to remove guesswork during service calls.
- Mount moisture and hydrocarbon traps as close to the GC as possible. Log pressure drops and change at defined delta-P.
- Keep the detector at or above 300 °C for dirty matrices to minimize fouling.
- Inspect the FID collector and insulator during liner changes. Clean with solvent and lint-free swabs.
- Verify EPC accuracy quarterly with an external flowmeter. Adjust or service if deviation exceeds specification.
Simple Diagnostics You Can Run
# Flow verification at FID exhaust 1. Disconnect exhaust line. 2. Place bubble meter at the detector exhaust outlet. 3. Measure each gas stream individually if the instrument allows test modes. 4. Compare measured flows to setpoints. Adjust EPC or regulators accordingly.
Water drop qualitative flame check
With flame reported "on", allow 60 s stabilization.
Hold a glass capillary with a 1–2 mm water droplet near the exhaust.
Droplet should flicker or evaporate rapidly if the flame is lit.Acceptance Criteria After Fix
- Ignition succeeds on the first attempt at baseline flows.
- Detector current rises to the instrument’s normal lit value within 10–20 s and stays stable.
- FID background noise and drift meet method control limits over a 30 min hold.
- Response factor on a standard matches historical control charts within predefined limits.
FAQ
Can I ignite with higher hydrogen to force the flame?
A slight increase can help cold starts, but too much hydrogen causes blow-off or rich quenching. Return to baseline as soon as the flame is lit.
Do I need makeup gas for capillary FID?
Yes for most systems. Makeup gas optimizes linear velocity and jet residence time. Without it ignition and stability degrade.
How often should I replace the jet and ignitor?
Replace annually under routine use or sooner if ignition becomes unreliable, deposits are visible, or baseline noise increases after cleaning.
Why does ignition fail after column maintenance?
Jet contamination during handling, improper jet type, disturbed fittings, and new leaks are typical. Re-verify flows, leaks, and jet identity after maintenance.
What detector temperature is best for dirty samples?
Operate near 300–320 °C to prevent condensation and polymerization on the jet and collector. Lower temperatures increase fouling and ignition failures.