Physics:Post column oxidation-reduction reactor

A post column oxidation-reduction reactor is a chemical reactor that performs derivatization to improve the measurement of organic molecules. It is used in gas chromatography (GC), after the column and before a flame ionization detector (FID), to make the detector response uniform for all organic molecules.

The reactor converts the carbon atoms of organic molecules in GC column effluents into methane before reaching the FID. The resulting detector response is uniform on a per-carbon basis and avoids the need for response factors and calibration standards for each molecule. It can improve the response of the FID to a number of molecules with poor/low response including, carbon monoxide (CO), carbon dioxide (CO₂), hydrogen cyanide (HCN), formamide (CH₃NO), formaldehyde (CH₂O) and formic acid (CH₂O₂), because these molecules are converted to methane.

History

The concept of using a post-column catalytic reactor to enhance the response of the FID was described by Kenneth Porter & D.H. Volman, for the reduction of carbon dioxide and carbon monoxide to methane using a nickel catalyst. This process was later refined by Johns & Thompson, and is now commonplace in many laboratories, colloquially referred to as a methanizer. This device is limited to the conversion of carbon dioxide and carbon monoxide to methane, and the nickel catalysts are poisoned by species such as sulfur and ethylene.

The use of two reactors in series for the subsequent combustion and then reduction of organic molecules is described by Takuro Watanabe's group and Paul Dauenhauer's group using separate reactors for oxidation and reduction. The authors demonstrate the effectiveness for this technique in qualifying traceable standards and the analysis of mixtures without calibrations.

The Polyarc reactor combines the combustion and reduction zones into a single microreactor using proprietary catalyst blends that efficiently convert organic molecules to methane.

Operating Principle

Chemical Reactions

The reactor operates by converting organic analytes after GC separation into methane before detection by FID. The oxidation and reduction reactions occur sequentially, wherein the organic compound is first combusted into molecules of carbon dioxide, which are subsequently reduced to methane molecules. The following reactions demonstrate the combustion/reduction process for formic acid.

HCO₂H + 0.5O₂ ↔ CO₂ + H₂O

CO₂ + 4H₂ ↔ CH₄ + 2H₂O

The reactions are faster compared to the time scales of typical chromatography, resulting in manageable peak broadening and tailing. Elements other than carbon are not ionized in the hydrogen and oxygen flames of the FID and thus do not contribute to the FID signal.

Effect on the FID

Only the CHO+ ions formed from the ionization of carbon compounds are detected. Thus, the non-methane byproducts of the reactions are not detected by the FID.

Since every compound goes through the catalyst bed in the reactor, it has the ability to alter certain substances that might be harmful or negatively affect the efficiency and durability of the FID into safer forms. For instance, cyanide is catalytically changed into methane, water, and nitrogen.

Advantages and Disadvantages

Advantages

• Uniform sensitivity to most organic molecules
• Increased accuracy of quantification by the elimination of errors resulting from multiple calibrations and standards
• Decreased cost of ownership and faster analysis times due to reduction in calibrations
• Quantification of mixtures where standards may not be available, provided retention times are known or can be guessed

Disadvantages

• Cost of reactor and replacements (each replacement unit costs ~$6000 USD)
• Addition of dead volume causes an increase in peak broadening depending on the GC column flow rates and molecule types (5-10% broadening is typical)
  • Heteroatoms and oxidation of Polyarc transfer lines increase this broadening
• Susceptible to sulfur, silicon, and halogen poisoning
• Requires constant feed of hydrogen
• Cannot be regenerated in-house; must be shipped back to manufacturer for a replacement
• Cannot be used with cryogenic oven temperatures, or for GCxGC
• Poor response for species containing C-F bonds
• May contribute to power overload on older GC models, preventing the use of other heated components such as valve boxes or auxiliary detectors

Benefits over Methanizers

• Converts all organic compounds to methane rather than just CO and CO₂ leading to uniform response and more sensitive detection for more organic species
• Resilient to poisoning by compounds containing nitrogen and oxygen
• Sharper peaks when compared with packed column versions of methanizer.

Operation and Data Analysis

The Polyarc reactor needs hydrogen and air, which are both gases used in any existing FID setup. Software for capturing and analyzing FID signals remains applicable, and no extra software is necessary for the device. Gas flows to the device are controlled using an external control box that must be calibrated manually for the desired flows of air and hydrogen. The detector's overall response can be analyzed either by an external or an internal standard method.

In the external standard method, the FID signal is correlated to the concentration of carbon separately from the analysis. In practice, this entails the injection of any carbon species at varying amounts to create a plot of signal (i.e., peak area) versus injected carbon amount (e.g., moles of carbon). The user should take care to account for any sample splitting, adsorption, inlet discrimination, and leaks. The data should form a line with a slope, m, and an intercept, b. The inverse of this line can be used to determine the amount of carbon in any subsequent injection from any compound.

[math]\displaystyle{ mol C = \frac {peak area - b}{m} }[/math]

This is different from a typical FID calibration where this procedure would need to be completed for each compound to account for the relative response differences. The calibration should be examined periodically to account for catalyst deactivation and other sources of detector drift.

In the internal standard method, the sample is doped with a known amount of some organic molecule and the amount of all other species can be derived from their relative response to the internal standard (IS). The IS can be any organic molecule and should be chosen for ease of use and compatibility with the compounds in the mixture. For example, one could add 0.01 g of methanol as the IS to 0.9 g of gasoline. The 1 wt% mixture of methanol/gasoline is then injected and the concentration of all other species can be determined from their relative response to methanol on a carbon basis,

[math]\displaystyle{ mol C/g = \frac {area}{area(IS)} \times mol C(IS)/g }[/math]

The effects of injection-to-injection variability resulting from different injection volumes, varying split ratios and leaks are eliminated with the internal standard method. However, inlet discrimination caused by adsorption, reaction, or preferential vaporization in the inlet can lead to accuracy issues when the internal standard is influenced differently than the analyte.

Any non-carbon species that would not be detected in a traditional FID setup (e.g. water, nitrogen, ammonia) will not be detected with Polyarc/FID. This detector can be paired with other detectors that give complementary information such as the mass spectrometer or thermal conductivity detector.