Methanizer is an appliance used in gas chromatography, which allows to detect very low concentrations of carbon monoxide and carbon dioxide. It consists of a flame ionization detector, preceded by a hydrogenating reactor, which converts CO2 and CO into methane CH4.
Contents
Chemical Reaction
On-line catalytic reduction of carbon monoxide to methane for detection by FID was described by Porter & Volman, who suggested that both carbon dioxide and carbon monoxide could also be converted to methane with the same nickel catalyst. This was confirmed by Johns & Thompson, who determined optimum operating parameters for each of the gases.
CO2 + 2H2 ↔ CH4 + O2
2CO + 4H2 ↔ 2CH4 + O2
Typical Design
The catalyst usually consists of a 2% coating of Ni in the form of nickel nitrate deposited on a chromatographic packing material (e.g. Chromosorb G). A 1½" long bed is packed around the bend of an 8"×1/8" SS U-tube. The tube is clamped in a block so that the ends protrude down into the column oven for easy connection between column or TCD outlet and FID base. Heat is provided by a pair of cartridge heaters and controlled by a temperature controller.
Hydrogen for the reduction can be provided either by adding it via a tee at the inlet to the catalyst (preferred), or by using hydrogen as carrier gas.
An alternative design has been suggested recently, which catalytically combusts all organic species to CO2 before reduction to methane. This has several benefits including the detection of many more organic molecules and resistance to poisoning. A commercial version of the device called the Polyarc reactor is available from Activated Research Company.
Start-up
Since the raw catalyst is supplied in the form of nickel oxide, it is necessary to reduce it to metallic nickel before it will operate properly.
The following procedure is recommended:
Inject a sample containing known amounts of CH4, CO and CO2 to check conversion efficiency and peak shape. The retention times of these compounds should be known. If not, and light hydrocarbons are present in the sample, there might be some confusion in identification. The user should be aware, also, that the FID does respond slightly to O2 so at high sensitivities an air peak might also be evident. As a very rough indication, 1% O2 gives a signal similar to that of 1 ppm CO or CO2.
If there is any doubt about retention times, the following pointers might be useful:
If necessary, adjust catalyst temperature to optimize conversion efficiency and peak symmetry. Also adjust the H2 flow to optimize sensitivity. The H2 flow through the catalyst and the ratio of H2 to catalyst and H2 to FID are not critical.
Temperature
Conversion of both CO and CO2 to CH4 starts at a catalyst temperature below 300 °C, but the conversion is incomplete and peak tailing is evident. At around 340 °C, conversion is complete, as indicated by area measurements, but some tailing limits the peak height. At 360-380 °C, tailing is eliminated and there is little change in peak height up to 400 °C.
Although carbonization of CO has been reported at temperatures above 350°, it is rather a rare phenomenon.
Range
The conversion efficiency is essentially 100% from minimum detectable levels up to a flow of CO or CO2 at the detector of about 6995500000000000000♠5×10−5 g/s. These represent a detection limit of about 200 ppb and a maximum concentration of about 10% in a 0.5 mL sample. Both values are dependent upon peak width.
Catalyst Poisoning
Some elements and compounds can deactivate the catalyst:
Troubleshooting
In general, the catalyst works perfectly unless it is degraded by sample components, possible minute amounts of sulfur gases at otherwise undetectable levels. The effect is always the same — the CO and CO2 peaks start to tail. If only CO tails, it might well be a column effect, e.g., a Mol. Sieve 13X always causes slight tailing of CO. If the tailing is minimal, raising the catalyst temperature might provide enough improvement to permit further use.
With a newly packed catalyst, tailing usually indicates that part of the catalyst bed is not hot enough. This can happen if the bed extends too far up the arms of the U-tube. Possibly a longer bed will improve the upper conversion limit, but if this is the aim, the packing must not extend beyond the confines of the heater block.
Catalyst Preparation
Dissolve 1 g of nickel nitrate Ni(NO3)2•6H2O in 4-5 mL of methanol. Add 10 g of Chromosorb G. A/W, 80-100 mesh. There should be just enough methanol to completely wet the support without excess. Mix the slurry, pour into a flat Pyrex pan and dry on a hot plate at about 80-90 °C with occasional gentle shaking or mixing. When dry, heat in air at about 400 °C to decompose the salt to NiO. Note that NO2 is emitted during baking — provide adequate ventilation. About an hour at 400 °C, longer at lower temperatures, will be needed to complete the process. After baking, the material is dark gray, with no trace of the original green.
Pour the raw catalyst into both arms of an 8"×1/8" nickel U-tube, checking the depth in both with a wire. The final bed should extend 3/8" to 1/2" above the bottom of the U in both arms. Plug with glass wool and install in the injector block.
Disadvantages
Methanizers are limited by their ability to react only CO and CO2 to methane and their deactivation by compounds commonly found in chemical samples. These include olefins and sulfur containing compounds. Thus, the use of methanizers typically require complex valve systems that may include backflush and heartcuts. These systems can work well, but can add cost and complexity, and the potential for leaks and adsorption in the chromatographic flow path.
Alternative Solution
An alternative methanizer variant that overcomes these limitations and allows for the direct injection of all compounds without backflush or heartcuts is a two-step oxidation and subsequent reduction reactor to convert nearly all organic compounds to methane. This technique enables the accurate quantification of any number of compounds that contain carbon beyond just CO and CO2, including those with low sensitivity in the FID such as carbon disulfide (CS2), carbonyl sulfide (COS), hydrogen cyanide (HCN), formamide (CH3NO), formaldehyde (CH2O) and formic acid (CH2O2). In addition to increasing the sensitivity of the FID to particular compounds, the response factors of all species become equivalent to that of methane, thereby minimizing or eliminating the need for calibration curves and the standards they rely on. The reactor is available exclusively from the Activated Research Company and is known as the Polyarc reactor.