Post-column oxidation–reduction reactor

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

The reactor contains catalysts that converts all of the carbon atoms of organic molecules in GC column effluents into methane before reaching the FID. As a result, all carbon atoms are detected equally, and therefore calibration standards for each compound are not needed. It can improve the response of the FID to many compounds with poor or low response, including carbon monoxide (CO), carbon dioxide (CO₂), hydrogen cyanide (HCN), formamide (CH₃NO), formaldehyde (CH₂O), and formic acid (CH₂O₂).

History

The concept of using a post-column catalytic reactor to enhance the response of the FID was first developed for the reduction of carbon dioxide and carbon monoxide to methane using a nickel catalyst. The reaction device, often referred to as a methanizer, is limited to the conversion of carbon dioxide and carbon monoxide to methane, and the catalysts are poisoned by sulfur and ethylene among others.

Using a combustion reactor prior to the reduction reactor allows other carbon-containing chemicals to benefit from enhancement in FID detection. In the combustion step, all carbon is converted to carbon dioxide, allowing it to be converted to methane for FID enhancement regardless of its original chemical form.

Operating principle

Chemical reactions

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

${\ce {HCO2H + 1/2O2 <=> CO2 + H2O}}$

${\ce {CO2 + 4H2 <=> CH4 + 2H2O}}$

The reactions are fast compared to the time scales typical of gas chromatography, resulting in manageable peak broadening and tailing. Elements other than carbon, as CH₄, are not ionized in the flame 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 passes through the catalyst bed in the reactor, certain substances that might be harmful, or that could negatively affect the efficiency and durability of the FID, are converted into safer forms. For instance, cyanide is catalytically changed into methane, water, and nitrogen.

Advantages and disadvantages

Advantages

• The reactor ensures uniform sensitivity to most organic molecules, leading to consistent and reliable detection across a wide range of analytes.
• By eliminating the need for multiple calibrations and standards, the reactor increases the accuracy of quantification, thereby reducing errors and enhancing the reliability of analytical results.
• Reduction in calibration requirements decreases the cost of ownership and saves time, making the analytical process more efficient.
• The reactor enables the quantification of complex mixtures even when standards are not available (provided retention times are known or can be estimated), thereby expanding the applicability of gas chromatography.
• Unlike traditional methanizers, which primarily convert CO and CO2, oxidation-reduction reactors can convert a broader range of organic compounds to methane, leading to a more comprehensive response and improved sensitivity for a wide variety of analytes.
• These reactors are more resistant to poisoning by compounds containing nitrogen and oxygen, which ensures consistent performance even in the presence of interfering substances.
• Compared to packed column versions of methanizers, oxidation-reduction reactors typically produce sharper peaks, which enhances resolution and improves the quality of chromatographic separation.

Disadvantages

• Cost of reactor and replacements. (Each replacement unit costs ~ $6000)
• The 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 the 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

Comprehensive Conversion: The reactor converts all organic compounds to methane, whereas traditional methanizers typically only convert CO and CO₂. This comprehensive conversion results in a more uniform response and more sensitive detection for a wider range of organic species.

Resilience to Poisoning: The reactor is more resilient to poisoning by compounds containing nitrogen and oxygen compared to traditional methanizers. This means that it can maintain its performance and efficiency even in the presence of potentially interfering compounds.

Sharper Peaks: When compared with packed column versions of methanizers, the reactor typically produces sharper peaks. Sharper peaks enhance resolution and can improve the accuracy and reliability of chromatographic analysis.

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.

$molC={\frac {peakarea-b}{m}}$

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,

$molC/g={\frac {area}{area(IS)}}\times molC(IS)/g$

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.