Accurate carbon monoxide (CO) and carbon dioxide (CO2) analysis is critical in many industries, ranging from food and beverage applications to a multitude of petrochemical methods. In most cases these gases are in the percent range and can easily be analyzed using techniques such as GC-TCD. However, if sample concentrations are below 100 ppm, they may be below the detection limits of a TCD. In this case, less common, more expensive detectors—such as BID, HID, or PDD—can be used instead. But, using a methanizer with an FID is a better alternative because FIDs are ubiquitous in labs around the world, and this combination allows for the analysis of CO and CO2 at ppb levels.
Methanizers are a simple, well-established way to catalytically convert CO and CO2 to methane (CH4), which can then be detected at very low levels using an FID. Here’s how it works: First, a gas sample is introduced into the GC and separated on the analytical column. Then, when the sample elutes from the column, it is passed over a hot nickel-based catalyst in the presence of hydrogen, where the main reactions are the reduction of CO and CO2 to CH4. Conversion is most efficient at 380 °C, so the catalyst tube is located within a heated, insulated chamber that maintains a constant temperature.
Using a methanizer for CO and CO2 analysis is quite simple, and they require very little effort to maintain. However, as with any piece of equipment, there are some limitations: for methanizers, care must be taken to prevent poisoning the catalyst. Analysts should be aware that the following elements and compounds can deactivate the catalyst and take appropriate precautions:
- Hydrogen Sulfide (H2S), Sulfur Hexafluoride (SF6), and Other Sulfur-Containing Gases
Very small amounts of H2S, SF6—and probably other sulfur-containing gases—cause immediate, complete deactivation of the catalyst. It is not possible to regenerate a poisoned catalyst that has been deactivated by sulfur using either oxygen or hydrogen treatment. If sulfur-containing gases are present in the sample, a switching valve should be used to bypass the catalyst or to backflush the column to vent after the elution of CO2.
- Air and Oxygen (O2)
Instances of oxygen poisoning a catalyst are reported from time to time, but in my experience, small amounts of air will not kill a catalyst; however, exposure to 5 cc/min or more will cause immediate, continual catalyst degradation. I have observed this firsthand on multiple systems during my 30 years of experience with a catalytic FID designed for analyzing U.S. EPA Method 25 and 25C samples.
- Unsaturated Hydrocarbons
Samples of pure ethylene will cause immediate, but partial, degradation of the catalyst, as evidenced by slight tailing of CO and CO2 peaks. The effect of a few samples might be tolerable, but since the effect is cumulative, such gases should be backflushed or bypassed. Samples of pure acetylene affect the catalyst even more severely than ethylene does. It is likely that with high concentrations of unsaturates some carbonization occurs, resulting in soot being deposited on the catalyst surface. (Aromatics would probably have the same effect.) Note that low concentrations of ethylene and acetylene have no effect.
- Other Compounds
Water has no effect on the methanizer catalyst, as is also the case with various Freon compounds and ammonia (NH3). Here again, with NH3, there is conflicting evidence: some users have observed degradation after several injections, but other researchers could not confirm it. As with sulfur-containing gases, NH3 can be backflushed to vent or bypassed if desired.
For laboratories interested in using a methanizer for CO and CO2 analysis, several options are available. Some GC systems are equipped with built-in methanizers; however, if you only need to use a methanizer occasionally or if you want to retrofit one into an existing GC, then a separate aftermarket methanizer may be a better solution. Restek has developed modern, user-friendly methanizers for most Agilent GC-FIDs and Thermo Trace 1300/1310 GC-FIDs that can be coupled with any analytical column (capillary, micropacked, or packed), making them suitable for a wide range of applications. Visit www.restek.com/CH4izer for more details.