All GCs, 6890, 6850, 7890, 7820, 8890 and 8860
This article is a troubleshooting guide for when the Flame Ionization Detector (FID) does not ignite, or the flame keeps going out.
If the FID is having ignition issues, it is due to the following causes:
- FID temperature: is too low.
- Lit Offset: is incorrect
- FID gas flows: if the air, hydrogen, or makeup gas flows are not correct (or not meeting setpoints).
- Gas quality: if the supplied air has a low proportion of oxygen, or gases from the generator includes significant levels of contaminants, such as water.
- The ignitor: it has corroded, is broken and/or no longer glowing.
- The jet is plugged: or the column is not installed correctly in the FID jet.
- A leak: the jet is not screwed in tight enough, the ferrule or column nut is leaking, the adaptable fitting is leaking (if present) or the collector base is leaking.
- Electronics: the collector is grounding (not insulated properly).
- Column flow: is too high, or abrupt change in column flow (e.g. valve systems).
We will cover how to troubleshoot each one of these potential causes below.
Before troubleshooting, first check if any recent maintenance has been performed on the FID, or if anything else has recently changed on the system or in the method parameters. Press the "status" button on the GC keypad to gather any more information about faults. If the FID is in error state ("front/back detector failed to ignite"), you will need to turn the flame OFF before continuing.
Ensure that the FID temperature is high enough for ignition (>150 °C), the GC will not attempt to ignite the flame at a temperature <150 °C. You should set the FID temperature to 20 °C above the highest oven temperature, depending on the column type. Alternatively, you can use the Agilent recommended value of >300 °C, which provides a good starting point for easier ignition, and minimizes water condensation.
The default lit offset for an FID is 2.0 pA. If the FID signal drops below this value, the GC thinks the flame has gone out and will attempt re-ignition. In some cases (e.g. the gases are extremely clean) the default offset value of 2.0 pA is too high, and the FID will struggle to stay lit. If you suspect this is the case, lower the lit offset and see if it helps. For 8890/8860 GCs, you can find the lit offset setting in Settings>Configuration>Detectors in the browser interface. For 7890 GCs, please press [Config][Front Det] or [Config][Back Det]> lit offset on the GC front keypad.
Check that the gas flow setpoints are sensible, and that the actual flow in the GC can meet the setpoint.
- Make sure that the final hydrogen-to-air ratio is between 8-12%, The default gas flows for an FID are:
- 30 ml/min for hydrogen/fuel
- 25 ml/min for makeup (check that the correct makeup gas is configured)
- 400 ml/min for air/oxidizer
- If required, these flow rates can be adjusted (in some cases, you need a higher hydrogen flow to get ignition).
- For checking the actual gas flows, turn all gas flows OFF and then turn ON each gas individually. The actual flow should meet the setpoint easily. If it does not, check the gas supply (it should be able to provide 80 psi supply pressure to the back of the GC. If the distance of the gas supply is far from your GC, you should install a pressure regulator at the back of GC to regulate the supply pressure. If the supply pressure is ok, it may be a faulty Electronic Pressure Control (EPC) that needs replacing. In this case, please contact your local Agilent service representative.
Synthetic air which has a low proportion of oxygen will lead to igniting failures. Agilent recommends using good quality compressed air (Air for flame detectors should be zero grade). All other gases used in the FID also need to be free from contaminants and moisture. Agilent recommends a carrier and detector gas purity of 99.9995% or better, and also recommends using traps to remove hydrocarbons, water, and oxygen.
If the ignitor has become corroded, it will no longer glow (or not glow as brightly) when trying to ignite the FID.
To check the ignitor:
Keep body parts at a safe distance from the FID chimney while performing this task. If using hydrogen, the FID flame will not be visible.
- Remove the detector top cover.
- Turn the FID flame On.
- Observe the ignitor plug through the FID chimney. The small hole should glow during the ignition sequence(see Figure 1).
If the ignitor is not glowing, replace it. In some cases, the ignitor will still glow weakly, but this damaged or aged ignitor will still cause ignition failures. In this case, replace the FID ignitor. To identify the correct part number for your ignitor, please see the relevant GC model user manual or the Agilent GC Parts Finder application on your GC Workstation PC.
