Q3 2009 / FID Maintenance – Key tips for the in-house detector doctor

Seal the detector inlet in the oven with a detector plug and ignite the detector. If the chromatogram noise disappears, then the source of the problem is contaminants in the carrier gas or bleed from the chromatography column, not a dirty FID.

To prevent this problem, you should always use a carrier gas purification system and columns prepared from chromatography quality stationary phases.

Second, check for gas-related noise sources 

A problem in the hydrogen or compressed air delivery system can be a source of noise. Measure the flow rates for both gases. As a general rule the air flow should be nearly ten times the hydrogen flow. However, check your instrument’s operators manual for the exact flow requirements of your model. An incorrect flow rate in either gas can cause noise, lack of sensitivity, and/or difficulty when igniting the flame. 

Also, check the connections on both systems. An electronic gas leak detector will quickly and easily pinpoint gas leaks via differences in thermal conductivity. If you do not find a major leak, check the gas pressure at the cylinders. If the pressure is less than 500 psig, replace the cylinder.

Figure 1. Silica, which appears as a white powder deposited on surfaces within the detector, causes noisy chromatograms, random spikes, and poor detector sensitivity. Use of a brass brush will not scratch metal or ceramic parts.

A contaminated gas cylinder could be the source of the problem, especially if the noise appeared several hours after a cylinder change. Check each cylinder for contaminants and replace if necessary.

Eliminating electrical noise

Electrical noise is another potential problem. It can cause symptoms similar to a dirty FID. A poor connection due to oxidized contacts can act as a small capacitor causing spikes and/or loss of sensitivity. If the FID has clip contacts to the collector or flame jet, clean them periodically with an emery cloth. Other electrical devices located near the chromatograph can also interfere with the instrument’s power supply and cause intermittent or random spikes.

To isolate this source of noise, disconnect the electrometer cable(s) from the FID. If noise persists, it is coming from the electrical system. In a two-detector system, electrical noise will appear on both detectors. If noise is present on only one channel, the electrometer or electrometer cable(s) are the problem.

Switch the cables at the electrometer and FID. If the noise now appears on the channel that was previously noise-free, replace the cable supplying the noise. If the noise does not change channels, check the electrometer on the malfunctioning channel by removing the electrometer cable at the FID. Do this by resting the cable connector on a surface to prevent contact with the metal frame of the GC. If the noise continues, the electrometer should be replaced. If the noise is eliminated, re-check the hydrogen and air systems.

Cleaning an FID

Properly cleaning an FID requires cleaning the collector assembly, the jets, the Teflon insulators, and the housing. If the detector has been cleaned recently, a simple light coat of silica or even a single silica flake could cause the trouble, and only a light cleaning is required. If the detector has not been cleaned recently, a thorough cleaning is necessary.

How to perform a light cleaning

Disassemble the detector and scrub the contamination from the components. The brass wire brushes will not scratch metal or ceramic parts. Use a nylon brush on Teflon parts and follow these steps.

1. Disconnect the power to the GC, and be sure the collector assembly is cool.

2. Remove the collector assembly and brush the collector to remove the deposits.

3. Replace the jet.

4. Clean the electrical contacts, using a fine emery cloth. (Be careful not to bend the contacts.)

How to perform a thorough cleaning

If a light cleaning does not succeed, the thorough cleaning method is required. To accomplish this start by unplugging the power cord from the electrical outlet, allow the detector to cool, disassemble it, and follow these steps.

1. Fill the basin in an ultrasonic cleaning device with a detergent that will effectively remove silica and other contamination from the FID (e.g., 10:1 water to detergent ratio will clean soiled parts). Immerse the FID parts, except the electrical contacts, and sonicate for two hours. You can brush the collector assembly and jets (with nylon or brass brushes) with the cleaning solution during the ultrasonic treatment. After the treatment, rinse the parts with distilled water to remove the detergent, and dry parts thoroughly. Use a fine emery cloth to clean the electrical contacts.

2. After you have cleaned all parts of the detector, check all O-rings and replace them if necessary. Worn-out O-rings will cause gas leaks, which can produce detector noise or an increase in detector contamination. Reassemble the FID, light the flame, and allow the detector temperature to equilibrate at 10°C – 50°C higher than the column will reach during typical operation. This will reduce the amount of phase condensing onto the detector parts. 

Do not exceed the maximum temperature limit of the stationary phase. Many columns fit far enough into the detector to expose the phase to these elevated temperatures. Set the proper flow rates for hydrogen and air (Figure 2), and ignite the flame. Turn on the electrometer and allow a few minutes for warm-up. The flame should now be stable and noise-free.

Figure 2. Manually checking flow rates can be useful to verify accuracy, even with electronic pressure or flow control systems.

Reducing detector noise and contamination

Most detector noise and contamination is the result of column bleed. The amount of bleed is greatest when the column is initially conditioned. Your detector will remain clean longer if you condition a new column before connecting it to the detector. By-products eluted during conditioning, potentially harmful to the FID, are voided into the oven.

Connect the column inlet to the injector as usual and set the optimal column flows. Purge the column with carrier gas at an ambient temperature for an hour before beginning the temperature program. Do not allow a combustible carrier gas such as hydrogen, and methane to exit the column into the oven.

Consult the column manufacturer for conditioning details, i.e., duration and temperature of conditioning. Do not routinely condition new columns at the maximum temperature limit of the stationary phase – this will reduce column life. Connecting a well-conditioned column to a clean FID should produce good sensitivity. If detector stability quickly degrades, evaluate the quality of the column and carrier gas.

By following these FID maintenance guidelines you will ensure that your test results remain accurate and reproducible.

Edward J. Zachowski

Edward J. Zachowski is President and founder of Alpha Omega Technologies, Inc. (AOT) in Brielle , NJ. He has a Masters Degree in Analytical Chemistry from the State University of New York at Buffalo, NJ and a Bachelor of Science Degree in Chemistry from Richard Stockton State College Pomona, NJ. Prior to starting AOT, Ed was employed by Waters Millipore of Milford MA & Warner Lambert/Parke Davis R&D Morris Plains, NJ.

Stan Paleologos

Stan Paleologos is Laboratory Director at Alpha Omega Technologies, Inc. (AOT) in Brielle, NJ. He holds a Bachelor of Science Degree in Chemistry from Rutgers University, New Brunswick, NJ. Prior to joining AOT, Stan served as Laboratory Manager for various firms including Environmental Management Services Div of Camin Cargo in Linden, NJ, Blue Marsh Laboratory and Chyun Associates, both in Princeton, NJ.