The Polyarc reactor^[1] is a scientific tool for the measurement of organic molecules. It is paired with a flame ionization detector (FID) in a gas chromatograph (GC) to improve the sensitivity of the FID and give a uniform detector response for all organic molecules (GC-Polyarc/FID).

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 allows the FID to have a good carbon sensitivity. GC-Polyarc/FID peak areas (integrated detector responses) are equivalent on a per carbon basis, thus eliminating the need for response factors and calibration standards. The GC-Polyarc/FID method also improves the response of the FID to a number of molecules with poor/low response including, carbon monoxide (CO), carbon dioxide (CO₂), carbon disulfide (CS₂), carbonyl sulfide (COS), 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,^[2] for the reduction of carbon dioxide and carbon monoxide to methane using a nickel catalyst. This process was later refined by Johns & Thompson,^[3] 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^[4][5] and Paul Dauenhauer's group^[6] 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 and resist poisoning by sulfur and other heteroatoms. ^[7]

Operating Principle

Advantages and Disadvantages

Operation and Data Analysis

The Polyarc reactor needs hydrogen and air, which can be obtained from the gases used in the FID. The existing software for capturing and analyzing FID signals remains applicable, and no extra software or control is necessary for the device. The detector's overall response can be analyzed either by an external or an internal standard method. Typically, the internal standard method is favored as it removes variability in the GC from one injection to another, although both methods are considered suitable.

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, or the calibration will be off. 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 because the detector response is uniform for all organic compounds.

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

This is different from a typical FID calibration where this calibration would need to be completed for every single different compound to account for the relative response differences. The calibration should be examined periodically to certify nothing has changed in the GC over time.

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,

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

The effects of injection-to-injection variability resulting from different injection volumes, varying split ratios and leaks are eliminated with the internal standard method leading to high analysis precision. 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.

The Polyarc/FID can be paired with other detectors that give complementary information such as the mass spectrometer or thermal conductivity detector. A multiple detector setup is possible using either a splitter (tee) to split the flow to two or more detectors at a time, or a switch/valve to allow for the selection of one detector at a time. In a splitter only a fraction of the flow goes to the detector, resulting in lower detection sensitivity. In addition, the split ratio can change as a function of temperature resulting in sample discrimination and inaccuracies in the analysis. A switch introduces a small amount of dead volume to the flow path and requires multiple injections if the analysis is to be repeated on multiple detectors.

Applications

The Polyarc system has been used for analyses in the following industries:

• Chemicals:^[9] The Polyarc system can be used to determine the purity of monomer feedstocks, determine total organic carbon content, study reaction byproducts, and more.

• Paints and Coatings:^[10] The Polyarc system can be used to save time by reducing calibrations for the GC/FID analysis of VOCs. A single internal (or external) standard can be used to quantify all other components in the mixture, without the need to first calibrate.

• Food, Flavor, and Fragrance:^[11] The Polyarc system coupled with a flame ionization detector (FID) can be used to accurately quantify complex mixtures using a single injection without using calibration standards. Furthermore, configuring the GC with a Polyarc/FID and MS split allows for accurate quantification and identification with a single injection^[12]

• Pharmaceuticals:^[13] The Polyarc system can be used to save time for the analysis of pharmaceutical products because it does not require calibration curves, allowing for quantitative information of new materials to be obtained quickly.

• Oil, Gas, and Biofuels:^[14] The Polyarc system can dramatically simplify the calibration process by reducing (or eliminating) the amount of calibration required. It will also increase the sensitivity of certain compounds in the FID, including oxygenates and other low sensitivity compounds (e.g., formic acid, formaldehyde, carbon dioxide, etc.).

• As a Methanizer:^[15] The Polyarc system can be used as a methanizer in applications such as TOGA/DGA as a replacement with advantages including capillary optimization, resistance to common catalyst poisons, full linear dynamic range, and minimized complicated valving.

• Other Industries: Life Sciences, Agrochemical, Environmental, Forensic, Academia, Defense, Consumer Goods & Safety, Wine, Beer, & Spirits, Halogens, and Nutraceuticals

References