The jet is plugged (or the column is not installed correctly)
If the FID jet is becoming plugged(either fully or partially), actual H2, Makeup, and Capillary carrier flows will be lower than the values indicated by the GC.
Do the "Jet Restriction Test" to check for a plugged FID jet:
- Turn OFF all the gas flows (hydrogen, makeup, and air).
- Turn on the hydrogen flow and set it to 75 ml/min (increase the H2 supply pressure as needed to achieve this flow rate setting).
- Monitor the makeup flow “Actual” reading. A makeup flow of more than 1.0 mL/min indicates that the jet is plugged or partially plugged (see figure 2).
Please turn the detector OFF and let it cool to room temperature before performing the following steps. There is a high risk of burns when touching hot surfaces if the FID is not allowed to cool fully before checking the column installation and removing the FID jet.
Use clean, lint-free gloves to handle detector parts to avoid contamination.
- A column installed too deep into the FID (i.e. not withdrawn 1~2 mm before tightening the column nut) will "block" the jet and cause ignition problems. In you think this may be the cause of the ignition issue please follow the following steps:
1. Turn the detector OFF and wait for it too cool to room temperature.
2. Trim the detector end of the column below the old ferrule to remove this ferrule and the part of the column previously inserted in the detector.
3. Place the appropriate column nut then a new ferrule on the column and carefully trim 1-2 mm off the end of the column again, leaving a clean square end to the column.
4. Reinstall the column correctly by inserting the column into the detector until it bottoms, then withdrawing 1~2 mm before tightening the detector column nut.
- If the jet is plugged by contamination. To check this, remove the jet from the housing and hold it up to a light source, check the holes in the jet for contamination), you should clean it, or for long term detector reliability, replace the jet with a suitable replacement. For a full list and details of FID supplies and accessories please click here.
If any of the detector supply gases are leaking out, it will prevent the FID igniting. Leaks can occur where the jet is screwed in, around the ferrule/column nut, the adaptable fitting (if present), or the collector base. If the FID has just had maintenance one of these areas is a likely place for leaks.
The structure of the FID assembly is shown below for reference:
Be careful! The oven and/or detector may be hot enough to cause burns. If the detector is hot, wear gloves to protect your hands or cool the oven and detector before performing the following checks.
To avoid contaminating the FID, wear clean, lint-free gloves when handling the collector assembly.
- The jet: make sure that the jet has been tightened into the FID base correctly by tightening the jet 1/6-turn past finger-tight (1/6-turn is one “flat” on a typical screwdriver handle or on the jet head).
- Ferrule/column nut: check the nut and ferrule have been tightened sufficiently (tighten the nut an additional 1/4 turn past finger-tight with a wrench). A gas leak detector can be used here to check for any leaks.
- Adaptable fittings: check this has been tightened sufficiently. A gas leak detector can be used here.
- Collector assembly: there is a quick test to see if the gasket under the collector is leaking (this test will also pick up a jet not screwed in tight enough)
- Cool the FID
- Set airflow to 400 ml/min
- Hold a septum tightly on the top of the FID (see Figure 4); you should feel the pressure building. If the hydrogen/makeup flows increase then there is no leak, if they do not increase there is a leak (and the gasket will need to be replaced).
If the collector is grounding (not insulated properly), then it will prevent the FID from igniting properly, it usually happens after maintenance on the FID. This will result in an extremely high output value of eight figures (e.g. 84597562) even when the flame is off. To fix this, check that the collector spring is seated in the collector body correctly and that both collector insulators are installed and in good condition.
If the column flow is very high, it can make the FID hard to ignite. Lower the column flow and then try to ignite the FID again. There have been cases on valve GCs where valve changes close to an FID will cause the FID flame to "blow out". If you suspect either of these issues, you can use a blanking nut or no-hole ferrule to blank off/isolate the FID from the rest of the system and see if the FID will ignite.
Hydrogen is a combustible gas, so do not run high hydrogen flows for extended periods with the FID flame off